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FundamentalsInstrumentation and Techniques of Sum Parameter Analysis

Fundamentals, instrumentation and techniques of Sum Parameter Analysis 2/ 42

Analytik Jena AG | Konrad-Zuse-Str. 1 | 07745 Jena / Germany | [email protected] | www.analytik-jena.com



Inhalt 1 Introduction...................................................................................................................5

1.1 Definition of sum parameters ................................................................................5

1.2 The aim of sum parameter analysis ......................................................................5

1.3 Advantages and limitations of sum parameter analysis.........................................6

2 Characteristics of the important sum parameters in brief .............................................8

2.1 Total Organic Carbon - TOC .................................................................................8

2.2 Adsorbable Organic Halogen Compounds - AOX .................................................8

2.3 Total Bound Nitrogen – TNb ..................................................................................8

2.4 Chemical Oxygen Demand – COD........................................................................8

2.5 Biological Oxygen Demand - BOD ........................................................................9

2.6 Chemical Oxygen Consumption/Oxidizable Substances – Permanganate Index .9

2.7 Electrical conductivity ............................................................................................9

2.8 Loss on ignition .....................................................................................................9

3 TOC and derived parameters .....................................................................................10

3.1 Definition and standardization .............................................................................10

3.2 The aim of the TOC determination ......................................................................13

3.3 Measurement principle ........................................................................................14

3.3.1 Thermal oxidation.........................................................................................14

3.3.2 Wet-chemical oxidation ................................................................................16

3.3.3 Detection of carbon dioxide..........................................................................17

3.3.4 Methods of determination.............................................................................19

3.4 Determination of POC .........................................................................................21

3.5 TOC and COD.....................................................................................................22

3.6 TOC in solids.......................................................................................................23

4 TNb – Total Bound Nitrogen .......................................................................................27


4.2 The aim of TNb determination..............................................................................27


4.4 TNb and the Kjeldahl method of nitrogen determination - TKN............................29

4.5 Typical measurement results for TNb determination in water samples of

various origins .....................................................................................................29



5 AOX/TOX and derived parameters.............................................................................30


5.2 The aim of AOX determination ............................................................................32


5.3.1 Sample preparation (enrichment, extraction) ...............................................32

5.3.2 Analysis........................................................................................................36

6 COD – Chemical Oxygen Demand.............................................................................40


6.2 The aim of the COD determination......................................................................40


7 BOD - Biological Oxygen Demand .............................................................................41


7.2 The aim of the BOD determination ......................................................................41


7.3.1 Electrochemical oxygen determination.........................................................41

7.3.2 Respirometric/ manometric method .............................................................41

7.3.3 Method at constant pressure........................................................................42



1 Introduction 1.1 Definition of sum parameters Sum parameters (otherwise known as composite parameters) describe similar chemical, physicochemical or biological characteristics of different substances, substance groups or mixtures of substances.

The products, substance groups and constituents, for example, might contain the same chemical element. The quantitative determination of this element allows the purity, quality, consistency etc. to be inferred.

TOC (Total Organic Carbon) or TNb (Total Bound Nitrogen) can be cited as examples here.

A shared chemical characteristic might also be the oxidizability of constituents with a defined oxidizing agent, e.g. by dichromate, as in with the case of the sum parameter COD – Chemical Oxygen Demand.

The electrical conductivity of aqueous samples as a shared physical-chemical property can also be seen as a sum parameter.

The toxicity towards defined organisms, e.g. fish toxicity, can be mentioned as a shared biological characteristic.

1.2 The aim of sum parameter analysis The demand for reliable, inexpensive analytical data on the state and quality of media and products, as well as for process control, has increased steadily over recent years. This applies to environmental monitoring (surface water, groundwater, soils), water processing (drinking water, wastewater), as well as to quality control in the chemical and pharmaceutical industry. Sum parameter analysis has become indispensable in these areas today.

The organic load of a water sample can be evaluated within a few minutes of sampling by using TOC determination, without requiring individual substance analyses that might not provide the required information. Based on the TOC determination result, individual substance analyses can then follow, for instance, to establish the causes for the high load in the water sample.

Sum parameter determinations are also particularly suitable for product quality control. For example, the sulfur content of diesel fuels can be reliably and quickly determined with the Total Sulfur (TS) parameter.

There is now a wealth of sum parameters and their associated determination methods available, most of which have been standardized.

The importance of the individual sum parameters and their application varies widely, however, and depends on the particular international, national and industry specifications.



Determination of the AOX parameter (Adsorbable Organic Halogen Compounds; X = Cl, Br, I) is controversial and is not mandatory or legally regulated in many countries, including those in the EU.

1.3 Advantages and limitations of sum parameter analysis The limitations of individual substance analysis become especially clear in very complex matrices, such as wastewater. Aside from the considerable costs, it is not always possible to detect the multitude of potential organic constituents of water, even with the most sophisticated chromatographic methods. The quantity of data obtained is also very large, and is not always sufficiently interpretable for evaluation. The advantages of sum parameters in comparison with individual substance analysis are presented as follows. Advantages of sum parameter analysis Fast screening methods with a cross-sectional character Detection of representative sample cross sections Inexpensive, with simple handling Analysis close to the process in real-time Important preliminary stage to applying specific analyses High degree of measurement comparability, since sum parameters are convention

parameters Rapid status evaluation, especially in the evaluation of hazard potential and as a

good early warning system in the environmental area

It should be emphasized here that both sum parameter analysis and specific individual analysis have their own roles to play and special, preferred fields of application. They should not be considered as mutual alternatives.

The limitations of sum parameter analysis result from the fact of their being convention parameters. This means that they are solely defined through the determination method. Two examples illustrate this.

TOC determination in solids has taken on major importance in the area of landfill disposal, where the relevant limit values are stipulated in the landfill ordinances. According to currently applicable methods of determining TOC in solids, elemental carbon is also included in the TOC value. Elemental (or inert) carbon has no ecological relevance since it is not degradable and therefore cannot contribute to acidification and mobilization of heavy metals or the formation of biogas. Thus, it can lead to a very high apparent TOC content, as presented in Figure 1 with the example of ash from a rotary tube furnace.



Figure 1: TOC and residual carbon (RC) determination in ash from a rotary tube furnace; the residual carbon essentially represents the elemental carbon fraction

An apparent TOC content of 9.9% was determined with conventional methods. However, the ecologically-relevant TOC content, that is the degradable organic carbon, amounts to just 0.6%. The largest fraction is residual carbon (RC), which has no ecological relevance.

The usual TOC determination would therefore lead to an incorrect evaluation of the ash from a rotary tube furnace.

Another example of the necessity for a critical evaluation of the results from sum parameter analysis is shown by the AOX determination of water samples containing PVC particles. These particles are detected by AOX determination as AOX-relevant, although they do not represent constituents in the sense of the AOX definition.

Falsely high AOX results are obtained if the AOX measurement is undertaken in samples containing chloride (c > 1g/L Cl-). The standard was therefore modified so that perceived high AOX load in strongly saline samples could be excluded.



2 Characteristics of the important sum parameters in brief 2.1 Total Organic Carbon - TOC TOC is one of the most important sum parameters used in environmental monitoring, in water treatment and increasingly in quality control and cleaning validation. The measurement principle involves the thermal, thermocatalytic or wet-chemical oxidation of the carbon compounds contained in the sample to carbon dioxide and its subsequent quantitative measurement by means of NDIR. Other detection methods are also used that are not CO2-selective, but these are of lesser importance.

2.2 Adsorbable Organic Halogen Compounds - AOX AOX (Adsorbable Organic Halogen Compounds X = Cl, Br, I) is a sum parameter for describing the organic halogen compound load in water, sewage sludge and soils. The main field of application is wastewater analysis. The relevant samples are treated with a defined quantity of activated carbon to adsorb the halogen compounds. The activated carbon is burned at 950 °C in a stream of oxygen, and the hydrogen halide formed is determined by microcoulometry (argentometric). The result is specified in mass units of chloride per unit volume or mass units of the sample.

Pure fluoroorganic compounds are not detected with the argentometric determination method.

2.3 Total Bound Nitrogen – TNb The TNb (Total Bound Nitrogen), an analytical parameter used only for water, has gained in importance over the last 15 years. It allows a rapid assessment of the total nitrogen compound load in a water sample; it can be determined simultaneously with TOC, and therefore offers an advantage over wet-chemical and photometric methods.

The nitrogen-containing substances in a water sample are thermocatalytically oxidized to NO at a temperature above 700 °C and the NO is determined with the aid of chemiluminescence, electrochemical or IR detectors. Although other methods of detection are permissible, chemiluminescence and electrochemical detectors have become well established. The nitrogen dissolved in the water is not detected. The TNb parameter has also been adopted in cleaning validation in recent times.

2.4 Chemical Oxygen Demand – COD COD (Chemical Oxygen Demand) is a measure of all constituents contained in water that are oxidizable under defined conditions. It is specified as the amount of oxygen in mg/L that would be required for oxidation.

The water sample is acidified with sulfuric acid, and then is heated with a defined amount of potassium dichromate, after which the quantity of dichromate consumed for oxidation is



determined titrimetrically or photometrically. The equivalent quantity of oxygen is calculated from this result.

2.5 Biological Oxygen Demand - BOD BOD (Biological Oxygen Demand) is a measure of the quantity of oxygen consumed by microorganisms in an aqueous sample within a defined period of time. It is therefore a parameter for the load of biologically degradable substances in the water. BOD is a control parameter for biological purification plants and is of significance for ecological analyses.

The measurement principle consists of determining the oxygen concentration at the beginning and end of a measurement period. The measurement period is usually 5 days and is specified as an index (BOD5). Shorter measurement times are sometimes also used. The BOD result is specified in mg oxygen per liter of sample. Chemical, electrochemical or physical (manometric) methods are used for the oxygen determination.

2.6 Chemical Oxygen Consumption / Oxidizable Substances – Permanganate Index

The COC parameter (Chemical Oxygen Consumption) has been almost entirely superseded by COD, and is now only of importance for determining oxidizable substances in pure water samples. The method is described in pharmacopoeias as an alternative to the TOC determination.

The measurement principle is the same as that of COD, however, potassium manganate is used in place of potassium dichromate.

2.7 Electrical conductivity As a sum parameter, electrical conductivity characterizes the salinity of aqueous samples, because it is a measure of the ion concentration in the water sample. This parameter is used in water purification plants for purity testing and in ecological monitoring.

2.8 Loss on ignition Loss on ignition is a parameter designed to describe the content of organic substances in solid samples, such as waste, soils, sediments, etc. Here the sample is heated to 550 °C in a muffle furnace and the percentage weight loss is reported. The loss on ignition is a problematic parameter, since the mass difference before and after the thermal treatment is not only attributable to the loss of organic components. For example, the discharge of water of crystallization or the decomposition of carbonates can also contribute to loss on ignition, as is known from the literature. The determination of loss on ignition is increasingly being replaced by the TOC determination.



3 TOC and derived parameters 3.1 Definition and standardization TOC (Total Organic Carbon) is a measure of the organic carbon compounds contained in a water sample. The method has been standardized worldwide through a large number of standards. Besides the ASTM and EPA standards, the international standard ISO 8245 and the European standard EN 1484 are the conventions most frequently considered. The scope of EN 1484 includes the determination of TOC in drinking water, ground water, surface water, lake water and wastewater. The method covers the measurement range 0.3 mg/L – 1000 mg/L. More highly concentrated samples should be diluted.

A series of abbreviations is commonly used in the determination of sum parameters for carbon compounds; these have become established in international linguistic usage, and their meaning is defined by the conventions of the standard methods.

TOC (Total Organic Carbon)

Total organically bound carbon, whereby determination in water samples includes both as the dissolved and undissolved organic constituents in the TOC.

Typical substances detected in the conventional determination of TOC are: sugar, mercaptans, oils, cellulose, polyethylene, as well as inorganic compounds such as cyanides, carbides and graphite.

DOC (Dissolved Organic Carbon)

Organically bound carbon from all organic compounds dissolved in water. This parameter has no relevance in the determination of TOC in solids.

For water samples in which there are no undissolved or suspended substances (e.g. drinking water), the DOC and TOC values are identical.

If DOC is to be determined in a sample containing undissolved or suspended substances (= particles), sample pretreatment is required: filtration of the water sample through a membrane filter of 0.45 µm pore size.

Every TOC analyzer can determine the DOC of a sample once filtration of the sample has been performed outside of the instrument.

Typical substances detected in the determination of DOC are: sugar and mercaptans, but also inorganic compounds such as cyanides.



TC (Total Carbon)

The total organically and inorganically bound carbon. Determination in water samples includes both the dissolved and undissolved carbon compounds.

Generally: TC = TOC + TIC

Typical substances detected in the determination of TC are: sugar, mercaptans, oils, cellulose, polyethylene, carbonates, cyanides, carbides and graphite

TIC (Total Inorganic Carbon)

Total inorganically bound carbon, which can be converted to CO2 when treating with a mineral acid (= convention).

Typical substances detected in the determination of TIC are: carbonates, but not cyanides, carbides or graphite

NPOC (Non-purgeable Organic Carbon)

Total non-purgeable organic carbon; when determined in water samples, NPOC includes both the dissolved and undissolved organic constituents.

In water samples that contain no purgeable (generally volatile) organic compounds, the NPOC and TOC values are identical.

Typical substances detected in the determination of NPOC are: sugar, cellulose, polyethylene (not mercaptans or volatile oils), but also inorganic compounds, such as cyanides, carbides and graphite

POC (Purgeable Organic Carbon)

Organically bound carbon in volatile organic substances that are purgeable under certain circumstances.

Generally: TOC = NPOC + POC

Typical substances detected in the determination of POC are: mercaptans or volatile oils (hydrocarbons)

In the past, the term VOC (Volatile Organic Compounds) was often erroneously equated with POC. The term VOC is of significance in air analysis, but not for water analysis. TOC analyzers are not suitable for determining VOC.



TC

=

TOC + TIC

=

NPOC + POC

Through the use of example substances, the following Table illustrates which substances are detected in a water sample in the determination of the respective parameters:

Detection by combustion > 800 °C as: Detection following acidification:

Substance class

Example substance TC TOC DOC NPOC POC TIC

Carbonates

Cyanides

Carbides ( ) ( ) ( ) ( )

Inorganic carbon compounds

Graphite/diamond

Sugar

Mercaptans

Oils (volatile)

Cellulose

Organic carbon compounds

Polyethylene

For improved clarity, only the detection of the individual substances for the TOC determination under high temperature conditions is shown in the table. If wet-chemical digestion (UV light and added oxidation agent) is used in the detection of TOC, some restrictions apply in regard to the digestion capability of particles and compounds that are hard to oxidize.



3.2 The aim of the TOC determination The determination of TOC facilitates a rapid overview of the load in a water sample containing organic substances (many of which are pollutants), and is therefore mainly used in environmental monitoring. Country-specific directives regulate the permissible limit values for this parameter in water samples of varying origin. The load limits in surface water, raw water for drinking water production, drinking water and wastewater generally vary markedly.

TOC, along with other parameters, is an important indicator for the efficiency of the plant with respect to wastewater treatment. Online analyzers that measure continuously are often used in this area, which allows timely intervention in cases where limit values are exceeded.

The classical field of application for the TOC parameter in environmental analysis is steadily widening in scope through its use in product control and cleaning validation. The quality of water produced by special cleaning processes is also monitored by determining the TOC contained. This synthetic water is primarily used as a raw material in the production of medical products (e.g. infusion solutions) in the pharmaceutical industry, or as a rinsing medium in the production of microelectronic components in the semiconductor industry. The quality demands placed on water as a product are very high in these areas (e.g. water for pharmaceutical purposes < 500 µg/L TOC), and special TOC analyzers with the appropriate detection limits are required for their fulfillment.

In the pharmaceutical industry it must also be ensured that the reactors/ fermenters are once again available in a “clean” state for their next use after a product run. The TOC parameter also plays an essential role in the cleaning validation process. Various methods are used here: the swab technique (wiping a defined reactor surface with a swab/cloth made of the most inert and TOC-free material possible) with subsequent elution or direct combustion of the swab, or the final rinse method. The results for cleaning validation are derived by a TOC determination performed in the final rinse water or in the eluate obtained by extraction of the swab with water. A TOC analysis by direct combustion of the swab is also possible, but is less practicable on account of the increased sources of error contributed by the blank value.



3.3 Measurement principle All methods and analyzers for TOC determination combine digestion of carbon-containing substances with subsequent selective detection of the carbon dioxide generated by digestion.

In principle, there are two digestion methods applicable for aqueous samples: Methods with wet-chemical oxidation, and methods with thermal oxidation.

Infrared spectroscopy has become established for the detection of CO2 with the majority of laboratory analyzers used, not least as a result of its outstanding suitability (detection sensitivity). 3.3.1 Thermal oxidation

The basic principle of all thermal oxidation methods is the oxidation of the organic constituents of water using oxygen at temperatures of approx. 800 °C to 1000 °C with the use of suitable catalysts. This digestion method is therefore frequently referred to as catalytic high temperature oxidation.

The catalysts used for complete oxidation are e.g. platinum (usually on a substrate material), cerium(IV) oxide and copper oxide, but also those based on cobalt, nickel or palladium. The catalyst is the main component of the combustion tube filling. Generally, the manufacturers of TOC instruments provide specific recommendations for the use of the catalyst best suited for the application, along with its temperature and the quantity used.

When the sample is introduced in the combustion system, the water evaporates instantaneously. As a result of the formation of water vapor and the associated effect on the carrier gas flow, the sample volume that can be used is usually limited to less than 1 mL for most instruments that operate thermally. Many analyzers on the market work with an injection quantity of just a few microliters (20 – 100 µL), so that one can no longer be assured of obtaining a representative sample under certain circumstances, especially in the analysis of wastewater samples that contain particles. However, very high sample volumes (< 5 mL) can also be dosed at very low sample input speeds. TOC analyzers that have an efficient method of compensating flow fluctuations without reducing the system sensitivity, such as with the retention-time coupled integration of the TOC signal (VITA), are preferred for the input of larger sample quantities.

R-C + O2 → CO2 + H2O + other products



There are two different techniques for sample input:

a) Direct injection – whereby the sample is usually dosed into the combustion system manually in a microliter syringe or directly in autosampler mode without additional valves or tubing

such as in the case of the multi N/C 2100/2100S TOC analyzer

b) Flow injection – whereby the sample is introduced indirectly for combustion with the aid of a precise syringe pump via tubing and valves and with the use of an auxiliary medium (ultrapure water).

such as in the case of the multi N/C 3100 TOC analyzer

Figure 2:

Left: Direct injection as in the multi N/C 2100/2100S TOC instrument

Right: Flow injection as in the multi N/C 3100 instrument TOC instrument



With their unrivalled high oxidation capacity, high temperature TOC instruments are widely used for analyzing wastewater, as well as drinking water and ultrapure water. Here the concentration spectrum of the samples analyzed ranges from a few µg/L TOC (above approx. 10 µg/L) up to the single-digit percent range (approx. 3% or 30,000 mg/L) 3.3.2 Wet-chemical oxidation

In this method, the sample is mixed with an oxidizing agent (or oxidizing agent mixture) in a reactor and is oxidized at a slightly elevated temperature (approx. 80 °C) in a stream of carrier gas. Peroxidsulfate in a sulfuric acid solution is used as the preferred oxidizing agent, although other oxidants, such as chromosulfuric acid, are also used.

Another wet-chemical method is the irradiation of the sample with ultraviolet light in a reaction vessel. Commercially available UV lamps with a wavelength of 254 nm are used here; a few systems can irradiate the sample with “harder” short-wavelength UV light (around 180 nm), which ensures more efficient sample digestion. Systems that combine UV irradiation with the addition of an oxidizing agent are widely used in order to achieve the most complete oxidation possible of all water constituents.

Wet-chemical TOC analyzers can process very large sample volumes (up to a few centiliters), as the stream of carrier gas remains unaffected by the dosage (no flash evaporation as with the thermal methods) and they are therefore also particularly well suited for TOC determination in ultrapure water, for example in the semiconductor industry. Sample input in this case can also be direct or via flow injection.

As with the high temperature systems, the scope of application of wet-chemical systems extends over a wide concentration range, from approx. 2 µg/L up to several thousand mg/L of TOC, and therefore facilitates TOC determination in all water samples that don’t contain solids. One limitation of the method is therefore the analysis of water samples that contain particles.



The following overview illustrates the advantages and disadvantages of the two digestion methods. Wet-chemical digestion Thermal digestion

Water samples that contain particles

Less suitable Very well suited

Ultra-trace determination

(< 10 µg/L TOC)

Very well suited Difficult due to the limited sample volume

Water samples that contain higher amounts of chloride

Less suitable due to the preferred oxidation of Cl- to Cl2 there is underdetection in TOC determination

Very well suited; however, increased wear on the combustion system is expected with raised saline loads

Water samples that contain salts (except chlorides)

Very well suited Also very well suited; however, increased wear on the combustion system is expected with raised saline loads

Difficult to oxidize compounds

Less well suited, especially at high concentrations

Very well suited

Cost per analysis Very low Higher maintenance due to aggressive digestion, regular replacement of catalyst / combustion tube required

Suitability for online determination

Very good, as it is low maintenance

More difficult

3.3.3 Detection of carbon dioxide

NDIR detection

The carbon dioxide produced from thermal or wet-chemical oxidation is generally transported into the CO2 measurement system with a carrier gas stream. A method of CO2 measurement with high selectivity and sensitivity that is preferred in practice is infrared spectrometry. Non-dispersive IR detectors (NDIR detectors) can be configured either as multi-channel detectors using different wavelengths or several cuvettes of different lengths, or also as wide-range detectors with just one cuvette. High quality wide-range detectors offer a wide dynamic measuring range and high linearity at the same time. A detector of this type makes TOC determination far easier for the user with water samples of unknown concentration, since a reliable result can be obtained without preselection of the measuring range or other interventions, such as retrospective dilution or post-calculations. Determination of CO2 using an NDIR detector takes place in the gas phase. Reliable signal evaluation therefore requires a constant flow rate through the cuvette of the



carrier gas that contains the analyte. Systems with the relevant compensation technique for flow fluctuations, e.g. retention-time coupled integration (VITA), demonstrate very stable, reliable results.

Figure 3: left; right

NDIR detectors are largely maintenance-free. The use of uncoated gas cuvettes made of a robust material is highly advantageous, since the unavoidable corrosion phenomena that occurs after long periods of use are easy and cheap to eliminate. This is not straightforward with cuvettes coated with noble metals; these also cause unavoidable corrosion effects to the coating, thereby necessitating costly replacement.

NDIR detection of CO2 is a relative measurement method; for this reason, the instruments must be calibrated. For this purpose, carbon-containing substances (e.g. potassium hydrogen phthalate) are first dissolved in a defined quantity in water (with as low a TOC as possible), then these standards are analyzed with the aid of a TOC measuring instrument. The different concentration levels of the solutions are compared relative to the CO2 area signals obtained. The calibration curves obtained in this way, the evaluation of which should also be undertaken over a wide concentration range, serve as the basis for quantifying the TOC content of unknown samples.



Other detection methods

In another purely physical CO2 measuring method, the carbon dioxide is firstly reduced to methane that is then detected with a flame ionization detector (FID).

Alongside the gas phase CO2 detection methods mentioned, there is a series of methods that operate in the liquid medium, such as coulometry, acidimetry, conductometry or the use of CO2-sensitive electrodes. These techniques are only rarely encountered in modern TOC analyzers on account of their high maintenance requirement. 3.3.4 Methods of determination

Since there are usually inorganic carbon components (carbonates) in an aqueous sample in addition to the organic substances, there are two basic alternatives for determining TOC. Direct (= NPOC) method and difference method. Depending on the carbon compounds that predominate in the sample, reliable TOC results will result from the selection of a suitable determination method. Both of the determination methods described as equivalent for the purposes of the standard methods (ISO, EN, ASTM etc.).

The analyzers on the market generally allow TOC determination by either method, and it depends on the discretion of the analyst to select a suitable method. Direct determination – NPOC method

The first step is to remove the inorganic carbon compounds (TIC) from the sample; the remaining component of the organic carbon is then determined by direct introduction into the digestion unit of the analyzer.

Removal of the TIC takes place by acidifying with mineral acids (most commonly with diluted hydrochloric acid) to a pH value 2, and subsequently purging the carbon dioxide generated with an auxiliary gas. This step can be executed externally, i.e. outside the TOC analyzer, or internally in a special part of the instrument, the TIC reactor. Several advantages arise in external acidification and purging of TIC, as under certain circumstances, this process can take place at the same time as the actual TOC determination of the previous sample. This can reduce the analysis time considerably. In internal TIC removal, the analysis times are far longer, since it is necessary to wait for completion of the purging phase for every single TOC determination. There is also a risk of non-representative sampling from the TIC reactor into the analyzer’s digestion unit for samples that contain particles, since the particles can adhere to the glass walls of the TIC reactor. This could mean that the results of the TOC determination are subject to a high degree of scatter.

During the TIC removal, volatile organic compounds can escape from the sample and are no longer available for the subsequent TOC determination. The result of direct determination is therefore formally termed as NPOC (Non Purgeable Organic Carbon). In



samples that do not contain volatile organic components, NPOC is the same as TOC. This is the case for most water samples.

The direct method has also become established in routine analysis as the most commonly practiced method of TOC determination due to its simple handling, speed and reliability. Difference method

In contrast to direct TOC determination, in which there is only one measurement of a sample that has previously been freed of inorganic bound carbon through acidification and purging, the difference method always requires two measurements. In the first step, part of the sample is transferred to the instrument’s TIC reactor, where the carbonate-derived CO2 fraction is detected as TIC in an acidic medium (usually through the addition of acid) and in a stream of carrier gas. In the next step, the total carbon (TC) is determined by inserting the untreated sample in the digestion unit of the analyzer. This converts both the organic and inorganic carbon compounds into carbon dioxide. The TOC is obtained by calculating the difference between these two results: TOC = TC – TIC.

Above all, the difference method is suitable for samples in which the TOC content is not significantly smaller than the TIC content, since this method necessarily leads to large inaccuracies if TIC > TOC. An example calculation serves to illustrate:

A sample X contains 1 mg/L TOC and 99 mg/L TIC, and therefore 100 mg/L TC.

Assuming that the TC and TIC determinations can be carried out with a measurement uncertainty of ± 2% RSD, the results actually obtained would lie within 97 mg/L and 101 mg/L for TIC and 98 mg/L and 102 mg/L for TC. If the respective worst case results are now subtracted from each other (e.g. 98 mg/L TC – 101 mg/L TIC = –3 mg/L TOC or 102 mg/L TC – 97 mg/L TIC = 5 mg/L TOC), an extreme fluctuation range for TOC of –3 mg/L to 5 mg/L is obtained, whereby the result deviates by several hundred percent from the true value (1 mg/L). TC [mg/L] TIC [mg/L] TOC [mg/L]

True value 100 99 1

Value obtained from example 1 98 101 -3

Value obtained from example 2 102 97 5

Due to the problems inherent in taking a difference, it is expedient to improve the accuracy of the TC and TIC values statistically by taking several individual measurements, to obtain the TOC content of the sample from the difference of the mean values and not from each individual pair of values. With the difference method, it should be kept in mind that the TIC content of alkaline samples can easily change under the influence of the laboratory atmosphere.



The following Table 1 once again summarizes which of the two TOC determination methods is advantageous and should be applied.

Table 1

Method Advantage Disadvantage

Difference: TOC = TC - TIC Also detects the volatile organic compounds

Unfavorable at high TIC concentrations

(TIC >> TOC)

Long measurement times

Direct: NPOC

Suitable at high TIC concentrations (TIC >> TOC)

Fast results

Does not detect any volatile organic compounds present in the sample

3.4 Determination of POC Like TOC, Purgeable Organic Carbon (POC) is an analytical convention parameter used to monitor water quality characteristics. It is a measure of the carbon content of the organic compounds contained in an aqueous sample, which are purged under defined conditions. It does not provide any indication as to the type of purged compounds.

The distribution of a substance between liquid and gaseous phase in equilibrium can be described by Henry’s law, and quantified with the Henry constant. It is generally the case that substances with a Henry constant > 100 Pa·m3/mol have a relatively high volatility and those with a Henry constant < 1 Pa·m3/mol have little volatility. Purgeability also depends on the matrix and the purging conditions. The basis for the POC determination is the purging of the volatile organic compounds and carbon dioxide under defined conditions. The separation of the purged CO2 takes place using an absorption tube filled with a suitable absorbent material (e.g. lithium hydroxide).

The organic compounds remaining in the gas stream are then fully oxidized to carbon dioxide, e.g. by combustion, addition of a suitable oxidation agent or by means of high energy irradiation (UV light). A CO2 measurement technique for determining the carbon dioxide formed by oxidation that is analogous to the method described for TOC determination is performed, e.g. using infrared spectrometry, conductivity measurement, coulometry, CO2-sensitive electrodes or using a flame ionization detector after reducing the carbon dioxide, e.g. to methane.

POC determination requires extreme caution in sampling and analysis, as analyte loss can easily occur due to degassing.

Analysis can usually be performed with commercially available TOC analyzers; special add-on modules may be required here for direct determination.



3.5 TOC and COD The importance of TOC determination is increasingly overtaking that of COD, which is still practiced in many places. The main reasons for this:

COD is more time and cost intensive

COD requires toxic chemicals

COD is not sensitive

COD cannot be automated as easily as TOC

The COD parameter can be replaced by TOC, because there is typically a relationship between these two parameters. In most wastewater samples, there is a constant factor of around 3 to 4 between COD and TOC, i.e. a TOC of 100 mg/L corresponds to a COD of approx. 300 mg/L to 400 mg/L.

Figure 4: Average distribution of COD/ TOC ratios for 69 industrial and municipal wastewater dischargers



Nevertheless, in extreme cases this factor can also be much smaller (approx. 2) or considerably higher (up to 7). This depends on the main organic compounds present in the water sample.

The cuvette test kits frequently used for COD determination, with their low sample volumes, certainly appear far more favorable at first glance than a TOC analyzer in terms of their procurement and disposal, but these costs even out markedly with an increasing number of samples. On account of its superior validity, the TOC parameter is increasingly used for applications where it is not yet stipulated by legislation.

3.6 TOC in solids In addition to the analysis of water samples, TOC is also used in the analysis of solids. TOC determination plays an important role, especially in waste management and the analysis of contaminated sites. TOC is analyzed with standardized methods in soils, sediments, construction waste, slurries, filter cake, ash, household waste and in other solids. These analysis results inform the decisions on the further utilization/treatment of waste, e.g. landfill disposal, composting or utilization as secondary fuel.

Together with loss on ignition, TOC is increasingly taken as the lead parameter in assessing the suitability of waste for landfill disposal. It provides information on the extent to which the solid is contaminated with organic pollutants, and consequently how the microbiological decay of the organic constituents can contribute to a change in the body of the landfill site or to the mobilization of heavy metals. A series of standardized methods describes the determination of TOC in solids, e.g. EN 13137. This method is applicable above a TOC content of more than 1 g/kg of dry matter (> 0.1%), and is applicable for utilization in wastes, slurries and sediments.

The definitions already established in water analysis (TC, TOC, TIC) are also common and valid for TOC determinations in solids.

Additional abbreviations commonly used in the context of sum determination of carbon compounds in solids are not defined by standardized methods to some extent:

EC (Elemental Carbon)

Graphite, diamond and soot particles that mainly exist in specific types of waste. Direct determination alongside other carbon species is difficult; the use of indirect methods (e.g., following pyrolysis) is preferred.

DOC (Degradable Organic Carbon)

DOC is the actual organic component (degradable by microorganisms) of the total organic carbon (TOC) in the determination, whereby, according to the convention, elemental carbon (such as soot, etc.) and several inorganic carbon compounds (such as cyanides)



are also detected. The DOC parameter plays an increasingly important role in waste management; with the aid of various methods (e.g., following pyrolysis), it can be determined indirectly or directly.

RC (Residual Carbon)

Residual carbon is principally a calculation parameter, which serves for quantification in the determination of DOC or activated carbon. RC represents the quantity of carbon compounds (elemental/inorganic) that can be directly determined following a preceding process step (e.g., removal of organic carbon compounds by pyrolysis).

Activated carbon (also called lustrous carbon)

This term is used in foundry technology. Activated carbon is mainly determined in foundry molding materials, which have to contain a certain amount of carbon to ensure a high surface quality of the molded part. Activated carbon can be determined in a similar way to DOC following a pyrolysis process. Cfree

Free carbon characterizes the amount of elemental carbon that may determined in certain products (for example carbides) using various, mainly indirect methods.

Thermal methods (dry combustion) with subsequent detection of CO2 by infrared spectrometry have become established as the method of determining TOC in solids.

Temperatures above 1000 °C are generally used in the combustion of the dry samples. Systems equipped with robust ceramic combustion tubes are especially well suited. These allow the use of higher combustion temperatures (up to 1500 °C) and are only subject to minimal wear. The combustion of solids in quartz combustion tubes often requires the use of a catalyst and is limited in its temperature, since quartz softens or melts at temperatures > 1100 °C depending on quality. The wear of quartz tubes is considerably higher than that of ceramics. Alkali and alkaline earth metals, which mainly present in high concentrations in soils and sediments, can modify quartz very quickly, first by an irreversible milky darkening, then cracks and breakage).

The homogeneity of the solid samples analyzed is of vital importance for a reliable result. Grinding samples to achieve a smaller particle size (< 200 µm) is essential for valid analysis. For some samples, especially from certain types of waste (residential waste, household waste), the process of homogenization is extremely difficult. Special mills, often cooled, are used here. A compromise between a small degree of grinding of the sample and as large a sample weight as possible is a workable alternative to ensure a reliable result. Instruments that can process large sample quantities (up to 3 g), and therefore can



ensure that the material analyzed is representative, are preferred to those that only allow the analysis of very small quantities (a few milligrams).

Analogous to TOC analysis in aqueous samples, either the difference method or the direct method can be used in TOC determinations of solids. In the direct method, the removal of TIC is performed by the addition of acid to the sample carrier (boat, crucible). The excess acid is then removed by drying at elevated temperatures and the pretreated sample is burned in an oxygen stream.

The difference method requires combustion of the homogenized original sample to determine TC and determination of TIC in a separate unit of the analyzer, again with the addition of acid and transfer of the CO2 generated into the detector.

The ratio between the organic and inorganic carbon components in the sample should again be taken into account when selecting a suitable determination method.

TOC in solids is increasingly superseding the determination of loss on ignition, since its validity in terms of the organic components in the sample is better. Table 2 uses examples of various types of waste to illustrate that the determination of loss on ignition often mimics a high organic load, which in fact is attributable to the decomposition / water release of inorganic species.

Table 2

Sample type TOC [wt%] Loss on ignition [wt%]

Abrasive slurry containing oil 6.3 13.0

Old paint, hardened 6.9 31.7

Sand screening residue 2.0 7.7

Recently an attempt has been made to counteract the mimicked higher TOC content that is caused by the unavoidable detection of elemental carbon, by determining DOC (Degradable Organic Carbon). (see Table 3)

Some types of waste, such as ash from specific combustion plants, certainly show considerable elemental carbon (EC) fractions, mainly due to soot particles. An extreme example serves to illustrate how far a TOC value determined according to standard methods (e.g. DIN 13137) can deviate from the actual hazard potential, and therefore lead to an incorrect assessment of the waste. Graphite crucible scrap – the formally determined TOC would amount to almost 100%, the actual degradable organic carbon (DOC) would tend towards zero.

One possible means of determining DOC is the pyrolysis method. In the first step, the total carbon (TC) and the residual carbon (RC) are determined after pyrolysis from two aliquots of the same sample. The organic components are degradable under the conditions of the



pyrolysis process in a stream of inert gas, and these will escape depending on the temperature and duration, whereas elemental carbon and other inorganic forms remain unaffected in the sample boat depending on the temperature selected. This fraction is detected as residual carbon in the subsequent oxidation in a stream of oxygen. In the second step, the inorganic component (TIC) of the carbon, which can be converted to CO2 under the influence of acid, is determined directly. The difference TC-RC-TIC is calculated from the three results obtained to obtain a value that correlates with the degradable organic carbon (DOC).

Table 3:

Sample type TOC [%] determined from standard methods (e.g. DIN 13137)

DOC* [%] determined with the pyrolysis method

Blast furnace sludge 31.2 4.26

Coke breeze 50.9 4.19

Boiler slag 0.61 0.35

Press cake 5.18 1.82

Soil 6.18 1.86

* Not corrected by matrix dependent calculation factor



4 TNb – Total Bound Nitrogen 4.1 Definition and standardization TNb (Total Bound Nitrogen) is a measure of the concentration of ammonia, ammonium salts, nitrites, nitrates and organic nitrogen components; dissolved nitrogen is not detected. The method is standardized by DIN 38409-H27 and DIN EN 12260. It applies for freshwater, seawater, drinking water, surface water and wastewater. The method covers the measurement range 0.5 mg/L – 200 mg/L. More highly concentrated samples should be diluted.

4.2 The aim of TNb determination If the concentration of nitrogen compounds exceeds that in the natural equilibrium, eutrophication occurs of the water in question with all its negative consequences. It is therefore necessary to monitor this concentration.

Until now, the individual determination of inorganic components – ammonia, ammonium, nitrite and nitrate – has been performed with common chemical analysis methods (photometric or volumetric). The organic nitrogen compounds were determined by the Kjeldahl method or with the Koreleff persulfate digestion, which also detects nitrogen in the form of nitrite and nitrate. This entails a considerable workload, is time-consuming and uses large amounts of chemicals.

The TNb determination allows the complete detection of all components in a single analysis procedure within a few minutes; apart from the catalyst, it uses no chemicals and it can be automated. This method also offers the option of simultaneous TOC/TNb analysis.

The recovery rates for inorganic and organic nitrogen compounds are generally almost 100%. Sodium azide is an exception, with just 54%.

The significant reduction in the time and work required makes TNb determination an analytically and economically superior method for measuring the nitrogen load in water analysis.

The financial investment for TNb determination is around 75% lower than for individual component analysis [Mehlhorn, UTA 5/96, p. 368].

In the recent past, the TNb determination has also found applications in cleaning validation.



4.3 Measurement principle In the TNb determination, the nitrogen-containing constituents in water are thermocatalytically converted to nitrogen monoxide at a temperature > 700 °C, which is quantitatively detected with a suitable method; the TNb in the sample is then calculated in mg/L. There are essentially two detection methods that have become established.

The NO formed is oxidized with ozone to excited NO2, which in turn falls back to its ground state and thereby radiates light (chemiluminescence). The intensity of radiation is proportional to the NO concentration. It is measured with a photomultiplier.

Figure 5: The principle of TNb determination with a chemiluminescence detector (CLD)

The NO formed is determined with an electrochemical sensor, also known as a chemodetector (CHD).

Figure 6: TNb determination with an electrochemical detector (CHD)

Other detection methods are of lesser importance.

Another option for determining TNb is the reductive digestion of the nitrogen-containing compounds with hydrogen to form ammonia, which can then be determined quantitatively. However, this method has not yet become established, since it does not allow the simultaneous determination of TOC/TNb.



4.4 TNb and the Kjeldahl method of nitrogen determination - TKN Since in contrast to TKN, TNb detects inorganic nitrogen components, as well as nitrite and nitrate, comparability only makes sense here if the matrix is taken into consideration.

Additionally, a series of organic compounds is incompletely detected by the Kjeldahl method. For example, the recovery of urea by standard Kjeldahl digestion exhibits an underdetection of approx. 7%. Only a modified Kjeldahl digestion using a mixed catalyst will give 46.03% N, which comes close to the calculated value of 46.65% N. It is similar for other nitrogen compounds, such as DL-tryptophan and other heterocyclic nitrogen compounds [P. Bornmann, Süddeutsche Kalkstickstoffwerke AG, Trostberg, Germany].

4.5 Typical measurement results for TNb determination in water samples of various origins

Water sample TNb [mg/L]

Landfill site 133.1

Textile manufacture 14.0

Mixed wastewater 27.2

Meat industry 9.6

Brewery 5.2

Dairy 6.3

Pharmaceutical production 42.0



5 AOX/TOX and derived parameters 5.1 Definition and standardization AOX (Adsorbable Organically Bound Halogens X = Cl, Br, I) is a measure of the organic bound halogens contained in a water sample. The term TOX is a common term in English-speaking countries for the parameter AOX. The method of determining AOX/TOX has been standardized worldwide through a large number of standards. Along with the ASTM and EPA standards, the international standard ISO 9652 is the most commonly used convention for determining AOX. The scope of application of ISO 9562 is the determination of AOX in all types of water samples. The method can be applied above a concentration of approx. 10 µg/L AOX and up to a concentration of inorganic chloride up to 1 g/L. Samples with a higher Cl- content (> 1 g/L) must be diluted prior to AOX determination. Alternatively, at higher chloride ion concentrations, the so-called SPE-AOX can also be determined in the filtered water sample. The result of an SPE-AOX determination can differ drastically from the result of an AOX determination. The solid substances (and possibly any bound AOX) contained in the water sample are also detected according to the definition. A differentiated determination of the dissolved AOX and AOX bound to particles is possible if the sample is filtered prior to determination.

A series of abbreviations is commonly used in the determination of sum parameters for organic halogen compounds; these have become established in international linguistic usage and their meaning is defined by the conventions of the standard methods.

AOX/TOX (Adsorbable Organically Bound Halogens)

The AOX (TOX) parameter represents the organically bound halogens - chlorine, bromine and iodine (but not fluorine) – contained in a sample – which can be adsorbed on activated carbon and detected after combustion of the loaded activated carbon in a stream of oxygen at ≥ 950 °C with the aid of argentometric titration (usually microcoulometry).

The result is specified as the mass concentration based on chloride.

AOX determination is predominantly used to analyze all manner of water samples (municipal and industrial wastewater, eluates, surface water and drinking water), but also to some extent in the analysis of solids (mainly sewage sludge).



EOX (Extractable Organically Bound Halogens)

EOX represents the organically bound halogens - chlorine, bromine and iodine (but not fluorine) - which can be extracted with a non-polar solvent (preferably n-hexane) and are detected after combustion of the extract in a stream of oxygen at ≥ 950 °C with the aid of argentometric titration (usually microcoulometry).


EOX determination is predominantly used in the analysis of solids (all types of waste, such as excavated soil, construction waste), but also to some extent in water sample analysis.

The determination of EOX is characterized in specific standard procedures (e.g. DIN 38414-17).

POX (Purgeable Organically Bound Halogens)

POX represents the organically bound halogens - chlorine, bromine and iodine (but not fluorine) - contained in a sample, which can be purged in the gas phase under defined conditions with an auxiliary gas, and are detected after combustion of the gas mixture in a stream of oxygen at ≥ 950 °C with the aid of argentometric titration (usually microcoulometry).


POX determination is predominantly used to analyze water samples.

The determination of POX is characterized in specific standard procedures.

SPE-AOX (Solid Phase Extraction AOX)

SPE-AOX represents the organically bound halogens - chlorine, bromine and iodine (but not fluorine) - contained in a sample, which are detected following solid phase enrichment, with subsequent elution by methanol and determination of the AOX in the methanol solution diluted with water. A styrene-divinylbenzene copolymerized resin is used as the solid phase. The result is specified as the mass concentration based on chloride.

SPE-AOX determination is predominantly used to analyze water samples that contain higher concentrations of inorganic chloride (up to 100 g/L) or DOC (up to 1000 mg/L), and in cases where AOX determination by a standardized procedure is not possible. The result of the SPE-AOX determination differs from that of the AOX determination.

The determination of SPE-AOX is characterized in specific standard procedures (e.g. Annex A of ISO 9562).



5.2 The aim of AOX determination Many organic halogen compounds are introduced into the environment through human activity; however, a not insignificant fraction is of natural origin and is therefore also subject to natural degradation processes. Although not all halogen organic compounds (HOCs) are permanently present in the environment, some HOCs are considered as pollutants. This results in the necessity of minimizing and monitoring the entry of these compounds into the environment. The AOX determination method can be used for this purpose. With the aid of the AOX parameter, the content of organically bound halogens can be analyzed promptly and inexpensively in surface water, wastewater, sewage sludge and soils.

5.3 Measurement principle Methods of OX determination combine a sample preparation step with the subsequent combustion and therefore mineralization of the organically bound halogens present to form gaseous hydrogen halides that can be quantified with the aid of argentometric detection methods.

Combustion and detection are performed with the aid of AOX analyzers, which, using certain add-on modules, can also be used for determining EOX and POX. 5.3.1 Sample preparation (enrichment, extraction)

AOX

A defined quantity of the sample is brought into contact with special AOX activated carbon to achieve enrichment of the species of interest on the activated carbon. Precise requirements apply to the activated carbon used in terms of its blank value, adsorption capacity and grain size.

Blank value: < 15 µg chloride per gram activated carbon

Adsorption capacity: iodine number > 1050

Grain size: 10 – 50 µm (for the batch method) or 50 – 150 µm (for the column method)

Commercially available AOX activated carbon usually satisfies these requirements.

As the adsorption capacity of the activated charcoal is limited and all the sample constituents compete for the “free spaces” on the carbon, it is important to investigate the sample prior to adsorption for two important parameters to achieve the best possible quantitative enrichment of the HOCs.

The concentration of the inorganic chloride should not exceed 1 g/L if possible otherwise the chloride cannot be removed during the subsequent washing of the activated carbon and will cause a positive-bias AOX result.



The concentration of the dissolved organic carbon compounds (DOCs) should be lower than 10 mg/L if possible to avoid negative-bias AOX results due to incomplete adsorption of the halogen organic compounds.

These constraints can usually be satisfied with an appropriate sample dilution.

There are basically two methods available for the enrichment on activated coal: the batch method or the column method.

The batch method can be used for AOX determination in water samples, slurries and other solids.

In the batch method, a defined sample aliquot (usually 100 mL) and a defined quantity of activated carbon (50 mg) are brought into contact for a defined period of time (1 h) by intensive shaking (commercially available laboratory shaker). The suspension is then filtered either through a special membrane filter made of polycarbonate or another filter material (e.g. ceramic wool) (negative or positive pressure filtration). The activated carbon filter cake is washed with a nitrate rinsing solution to remove inorganic chloride, and is then available as a sample for subsequent determination in an AOX analyzer.

The column method can be used for determining AOX in water samples. A defined sample volume (usually between 50 and 100 mL) is pumped through two vertically configured columns arranged in series and filled with activated carbon (50 mg each). Here the sample throughput rate is defined as 3 mL/min. The loaded charcoal is then washed with a nitrate rinsing solution to remove inorganic chloride. Special apparatus is available for enrichment according to the column method; this usually allows several samples to be processed in parallel or sequentially. The degree of automation of the column method is therefore significantly higher than that for the batch method. The column method also has some advantages over the batch method from the analytical perspective.

Figure 7: Triplex column (one prefilter, two activated charcoal columns) for the enrichment of particle-rich samples



Separate evaluation of the two columns configured in series allows information on the completeness of adsorption of halogen organic compounds to be obtained by simple comparison of the columns. The upper column should show a higher AOX content in every case, the lower column should only have a low AOX load. If the two columns show almost the same level of AOX content, it can be safely assumed that the capacity of the activated carbon was insufficient to adsorb all the HOC quantitatively – this case is described as a “breakdown” of the columns. Another advantage of the column method is the reduced risk of contamination from the laboratory atmosphere, since the activated carbon is in more or less closed columns and is handled with only a small contact area. Lower AOX blank values and a lower blank value scatter are the welcome outcome; these ultimately contribute to the attainment of overall lower detection and determination limits for the AOX method.

Samples rich in particles can also be enriched with the aid of the column method – this is possible through the use of a pre-filter column filled with ceramic wool that therefore effectively avoids blockage of the activated carbon columns.

Table 4 below compares the column and batch method with their respective advantages and disadvantages.

Table 4

Column method Batch method

Risk of contamination Low High

Control of completeness of adsorption

Provided with comparison of columns

Not possible, or verification by analyzing various diluted samples

Degree of automation Very high Low

Suitable for Water Water, slurries, solids



EOX

Sample preparation for EOX determination is based on extraction of the sample using a non-polar solvent. For aqueous samples, liquid-liquid extraction is practiced by shaking out in a separating funnel; for solid samples a Soxhlet extraction with multiple extraction cycles should be used as an efficient method. Simple stirring methods or ultrasonic extraction are also used in routine laboratory work on account of their simple practicability. The completeness of extraction should be critically evaluated here.

The solvent extract obtained is usually reduced to a suitable volume and concentrated with the aid of a vacuum rotary evaporator. This step is also necessary to detect even very small EOX contents.

Smaller quantities (approx. 200 µL) of the concentrated extract then undergo combustion and detection in an AOX analyzer with a suitable dosing device for the optimal injection speed of the organic extract.

POX

Generally, no special sample preparation is necessary for POX determination. Normally a purging device coupled directly to the AOX analyzer can transfer the volatile halogen organic components with the aid of a stripping gas to the combustion unit of the AOX instrument where it is detected.

SPE-AOX

Determination of solid phase AOX is only possible in practice at great expense and is therefore only used for aqueous samples, which in addition to having an extremely high chloride load, also show a very small AOX content. Given this constellation, sample dilution and subsequent determination of “normal” AOX is impossible. An example of this is the determination of AOX in hydrochloric acid.

In the first step of SPE-AOX determination, the filtered water sample is brought in contact with the solid phase (styrene-divinylbenzene copolymerized resin). Commercially available SPE cartridges that are prefilled with the appropriate material are used for this purpose. Special conditioning with methanol is required. A vertical configuration and a defined sample flow rate through the solid phase material must be maintained. Conveniently, the same enrichment apparatus is normally used here as is used for the column method of AOX determination. Following enrichment of the organic halogen compounds on the solid phase, this is rinsed with nitrate washing solution to completely remove the inorganic chloride and is then eluted with methanol. The methanol eluate is collected in a volumetric flask and diluted with water for further determination. Now the same pretreatment of the methanol/ water mixture takes place (either column or batch method) as for determining the “normal” AOX: Absorption on activated carbon, washing the activated carbon, analysis.



Figure 8: SPE enrichment

5.3.2 Analysis

The actual analysis of the samples following the relevant pretreatment of the samples takes place in an AOX analyzer. AOX analyzers have four main components: sample introduction, combustion, gas drying and detection.

Sample introduction

Every AOX instrument is suitable for introduction and combustion of solid samples (activated carbon) in its basic configuration. The loaded charcoal can be introduced directly for combustion in the form of columns (tubes of quartz glass filled with the loaded carbon), or after discharge of the activated carbon onto a sample carrier, or in the form of a filter or frit loaded with activated carbon.

Microliter syringes and an appropriate dosing module are usually offered as add-on modules to the AOX analyzer for the introduction of liquid samples (i.e. extracts from the EOX sample preparation), which allow injection at an optimal rate. AOX instruments which allow injection of a solvent extract on a “cold” sample boat (boat injection) that is then slowly introduced for combustion, are advantageous over syringe injection systems. The risk of blockage of injection needles with the condensation products generated during combustion does not arise for boat injection.

The introduction of gaseous samples (volatile sample components such as POX) can best be performed with a purging module directly coupled to the AOX instrument that is also equipped with a temperature control option. In this case, the gaseous sample components are transferred to the combustion space with the aid of a carrier gas.

Sample introduction can generally be automated for AOX and EOX samples.



Combustion

The combustion tubes are made of quartz glass, and a typical combustion temperature is 950 °C. If the loaded activated carbon is introduced into an atmosphere of oxygen at this temperature, the halogen organic compounds adsorbed on it are converted to HX.

In principle there are two types of combustion systems available: vertical or horizontal. Vertical combustion systems are very easy to automate by utilizing gravity.

Gas drying

Immediately after combustion, the combustion gas mixture (HX as analyte, CO2, possibly other oxides and H2O) that is generated must be freed from water to avoid condensation on the cooler parts of the instrument, which would lead to the loss of analyte due to the formation of acid. Concentrated sulfuric acid, ideally in a suitable drying vessel that is directly coupled to the output of the combustion tube without any additional transfer path, is best suited for this purpose. Concentrated H2SO4 has a high capacity for absorbing water without retaining the other components of the analyte (HX). With increasing water absorption and consequent dilution of the sulfuric acid, the absorption of HX also increases (at a concentration < 85% H2SO4). The acid reserve has to be replaced regularly for this reason; some systems have an automatic sulfuric acid replacement unit for this purpose.

All AOX analyzers on the market employ sulfuric acid drying.

Detection

After drying, the analyte (HX) is transferred to an absorber solution (also electrolyte) that is part of the detection device. The principle of HX detection and quantification is argentometric titration. Here, the exact quantity of silver ions that is necessary for a quantitative reaction with HX is made available and is calculated. This occurs by utilizing the fact that silver halides are highly insoluble salts and therefore show a very low solubility product. Silver fluoride is an exception, so that the quantitative determination of fluoroorganic compounds is not possible.



Measurement principle

Coulometric titration

Reaction equation

Ag+ + X- → AgX X = Cl, Br, I

Electrolytic silver ion generation at a silver anode

Ag → Ag+ + e-

Silver ion count:

Q = I • t (quantity of charge)

Q = n • F • z

m = M • Q F • z

Microcoulometry as an argentometric method is the most commonly applied detection principle in AOX analyzers. The silver ions required for the reaction are generated electrolytically at an anode. From the quantity of charge required, the mass of silver ions liberated from the anode can be calculated according to Faraday’s law. The mass of silver is converted to the equivalent mass of chloride ions, and is reported as the result for the AOX determination. Differentiation between the individual halide species (Cl, Br, I) is not possible with argentometry/microcoulometry.

To identify when a sufficient amount of silver ions have been provided/generated for the required reaction with HX, a so-called indicator electrode pair is used in microcoulometry. Either the potentiometric or amperometric “end point indication” can be used. The current or voltage change in the electrolyte is directly proportional to the HX input. If the generation of silver ions is switched off (in the resting state, when no HX enters the detection cell), the potential and the indicator current is at a certain level. If halide ions then enter the measuring cell, a change in the potential or indicator current is a sign to start generating silver ions. Generation takes place until the potential or current has once again reached its original resting level.



Thus, a microcoulometer requires two electrode pairs (= four electrodes): a generator electrode pair and an indicator electrode pair. In particularly low maintenance coulometer systems, some of the required electrodes are advantageously combined in a single electrode body (= combination electrode). This means minimum maintenance and simple handling for the user.

As microcoulometry is an absolute method of measurement, calibration using standard solutions of different concentrations is not required. As part of analytical quality assurance, blank values and standards are analyzed and monitored every working day.

Figure 9: Coulometer cell of AOX/ TOX analyzer multi X 2000

Fundamentals, Instrumentation and Techniques of Sum Parameter Analysis 40/ 42

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6 COD – Chemical Oxygen Demand 6.1 Definition and standardization COD (Chemical Oxygen Demand) is the volume-related mass of oxygen that would be required for complete oxidation of the constituents of a water sample under defined reaction conditions. Potassium dichromate is used as the oxidizing agent. It is a measure of the pollution load of a water sample, and encompasses biologically degradable and non-degradable constituents alike. The method is standardized through national and international standards according to the respective method used and the COD value to be measured. The following standards should be mentioned: DIN 38409-41/-43/-44, DIN ISO15705, ISO 6060.

6.2 The aim of the COD determination COD serves to qualify the load of wastewater with organic substances; however, inorganic oxidizable constituents are also detected. COD is predominantly used for monitoring purification plants. It allows the organic substance flows in purification plants to be described. This allows the oxygen requirement, e.g. in activation basins, to be estimated.

It is a historically older parameter than TOC; it is also not carbon-specific, and requires toxic and environmentally hazardous chemicals for its performance. Its replacement by TOC has been under discussion for many years.

It is of relevance to water legislation in Germany. The first step has been taken in that a factor of 4 times the TOC value can be taken to correspond with the COD value.

Despite the restrictions mentioned, COD has the advantage that it can use cuvette tests, which can be used in small purification plants without a laboratory.

6.3 Measurement principle A COD reagent solution is added to the water sample in accordance with the applicable standard. Any chloride ions present will be masked by mercury ions. The sample prepared in this way is heated, whereby potassium chromate is consumed by the oxidation of the constituents.

The dichromate consumed is ascertained by titrating the excess dichromate and the COD is calculated as mg oxygen / mL of sample.

The cuvette test kits are evaluated photometrically.



7 BOD - Biological Oxygen Demand 7.1 Definition and standardization BOD (Biological Oxygen Demand) is the quantity of oxygen needed for respiration by microorganisms within a defined period of time and at a constant temperature in an aqueous sample. The mass of oxygen is related to a liter of sample, and the measurement duration over n days is specified as BODn. The method is standardized through DIN EN1899-2 and ISO5815-2.

7.2 The aim of the BOD determination BOD provides information on the effect of organic constituents in water on the oxygen content of water samples, on the biodegradability of these constituents and on the functionality of biological purification plants.

It is a control parameter for biological purification plants and consequently is included in the administrative regulations related to water legislation. Along with other sum parameters, BOD is an important parameter in ecological water monitoring and in limnological and marine research.

7.3 Measurement principle All of the methods used have the common feature of measuring the oxygen consumed by microorganisms in the water sample. The following methods are employed:

7.3.1 Electrochemical oxygen determination

The water sample is placed in a BOD bottle and the oxygen concentration is determined with an oxygen electrode.

The sample is then stored in a BOD cabinet at 20 °C, and the oxygen measurement is repeated generally after 5 days. The BOD is determined from the difference. 7.3.2 Respirometric/ manometric method

In this method, the sample is filled into a bottle containing sufficient oxygen. The bottle is connected with a manometer / pressure sensor.

As the oxygen is consumed, additional oxygen is supplied from the gas phase, and this causes a drop in pressure in the bottle. The BOD can then be calculated from the pressure difference.

The carbon dioxide formed by the respiration of the microorganisms is bound by caustic soda to avoid falsification of the pressure drop.



7.3.3 Method at constant pressure

In this method the air pressure above the sample is kept constant by electrochemically generating the oxygen consumed by the microorganisms and supplying it retrospectively. The quantity of oxygen generated electrochemically is calculated by coulometry. It is the same as the mass of oxygen required by the microorganisms and so allows the BOD to be calculated.

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