air, water and land pollution chapter 6: common operations and wet chemical methods in environmental...

Air, Water and Land Pollution

Chapter 6:Common Operations and Wet Chemical Methods in Environmental Laboratories

Copyright © 2010 by DBS

Contents

• Basic Operations in Environmental Laboratories• Wet Chemical Methods and Common Techniques in Environmental Analysis• Analytical Principles for Common Wet Chemical Methods

Common Operations…Basic Operations in Environmental Laboratories


• Classical and modern techniques• ‘Classical’ gravimetric or volumetric based wet chemical methods are routinely

employed in many laboratories• Still required for preparation prior to modern instrumental analysis (e.g. calibration

standards)


“A true chemist, or analytical chemist, has several characteristics. He or she has knowledge of the methods and instruments used for analysis, and understands the principles of the analysis. Laboratory analysts should have a chemistry background adequate to understand and correctly apply all of the laboratory rules and to evaluate and interpret the results of their analysis. They have to know how to plan and organize laboratory work so the time is used efficiently. Thus, a laboratory technician should be a skilled, well-trained chemist – in sharp contrast with the so-called ‘determinators’ who simply twist the dials of an instrument or follow ‘cookbook’ analytical procedures”

Csuros, 1997


Labware Cleaning Protocols for Trace Analysis

• Critical for quality data• Particularly true for trace analysis• Increased use of disposable labware

(e.g. pipet tips, etc.)• Lab cleaning can account for up to

one-third of an analyst’s time• In multi-use labs it is imperative not

to share glassware• Shared glassware may contain enough

residue to completely overwhelm a trace analysis in an environmental sample


• Cleaning technique varies depending on analyte of interest• General rule of thumb is:

(i) an acid wash for trace metals

(ii) acetone wash for trace organics


Basic guidelines:

1. Soak dirty glassware in 2 % hot detergent solution (lab grade, e.g. Alconox/Liquinox)

2. For metals acid wash in 10-15 % HNO3 (except for N analysis), or 10-15 % HCl (should not be used for metallic items)

3. For organics use wash bottles filled with pesticide-grade isopropanol, acetone or methanol

4. Rinse in-between with hot running water detergent and acid/acetone wash, filling each item with water, shaking and emptying at least three times. After acid/acetone was, use reagent water to rinse at least three times.

5. Once clean, dried and stored in a location that prevents contamination from airborne dust or other particles. If oven drying is used do not exceed 140 ºC for glass or 60 ºC for plastic. Never oven dry burets, pipets, or volumetric flasks.

6. In biological work, glassware contaminated with blood clots or bacteria must be sterilized with 2 % disinfectant solution or boiling water prior to cleaning. If virus or bacteria are present, autoclaving is absolutely necessary.


Chemical Reagent Purity, Standard, and Reference Materials

1. Reagent Water• Most widely used analytical solvent for preparation of analytical blanks, samples and

standards.• Preparations: Type I, II, III and IV



1. Reagent Water• Type I – highest quality, prepared by distillation, deionization or reverse osmosis• Followed by polishing with a mixed bed deionizer and a 0.2 µm pore size membrane

filter• Polishing removes trace organics, particulates, and bacteria



1. Reagent Water• Type II – by distillation or deionization



1. Reagent Water• Type III – used for glassware washing, preliminary rinsing of glassware and

feedwater for production of higher purity waters

µmho/cm = µS/cm, megohm-cm is the unit for resistivity



• Show that a conductivity of 0.06 µS/cm is equal to a resistivity of 16.67 megohm-cm

µmho/cm = µS/cm, megohm-cm is the unit for resistivity



1. Reagent Water• Common indicator is resistivity > 10 megaohm-cm at 25 ºC / low conductivity < 0.1

µmho/cm at 25 ºC



2. Common Acid and Alkaline Solutions• Commercial acids and bases commonly used in environmental laboratories



3. Reagent Purity• Grade or degree of purity decided based on analytical purpose• Highest grade for preparation of standard solutions• ACS grade, reagent and technical grades• Specified impurities are typically 10-1,000 ppm



4. Standard and Reference Materials• Materials containing a known concentration of analyte• Provide reference to determine unknown concentrations or to calibrate analytical

instruments• Titration Standards

– Primary standard prepared from a reagent that is pure, stable, and no water of hydratione.g. sodium carbonate, potassium hydrogen phthalate, potassium dichromate etc.

– Secondary standard is a reagent standardized against a primary standarde.g. sulfuric acid, hydrochloric acid, sodium hydroxide

• Standard Reference Materials (SRM)– Purchased from a reputable vendor or certifying organization– EPA requires SRMs of known purity and traceability


Volumetric Glassware and Calibration

• Subject to error if used improperly• Volumetric flasks, burets, and pipets for measuring volume

• Transfer Pipets:– Come in two main size ranges:

(a) micropipets smaller than 1 mL (1, 10, 50, 100, 500 µL)

(b) regular glass or plastic pipets (1, 2, 5, 10, 25, 50, 100 mL)



• Volumetic pipets deliver fixed volumes (1, 2, 5, 10, 25, 50, 100 mL). Volumetric pipets are allowed to drain and should never be blown out

More accurate than:

• Measuring pipets which have graduation lines, may be serological (graduations continue to the tip, may be blow-out type) or Mohr (graduations end before tip)



• Volumetic glassware is calibrated wither to contain (TC) or to deliver (TD)


Laboratory Health, Safety, and Emergency First Aid

• Minimum standard of practice for health and safety activities is detailed in government documents OSHA 29 CFR 1910.1450

• Laboratory Hygiene Plan (LHP) or Chemical Hygiene Plan (CHP)• Chemical, physical and biological hazards


Waste Handling and Disposal

• Source reduction and waste minimization• Separation of wastes• Special rules for highly toxic or reactive chemicals, biological wastes, and radioactive

chemicals

Common Operations… Wet Chemical Methods and Common Techniques

Gravimetric and Volumetric Wet Chemical Methods

• Gravimetric – weighing with an analytical balance– Used for the following parameters: Solids, moisture, sulfate, oil and grease

• Volumetric – measuring volumes (titration)– Classical using indicators– Electrochemical using potentiometry/conductivity– Types: acid-base, oxidation-reduction, complexation, precipitation


Commonly used wet chemical methods in environmental analysis


Common Laboratory Techniques

Filtration• Used to collect materials from fluid (air and water) matrices• Gravity filtration and vacuum (suction) filtration• Water: e.g. analysis of suspended solids in water or dissolved phase after removal of

solids• Air: e.g. filter media connected to a vacuum sampling pump to measure particulate

matter• Filter media come in various materials (glass fiber, membrane etc.) shapes and sizes



Centrifugation• Centrifuge is used to separate solids or liquid particles of different densities or sizes

by rotation at high speeds• Ultracentrifuge > low speed centrifuge > microcentrifuge• Precautions:

– Must be balanced– Use correct centrifuge tubes– Rotors must be secured before operation



Distillation• Required for ammonium, cyanide, sulfide, and phenol analysis• Distillation flask, condenser, and receiving flask



Reflux• Used in COD analysis where organic compounds are oxidized by a strong oxidizing

reagent under acidic conditions• Reflux apparatus allows samples to be heated for an extended time without water

loss



Concentration• Kuderna-Danish (K-D) evaporative concentrator or rotary evaporator (rotavap)• Remove excess solvent from the mixture of analyte and solvent• Concentration of analytes will be sufficiently high to be detected



Digestion• Used for metals, total phosphorus, and total nitrogen• Procedure breaks down organically bound substances and converts the substance to

the analyzed form by using liquid oxidizing agents (e.g. H2SO4, HNO3, HClO3, HCl)

• If total decomposition of silicates is needed HF is used



Extraction• Chemicals have different solubilities in different solvents which can be used to

selectively remove a solute from a mixture• Used primarily to concentrate trace organic samples• Liquid-liquid (L-L) extraction using a separatory funnel for liquids• Soxhlet extraction for solid samples

Common Operations… Analytical Principles for Common Wet Chemical Methods

Moisture in Solid and Biological Samples• Moisture content of solid (soil, sediment, sludge) and biological samples is required

so that the contaminant concentration can be reported on a dry basis• Involves weighing a portion of sample in a tarred crucible, drying to constant weight at

104 ºC

% moisture = (wet weight – dry weight) x 100 %

wet weight

Concentration (dry weight basis) = concentration on wet basis

100 - % moisture


Solids in Water, Wastewater, and Sludge (TS, TSS, TDS, VS)

Total Solids (TS)• TS refers to material left behind in a vessel after evaporation and subsequent oven

drying

TS = TSS + TDS

Total Suspended Solids and Total Dissolved Solids• TSS is the portion of solids retained by a 0.45 μm filter• TDS is the portion that passes through the filter• Temperature at which the residue is dried is important due to loss of volatile material,

water of crystallization, and gases


Solids in Water, Wastewater, and Sludge (TS, TSS, TDS, VS)

Volatile Solids (VS)• VS is the difference between dried solids and the same sample further ignited at 550

C in a muffle furnace

TVS = TS – TSash, TVSS = TSS – TSSash, TVDS = TDS – TDSash

• Ash is the residue at 550 ºC


Acidity, Alkalinity, and Hardness of Water

1. Acidity

• Natural sources include CO2, H2PO4-, H2S, protein, fatty acids, and salt of trivalent

metals such as Fe3+ and Al3+ (e.g. Al(H2O)63+ Al(H⇌ 2O)5OH2+ + H+)

• Man-made sources include mineral acids from metallurgical industry, acid mine drainage, acid rain and organic acid waste

• Measurement – alkaline solution (NaOH) and indicator• Mineral acidity or methyl orange acidity – red to yellow at pH 3.7



1. Acidity

• Measurement – alkaline solution (NaOH) and indicator

Mineral acidity or methyl orange acidity – red to yellow at pH 3.7

Total acidity or phenolphthalein acidity – colorless to pink at pH 8.3



1. Acidity

• To calculate acidity NaOH must be converted to the equivalent amount of CaCO3 (mg/L)

Acidity (mg CaCO3/L) = N x V x 5 x 104

Vs

• Where N = normality of base (=concentration in this case), Vs = volume of water sample (mL), V = volume of base (mL)

• Note 1 N NaOH is 1 M wrt OH-, 2 N H2SO4 is 1 M in H+

Units

• Units are mg L-1 CaCO3 MW = 100 g/mol

CaCO3 + 2H+ ⇌ H2CO3

H+ + OH- → H2O

mg CaCO3 / L = mols OH- x 1 mols CaCO3 x 1 x 105 mg 2 mols OH- mol CaCO3

Volume of sample (L)



2. Alkalinity

• Alkalinity is the capacity of water to neutralize an acid

• In natural waters alkalinity is due to H2CO3 (dissolved CO2)

• In polluted waters minor amounts of NH3, salts of weak acids, and organic acids contribute to alkalinity

• Measured using titration with acid as titrant

– Phenolphthalein alkalinity pH 8.0 - 9.6 (due to OH- and CO32-); and

– Total alkalinity (methyl orange alkalinity) pH 3.1 - 4.4 (due to OH- and CO32-), and

HCO3-)

• Reported as mg CaCO3/L using previous equation



3. Hardness• Water hardness due to Ca2+ and Mg2+

• Adverse effects for domestic and industrial uses (scale in hot water pipes, heaters, and boilers)

Ca2+ + 2HCO3- →CaCO3(s) + CO2(g) + H2O

• Require large amounts of soap to make foam

Hardness (mg/L CaCO3) = Σ M2+ (mg/L) x 50/EW of M2+

• Where equivalent weight (EW) of Ca2+ and Mg2+ are 20.0 and 12.2 respectively• Titration, FAAS or ICPAES/MS



3. Hardness


• Where equivalent weight (EW) of Ca2+ and Mg2+ are 20.0 and 12.2 respectively

e.g. water has a calcium content of 51 mg/L, what is the calcium hardness as CaCO3?


= 51 x 50/20.0 = 128 mg/L CaCO3



3. Hardness


• Where equivalent weight (EW) of Ca2+ and Mg2+ are 20.0 and 12.2 respectively

e.g. water has a calcium content of 28 mg/L and a magnesium content of 9 mg/L, what is the calcium hardness as CaCO3?


= (28 x 50/20.0) + (9 x 50/12) = 107 mg/L CaCO3


Oxygen Demand in Water and Wastewater (DO, BOD, COD)

1. Dissolved Oxygen

• DO originates from atmospheric O2 (21 %)

• DO in water is typically around 8 mg/L• Amount depends on aeration, wind, water flow, algae, temperature, pressure, organic

compounds, salt content, bacteria, etc.• Important water quality parameter• Most common oxygen consuming pollutants are organic compounds (protein, sugar,

fatty acids) which are readily degraded by aerobic bacteria



1. Dissolved Oxygen• Two analytical methods:

– Classical Winkler Method (primary)– Polarographic membrane electrode method (secondary)

Common Operations… Wet Chemical Methods and Common Techniques Method

• DO is fixed after sampling by reaction with Mn2+ (MnSO4) together with alkaline iodide/azide mixture

Mn2+ + 2OH- + ½ O2 → MnO2(s) + H2O

• I- is needed for titration, N3- prevents NO2- and Fe3+ interference (production of excess I2 from

KI)• After transport to lab sample is acidified with H2SO4

(dissolves Mn4+ floc), and Mn4+ oxidizes iodine ions:

MnO2 + 2I- + 4H+ → Mn2+ + I2 + 2H2O

• I2 is then titrated with thiosulfate and starch indicator

I2 + 2S2O32- → S4O6

2- +2 I-

• Over all reaction:

2S2O32- + 2H+ + 1/2O2 → S4O6

2- + H2O

Common Operations… Wet Chemical Methods and Common Techniques Method

• Over all reaction:

2S2O32- + 2H+ + 1/2O2 → S4O6

2- + H2O

• 1 mol of O2 is equivalent to 4 mols of thiosulfate

DO (mg/L) = N x V x 8000

Vs

• Where N and V are the normality and volume (mL) of Na2S2O3 respectively, Vs is the sample volume

e.g. 1 mL of 0.025 M S2O32- is required to reach the blue starch end-point of a 200 mL

sample, calculate the DO level (mg/L)

0.025 M S2O32- = 0.025 N S2O3

2- DO = 0.025 N x 1 mL x 8000 / 200 mL = 1 mg/L

Example Calculation

1 mL of 0.025 M S2O32- is required to reach the blue starch end-point of a 200 mL sample. Calculate the moles of

O2 dissolved in the sample, and the mg/L DO.

1 mol O2 = 4 mols S2O32-

mols S2O32- x 1 mol O2 = mols O2

4 mols S2O32-

0.025 mol/L x 1 mL x 1 L / 1000 mL = 2.5 x 10-5 mols S2O32-

2.5 x 10-5 mols S2O32- x mol O2 = 6.25 x 10-6 mols O2

4 mols S2O32-

There are 6.25 x 10-6 mols of O2 in the 200 mL sample

6.25 x 10-6 mol = 3.125 x 10-5 mol O2 / L x 32 g / mol = 1 x 10-3 g /L x 1000 mg/g = 1 mg O2 / L =

0.2 L

= 1 ppm DO



2. Biochemical Oxygen Demand (BOD)• BOD refers to the amount of oxygen consumed if all the biodegradable organics in

one liter of water were oxidized by aerobic bacteria

• Standard 5-day BOD5 test – 300 mL BOD bottles are used

• Initial DO reading is made with sample saturated with DO, parallel samples are incubated at 20 ºC in the dark for 5 days

BOD5 (mg/L) = DO0 – DO5



3. Chemical Oxygen Demand (COD)• ABOD provides closest measure of the processes occurring in natural waters• Test is time consuming• COD test is used as a measure of the oxygen equivalent of the organic matter

content of a sample susceptible to oxidation by a strong chemical oxidant• Results in 2 hr or less, provides good estimate of BOD• Disadvantage is that it may oxidize materials not readily oxidized in nature


3. Chemical Oxygen Demand (COD)

• Dichromate ion, Cr2O72- dissolved in sulfuric acid is a powerful oxidizing agent. It is used as

an oxidant to determine COD

6Fe2+ + Cr2O72- + 14H+ → 6Fe3+ + 2Cr3+ + 7 H2O

• Excess dichromate is added to achieve complete oxidationBack titration with Fe2+ gives the desired endpoint value

COD (mg/L) = N x (V0 – V) x 8000

Vs

• Where N is the normality of Fe(NH4)2(SO4)2, Vs is the volume of water sample (mL) and V0 and V are the volumes (mL) of Fe(NH4)2(SO4)2 for the blank and sample, respectively


Oil and Grease in Water and Wastewater

• “Oil” represents a wide variety of substances from low to high molecular weight hydrocarbons found in petroleum to light gasoline, heavy fuel, and lubricating oils

• “Grease” represents higher molecular weight hydrocarbons and glycerides of animal and vegetable origin

• Defined by operational procedure• Partition-gravimetric method: acidification is required if fatty acids are present to

increase their solubility, hexane is used to extract the oil and grease and removed via evaporation, final extract is weighed


Residual Chlorine and Chloride in Drinking Water

1. Residual Chlorine• Chlorine gas is most widely used disinfectant for water supplies

• Cl2 hydrolyzes to hypochlorous acid and hydrochloric acid

Cl2 + H2O HOCl + HCl⇌

• HOCl is a weak acid that dissociates into hypochlorite (OCl-)

HOCl H⇌ + + OCl-

• Hypochlorous acid and hypochlorite are called free chlorine residuals



1. Residual Chlorine• 3 titrimetric methods and other instrumental methods for measuring residual chlorines

(a) iodometric titration – for concentrations > 0.1 mg/L

(b) iodometric back titration – primarily used for wastewaters

(c) DPD-FAS titration



1. Residual Chlorine

• Iodmetric Titration – free chlorine species (OCl- and HOCl) liberate iodine (I2) from potassium iodide at pH 4 or lower

• I2 is titrated with sodium thiosulfate using a starch indicator

Cl2 + 2KI → I2 + 2KCl

I2 + starch → blue color

I2 + 2Na2S2O3 → 2NaI + Na2S2O6

• Iodometric Back Titration – liberated I2 is reacted with excess Na2S2O3, remaining Na2S2O3 is back titrated with I2 as the titrant instead of using Na2S2O3 as the titrant

I2 + 2Na2S2O3 (excess) → 2NaI + Na2S2O6



1. Residual Chlorine• DPD-FAS Titration – N,N-diethyl-p-phenylenediamine (DPD) reacts with free

chlorine residuals to produce a red color• At pH 6.2 to 6.5 reaction is rapid and reversible• Solution is titrated with ferrous ammonium sulfate (FAS) until disappearance of red

color



2. Chloride (Cl-)• High chloride concentration may be due to seawater intrusion, evaporated salts,

discharge of human wastes• Cl- is commonly used for tracer studies• Argentometric (Mohr) Method – In neutral or slightly basic solution, potassium

chromate (CrO42-) indicates the end-point of a silver nitrate titration

AgNO3 + Cl- AgCl + NO⇌ 3-

2Ag+ + CrO42- Ag⇌ 2CrO4 (orange)



2. Chloride (Cl-)

• Mercuric Nitrate Method – Cl- is titrated with mercuric nitrate (Hg(NO3)2). At the pH range 2.3 to 2.8 diphenylcarbazone (DPC) indicates the end-point via the formation of a purple complex with excess mercuric ions

Hg2+ + 2Cl- HgCl⇌ 2


Ammonia in Wastewater

• Ammonia is produced by decomposition of organic nitrogen containing compounds and by the hydrolysis of urea

• Ammonia is titrated with H2SO4 using mixed indicator (methyl red and methylene blue)

• Sample is titrated until it turns pale lavender

2NH4+ + H2SO4 → 2H+ + (NH4)2SO4


Cyanide in Water, Wastewater and Soil Extract

• Cyanide refers to all cyanide groups in cyanide compounds (CN-)• Used in plastics, electroplating, metallurgy, synthetic fibers, and chemicals• Titrimetric method detection limit of 1 mg/L• HCN is liberated from an acidified sample by distillation and purging with air through a

NaOH scrubbing solution• CN- in scrubbing solution is the titrated with silver nitrate to form a soluble cyanide

complex (Ag(CN)2-)

• As soon as all CN- is complexed and a small amount of Ag+ has been added, excess Ag+ combines with an indicator (p-dimethyl-aminobenzal-rhodamine, C12H12N2OS2) which turns the solution from yellow to brownish-pnk

2CN- + Ag+ → [Ag(CN)2]-


Sulfide in Water and Waste

• Sulfide (H2S, HS-, and S2-) present due to reducing conditions

• Waste and wastewater sulfide comes from anaerobic decomposition (bacterial reduction of sulfate)

• Determined via titration – sulfide samples are preserved by addition of zinc acetate and NaOH, ZnS precipitate is oxidized to sulfur with addition of excess iodine (I2) in an acidic solution, remaining iodine is determined via titration with sodium thiosulfate

References

• Csuros, M. (1997) Environmental Sampling and Analysis: Lab Manual. Lewis Publishers, Boca Raton, Fl.

• HACH Company (1992) HACH Water Analysis Handbook, 2nd Edition, Loveland, CO (Procedures available: http://www.hach.com).

• US EPA (1983) Methods for Chemical Analysis of Water and Wastes. EPA/600/4-79/020.

Questions

11. 100 mL water sample was used for SS measurement. The mass of the weighing bottle plus filter paper was 25.6257g before filtration and 25.6505 g after . What is the TSS in mg/L?

17. If 3.0 mL of 0.02 N H2SO4 is required to titrate 200 mL of sample to phenolphthalein end point, what is the phenolphthalein alkalinity as mg/L CaCO3? If additional 20.0 mL of H2SO4 is needed to reach the methyl orange point, what is the total alkalinity as mg/L CaCO3.

22. Write all chemical reactions involved in the measurement of DO using Winkler method.

28. (a) Explain why the conversion factor in eqn. 6.13 is 8000. (b) If 200 mL sample was used for DO measurement, it requires 8.6 mL of 0.0275 N Na2S2O3 solution. What is the DO in mg/L?

32. Describe the difference between BOD and COD in terms of the chemicals each method represents.

air, water and land pollution chapter 6: common operations and wet chemical methods in environmental...

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