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- 3.6.FIELD AND LABORATORY WORK

3.6.2. Laboratory work

3.6.2. Laboratory work (optional)

Peter Kelderman

UNESCO-IHE Institute for Water Education

Online Module Water Quality Assessment

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PHYSICO-CHEMICAL ANALYSIS

• Conventional, cheap methods (titrations, colorimetry..)

• Advanced, expensive methods (AAS, GC..) àspecialised, central labs

• Compare (relative) investment and operationalcosts; labour time (ECU~ US$)

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PHYSICO-CHEMICAL ANALYSIS

• Conventional, cheap methods (titrations, colorimetry..)

• Advanced, expensive methods (AAS, GC..) à specialised, central labs. These instrumentations need good facilities, much manpower, expertise and maintenance, and good quality control .

•See example of price list (2009) of analyses in an Asian Certified lab:

• Nutrients: 5-10 US$/analysis

• Heavy metals: 35-75 US$/sample (multiple HMs)

• Pesticides: 50-100 US$ (per group of pesticides)

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Certified Water Laboratory Nusantara, Manado, Indonesia

Clockwise, starting

• Electrodes for pH, EC, ..

• AAS for heavy metals

• Colorimeter for phosphate, ammonia, nitrate, etc.

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CONVENTIONAL CHEMICAL ANALYSIS TECHNIQUES

• Titrations: used for e.g. chloride, Mg2+, Ca2+, alkalinity….

Adding known amount of chemical A to unknown amount B à from reaction Aà B you can calculate concentration of B.

• Colorimetry/spectrophotometry: used for phosphate, nitrate, ammonia, iron, etc.

By adding specific chemicals you can make coloured solution (e.g.phosphate à blue). Colour intensity is proportional to concentration à measure in spectrophotometer

• Electrodes: for pH, dissolved oxygen, conductivity, fluoride,.. ; calibration necessary at known values.

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MEASUREMENT OF pH

pH can be measured with the help of a glass electrode:

• Diffusion of H+ ions across the glass membrane

• Difusion rate proportional to pH of solution à redoxreaction à electrical current à reading

• Calibration is essential: e.g. at pH = 4.0; 7.0; 10.0

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ELECTRICAL CONDUCTIVITY (EC)

• EC is reciprocal of electrical resistance; it indicates presence of cations (K+; Na+,Ca2+….) and anions (Cl- ; HCO3- ; SO42- …)

• Unit: μS/cm or mS/m (S=Siemens). Values in most natural waters: 10-1000 μS/cm

• Measure electrical current in the solution by using electrical circuit with two platinum plates.

• EC is often used as (fully automated) water quality check for TDS (total dissolved solids), e.g. for drinking water.

In agriculture for quality of the irrigation/drainage water.

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TURBIDITY• Indicates “Total suspended solids” (TSS) in water

• High turbidity in e.g. monsoon period

“Turbidimeter”: measures light scattered at 900

Turbidity unit: JTU = FTU= NTU: Nephelometric turbidity units.

WHO guideline 5 NTU. In some (tropical) rivers during monsoon:1000 NTU!

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REDOX POTENTIAL (Eh)

• Connected with “aerobic” vs “anaerobic” conditions of surface/groundwaters

• Expressed in (milli)Volts. - At low redox potential (e.g. : -200 mV), we can expect

presence of “reduced” components: S2- ; CH4; Fe2+ ; NH4+, …- At high redox potential (e.g.: +500 mV), we can expect presence of “oxidized” components: O2 ; NO3- ; Fe3+ ; SO42- ..

• Measurement: with Eh electrode

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DISSOLVED OXYGEN

• Alternative (especially at very low DO): “Winkler titration”

• Oxygen is measured with oxygen electrode

O2 diffuses through membrane at a rate proportional to [O2]; inside electrode redox reaction à "volts" on meter (calibration needed !)

Solubility of dissolved oxygen in fresh water

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NUTRIENTS• NH3/NH4+

• NO3-, (NO2-)

• Phosphate (PO43-, HPO42-,..)

• Analysis: colorimetric or with I.C. (“ion chromatography”; can measure “all” ions); see slides on “Gas chromatography” à

• Total N and P: by “destruction” in strong, boiling acid and measure as nitrate /phosphate

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• Often very low concentrations --> accurate working; clean glassware, very pure chemicals and distilled water!

• Also good “Quality Control” is vital !

Absorption wavelength à specific for elementIntensity of absorption à concentration

MEASUREMENT OF TRACEMETALS WITH ATOMIC ABSORPTION SPECTROMETRY (AAS)

AAS : Atom M à M* (“excited state”)light absorption

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Work with a flame of 2000-3000 0 C to get the metal atoms

ANALYSIS MOSTLY BY GAS CHROMATOGRAPHY

Injection mixture of organics in gas stream

Separation in a column with stationary medium (adsorbent)

• Identification by “retention time” or GC-MS

• Concentration: peak height or area

ORGANIC MICROPOLLUTANTS (DDT, PCBS, ..)

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MICROBIOLOGICAL INDICATORS

• Human faeces: Salmonella; Shigella; Escherichia Coli; …; extreme health risks (water-borne diseases)

• Separate analysis very costly; therefore often E-coli taken as “indicator organism”; E-coli WHO standard: ca. 100/100 mL forrecreation water ; 0 (zero) for drinking water

• In wastewater: E-coli > 108/100 mL; 90-99% reduction in conventional wastewater treatment

• Analysis by counting colonies on plate in lab, after

few days at 37 0CàMPN: “most probable number”.