Laboratory waterquality analyses

Although there are now instruments to measure some physico-chemical properties directly in the field, for example, electrical conductivity monitors, most measures of the physico-chemical content of water are still made by laboratory analyses of samples. It is far beyond the scope of this chapter to describe the analytical techniques for all substances likely to be found in river water, but a knowledge of the main types of analysis used would benefit the hydrologist. The analytical methods may be grouped under the headings: gravimetric, titrimetric, colorimetric, chromatographic, mass spectrometry and electrode-based.

Gravimetric analysis is used to determine the suspended sediment (SS) concentration. With this method, a known volume of water sample is filtered through a pre-weighed filter paper with a pore-size of 0.45 /m. The SS concentration is given by the increase in weight at drying at 105° C. Just prior to both weighing stages, the filter papers should be placed within a desiccator.

Titrimetric analysis, the well-known balancing of reactions using coloured indicators can give satisfactory results down to 1mgL—1 in determining, for example, DO or alkalinity. This approach is also described as volumetric analysis, as it depends on the measurement of volumes of liquid reagent of known strength.

Colorimetric analysis involves the measurement of colour intensity when a reagent is combined with a water sample. There are several experimental techniques used in col-orimetry that are reliable for measuring such chemicals as ammonia, phosphorus and chlorine as long as they are not mixed with other compounds with similar coloration. The colour can be measured by visual methods of comparison (or Nessler) tubes or

Fig. 8.4 A double-active piston pump used for extracting water samples from piezometers. (Reproduced by permission of Nielsen, D. M. (2006) Practical Handbook of Environmental Characterisation. Taylor & Francis, Boca Raton.)

colour papers/discs, but more commonly, instrumental methods employing a colorimeter or spectrophotometer are used. A colorimeter uses a photoelectric cell to detect a light source passed through the sample. In contrast, a prism producing monochromatic light of a specific wavelength is used within a spectrophotometer. With an atomic absorption spectrophotometer, the colour of metal ions burnt within a flame is measured and compared with the light intensity from a known standard solution of the metal ions. With this approach, metals such copper, lead, and zinc are measured.

Chromatographic analysis involves the separation of analytes by their interaction with a microscopic layer of chemicals on the walls of a chromatographic column. Gas chromatography (GC) or strictly gas-liquid chromatography is used for measuring phenols and VOCs within water. With this method, the sample is first vaporised and a carrier gas (e.g. helium, nitrogen, argon) is used to transport the sample through the column coated with a microscopic layer of a liquid or polymer. Different ana-lytes within the sample produce eluates that emerge at different times and these are detected with for example a flame-ionization detector. With column chromatography, notably high-performance liquid chromatography (HPLC), a water sample is mixed with a solvent and passed through the HPLC column, and the resultant eluates are commonly detected with a ultra-violet absorption detector. HPLC can be used to measure pesticides and polycyclic aromatic hydrocarbons (PAHs) within water.

Mass spectrometry analysis, notably inductively coupled plasma mass spectroscopy (ICP-MS) involves the decomposition of water samples to neutral elements in a high-temperature argon plasma, with analysis based on mass to charge ratios. This method is used for measuring trace metals within waters samples. Mass spectroscopy is also used for determining the relative proportions of the stable isotopes of water ^H, 2H, 16O, 17O and 18O; DeGroot, 2004) for tracing water pathways or calculating the residence times of different water sources (Kendall and McDonnell, 1998; McGuire and McDonnell, 2007).

Electrode-based analysis is routinely used to measure dissolved oxygen and pH in the laboratory, but can be used to measure ions such as nitrate, potassium, ammonia, cadmium and lead. Details of these analyses are given in the following section on continuous field monitoring. Within laboratory, dissolved oxygen probes can be used to measure the biochemical oxygen demand (BOD). This is undertaken by measuring the dissolved oxygen before and after incubation of water samples held at 20° C for 5 days within sealed glass bottles.

Nollet (2007) explains the details of these and other methods of water analysis in considerably more depth.

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