The concentrations of impurities found in waters and wastewaters are relatively low compared with normal chemical solutions, so that analytical techniques appropriate to dilute aqueous systems are necessary. In general, concentrations of impurities are expressed in terms of milligrams per litre (mg/l), and analytical procedures must be carefully carried out to obtain accurate information. To permit comparison of data obtained by different analysts it is important to use standard analytical methods. There are four main types of physical and chemical analyses which are commonly employed in the examination of waters and wastewaters. These are gravimetric, volumetric, colorimetric, and the use of electrodes. Specialized microbiological analyses are used to detect and enumerate bacteria and other microorganisms in samples.
Gravimetric analysis:
This form of analysis relies on weighing solids obtained from a known volume of sample after evaporation, filtration or precipitation. The main uses of gravimetric analysis are found in the determination of total solids by evaporation and of suspended solids by filtration through a 0.45 μm paper. A sensitive analytical balance, drying oven and desiccator are essential for gravimetric determinations, which are not feasible for field use.
Volumetric analysis:
Many determinations in water quality can be rapidly and conveniently carried out by volumetric analysis, which depends upon the measurement of volumes of a known strength liquid reagent which reacts with the constituent being determined. The apparatus required is simple and the analyses can usually be carried out in the field if necessary.
Colorimetric analysis:
For a wide range of constituents in natural waters and in wastewaters it is possible to utilize the formation of a soluble coloured compound following the addition of a special reagent as a means of analysis. The coloured solution must be such that light absorption through it increases exponentially with the concentration and also that light absorption increases exponentially with the length of the light path through the solution.
The colour intensity of a solution can be measured by visual comparison with standards, or more usually by instruments provided with colour filters or prisms to produce the appropriate wavelength of light for greatest sensitivity. Such techniques are widely used for portable analytical systems for field use and also for continuous monitoring of some constituents. Colorimetric analysis is inaccurate in the presence of suspended matter since this also absorbs light and results in a false indication of solution concentration.
Nephelometry, which measures the amount of scattered light when a beam is passed through a liquid sample, is used to determine the presence of colloidal solids in a liquid which are responsible for turbidity.
Electrodes:
For many years pH, which expresses the intensity of acidity or alkalinity of a solution, has been measured using a glass electrode which is sensitive to hydrogen ions in solution. More recently many other electrodes have become available to measure specific ions such as ammonium, nitrate, chloride, calcium, sodium, etc. Dissolved oxygen is also easily measured by means of a special electrode which is particularly suited for field use. All electrodes require careful, and sometimes frequent, calibration and regular cleaning if left in position for continuous monitoring purposes.
Microbiological analyses:
Most determinations in this area are concerned with enumerating bacteria and are based on total viable cell counts on a general-purpose medium or specific medium and incubation conditions for the normal indicator organism Escherichia coli, whose presence is taken as positive evidence of human faecal pollution.
Specialized analytical:
When analysing samples for specific organic or inorganic constituents, particularly at low concentrations, it is often necessary to employ sophisticated instruments such as atomic absorption spectrophotometers, fluorimeters, gas chromatographs, and mass spectrometers. These instruments are costly to purchase and maintain and require skilled operators to obtain reliable results.