Analytical Notes

The significance, methods and instruments of water analysis and monitoring

Drilled down to basics, water quality may be defined as ‘a measure of the suitability of water for a particular use based on selected physical, chemical, and biological characteristics.’ Awareness about water quality, be it for domestic or industrial purposes, has taken off in a big way in the past few years with water contamination emerging as a serious problem in both the developing and developed worlds. Rapid expansion of agricultural, industrial, residential, commercial activities as well as climate change are impacting the availability and quality of water resources. For example, according to a study by the Ralph Nader Study Group, the US drinking water reportedly contains more than 2,100 toxic chemicals that can cause cancer. The World Health Organisation’s (WHO) Guidelines for Drinking Water include an assessment of the health risks presented by the various microbial, chemical, radiological and physical constituents that may be present in drinking water, which if some accounts are to be believed, number up to 200 or more.

Water has become contaminated with fertilisers, pesticides, drugs, hormones, heavy-metal compounds, body care and synthetic products due to its wide-spread use as source of food and energy, as a solvent, cleaning agent, coolant, means of transportation and discharge system for effluents. Faced with this growing problem, countries around the world are extending existing or developing new legal requirements, often stringent, on the qualities of various types of water. These requirements call for the measurement of many different substances, a large proportion of which must be measured and controlled at very small concentrations. Most environmental analyses are measuring very low concentrations of substances, in milligrams per litre or mg/L. Since a milligram is one thousandth of a gram, and a litre of water weighs about a thousand grams, a mg/L is approximately equal to one part per million by weight. A part per million (ppm) is only one ten thousandth of one per cent. For toxic metals and organic compounds of industrial origin, measurements are routinely made in the part per billion (microgramme per litre) range or even lower. At such low levels, sensitive equipment and careful technique are clearly necessary for accurate results. In fact, there have been considerable developments in the analytical techniques and procedures applied to make such measurements, with automatic procedures –semi or fully–being increasingly employed to meet the challenge of greater numbers of samples.

Parameter watch

The most frequently measured parameter of aqueous solutions, ranging from measurement in drinking water, surface water, groundwater and wastewater through to precise measurement for pharmaceutical use is the pH value, a measure of how acidic or alkaline a solution is. A pH of 7 is neutral; a pH above 7 is alkaline (basic) while below 7 is acidic. The scale runs from about zero, which is very acidic, to 14, which is highly alkaline. Although there are some microorganisms which can function at extreme pHs, most living things require pHs close to neutrality. In waters with low dissolved solids, which consequently have a low buffering capacity or low internal resistance to pH change, changes in pH induced by external causes may be quite dramatic. Besides the harm to aquatic life in natural waters, pH imbalances can inhibit or completely wipe out biological processes in wastewater treatment plants, resulting in incomplete treatment and pollution of the receiving waters. Low (acidic) pHs cause corrosion in sewers systems and increase the release hydrogen sulphide gas. Apart from the above, pH values govern the behaviour of several other important parameters of water quality like ammonia toxicity, chlorine disinfection efficiency, and metal solubility. In fact, chlorine becomes toxic as the pH level of the water drops, and it becomes even more toxic when it is combined with other toxic substances such as cyanides, phenols and ammonia. Higher the pH and the warmer the water temperature, the more toxic the ammonia.

Selected standards relating to water analysis (Click on the image for larger view)

While pH measures the strength of an acid or base; alkalinity indicates a solution’s power to react with acid and “buffer” its pH — that is, the power to keep its pH from changing. Alkalinity is a total measure of the substances in water that have ‘acid-neutralising’ ability. The main sources of natural alkalinity are rocks, which contain carbonate, bicarbonate, and hydroxide compounds. Borates, silicates, and phosphates may also contribute to alkalinity. The parameter is of interest to water engineers in that it is a factor concerned in the computation of the Langelier ‘Saturation Index’ which relates to the corrosion of or deposition of scale in distribution networks.

Chlorine is a universal and cost- effective drinking water disinfectant. It is also used as a disinfectant in wastewater treatment plants and swimming pools, as a bleaching agent in textile factories, paper mills and an important ingredient in many laundry bleaches. Free chlorine (chlorine gas dissolved in water) is toxic to fish and aquatic organisms, even in very small amounts. However, chlorine reacts quickly with other substances in water (and forms combined chlorine) or dissipates as a gas into the atmosphere. But if the water contains a lot of decaying materials, free chlorine can combine with them to form compounds called trihalomethanes or THMs. Some THMs in high concentrations are carcinogenic to people. Unlike free chlorine, THMs are persistent and can pose a health threat to living things for a long time.

Nitrite exists normally in very low concentrations, and even in waste treatment plantS, effluents levels are relatively low, principally because the nitrogen will tend to exist in the more reduced (ammonia) or more oxidised (nitrate) forms. Nitrites are relatively short-lived because they’re quickly converted to nitrates by bacteria. Nitrates can be traced to organic and inorganic sources, the former including waste discharges and the latter comprising chiefly artificial fertilisers. Because nitrite is an intermediate in the oxidisation of ammonia to nitrate, because such oxidation can proceed in soil, and because sewage is a rich source of ammonia nitrogen, waters which show any appreciable amounts of nitrite are cause for suspicion of past sewage pollution or of excess levels of fertilisers or manure slurries spread on land. Most importantly, high nitrate levels in waters to be used for drinking will render them hazardous to infants as they induce the Blue Baby syndrome. In addition, nitrites can give rise to the presence of nitrosamines by reaction with organic compounds and there may be carcinogenic effects. Because it is the nitrite rather than nitrate which is the direct toxicant, there is a stricter limit for nitrite in drinking waters.

Aluminium is monitored in boiler make-up water, where aluminium sulphate or alum has been used (alum is typically used for colour- and colloid-removal in the treatment of water) to determine whether aluminium is present after pre-treatment. Residual aluminium may consume ion exchange capacity or consume boiler water treatment chemicals added to stoichiometrically chelate hardness ions (calcium and magnesium) in boiler feed water. Aluminium is monitored in cooling water make-up, since its presence may result in deactivation of anionic substances in scale or corrosion inhibitor treatment chemicals, or both. Deactivation may result in decreased performance of inhibitors.

Ammonia is generally present in natural waters, though in very small amounts, as a result of microbiological activity which causes the reduction of nitrogen-containing compounds. When present in levels above 0.1 mg/l , sewage or industrial contamination may be indicated. Ammonia is toxic to fish and aquatic organisms, even in very low concentrations and when water contains very little dissolved oxygen and carbon dioxide. High ammonia levels interfere with chlorination processes in water treatment. The formation of chloramine compounds (which are much less potent disinfectants than free chlorine) by reaction between the added chlorine and the ammonia present in the water necessitates an increased use of chlorine if disinfection efficiencies are to be maintained.

Phosphates (chemical compounds containing the element, phosphorous) are found in nearly all fertilisers. Phosphate is a major constituent of detergents, particularly those for domestic use. The significance of phosphorous is principally in regard to the phenomenon of eutrophication (over-enrichment) of water bodies. If too much phosphate is present, algae and water weeds grow wildly, choke the water body and use up large amounts of oxygen. Many fish and aquatic organisms may die.

Cyanide is a common constituent of industrial wastes, especially in mining, metal finishing and plating industries because of its ability to bind very strongly to metals to form water-soluble complex ions. This same property makes it highly toxic to living things because it prevents the normal activity of biologically important, metal-containing molecules. It is, however, biodegradable by some bacteria in low concentrations; and they can become acclimated to higher concentrations if given enough time.

Heavy metals contamination of water can be traced to effluent discharges, or from distribution piping or geological formations. Heavy metals imply antimony, arsenic, beryllium , cadmium, chromium, cobalt, copper, lead, mercury, molybdenum, nickel, selenium, silver, thallium, tellurium, tin, titanium, uranium, vanadium, zinc. They are toxic to humans (to a degree varying greatly from metal to metal) and to fish (the hazard levels for which are generally very much lower). Because they are easily accumulable in fish and other tissue, they are liable to enter the food chain. The US EPA lists nine metals (arsenic, cadmium, chromium, copper, lead, mercury, nickel, silver, and zinc) as toxic ‘priority pollutant’ metals.

Boron is a naturally occurring trace element, and is present in seawater around 5 mg/l. The element is not considered a problem in drinking water but could endanger crops when present in irrigation water at the 1-2 mg/L concentration range. Bromate, which is formed when bromide ions present in water are oxidised by ozone and some other oxidising agents (including chlorine), can be both carcinogenic and mutagenic.

Analytical methods

In volumetric titration, chemicals are analysed by titration with a standardised titrant. The titration end-point is identified by the development of colour resulting from the reaction with an indicator, by the change of electrical potential or by the change of pH value.

Gravimetric analysis is more common with water solutions that are more concentrated such as chemicals used in water or wastewater treatment. Analytical balances routinely used for gravimetric analysis are sensitive to one tenth of a milligram, or one ten-thousandth of a gram. Most laboratories use electronic balances with direct digital readouts.

Colorimetric methods are based on measuring the intensity of colour of a coloured target chemical or reaction product. The optical absorbance is measured using light of a suitable wavelength. The concentration is determined by means of a calibration curve obtained using known concentrations of the determinant. The UV method is similar to this method except that UV light is used.

Atomic Absorption Spectrometry (AAS) is used for determination of metals. It is based on the phenomenon that the atom in the ground state absorbs the light of wavelengths that are characteristic to each element when light is passed through the atoms in the vapour state. Because this absorption of light depends on the concentration of atoms in the vapour, the concentration of the target element in the water sample is determined from the measured absorbance. The Beer-Lambert law describes the relationship between concentration and absorbance.

For ionic materials, the ion concentration can be measured using an ion-selective electrode. The latter are part of laboratory electrochemistry instruments for water analysis that also include titrators, pH meters, conductivity meters as well as other meters or probes for measuring specific analytes, as well as dissolved oxygen (DO), chemical oxygen demand (COD) and biochemical oxygen demand (BOD). In-field and process electrochemistry techniques include systems to measure pH, oxidation reduction potential conductivity, DO, and selected ions. The measured potential is proportional to the logarithm of the ion concentration.

Electrochemical procedures involve placing electrodes in a water sample and measuring either an electrical potential (voltage), in millivolts, or a current, in milliamperes, which is related to the concentration of analyte. Depending on what they are designed to measure, electrodes can be simple pieces of metals such as gold, silver or they may be elaborate systems with semi-permeable membranes and several internal electrodes and filling solutions. The instrumentation may be capable of reading out directly in concentration units. Usually, some sort of calibration procedure is necessary, using one or more standard solutions of known concentration. Electrochemistry techniques make up the largest segment of the laboratory and process water testing and analysis market.

Biochemical Oxygen Demand (BOD) is a test for measuring the amount of biodegradable organic material present in a sample of water. Depletion of oxygen in receiving waters has historically been regarded as one of the most important negative effects of water pollution. The (five-day) BOD of water is the amount of dissolved oxygen taken up by bacteria in degrading oxidisable matter in the sample, measured after five days incubation in the dark at 20°C. The BOD is simply the amount by which the Dissolved Oxygen (DO) level has dropped during the incubation period. This technique is the basis of BOD analyses for all types of sample even though considerable extensions of procedure are necessary in dealing with wastewaters and polluted surface waters.

Monitoring BOD removal through a treatment plant is necessary to verify proper operation. However, because the test takes too long to be useful for short-term control of the plant, the chemical or instrumental surrogate tests are often used as guides. The five-day BOD value in a properly conducted test usually amounts to some 65% of the total carbonaceous oxygen demand. To measure the latter in the BOD test would take some four times as long and would involve special measures to counter the side-effects of oxidation of nitrogenous matter. So chemical methods have been devised to obtain a rapid, accurate measurement of the total carbonaceous oxygen demand. In any such method, the only organic compounds affected will be those amenable to oxidation by the particular chemical agent used. Chemical Oxygen Demand (COD) refers to the test in which potassium dichromate is used to carry out the oxidation. The COD test procedure involves the use of additional reagents to catalyse the oxidation of organic matter and to suppress the effects of interfering substances such as chloride, and, as a result, in many cases the oxidation achieved is at or very near the maximum level. Application of the COD/BOD ratio to the results of a quickly performed COD test is very useful for the analyst and for the plant manager.

The determination of total organic carbon (TOC) is complementary to the oxygen demand analyses (BOD and COD) and is regarded as a better indicator of organic content in that it is a direct measurement of the key element. Also, it is theoretically independent of the form in which the carbon exists in the water and the analyses should therefore be comparable for a wide range of organically polluted waters. The TOC is done instrumentally. The organic carbon is oxidised to carbon dioxide by burning or by chemical oxidation in solution. The carbon dioxide gas is swept out and measured by infrared spectrometry or by re-dissolving it in water and measuring the pH change (the gas is acidic.) Both COD and TOC can often be correlated with BOD for a specific wastewater sample

Chromatography is a separation method based on the affinity difference between two phases, the stationary and mobile phases. This technique got its name, which means ‘colour picture,’ because it was first used to separate coloured pigments from a single spot on a piece of paper. A solvent, such as alcohol, is allowed to move slowly across the paper, and the different components of the pigment travel at different rates. The result is a series of separated spots of different colours. They move at different rates because of differences in the pigments’ relative attraction to the paper (the ‘stationary phase) and their solubility in the solvent (the mobile phase). This principle is used in modern instrumentation to separate mixtures of organic chemicals or inorganic ions. The components can be identified by their retention times or how long it takes them to pass through the instrument, and detectors can be used to measure the amount of each component.

Gas Chromatography (GC) permits the identification and quantification of trace organic compounds. In GC, gas is used as the mobile phase, and the stationary phase is a liquid that is coated either on an inert granular solid or on the walls of a capillary column. When the sample is injected into the column, the organic compounds are vaporised and moved through the column by the carrier gas at different rates depending on differences in partition coefficients between the mobile and stationary phases. The gas exiting the column is passed to a suitable detector. A variety of detectors can be used, including flame ionisation (FID), electron capture (ECD) and nitrogen–phosphorous. Since separation ability is good in this method, mixtures of substances with similar structure are systematically separated, identified and determined quantitatively in a single operation.

A more positive identification is possible using a mass spectrometer (MS) as the detector. In a MS, an ionised vapour is passed between magnets or radio frequency coils which separate the ions by mass (actually by charge to mass ratio). The pattern produced is characteristic of the particular substance, which can be identified by comparison with computerised ‘libraries’ of mass spectra. In the GC/MS method, as the gas emerges from the end of the GC column opening, it flows through a capillary column interface into the MS. The sample, then, enters the ionisation chamber, where a collimated beam of electrons impacts the sample molecules, causing ionisation and fragmentation. The next component is a mass analyser, which uses a magnetic field to separate the positively charged particles according to their mass. Several types of separating techniques exist; the most common are quadrupoles and ion traps. After the ions are separated according to their masses, they enter a detector.

For substances which cannot easily be vaporised because of high boiling point or instability at higher temperatures, there is a liquid version of this technique called High Performance Liquid Chromatography or HPLC. Organic solvents are used as the mobile phase. Detection of the separated compounds is achieved through the use of absorbance detectors for organic compounds and through conductivity or electrochemical detectors for metallic and inorganic compounds. UV light absorption is often used for detection.

In Ion Chromatography (IC), the target analytes are charged inorganic or organic substances. The mobile phase is an aqueous (water-based) solution, and the stationary phase is made up of an ion-exchange resin. Colorimetric, electrometric or titrimetric detectors can be used for determining individual anions. This technique can be used to measure the concentrations of several important inorganic anions, such as fluoride, sulphate, phosphate, and nitrate all in one analysis. IC provides for both qualitative and quantitative determination of anions n the mg/L range from a single analytical operation requiring only a few millilitres of sample and taking approximately 10 to 15 minutes for completion. IC can be used to monitor suspected carcinogenic by-products formed by oxidation of the halides–chlorate, chlorite and bromate– when drinking water and mineral water are disinfected

Voltammetric trace and ultratrace analysis of drinking water, groundwater, surface water, seawater and wastewater is used to determine electrochemically active inorganic ions. It is frequently employed to complement and validate spectroscopic methods. Its features are compact equipment, relatively low investment and running costs, simple sample preparation, short analysis times and high accuracy and sensitivity. In addition, unlike the spectroscopic methods, voltammetry is able to distinguish between different oxidation states of metal ions (speciation) as well as between free and bound metal ions. This provides important information regarding the bioavailability and toxicity of heavy metals. Important fields of application include environmental monitoring, limnology, hydrography, oceanography, marine biology and soil science. Many toxic transition metals and a few anions can be determined voltammetrically with a high degree of sensitivity and without prior sample preparation in drinking water and groundwater. These include zinc, cadmium, lead, copper, thallium, nickel, cobalt metal ions and even uranium (For adults, the World Health Organisation (WHO) recommends a drinking water limit of 15 μg/L for uranium, which is radioactive and highly toxic.) Apart from heavy metals, voltammetry can also be used for trace analysis of a few anions. For example, free cyanide in a concentration range of 0.01 – 10 mg/L can be determined easily and reliably even in sulphide-containing solutions with a large excess of phosphate, nitrate, sulphate and chloride. Apart from its use to determine total metal concentration, voltammetry makes it possible to distinguish between the different oxidation states and also between free and bound metal ions. Important applications in seawater analysis include the determination of a series of transition metals, some of them toxic like chromium, cadmium, lead, copper and iron.