Water testing by simple UV/Visible spectrophotometric analysis
- Table 1. The most common parameters which can be determined in water by UV/Vis spectrophotometry.
The quality of water, be it water for domestic use, bottled water or even industrial effluent is subject to tight regulatory control. It is the responsibility of the producer of the water to ensure that the levels of certain analytes all fall within the limits specified in the relevant legislation. Many water producers think that checking that their water is within the statuory limits must require a full battery of sophisticated, high-tech analytical instrumentation.In many cases this is not so. This article describes how a UK cardboard manufacturing company, with neither specialised chemists nor a chemistry laboratory, used a straightforward spectrophotometer to check that the levels of Cu(II) in their effluent were within limits.
Authorities providing domestic water supplies have a statutory obligation to ensure that the quality of their final product is safe for consumers, who generally take the quality of the water from their tap for granted. Depending on the original source of the water, it can however contain many contaminants which, above certain concentrations, can be downright dangerous. Arsenic, for instance, is an obvious hazard to human health; it can be present in water if the source water has filtered through arsenic-rich rocks. Arsenic can also contaminate the water supply as a result of mining or industrial activities in the area from which the water is sourced. Whatever its origin, the level of arsenic must be strictly controlled. Another common polluant in water is lead, which is unfortunately widespread in the environment and can enter the water supply from a variety of sources such as vehicle exhausts and old lead-containing paint. Lead levels can easily reach unacceptable levels in water. Yet another common contaminant is antimony; the major sources of antimony are waste incinerators, metal processing works, mine workings and as by-products of coal and oil combustion. Aluminium is widely present in the soil and is therefore frequently found at toxic levels in untreated sources of drinking water. Aluminium compounds are also used to remove impurities at water treatment works; it must be ensured that such processes do not introduce an unacceptable level of aluminium contamination into the very water being treated. Through so-called run-off pollution from agricultural land, several constituents of fertilisers, such as nitrates and nitrites, can reach levels in water that are toxic, especially for infants. These are just the most obvious contaminants — there is a long list of potentially toxic substances for which drinking water must be tested.
The statutory testing limits do not only apply to potentially dangerous substances in source water. Even relatively harmless components such as minerals in bottled waters must be determined. Likewise effluent waters must be carefully monitored, before they are discharged into public water systems.
A wide variety of analytical techniques and instruments have been used for the determination of the overall chemical quality of waters. These techniques range from simple pH measurements, ion selective electrodes, ion chromatography and UV/Visible spectrophotometry through to mass spectrometry and its hyphenated variants, as well as chromatographic procedures such as HPLC, GC, etc. However tests for water safety should ideally be easy, fast, accurate, sensitive, reliable and reproducible. In addition, any method chosen should be able to carry out selective testing of a very wide range of common analytes. UV/Visible spectrophotometry fulfils most of these criteria and is perhaps the simplest technique to use.
The basic principle of UV/Vis spectrophotometry is described by the well-known Beer-Lambert law which can be used to determine the concentration of a specific analyte in a sample at a specific wavelength:
A = e x l x c, where, at a specific wavelength, A is the measured absorbance, e is the molar absorbtivity or extinction coefficient (M-1 cm-1), l is the path length (cm), and c is the molar analyte concentration (M). Provided that the sample is measured at a level at which the absorbance of the analyte or its derived chromophore varies linearly with concentration, the Beer-Lambert law can be reliably applied in practice.
There are many parameters that can be determined using the simple application of Beer-Lambert in UV/Visible spectrophotometry. Table 1 provides a list of the most common parameters that can be determined in water by UV/Visible spectrophotometry. Since most of the analytes cited do not naturally absorb UV/Visible light, they must first be derivatised to produce a UV/Visible chromophore complex. After derivatisation, the concentration of the majority of the parameters listed can easily be determined by measurement in the visible wavelength region.
- Figure 1. The Cecil AquaQuest spectrophotometer.
- Figure 2. Molecular structure of the complex formed by the interaction of Cu(II) ions with cuprizone. The absorbance of the complex at 605nm is directly proportional to the concentration of Cu (II) ions in the original sample.
Case study: effluent from cardboard manufacture
Some months ago, a UK cardboard manufacturing company was found to be inadvertently discharging cupric ions Cu (II) into the local water course. The resultant environmental pollution occasioned a heavy legislative fine. Although the manufacturing company in question had modern laboratory facilities for testing packaging, it had no analytical chemists or specialised chemical testing instrumentation.
The precise source of the contamination was not known but it was suspected that some of the cardboard manufacturing processes may have produced copper-containing effluent. Procedures had to be implemented so that, prior to discharge into the local water course, effluents could be quickly monitored for copper content. The employment of a dedicated analytical chemist or the setting up of a specialised analytical laboratory could not be justified economically. It was therefore necessary to find an easy, fast, accurate, sensitive, reliable, reproducible and selective method of determining copper (II) content, which could be safely used by non-chemists. As the data would be being reviewed by regulatory authorities, any chosen analytical method would have to provide timed and dated test results that would be recorded and saved in an easy but tamperproof manner.
After consultation, it was decided to use a Cecil AquaQuest visible spectrophotometer for copper determination [Figure 1]. This spectrophotometer contains an integral printer and can be used with a PC. Not only did the instrument meet all of the necessary criteria for Cu(II) determination, but in addition, its use could be expanded at any time to cover the determination of other analytes. In addition, if the cardboard manufacturing company were to expand in the future, the robust and durable instrument could be used by an analytical chemist for both routine and R&D lab work.
Determination of copper (II)
The test principle is based on the quantitative formation of a blue complex when Cu (II) ions react with cuprizone (oxalic acid bis cyclohexylidene hydrazide) in an ammoniac solution [Figure 2]. The absorbance of the complex can be measured photometrically at 605 nm and the accuracy of the results is greater than 90%. A proprietary reagent test kit, validated over the measurement range 0.1 mg/L - 6 mg/L of copper (II) was used. Copper (I) and/or total copper could also have been measured, by additional sample pre-treatments. The chosen reagent test kit is specific for copper (II) even in the presence of known concentrations of other
analytes within the sample matrix.
In addition to the spectrophotometer, glass cuvettes and the proprietary reagent test kit, the cardboard manufacturing company only required a few common laboratory reagents and basic equipment, such as an automatic 5 mL dispensing device, deionised water, universal indicator paper, 1M sodium hydroxide and test tubes to carry out the determinations.
With the aid of clear instructions provided with the reagent test kit, indicator paper is used to check the pH of a sample of the normally clear effluent prior to testing. Two test reagents are then sequentially added in excess to a 5mL aliquot of the sample; the reaction is left to stand for five minutes. Deionised water is used as control.
After following the screen-prompted test menu, automatic readings are taken at 605 nm by the spectrophotometer and the results of the analysis are printed out in the required units.
In this case, a Merck Spectroquant was chosen as test kit since the Cecil AquaQuest specrophotmeter is pre-programmed with some two hundred Merck Spectroquant test methods. Of course, had the cardboard manufacturing company wished to use other proprietary reagent test kits or use their own appropriate ‘off the shelf’ reagents, then the automatic calibration curve and method-creation facilities of the spectrophotometer could have been used. Once validated and approved, the new method is simply stored in memory for subsequent routine use.
The staff of the cardboard manufacturing company required only minimal training before they were able to produce reliable and accurate results. The use of the AquaQuest spectrophotometer to monitor the levels of copper in the effluent before discharge into the water course resulted in a complete halt to the environmental pollution.
Cecil Instruments Limited
Milton Technical Centre,
Tel: +44 1223 420821