Analytical determination of ions in water
Water analyses are carried out by many different laboratories for a whole variety of constituents for as many different reasons. Requirements include ensuring potable water meets current standards, checking that purification processes have been successful, the environmental testing of lakes and rivers and confirming the purity of purified water for different applications. Concentration ranges of interest cover sub-ppt concentrations to % levels.
Techniques that involve the direct determination of ions present in the water play key roles in these analyses. The measurement of specific ionic constituents in water is used in two ways:
- To provide direct determination of the speciation of various molecular or valence forms of an element, and
- To provide elemental analysis either directly or after chemical conversion into a measurable form.
The major techniques used involve direct species determination by voltammetry, potentiometry, photometry or separation of the ions chromatographically and sequential measurement by a non-specific detector, such as conductivity.
This approach has been further extended by combining separation methods with selective elemental analysis techniques such as AAS, ICP-OES, MS and ICP-MS to provide information on specific components. The types of method all depend on the availability of purified water for the preparation of reagents, standards and blanks, for sample pretreatment and for rinsing components.
This paper reviews the principles, sensitivities and particular features of each type of analysis and assesses their specific water purity needs.
Ion selective electrodes
The most frequently determined ion in water is the hydrogen ion. The correct pH is clearly critical in many applications as it has a major effect on chemical equilibria. The ubiquitous glass electrode is now being supplemented by more robust alternatives.
The usefulness of pH is very dependent on the ionic content of the solution. At very low overall levels of ions, as, for example, in highly purified water, the pH can only be measured reliably with special electrodes in a flow-through cell to prevent contact with the atmosphere.
The addition of potassium chloride to provide a higher ionic strength environment can give greater stability but is still prone to contamination.
Perhaps fortunately, pH has little real significance in high purity water where the quantity of acidity or alkalinity is bound to be negligibly low for virtually all laboratory applications. Only 1.0 ppb hydrogen ion is needed to give a pH of 6.0 in pure water. Clearly, other ion selective electrodes are also used for the direct determination of ions but these are mainly in monitoring applications or for the comparison of large numbers of samples. They depend on the complex interactions between the electrode surface and the solution and so are prone to interference and drift from species that can affect this eg, fatty organics and similar ions. The electrodes for fluoride and ammonium have proved particularly useful.
The well-established titrations for alkalinity, hardness and chloride are still widely used. Potentiometric end-point detection with pH or ion selective electrodes has tended to replace indicator colour changes with the greater ease of automation and consistency. With increasing regulation any means of reducing the scope for human variability and judgement is popular.
With large volume samples detection limits can be reduced to ppm levels in some cases. This technique can be applied to chemical additions to water, for example, boric acid in the primary water circuit of light water reactors and mixtures of hypochlorite, chloride and chlorate.
Spectrophotometric analyses using colour reactions of specific ions are also still popular. With automated analysers they can prove quicker and more cost effective than, for example, ion chromatography.
Most ions can be determined in most types of water at ppm or lower concentrations. In some cases spectrophotometry offers particular advantages over other techniques. For iron the detection limit is lower than, for example, flame AAS, while for silica the colour reaction is extremely sensitive and also specific to soluble silicates, enabling them to be distinguished from total silicon, as measured by AAS or ICP-OES.
A suite of electrochemical techniques exists which can be applied to the determination of ions. Their particular strengths lie in their modest cost, their suitability for direct ion measurement in difficult matrices such as sea-water and the scope they provide for speciation.
In the basic form of voltammetry an electrical potential applied to a working electrode is varied in small steps and the resulting current recorded as a function of potential. This approach is suitable for concentrations at or above ppm levels.
The potential can be varied in steps, pulses or both, giving Staircase or Normal Pulse Voltammetry or Polarography (with a dropping mercury electrode) which are suitable for qualitative analysis and Differential Pulse or Square Wave Voltammetry for quantitative work. Greater sensitivity can be achieved using a stripping step to concentrate the trace components of interest.
Stripping analysis involves two stages, electrolysis followed by stripping. During electrolysis a specific potential is applied to the working electrode for a certain period of time during which the electroactive species in the sample are deposited on the working electrode. The longer the time the greater the sensitivity. The working electrode is glassy carbon covered by a thin film of gold or mercury, a hanging mercury drop electrode or a solid electrode.
Film electrodes offer the lowest detection limits. Stripping, during which deposited species are redissolved into the solution, is performed electrochemically or by chemical oxidation. Elements are identified by their characteristic redox potential.
Potentiometric stripping analysis
PSA uses chemical oxidation by dissolved oxygen or mercury (2) ions to determine metals such as lead, copper, cadmium and zinc.
The electrode potential is measured against time during stripping. The rest time at each metal's redox potential is proportional to the amount of the metal in the sample.
Constant current stripping analysis
CCSA uses the same measuring principle but applies a constant current to control stripping. Applications include the determination of mercury, nickel, cobalt, iron and arsenic.
Voltammetric stripping analysis
In ASV/CSV the electrode potential is gradually changed and the resulting current measured. The stripping current is proportional to the amount of species deposited. Most metals can be determined. Metals which form amalgams such as lead, copper, cadmium, zinc, tin, thallium, manganese, bismuth and gallium can be measured using a mercury film or drop electrode.
Similarly, the technique can be used for metals which form complexes that can be potentiostatically adsorbed on the electrode surface. For example, these include Ni2+ and CO2+ as their dimethylgloxime complexes.
Mercury, arsenic, antimony, tin, selenium and copper can be determined on gold film or gold electrodes.
Voltammetry can differentiate between oxidation states of an element and so can be used for environmentally significant assays such as the separate determination of Cr6+ and Cr3+ at ppb levels in aqueous samples by differential pulse polarography. The peak for chromium (6) is at -140 mV and that for total chromium is at -1440 mV.
A range of anions can also be determined at sub-ppm levels. These include cyanide, iodate, nitrite, thiocyanate, thiosulphate, sulphite and sulphur dioxide.
The mechanism relies on the formation of insoluble mercuro-compounds on the electrode surface during electrolysis and their subsequent reduction during stripping.
Electrochemical techniques, while lacking the wide elemental range and long linear response of some atomic and mass spectrometry technologies, offer a valuable alternative in a number of specific applications and have particular advantages for direct speciation and anion determination.
Ion chromatography (IC)
Ion chromatography is very well established as a major technique for ion determination. Over the years the technologies have continued to develop with a series of innovations to enhance sensitivity and selectivity and to simplify operation.
A mixture of ions is injected into an eluent stream. They are separated by their relative interactions with a column of ion exchange resin and detected sequentially, usually by conductivity.
Different columns are used for the separation of anions and cations. The major step forward in the applicability of the technique was the introduction of chemical suppression. By passing the eluent from an anion exchange column through a cation exchanger in the hydrogen form the salts that make up the sample peaks are converted into more highly conducting acids and the eluent is converted into weak acids or water. The overall effect is a substantial increase in sensitivity.
Cation sensitivity can be similarly enhanced with an anion suppressor. The suppression can now be carried out by electrical generation of the suppressor hydrogen or hydroxyl ions.
While most IC analyses are carried out with conductivity detection, amperometric UV/Visible spectrophotometric and fluorescence detectors are available for selective and more sensitive analyses.
A wide variety of IC columns have also been developed with different selectivities and capacities. They include highly specialised forms of ion exchanger with generally high surface material on the outer layers of solid spheres to give enhanced kinetics.
Standard ion chromatography with injections of 20 ÂµL of sample and separations of 5 to 15 minutes can be used to determine sub-0.1 ppm levels of common anions (fluoride, chloride, bromide, nitrite, nitrate, phosphate and sulphate) and cations (lithium, sodium, ammonium, potassium, magnesium and calcium) in a wide variety of water types.
The scope of the methodology is considerable; by varying the column, the pre- and post-treatment and the detector, most anions and a wide range of cations can be determined. Tighter specifications for impurities in drinking water have lead to the development of more sensitive methods using large volume injections and special columns.
For the analysis of highly purified water the use of preconcentration is needed to extend detection limits to ppt concentrations.
A large sample volume is passed through a concentrator column, which collects the anions. These are subsequently eluted with eluent and analysed in the usual way. In this way, volumes of up to 100 mL or more can be analysed.
By using a complexing agent such as pyridine-2, 6-dicarboxylic acid as eluent and adding a colorimetric indication such as 4-(2-pyridylazo) resorcinol after separation, transition metals can be detected colorimetrically. This approach also provides scope for speciation. ppt levels of common transition metals can be determined in one chromatogram. Silicate can also be detected at sub-ppb levels colorimetrically.
Purified Water Requirements
All the techniques described in this review require purified water for their successful application.
For trace ion chromatography and voltammetry the use of ultra-pure water is specified for the preparation of samples, blanks and standards but also for eluents and other reagents. The use of water with minimum contaminant levels of all types removes one area of potential variability in the analysis.
Impurities in the water can cause various problems ranging from direct interference from the presence of an ion being measured to overlapping peaks, shifts in background or inhibition of electrode or column activity. These can be caused by ionic, organic, colloidal or microbiological impurities. All need to be controlled and reduced to acceptable levels. In the Elga Purelab Option-E, potable water is treated sequentially by filtration, activated carbon to remove chlorine, reverse osmosis (RO) to remove over 95% of all types of impurities, ionic and organic and all particulates and micro-organisms. The water is then recirculated through an electro-deionisation stack to remove the remaining ionic impurities and an ultraviolet irradiation chamber to break down bacteria and organic molecules. The resultant water contains sub-ppb levels of ionic impurities and low ppb levels of total organic carbon (TOC).
The Purelab Maxima HPLC further extends the degree of water purification. It polishes water that has been pretreated with activated carbon and RO, by recirculation through activated carbon, the highest purity ion exchange resins, a UV photo-oxidation chamber and a 0.05 Âµm micro-filter or an ultrafilter to achieve water with a purity approaching that of absolutely pure water.
Impurities are at or below detection limits by even the most sensitive techniques.
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