This article discusses the application of techniques to quantify ions present in the mg l-1 range. Many of the instrumental methods for ions within the mg l-1 concentration range need little sample preparation. The instrumental method then becomes just one part of a more complex analytical procedure.
A Complete Guide for Analyzing Common Ions Present in Water
Ultraviolet and Visible Spectrometry:
This spectroscopic technique is based on Beer-Lambert law.
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At sufficiently low concentrations, the Beer-Lambert law is followed:
A=εcl … (16.24)
Where, A is the absorbance of radiation at a particular wavelength [= log(I0/I)], I0 the intensity of the incident radiation, I the intensity of the transmitted radiation, ε the proportionality constant (molar absorptivity [1 mol-1 cm-1)], c the concentration of the absorbing species (mol 1-1) and 1 the pathlength of the light-beam (cm).
The instruments used to measure the absorption of light can range from sophisticated laboratory instruments which can operate over the whole ultraviolet/visible range to portable calorimeters employing natural visible light, which are used as field instruments. This makes absorption spectrometry one of the most useful and versatile techniques for an environmental analyst.
After all, none of the common ions in water absorb light in the visible region of the spectrum. In addition, the only ions commonly found in water which absorb in the ultraviolet range above 200 nm are nitrate and nitrite.
The main use of the technique involves the analysis of light-absorbing derivatives of these ions. This can be carried out for almost all of the common anions (except sulphate), as well as ammonia.
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We can summarise as follows:
Analysis by direct absorption of: nitrate.
Analysis after formation of derivative:
(i) Chloride,
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(ii) Fluoride,
(iii) Nitrate,
(iv) Nitrite, and
(v) Phosphate.
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As an example of such an approach, the procedure for phosphate involves the addition of a mixed reagent (sulphuric acid and ammonium molybdate, ascorbic acid and antimony potassium tartrate) to a known volume of sample, making up to the working volume, shaking and leaving for 10 minutes. A blue-coloured phospho-molybdenum complex is produced and its absorbance is measured at 725 nm.
Ultraviolet/visible spectrometry is the first technique where, at low concentrations, there is a simple linear correlation between the instrument response and the concentration of the unknown.
This technique for a quantitative analysis can make up a series of standard solutions of known concentration of the unknown and from this construct a calibration curve. The concentration should be within the range over which the Beer-Lambert law applies and thus a straight-line graph will be produced.
Above this range, the calibration will no longer be linear and the solutions should be diluted. The ‘best- fit’ calibration line can readily be calculated by using the method of least-squares found on standard PC spreadsheets or even the most basic scientific calculators. The absorbance of the unknown can then be measured and from this the concentration calculated.
This procedure is known as calibration by external standards.
You will find a great diversity of instrumentation based on these chemistries for high-throughput laboratory analysis. A number of instruments are based on continuous flow, with a schematic of a typical system being shown in Fig. 16.9.
Instead of prior mixing of the reagents for each analysis, streams of each reagent (segmented by air bubbles to diminish premature mixing effects) in narrow-bore tubes are mixed by combining the flows at a T-junction or within a (mixing) cell.
A sample is introduced from an automatic sampler as a continuous flow into the reaction stream. The combined flow is then led into a spectrophotometer and the absorption measured. The flows of all of the reagents and samples are controlled from a multi-channel peristaltic pump.
Other instruments are based on flow-injection techniques where individual aliquots of sample are injected into a continuous flow of water. Colour-forming reagents are then added also via a continuous- flow system. The mixing of the reagents and samples is dependent on the length and diameter of the tubing.
After time being allowed for the formation of the colour, the absorbance of the solution at a specific wavelength is then measured. The response of the instrument is in the form of a peak, with the peak height being proportional to the sample concentration.
If there is a requirement to analyse more than one of the ions, then discrete analysers may be used. In such systems, the samples are introduced into vials on a rotating carousel. As the carousel rotates, reagents are added and mixed, time is allowed for colour development and the light absorbance at a specific wavelength is then determined. If the instrument is suitably configured, several different analyses can be performed simultaneously on samples by one instrument.
Field Techniques:
Field techniques are becoming increasingly important for giving immediate measurement of ion concentrations. Un-manned field stations can be set up by using the automatic procedures. Alternatively, portable (often hand-held) instruments may be used.
The procedure for the colour-forming reaction has to be made simple. No one wishes to perform complicated analytical routines on a muddy riverbank. Calibration of the instrument should avoid the use of standard solutions, which, once again, are inconvenient in the field. The optical components of the instrument should be minimised or at the very least, be made robust.
Each manufacturer has a different approach to such modifications. Colour-forming reagents may be pre-measured in the form of tablets, or in solution. As a further simplification, one manufacturer seals the reagents under vacuum in an ampoule.
Breakage of the top under water automatically draws the correct sample volume into the ampoule. Coloured glass or moulded plastic standards are often used rather than solutions.
These can be in the form of a disc. One manufacturer’s design contains glasses of different optical density. The disc is rotated through the light beam until the colour of the standard glass matches that of the unknown.
Alternatively, a moulded plastic cube may be used which has a stepped side to provide a number of possible pathlengths (and hence absorbances). The most simple procedure for quantification is by visual comparison of the colour of the standards and the unknown using available sunlight.
Alternatively, portable spectrometers are available, often housed in briefcases, along with titration equipment and pH and conductivity electrodes—these are known as ‘water quality’ test kits.
The most obvious way of expressing the concentration of ions is as the mass of the ion per unit volume (mg l-1), but sometimes you will find other units, most notably as the concentration of the major element within the ion. This alternative method is most common for the nitrogen-containing ions.
Nitrate, nitrite and ammonium are often expressed as mg l-1 of NO–3, NO–2 and NH4+, respectively, but can all be expressed as mg l-1 of nitrogen (mg l-1 N). It then becomes easy to compare the relative concentrations of species without having to use molarities.
If all of the ammonia in a water sample which contains a concentration of 2 mg l-1 (expressed as nitrogen) is totally converted into nitrate, then the water will contain a nitrate concentration of 2 mg l-1 (also expressed as nitrogen).
This is easier for a non-specialist to understand than by saying that 3.09 mg l-1 NH+4 will produce 8.86 mg l-1 NO–3. Difficulties can arise because the two systems are sometimes used in parallel.
Emission Spectrometry (Flame Photometry):
Emission spectrometry relies on the principle that, for some metals at low concentrations, the intensity of light emitted from an electronically excited atom (usually produced by introduction of the sample into aflame) is proportional to the concentration of the excited species. Simple and inexpensive instrumentation is available, often known as ‘Flame Photometers’.
Flame photometry seems almost ideally suited to the analysis of environmental water samples.
1. Although the use of flame photometry is limited to a few alkali metal and alkaline-earth ions, this includes sodium, potassium and calcium, three of the four major cations present in water.
2. The linear concentration ranges [0-10 mg l-1 (for sodium and potassium) and 0-50 mg l-1 (for calcium)] are within that expected for environmental water samples. Little sample preparation is needed.
3. The instrument is simple to use and the only laboratory requirements are a gas supply (natural gas is adequate) and a source of vacuum. This can be easily installed in temporary laboratories for analysis close to the sampling site.
It is a pity that flame photometers cannot be used to analyse the fourth common ion, i.e. magnesium, as all of the routine analytical requirements for metal ions could then be satisfied by this simple method. Analysis for magnesium is usually carried out by using atomic spectrometry.
The major disadvantage of flame photometry is the variation of the response of the instrument with time (i.e. drift). Great care has to be taken to ensure that calibration of the instrument and the analytical measurements are performed quickly after each other. It is also good practice to repeat the calibration after the analysis to check that no variation has occurred.
Ion Chromatography:
The methods we have looked at so far have been for the analysis of individual ions, but sometimes a complete analysis of all of the ions in the sample is needed. Chromatographic separation of the ions is an obvious approach. Liquid chromatography would seem particularly useful since the species to be analysed are already in solution.
From your reading elsewhere, you will be familiar with the principles of High Performance Liquid Chromatography (HPLC) and how its application over the last three decades has expanded to include virtually all soluble ions and compounds.
The major application in environmental analysis has been for inorganic anions. Several variations of the liquid chromatographic technique have been developed which normally use specialised ‘ion chromatographs’. The most sensitive systems are often those which use a technique known as ion suppression.
The separation of the anions is achieved by using an ion-exchange column (length 10-25 cm, 3-4.6 mm i.d.), usually based on poly(styrene-divinylbenzene) or another organic polymer, with an eluent typically containing sodium hydroxide or a sodium carbonate/ hydrogencarbonate buffer.
Detection of the analyte ions is achieved by monitoring the increase in conductivity of the eluent produced by the ions as they pass through the detector. In order to maximise detection sensitivity, prior to passing to the detector, all buffer ions have to be removed from the eluent as these would contribute to the background conductivity.
The sodium ions in solution are replaced by hydrogen ions. The hydroxide ions react to form water (Eq. 16.25). Carbonate and hydrogencarbonate react to form carbon dioxide Eqs 16.26 and 16.27, which has little conductivity in solution.
Hydroxide eluents:
OH– + H+ ® H2O … (16.25)
The suppressor has to provide, uninterruptedly, precisely the correct number of protons for the neutralisation. There are a number of methods used to achieve this. All of these are based on the ion- exchange process.
One manufacturer uses a continuous suppression system, as shown in Fig. 16.14. The eluent passes between cation-exchange membranes, through the detector cell, and is finally recycled on the outside of the membranes. The H+ ions necessary to replace the Na+ ions in the fresh eluent are generated by electrolysis of the recycled eluent, with the H+ ions being generated at the cathode.
Other manufacturers use cation-exchange columns which need periodic regeneration. This can be achieved without interruption of the analytical operation of the chromatograph. In one system, there is a carousel of regeneration columns with one column being regenerated while another one is in use. Another system regenerates the column while the following sample is being loaded. Disposable regeneration columns may also be used.
The response plotted in the chromatogram is conductivity. As the latter is directly proportional to the concentration of the ion, quantification can be simply carried out by comparison of the peak area of the unknown with that of a standard of similar concentration, i.e. by external standards.
For ions of interest in environmental water found at mg l-1 concentrations and with a suppressed system the sample would need to be diluted before injection. This, along with filtration, is often the only sample preparation necessary and common ions in water can be determined within the space of a few minutes (Fig. 16.15).
Non-suppressed ion chromatographs monitor the conductivity of the eluent directly, i.e. without the suppressor. Although the sensitivity is lower than that of suppressed systems, it is still sufficient to determine ions at mg l-1 concentrations.
Non-suppressed systems have the advantage of being less complex instruments than suppressed chromatographs. Separation columns can be used with a wider range of eluents.
The instruments resemble conventional liquid chromatographs (the components being simply pump, analytical column and detector) but often they contain no metal components in contact with the eluent and with the pumps designed to operate at lower pressures than is necessary for conventional HPLC. This reflects the lower back pressures found with ion chromatography when compared with reversed phase liquid chromatography.
Although most analysis nowadays would use specialised ‘ion chromatographs’, conventional high performance liquid chromatography may still find some application. Methods developed for conventional HPLC can use either a reversed phase column and ion-pair techniques or an ion-exchange column.
Ultraviolet absorbance and conductivity detectors are used. When a conductivity detector is employed, the system becomes similar to the specialised chromatographic set-up without ion suppression which has already been described. The sensitivity is lower in comparison with the specialised system, although it is still sufficiently high to analyse common anions at mg 1-1 concentrations.
Although the most common use of chromatography is for anions, similar methods have been developed for specialised ‘ion chromatographs’ for the separation of the common cations (Na+, K+, NH+4, Ca2+, Mg2+ etc.) in a single isocratic run.
A typical eluent would be methanesulphonic acid. This would allow ion suppression similar to that used for anions, although this may not be necessary at typical natural water concentrations.
Examples of the Use of Other Techniques:
We have now discussed the most widely used methods for analysis of the common ions. There are, however, a few frequently used techniques which have not yet been covered. We will look at the analysis of three species—ammonia, fluoride and sulphate—to exemplify these techniques.
Ammonia:
Ammonia is the only alkaline gas commonly found in environmental water. If extracted from the sample, the ammonia can be determined by a simple acid-base titration. Magnesium oxide is added to the sample to make it slightly alkaline. The ammonia is then present predominantly in the form of NH3, rather than the less volatile NH+4. Ammonia is then distilled off and absorbed into boric acid solution.
For rapid screening of samples, it is possible to use an ion-selective electrode (i.e. an electrode whose potential, measured with respect to a reference, is proportional to the log of the activity of one particular ion).
A combination pH electrode is simply an ion-selective electrode responsive to hydrogen ions and a reference electrode housed in a single body. Ion-selective electrodes are available for most common ions and gases which dissolve as ionic species, although they do have some limitations.
Many are prone to interference from other species and thus have poor precision. Even the pH electrode has taken many years of development to produce reliable responses. All of them respond to ionic activity rather than concentration, and so it is essential to add a large excess of an ionic salt to both the standard solutions and the unknown in order that the ionic strength of each solution is identical.
Ammonia electrodes are of the gas-sensing type. The ammonia diffuses through a permeable membrane and causes a pH change in a small volume of internal solution, which is sensed by a ‘glass’ electrode. Prior to measurement, concentrated sodium hydroxide solution is added to the samples and standards.
This serves to increase the pH to above 11 to ensure that the ammonia is in the unprotonated form, and also to provide a constant ionic strength. The ammonia electrodes respond only to gaseous alkaline gases. For most environmental applications (except in the analysis of heavily polluted water), there will then be little possibility of interference. Calibration is by external standards.
A second electrode which has found widespread use for water analysis is that which detects fluoride ions. This is a solid-state electrode where the electrical potential is generated by migration of the ion through a doped lanthanum fluoride crystal. This once again gives extremely high specificity to the analyte ion, with the only pre-treatment necessary being the addition of buffer solution to maintain constant pH and ionic strength. Alternative techniques for fluoride determination are spectrometry and ion chromatography.
There is no direct calorimetric method available for sulphate and ion-selective electrodes for the ion are not very reliable; in fact, the only direct instrumental method is by using ion chromatography. Virtually every other method is based on precipitation of an insoluble sulphate.
Barium or 2-aminoperimidinium salts are used for the precipitation. The precipitate formed may then be weighed for a direct determination of the sulphate. This represents one of the few remaining important applications of gravimetric analysis.
Other methods using insoluble salt precipitation are indirect, estimating the excess cation after precipitation of the sulphate. Excess barium may be determined by titration or by atomic absorption spectrometry Excess 2-aminoperimidinium ions may be estimated by visible spectrometry.
None of these methods would appear ideal for a high-throughput laboratory. For most samples, sulphate would be the only major sulphur-containing species. Total sulphur in solution, as determined by an elemental sulphur analyser, will then give a good estimate of the sulphate concentration.
Trace Pollutants in Water:
Several trace elements (few ppm or less) are found in polluted water. The most dangerous among them are the heavy metals, e.g. Pb, Cd, Hg and metalloids, e.g. As, Se, Sb, etc. The heavy metals have a great affinity for sulphur and attack sulphur bonds in enzymes, thus immobilising the latter.
Other vulnerable sites are protein carboxylic acid (COOH) and amino (-NH2) groups. Heavy metals bind to cell membrane, affecting transport processes through the cell wall. They also tend to precipitate phosphate biocompounds or catalyse their decomposition.
The trace elements in natural waters and waste-waters are summarised in Table 16.3. The sources of heavy metals in surface waters are shown in Fig. 16.17. Street dust containing heavy metals, e.g. Pb, represent an important source of metal input to surface waters.
Metals are contributed by industrial effluents as domestic sewage. All these sources may be routed by way of sewage treatment works which reduce significantly the amount of metal discharged. In developing countries, however, such sewage treatment works hardly exist or function.
Another major cause of concern is the presence in water of a number of non-bioaccumulative organic compounds with adverse toxicological properties. For many years, there has been much concern over compounds suspected of being carcinogens.
A typical example would be chloroform which can be produced in trace quantities during the disinfection of water by chlorination and which is thought to be harmful at μg l-1 concentrations. Of more recent concern is the large number of compound types considered to be endocrine disruptors. These compounds can range from pesticides, through components of common plastics, to active ingredients in the contraceptive pill.
In the early days of instrumental analysis the concentrations would have been beyond the capabilities of the available instrumentation and techniques but developments since then have made such analyses routine.
This is partly due to the development of more sensitive instrumentation, but also through the development of suitable pre-treatment processes. This is required to remove potential interferences and, for many techniques, to increase the analyte concentration to within the instrument sensitivity.
Organic Trace Pollutants:
The range of organic compounds which may be found in environmental waters includes the following:
1. Naturally occurring compounds from decaying organic material.
2. Pollutants discharged or escaping into the environment.
3. Degradation and inter-reaction products of the pollutants.
4. Substances introduced during sewage treatment.
Analysis of Compounds Present in Water:
Typical analyses could include:
1. Analysis of individual compounds or groups of compounds of environmental concern.
2. Total analysis of all organic components above the limit of detection. This is an enormous task and at the lower end of the concentration range there will almost invariably be unidentified components.
3. Field screening for specific pollutants prior to laboratory analysis.
4. Qualitative identification of trade products in spillages or discharges.
The properties of compounds causing widespread environmental problems include toxicity, slow biodegradation and the ability to bioaccumulate within organisms.
The types of organic compound may be:
1. Pesticides, particularly those containing chlorine.
2. Chlorinated solvents.
3. Polychlorinated biphenyls.
4. Dioxins.
5. Endocrine disruptors.
For more localised pollution problems, we could extend our list of concern to include virtually every organic compound currently in use or production, together with their reaction and degradation products.
For the purpose of grouping into suitable analytical techniques, organic pollutants are often classified as being either ‘volatile’ (e.g. chloroform) and semi-volatile (e.g. most pesticides). The two groups may have different extraction and clean-up methods.
Analysis of complex mixtures of organics would normally involve the chromatographic separation of the components. As most organic compounds have significant volatilities even at room temperature, gas chromatography would be expected to be a useful technique.
The alternative of high performance liquid chromatography is used only where there are advantages over established gas chromatographic methods, although the number of applications of this technique is increasing.
A major area where non-chromatographic methods are used is in the determination of groups of compounds such as phenols and also of classes of detergents, where the total concentration of the group of substances is required rather than the concentration of individual compounds.
Groups of organic compounds can be analysed by ultraviolet/visible absorption spectrometry which appears ideal. Absorptions are broad and the molar absorptivities often vary little between compounds within groups.
A single absorption measurement could be used to determine the total concentration of the group. Although there may be suitable volumetric techniques for individual groups of compounds they would not be sufficiently sensitive for concentrations in the μg l-1 range.
The current desire for field screening has lead to novel approaches which may not have widespread use in other areas of chemical analysis. These include the use of immunoassays.