Voltammetric Methods for Trace Analysis of Chromium Essay Example for Free

Voltammetric Methods for Trace Analysis of Chromium Essay Voltammetric methods of analysis, which have been used since the invention of polarography in 1922, witnessed a serious decline in use and was even threatened with extinction with the development of Atomic Absorption Spectrometry (AAS) in the mid-1960s (Bond, 1980, pp. 2-3). The remarkable detection limits of AAS, coupled with its ability to determine almost all the metallic elements, was beyond the reach of classical polarography, which had come to be regarded as a very unattractive technique due to its clumsy instrumentation. However, there has been a resurgence of interest in the electroanalytical techniques during the past years, mainly as a result of the appearance of vastly improved, commercially available instrumentation which has taken full advantage of the electronic revolution. In parallel with the instrumental developments, there have also been accompanying advances in the theoretical aspects of electroanalytical techniques with the development of, for example, ax, pulse and stripping techniques. As a consequence, voltammetry is now established as an extremely versatile, sensitive, rapid and inexpensive analytical technique which has found applications in most areas of analytical chemistry. The fundamental principles of polarography are described by Bond (1980), though he recounts developments in polarographic techniques that have led to the renaissance and widespread adoption of voltammetry. Over the last 15-20 years, there has been a revolution in the existing data regarding the distributions and chemical behavior of trace elements in natural waters. This revolution has been brought about by the realisation that any analytical methodology has to account for the risks of contamination as well as analyte losses involved during the sampling and sample handling steps. Thus clean techniques have been developed and adopted for the collection, preservation, storage and analysis of water samples for trace analysis. This, coupled with the advent of extremely sensitive techniques, has resulted in concentrations of trace elements in seawater being shown to be factors of 10 to 1000 times lower than those previously accepted (Donat, et al. , 1995, p. 247). This in turn has led to a demand for more accurate data to be generated at lower concentrations. The focus of this paper is to discuss voltammetric methods for the analysis of one of the biogeochemically important trace metals in natural water: chromium. Voltammetric Methods AAS (especially Electrothermal AAS) techniques are generally regarded as the ultimate methods of detection for ultra-trace analysis because of the detection limits attainable by these techniques. However, in the form of Anodic Stripping Voltammetry (ASV), voltammetry offers a technique that, in specific cases, can rival these techniques with respect to detection limits, reproducibility and ease of operation. The extreme sensitivity of ASV is due to the analyte preconcentration step inherent to the technique, whereas the spectrometric techniques rely on a prior analyte preconcentration step. Another advantage that ASV offers is that it can speciate the analyte species on the basis of their lability in the natural medium (Florence, 1986) The high sensitivity of ASV allows for the determination of metal speciation in natural waters without the necessity of external pre-concentration. ASV involves two steps: deposition step, which is an internal pre-concentration, during which a negative potential is applied at the mercury drop (i. e. the working electrode) and the metal ion is reduced to the metal which dissolves in the mercury drop forming an amalgam, followed by the stripping step, during which a positive-going potential scan causes re-oxidation of the metal in the amalgam. Thus, the amalgamated metals are stripped out of the mercury electrode and give rise to anodic peak currents, whose heights are proportional to the ASV-labile (i. e. ASV-measurable) metal species (Willard et al. , 1988, p. 719). The applicability of ASV is contingent on the metal to be determined being soluble in mercury to form an amalgam. This requirement severely limits its widespread application in environmental analysis and ASV has remained more or less confined to the determination of Cu, Pb, Cd and Zn. In this respect, the applicability of ASV is very restrictive, in contrast to the capability of AAS or ICP techniques, which are readily applicable for the determination of most of the elements in the Periodic Table (Willard et al. , 1988). In parallel with ASV, Cathodic Stripping Voltammetry (CSV) techniques have also been used for trace element analysis. Until relatively recently, this technique was viewed as the mirror image of ASV (Wang, 1985). In classical CSV, the analyte species is electrolytically preconcentrated as an insoluble Hg species on the electrode by the imposition of a relatively positive, constant potential during the deposition stage. The applied potential results in the formation of Hg22+ ions on the electrode surface. Analyte species capable of forming insoluble Hg compounds react with the Hg22+ to form an insoluble film on the surface of the electrode. During the stripping stage, a negative potential scan is applied on the electrode, resulting in the reduction of this insoluble compound to Hg0 and the original analyte ion. The faradaic current resulting from this reduction forms the analytical signal. In this preconcentration mode, CSV is applicable to the analysis of mainly anionic species and has been used for the analysis of halides, cyanide, sulphide and a variety of organic compounds (Wang, 1985). The applicability of CSV has now been extended to the determination of metallic species following considerable research into a new, non-electrolytic method of preconcentration during the last decade. This preconcentration method is based on the observation that many organic compounds exhibit surface active properties that are manifested by their adsorption from solution onto the surface of a solid phase. Adsorption has been regarded as an undesirable adverse effect in polarography for a long time but enhancements in polarographic waves had been observed and attributed to adsorption since the early days of polarography (Bond, 1980). Pihlar et al. (1981) were the first to exploit adsorption of the dimethylglyoxime complex of Ni on the Hg electrode for the preconcentration of Ni before its stripping. Since then, procedures for the determination of a large number of trace elements have been developed and applied to environmental samples. Wang (1989) provides excellent review on the development, potentials and applications of CSV, which contains a detailed treatment of the fundamental principles of CSV, the mechanisms of complex adsorption and of the stripping step. The principle behind the new method is very simple: under optimized solution conditions, the analyte (generally metal ions) reacts with an added ligand to form a complex which is adsorbed on the surface electrode during the preconcentration stage. This complex is then reduced during the stripping stage, which consists of the application of a negative potential scan on the electrode. During the stripping stage, the reduction process producing the peak current may be due to the reduction of the metal ion, the reduction of the ligand or the simultaneous reduction of both the ligand and the metal ion. The selectivity of the method is determined by the judicious choice of the complex-forming ligand and, since the reaction between the ligand and the analyte is usually dependent on the oxidation state of the analyte species, speciation analysis is generally achieved (Wang, 1985). A comprehensive review of ligands used in, and metals determinable by CSV is given by (Paneli, 1993). It can be conceived that with the choice of a proper ligand, any metallic species should be amenable to CSV determination, opening up the whole Periodic Table to this extremely sensitive, selective and inexpensive analytical technique. The reduction of the ligand can be used for the determination of metals which are reduced at very negative potentials. It is no wonder therefore that so much activity has been channeled towards the search for new ligands for CSV of trace metals in environmental samples. Almost two decades after the technique was first used for the determination of nickel, there is some continuing debate as to the name of the technique. Since the adsorption phenomenon is utilized for preconcentration of the analyte species, the technique has also been referred to as Adsorptive Stripping Voltammetry, (AdSV), as well as Adsorptive Cathodic Stripping Voltammetry (AdCSV), whereas many workers simply refer to it as CSV based on the direction of the current flow during the reduction. Following a discussion on the pros and cons of the different names used for the technique, Fogg (1994) reached the conclusion that the term cathodic stripping voltammetry with adsorptive accumulation would be more informative. However, he acknowledged that the term cathodic stripping will continue to be used. In contrast to the analytical methods, electrochemical methods for trace metal analysis are very fast and require relatively simple and inexpensive instrumentation. If the complexing ligand is chosen such that the reaction occurs selectively between the ligand and the analyte in a given oxidation state, speciation is achievable without lengthy separation steps and the preconcentration inherent to the technique precludes the need for a potentially contaminating preconcentration step (Wang, 1985). The whole analytical procedure can generally be carried out within the confines of a clean bench, which is a major asset in trace analysis. The fact that the material adsorbed on the mercury electrode is readily accessible for instantaneous reduction during the stripping stage leads to the flow of a large current, which is the analytical signal. Hence high sensitivities, i. e. , extremely low detection limits, can be achieved. In CSV, detection limits in the sub-Â µg/L level are routinely achieved using preconcentration times of 1-3 min (Wang, 1985). All these assets make CSV potentially the most appropriate technique for environmental, and, specifically, natural water analysis. Voltammetric Analytical Methods for Chromium Chromium occurs principally in nature as the extremely stable mineral chromite, FeO. Cr2O3. In most soils and bedrocks, it is similarly immobilized in the trivalent state; however, the environmental concentrations of chromium are significantly in excess of the natural mobilization of the element by weathering processes. This is because chromium and its compounds have widespread industrial applications, resulting in large quantities of the element being discharged in the environment (Bowen, 1979). The chromium concentrations encountered in natural waters are very low; concentrations vary from 0. 1 to 0. 3 Â µg/L in seawater and from 0.3 to 6 Â µg/L in unpolluted surface waters (Bowen, 1979). The study of the chemical speciation of chromium in natural waters has been a topic of great interest for 40 years. The speciation studies have almost exclusively focused on the distribution of chromium between Cr(III) and Cr(VI) (Fukai, 1967, p. 901). Polarographic methods for the analysis of chromium have long been established but the detection limits do not permit their application to natural waters. However, it was during the polarographic study of Cr in supporting medium containing EDTA and nitrate ions that an important observation was made by Tanaka and Ito (1966). These authors found that the Cr polarographic waves were unusually high in this medium and attributed it to the catalytic re-oxidation of an intermediate Cr(II)-EDTA complex by nitrate ions. Golimowski et al. (1985) were the first to recognize the role of adsorption in the polarographic determination of Cr in the presence of DTPA as supporting electrolyte. They showed that the Cr-DTPA is adsorbed on Hg whereas Cr-EDTA is not, hence the notion that DTPA is more suitable than EDTA for the polarographic determination of Cr. Golimowski et al. (1985) exploited the adsorption of the Cr-DTPA complex for the preconcentration of the analyte at a Hanging Mercury Drop Electrode and thus published the first CSV method for chromium. DTPA was used as the complexing ligand and the catalytic effect of nitrate ions was used for enhancement of the reduction currents. In what would be the first application of a voltammetric technique for the determination of chromium at levels prevalent in natural waters, they reported a detection limit of 20 Â µg/L for a 2-min deposition time. The superiority of this analytical method vis-a-vis the non-electrochemical methods was unquestionable. The CSV method provided not only the required detection limit, but it did so without the need for any separate sample pretreatment steps (Golimowski et al. , 1985). However, Golimowski et al. (1985) failed to consider that the sensitivity of Cr(III) was less than that of Cr(VI), although this observation had already been made by Zarebski in 1977. These authors also failed to observe that the response of Cr(III) was transient (see below). According to Golimowski et al. (1985) therefore, the method was applicable for the determination of total chromium and they claimed success in its application for the determination of chromium in river, lake, sea and rain water. Given the view of Golimowski et al. (1985) regarding the applicability of the DTPA method for the determination to Cr(III), Torrance and Gatford (1987) made a very thorough study of the CSV of the Cr-DTPA complex and confirmed that the responses of Cr(III) and Cr(VI) were indeed different. They found that the Cr(VI):Cr(III) response ratio was 14:1 at 0. 1 Â µg/L and 1. 2:1 at 1 Â µg/L of Cr respectively. These authors also found that with both Cr(III) and Cr(VI) there was a kinetic effect that produced a decrease in peak current with time; this decrease was more severe for Cr(III), with a decrease of 15% in the first 5 min after the addition of DTPA. Therefore it was concluded that Cr(III) and Cr(VI) cannot be determined in a solution unless all Cr(III) is oxidized to Cr(VI). They achieved this by heating the sample solutions with bromine water and attained detection limits of 0. 023 Â µg/L Cr as Cr(VI) (Torrance and Gatford, 1987). Scholz et al. (1990) also confirmed that the DTPA method works reliably only for Cr(VI) and proposed that, for the speciation of chromium, total chromium be determined as Cr(VI) after prior conversion of Cr(III) to Cr(VI) by uv-irradiation. Cr(VI) only was determined after a prior step in which the Cr(III) was removed from solution by coprecipitation with AI(OH)3. Cr(III) could then be obtained by difference. The use of DTPA as the complexing ligand in the determination of chromium was further studied by Boussemart et al. (1992), who devised and optimized a method for the speciation of chromium in natural water. These authors observed that the sensitivity for Cr(III) was about 70% of the Cr(VI) sensitivity. They also found that the response for Cr(III) was transient, disappearing completely in about 30 min. They therefore devised a method whereby the CSV peak current was recorded under optimized conditions immediately after the addition of DTPA to the voltammetric cell. The peak current at this time would be equivalent to the response due to Cr(III) and Cr(VI). Then, after 30 min (when the Cr(III) was believed not to be responding), they carried out a determination of Cr(VI) by a Cr(VI) standards addition. The concentration of Cr(III) was estimated from the initial response of Cr(III) plus Cr(VI). Thus, they reported a detection limit of 0. 1 nM (ca. 5 ng/L) for a 2-min deposition time. Although they used this method for the speciation of Cr in natural water, it is deficient in that the Cr(III) can only be estimated (Boussemart et al. , 1992). Apparently, these authors failed to consider the findings of Torrance and Gatford (1987) regarding the differing ratios of Cr(VI):Cr(III) responses at different concentrations as well as the rapidly decreasing response of Cr(III). The rate of decrease of the Cr(III) response is such that by the time the solution is purged and the first voltammetric run completed, there already is a substantial loss in signal. If, as is normal practice, voltammetric runs are carried out in triplicate and, as proposed, a deposition time of 2 min is chosen, it would be impossible to quantify the initial response due to the Cr(III). However, this method is very useful because it enables total Cr(VI) to be determined without any sample pretreatment step. Probably having realized the deficiencies of the above method, Boussemart and van den Berg (1994) later published another method for the determination of Cr(III) in natural water. In this case, the Cr(III) was preconcentrated by adsorption on silica. The adsorbed Cr(III) was later released by converting it to Cr(VI) by uv-irradiation and this Cr(VI) was determined by CSV, with DTPA as the complexing ligand. Conclusion From the discussion above it can be seen that analytical methods with the required sensitivity for the speciation determination of chromium in natural water can be based on electrochemical techniques with better attainable detection. Additionally, the electrochemical techniques generally involve less sample pretreatment and are faster and cheaper to perform. For these reasons, electroanalytical methods are preferable for the determination of chromium. Of the stripping techniques discussed, the method based on DTP A seems best suited to the determination of Cr(VI) in natural water, because Cr(III) does not respond. However, the difficulty faced in determining Cr(III) is a major drawback. Considering the methods described above, the complete speciation of chromium would need the complete oxidation of Cr(III) to Cr(VI) or the physical separation of the Cr(III) species as done in the methods by Boussemart and van den Berg (1994), or Scholz et al. (1990). These pretreatment steps are lengthy and are potentially likely to introduce analyte losses as well as contamination in the analytical method. The incorporation of sample pretreatment steps seems to be contrary to the spirit of electroanalytical techniques where excellent sensitivity coupled with simplicity and minimal sample handling is lauded as the great asset of the technique. The complete speciation of chromium could in principle be achieved without any need for sample pretreatment by the use of two different complexing ligands, for example, DTPA for Cr(VI) only and then cupferron or 2,2-bipyridine for total chromium (Cr(III) plus Cr(VI)). The difference between total chromium and Cr(VI) would then be equivalent to Cr(III). However, adoption of such a speciation scheme has apparently not yet been investigated, probably because it would entail undesirable additional time and costs (costs and purification of additional chemicals etc. ) in the overall process. References Bond, A. M. (1980). Modern Polarographic Methods in Analytical Chemistry. New York: Marcel Dekker. Boussemart, M. , van den Berg, C. M. G. , Ghaddaf, M. (1992). The determination of the chromium speciation in sea water using catalytic cathodic stripping voltammetry. Anal. Chim. Acta, 262, 103–115. Boussemart, M., van den Berg, C. (1994). Preconcentration of chromium (III) from seawater by adsorption on silica. and voltammetric determination. Analyst, 119, 1349-1353. Bowen, H. J. M. (1979). Environmental Chemistry of the Elements. Academic Press. Donat, J. R. , Bruland, K. W. (1995). Trace Elements in the Oceans, in Salbu, B. and Steinnes, E. (Eds. ), Trace Elements in Natural Waters. CRC Press. Fogg, A. G. (1994). Adsorptive stripping voltammetry or cathodic stripping voltammetry? Methods of accumulation and determination in stripping voltammetry. Anal. Proc. , 31, 313-317.