ANALYST, DECEMBER 1986, VOL. 111 1355 Simultaneous Determination of Zinc, Cadmium and Lead in Manganese Sulphate Electrolyte by Differential-pulse Anodic-stripping Voltammetry Samuel B. Adeloju Trace Analysis Unit, Division of Chemical and Physical Sciences, Deakin University, Victoria 32 17, Australia and Tam Tran BHP Central Research Laboratories, PO Box 188, Wallsend 2287, Australia Conditions are described for the direct simultaneous determination of zinc, cadmium and lead in process manganese sulphate electrolyte by differential-pulse anodic-stripping voltammetry. The suitability of the technique in this medium is influenced by the solution pH, scan rate and pulse height. Reliable determinations of the elements are accomplished in the concentrated electrolyte at pH 4.1 with a scan rate of 8 mV s-1 and a pulse height of 50 mV.Under these conditions, the limits of detection with a deposition time of 30 min are 0.05, 0.03 and 0.10 pg I-’ for the three elements, respectively. The precision of the method is also satisfactory with relative standard deviations of 0.8, 1.5 and 1.2% for zinc, cadmium and lead, respectively, with 10 samples at the 1 pg 1-1 level. However, the high manganese sulphate background is inadequate for copper determination at concentrations less than 50 pg 1-1. Keywords; Anodic-stripping voltammetry; cadmium determination; lead determination; zinc determination; manganese sulphate electrolyte Electrolytic manganese dioxide (EMD) is widely used as a cathode active material in dry cells and is usually deposited from manganese sulphate solution.Unfortunately, the heavy metal impurities in such solutions may be co-deposited or adsorbed on to the EMD during its production and may, consequently, be leached out into the battery electrolyte during storage. These impurities will, in turn, corrode the anode and eventually decrease the battery performance. For these reasons, stringent specifications have now been imposed on the EMD manufacturers to produce high-quality cathode active materials that contain relatively low concentrations of most heavy metal impurities. 1 This requires extensive purifi- cation of the process manganese sulphate electrolyte prior to the electrodeposition of the EMD .* Successful purification will rely on the availability of sensitive analytical techniques that can reliably determine ultra-trace amounts of impurities in the electrolyte. The reported levels of some of the heavy metal impurities in manganese sulphate electrolyte and EMD products are often less than the detection limits (1 mg 1-1 or 1 pg g-1) of the techniques currently used for analysis.3 Evidently, there is a need for the development of sensitive and selective analytical techniques that can be used in conjunction with the purifica- tion process in order to lower the ultimate concentrations of the impurities in the process electrolyte, prior to the EMD production.Ideally, such techniques should be amenable to both plant monitoring and the reliable determination of the heavy metal impurities in the EMD products. The only restriction in this regard is the high manganese sulphate background, which may cause severe interference problems with some of the available sensitive analytical techniques.The possible exception to this type of interference is voltammetry, which may be able to utilise the high background as a suitable “supporting electrolyte” for such determinations. However, no previous application of voltammetric techniques to the determination of heavy metal impurities in manganese sul- phate electrolyte has been reported. In the work reported in this paper, the suitability of differential-pulse anodic-stripping volt ammetry (DPASV) for the simultaneous determination of zinc, cadmium and lead in process manganese sulphate electrolyte was investigated. In particular, the critical dependence of the reliability of this method on pH and some instrumental parameters was carefully examined.Experimental Reagents and Standard Solutions The acids and ammonia used were of Aristar purity (BDH Chemicals) and the other reagents were of analytical-reagent grade. Distilled, de-ionised water, prepared as previously described,4 was used for all sample and solution preparations. Ammonia buffer (1 M, pH 9.5) was prepared by mixing equal volumes of 4 M ammonia and 2 M acetic acid solutions. Stock solutions (1 g 1-1) of zinc, cadmium, lead and copper were prepared by dissolving appropriate amounts of the chloride and nitrate salts in 1 M hydrochloric acid. The required daily standard (1 mg 1-1) was prepared weekly by dilution of the stock with 0.1 M hydrochloric acid.Instrumentation An EG & G Princeton Applied Research microprocessor- based polarographic analyser (PAR Model 384), equipped with a PAR Model 303 static mercury drop electrode and a PAR Model 305 stirrer, was used to record all stripping voltammograms. The electrode compartment consisted of a hanging mercury drop electrode (HMDE), a silver - silver chloride electrode (saturated KCI) and a platinum wire electrode as its working, reference and auxiliary electrodes, respectively. Solution pH measurements were made on an Activon Scientific (Sydney, Australia) portable pH/mV meter. Standard solutions of zinc, cadmium and lead were added to the polarographic cell with fixed volume Soccorex micropipettes with disposable tips. Glassware All glassware and polyethylene bottles were soaked in 2 M nitric acid for at least 7 days and rinsed several times with distilled, de-ionised water prior to use.Between experiments, the used glassware and bottles were soaked in 2 M nitric acid for at least 12 h and again rinsed several times with distilled, de-ionised water before use. Plant Process Electrolyte For preliminary investigations, a solution of 1 M manganese sulphate was prepared by dissolving appropriate amounts of1356 ANALYST, DECEMBER 1986, VOL. 111 the analytical-reagent grade salt in distilled, de-ionised water. This solution (1 1) was then purified by adding 0.5 g of calcium sulphide, stirring with a glass rod to mix thoroughly, and was left to stand for 1 h before finally being filtered through a Whatman No. 541 hardened ashless paper.The resulting pink solution was left overnight, filtered again the next day and 100 yl of concentrated HN03 were added before aliquots for voltammetric measurement were taken. The ammonia buffer was used to adjust the solution pH to the desired value for the ASV determination. The process manganese sulphate electrolyte used in the production of EMD was provided by Broken Hill Proprietary Ltd. (Wallsend, NSW, Australia). The electrolyte contained about 1.2 M manganese sulphate and had a pH of 3.0. Stripping Voltammetric Determinations An aliquot (5 ml) of the electrolyte was transferred into the polarographic cell, de-oxygenated for 5 min and maintained under a flow of nitrogen during the experiment. The three elements were then determined by ASV at the HMDE using the following conditions: operating mode, DPASV; deposi- tion potential, -1.15 V vs.Ag - AgCl (saturated KC1); final potential, -0.25 V; scan rate, 8 mV s-1; duration between pulses, 0.5 s; modulation amplitude, 50 mV; deposition time, 285 s (stirred); and equilibration period, 15 s (unstirred). The deposition of the three elements on to the mercury electrode was achieved by using a fast stirring rate and a medium-sized drop with a surface area of 0.015 cm2. The concentrations of the three elements in the electrolyte were determined by the standard additions method using the peaks that appear at about -1.0, -0.7and -0.4Vvs. Ag-AgClatpH4.1forzinc, cadmium and lead, respectively. The solution was de-oxygen- ated after each addition for 30 s prior to the ASV measure- ment.Working Area All reagent preparations and sample manipulations were carried out in a Class 100 clean room, controlled at a temperature of 22.5 k 0.5 "C. All stripping voltammetric determinations were carried out in a Class 1000 clean room under similar temperature control. Both of these laboratories form part of the Deakin University Trace Analysis Unit. Results and Discussion pH Dependence The reliable determination of zinc, cadmium and lead in the manganese sulphate electrolyte by ASV is dependent on the pH of the solution. The results obtained in this study indicate that the sensitivity and resolution of the stripping peaks for the three elements varied considerably with increasing solution pH. Generally, as can be seen from the data in Table 1, there is no significant difference in the sensitivity of the cadmium peak at pH 22.9, whereas the sensitivities for the lead and zinc peaks varied considerably with increasing pH.Careful exam- ination of these data reveals that the optimum sensitivities for lead and zinc were obtained at pH 4.1 and 7.7, respectively. The data also indicate that the lowest sensitivity for lead was obtained at pH 7.7, whereas the zinc peak was reasonably sensitive at pH 4.1. From these observations, it was concluded that pH 4.1 is adequate for the reliable simultaneous determination of the three elements in the manganese sulphate electrolyte by ASV. The data in Table 1 also indicate that the chosen deposition potential has some influence on the sensitivities of the zinc and lead peaks between pH 1.8 and 5.1.The more negative deposition potential (- 1.15 V) gave a better sensitivity for the zinc peak, whereas the lead peak seems to be influenced by both the solution pH and the chosen deposition potential. Evidently the use of a deposition potential of - 1.15 V is useful in maintaining an adequate sensitivity for the three elements in solution between pH 1.8 and 5.1. However, beyond pH 7.0 the use of a deposition potential of -1.2 V is necessary to obtain an adequate stripping peak for zinc, owing to the negative shift in its peak potential with increasing solution pH. The observed reduction in the sensitivity of the lead peak beyond pH 4.1 may be associated with the increasing tendency to precipitate manganese hydroxide from the solution.This view is supported, to some extent, by the sudden change in the solution colour from pink to orange at pH > 5.0. Eventually, at pH > 7.7, a brownish orange precipitate is formed on the addition of more buffer solution, or when left overnight. It is also likely that the increasing tendency to form manganese hydroxide in the electrolyte at the higher pH aids the precipitation of some of the lead and consequently results in the decreasing peak current measurements for this element. Another interesting observation in the ASV determination of the three elements in the manganese sulphate electrolyte was the absence of a voltammetric response for copper. The only response for the element was noted at pH 2.9 (Fig. l ) , but no increase in the peak current was observed on the addition of 4 pg 1-1 of the analyte.Under normal conditions (in the absence of such a high background), less than 4 pg 1-1 of the element can be determined in a supporting electrolyte such as 0.1 M hydrochloric acid or acetate buffer solution by ASV. It is conclusive, therefore, that the high manganese sulphate background interferes seriously with the ASV determination of copper, possibly via intermetallic compound formation. Copper is known to form intermetallic compounds with most elements.5 However, a definite and quantitative r zsponse was obtained for the element, as shown in Fig. 1, at concentrations 250 yg 1-1. This level is considerably higher than the usual detection limit (0.1 yg 1-1) obtained in other supporting electrolytes.6 Influence of Scan Rate and Pulse Height The sensitivity and resolution of the stripping peaks obtained for zinc, cadmium and lead in the manganese sulphate electrolyte were also influenced by a number of instrumental parameters.In particular, the chosen scan rate and pulse height had a considerable influence on the sensitivity and resolution of the stripping peaks. Fig. 2 shows that the resolution of the peaks decreased with increasing scan rate. However, no significant difference was observed in either the resolution or sensitivity of the stripping peaks with the slower scan rates (S4 mV s-1). Even with the use of a scan rate of 8 mV s-1, only slight decreases in peak currents were observed for cadmium and lead, but resolution was still maintained. In contrast, the use of a faster scan rate (310 mV s-1) resulted in a considerable reduction in the peak currents and affected the resolution for both elements.From these observations, it was concluded that the resolution and sensitivity obtained with a Table 1. Influence of the pH of manganese sulphate electrolyte on the sensitivity of the stripping peaks for zinc, cadmium and lead SensitivityhA (pg l - I ) - l * PH Ag - AgCl Zn Cd Pb EdJV V S . 0.9 1.8 2.9 3.8 4.1 4.7 5.1 6.3 7.7 -1.10 -1.10 -1.15 -1.10 -1.15 -1.10 -1.10 -1.15 -1.10 -1.15 -1.15 -1.20 - 3.0 5.1 9.8 9.8 10.3 13.3 10.0 15.0 15.3 17.0 21.5 22.8 23.0 28.5 28.5 29.8 29.8 29.3 29.5 30.8 32.3 29.8 4.0 4.5 4.5 5.0 8.3 8.3 8.8 9.0 7.3 6.5 4.9 1.9 * Based on addition of 4 pg I-' of the analytes, t, = 300 s, 8 mV s-I.ANALYST, DECEMBER 1986, VOL.111 I 1357 I scan rate of 8 mV s-1 are adequate for the reliable and rapid determination of the three elements. Consequently, this scan rate was used for all determinations in this work. Similarly, the applied pulse height (or modulation ampli- tude) also influenced the sensitivity and resolution of the stripping peaks for zinc, cadmium and lead in the manganese sulphate electrolyte. The results in Fig. 3 show that the peak currents for the three elements increased with increasing pulse height, except at values >50 mV where the sensitivity of the lead peak was reduced. In addition, the resolution of the three stripping peaks was affected by the use of high pulse heights. On this basis, a pulse height of 50 mV was chosen as the optimum with respect to the sensitivity and resolution of the three stripping peaks.Cd Zn I 1.15 0.35 - Etv Fig. 1. Simultaneous determination of zinc, cadmium, lead and copper in manganese sulphate electrolyte by DPASV. Electrolysis time ( t e ) , 120 s; pH, 4.1; concentration of analytes added, 205 pg 1--1; other conditions as described under Experimental Application to Process Electrolyte The ultimate adequacy of the established ASV conditions for the reliable determination of zinc, cadmium and lead in the process plant electrolyte is dependent on the linear concentra- tion ranges for the three elements. It can be expected that the concentrations of the elements in such samples will vary bepending on the effectiveness of the chemical purification step.It is therefore desirable to incorporate some flexibility in the method for handling the possible variations in the concentrations at various stages of the purification process. Fig. 4 shows that the use of a deposition time of 120 s with the established ASV conditions gave linear calibration graphs for zinc and cadmium up to 200 pg 1-1, and up to 100 pg 1-1 for lead. The observed increase in the lead peak currents at concentrations >lo0 pg 1-1 (Fig. 4) suggest that the matrix or intermetallic effects on the ASV determinations of the element were progressively reduced at the higher concentra- tions. Nevertheless, the linear concentration range for lead can also be extended to 200 pg 1-1 by use of a 60 s deposition time. However, as indicated by the data in Table 2, the detection limit for lead was also affected by the chosen deposition time.In general, the use of a 60 s deposition time is adequate for the reliable determination of ultra-trace amounts of the three elements in the concentrated manganese sulphate electrolyte. Providing that the process plant electrolyte can be adequately purified, as little as 0.05 pg 1-1 of Zn, 0.03 pg 1-1 of Cd and 0.10 pg 1-1 of Pb can be reliably determined by the ASV method. These estimated limits of detection are compar- able to those obtained in various supporting electrolyte^^^^ and, hence, indicate that the high manganese sulphate background did not interfere to any great extent with the simultaneous determination of the three elements by ASV. At concentrations greater than 200 pg 1-1, the three elements were rapidly determined in the manganese sulphate elec- trolyte by differential-pulse polarography (DPP).This pro- l 1.15 0.35/1.15 0.35 - E N Fig. 3. Influence of pulse height on the resolution and sensitivity of zinc, cadmium and lead stri ping peaks. Pulse heights: ( a ) 10 mV; ( b ) 25 mV; ( c ) 50 mV; and (8 100 mV; scan rate, 8 mV s-1; analyte concentration and pH as in Fig. 21358 ANALYST, DECEMBER 1986, VOL. 111 2 1 B A 1.0 0 50 100 150 200 Concent ra t i on/pg I-’ Fi . 4. Calibration graphs obtained for (A) zinc, (B) cadmium and (C! lead in manganese sulphate electrolyte by DPASV. Conditions as in Fig. 1, except for analyte concentrations 1.2 0.8 0.9 -EN 0.3 Fig. 5. Sequential simultaneous determination of zinc, cadmium and lead in process plant electrolyte by ( a ) DPP and (b) DPASV. Conditions for ( a ) : pulse height, 100 mV; 4 mV s-l; drop time, 1 s .Standard additions: (A) 0; (B) 1; (C) 2; and (D) 3 mg 1-1 of Zn. Conditions for ( b ) : te = 120 s. Standard additions: (A) 0; (B) 1; (C) 3; and (D) 5 pg 1-l (Cd, Pb). pH 3.0; other conditions as described under Experimental Table 2. Estimated limits of detection* for zinc, cadmium and lead in manganese sulphate electrolyte with different deposition times Deposition time/s Zn/pg 1- Cd/pg I-* Pb/pg 1- 1 60 2.0 1 .o 3.0 120 1 .o 0.5 2.0 300 0.4 0.2 0.6 600 0.2 0.1 0.3 1800 0.05 0.03 0.1 * Estimated from the data in Table 1 and Fig. 5 , pH 4.1. vides suitable flexibility in handling possible variations in the analyte concentrations during the chemical purification of the process electrolyte.The application of the ASV method to the determination of the three elements in a process plant electrolyte supplied by Broken Hill Proprietary Ltd. revealed that zinc is present at excessively higher concentrations than cadmium and lead. Based on the peak current measurements it was determined that the zinc concentration in the electrolyte is >500 pg 1-1. The use of ASV for such high concentrations is not recom- mended as it may result in serious contamination of the electrode.8 As a result, zinc determination in the electro- lyte was accomplished by DPP, whereas cadmium and lead were determined by ASV. Fig. 5 shows the utilisation of both techniques for the reliable determination of the three elements in a single electrolyte sample.The concentrations of the three elements in the plant electrolyte determined by this approach with standard additions were 625 2 5 pg 1-1 of Zn, 1.35 k 0.02 pg 1-1 of Cd and 4.63 k 0.06 pg 1-1 of Pb. Alternatively, the zinc in the sample may be determined by ASV after dilution, but such an approach is more time consuming and requires the use of a separate sample aliquot for the direct determination of cadmium and lead. Fig. 5 also shows that it was not necessary to adjust the pH of the process electrolyte to the recommended value (pH 4.1) for the cadmium and lead determinations as the data in Table 1 indicate that there is no significant difference in the sensitivi- ties obtained for both elements between pH 2.9 and 4.1. Evidently, the ASV method is adequate for the reliable determination of ultra-trace amounts of the three elements in manganese sulphate solution and can readily be combined with other less sensitive voltammetric techniques such as DPP to handle possible variations in analyte concentrations during the chemical purification of the electrolyte.Conclusion The high manganese sulphate background in the process plant solution was suitable as a “supporting electrolyte” for the simultaneous determination of zinc, cadmium and lead by ASV. Under the established optimum conditions, as little as 0.05, 0.03 and 0.10 pg 1-1, respectively, can be reliably determined. The precision of the method was also satisfactory with a relative standard deviation between 1 and 2% for the three elements. However, the determination of copper in the electrolyte was considerably affected, possibly as a conse- quence of an intermetallic interference, but its determination was still possible at concentrations >50 pg 1-1. More work is being undertaken to improve the sensitivity and detection limit of the ASV method for this element. The authors are grateful to Broken Hill Proprietary Ltd., NSW (Australia), for providing the funds for this research and giving permission for publication. 1. 2. 3. 4. 5. 6. 7. 8. References Deane, M. E., Inst. Min. Metall. Trans., Sect. A, 1985, 94, A169. Kozawa, A., in Kordesch, K. V., Editor, “Batteries, Volume One, Manganese Dioxide,” Marcel Dekker, New York, 1974, Chapter 3, p. 385. Toyo Soda Manufacturing Co. (Japan), in Kozawa, A., and Nagayome, M., Editors, “Proceedings of the IBA Symposium, Brussels, 1983,” International Battery Material Association, Cleveland, OH, 1984, pp. 275-279. Adeloju, S . B., Bond, A. M., Briggs, M. H., and Hughes, H. C., Anal. Chem., 1983, 55, 2076. Wang, J., “Stripping Analysis,” VCH, Deerfield Beach, FL, Adeloju, S. B., Bond, A. M., and Hughes, H. C., Anal. Chim. Acta, 1983, 148, 59. Adeloju, S. B., and Bond, A. M., Anal. Chem., 1985,57,1728. Adeloju, S. B., Bond, A. M., and Briggs, M. H., Anal. Chem., 1985, 57, 1386. Paper A61155 Received May 21st, 1986 Accepted July 3rd, 1986 1985, pp. 93-98.