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The determination of copper, lead, cadmium, nickel, zinc and cobalt in natural waters by pulse polarography
4nalytica ChimicaActa Elsevier PublishingCompany,Amsterdam Printed in The Netherlands
283
THE DETERMINATION OF COPPER, LEAD, CADMIUM, NICKEL, ZINC AND COBALT IN NATURAL WATERS BY PULSE POLAROGRAPHY
M. I. A B D U L L A H aND L. G. ROYLE
Department of Oceanography , Liverpool University, Liverpool L69 3BX (England) (Received 20th September 1971 )
The application of electrochemical methods to the estimation of trace elements in natural waters has received little attention, principally because instruments with sensitivities in the 10 -4 10 -s g 1-1 range have not been widely available. Even the pulse polarograph, with its much greater sensitivity, is unsuitable for the direct determination of trace metals in sea waters. Indeed only uranium has been determined by this technique after pre-concentration 1. However, the pulse polarographic technique has the advantage not only of sensitivity but also of selectivity, so that only minimal chemical separation is required for the simultaneous determination of a number of components. The only method to have been used successfully for the direct determination of trace metals in sea water 2 5 is anodic stripping voltammetry. However, this method is time-consuming, so that it is unsuitable as a routine analytical method, and its range of application is rather limited. Despite the higher sensitivity of modem polarographic instruments, pre-concentration is usually necessary before trace elements can be determined in natural waters. Several different techniques have been used for this preconcentration; these include co-crystallisation6, solvent extraction of the metal complex with an organic reagent such as pyrollidine dithiocarbamate 7,s, and for fresh water, evaporation 9. The use of chelating resins 1° offers a simple procedure for the concentration of trace metals from natural water samples with a minimal use of reagents. By appropriate choice of the counter-ion of the resin, the concentrate can be obtained along with sufficient supporting electrolyte for the subsequent polarographic analysis. The present paper describes a procedure for the determination of copper, lead, cadmium, nickel, zinc and cobalt in natural waters on a single aliquot of the sample; pulse polarography is applied after concentration of the metals by means of chelating resins. Concentration of trace metals from natural water Chelating resins in the ammonium form have been used to concentrate several trace metals from sea water 1°. Experiments have shown that these resins also take up copper, lead, cadmium, nickel, zinc and cobalt quantitatively from fresh waters provided that the pH of the water sample is between 4 and 8. When the resins are eluted with 2 M nitric acid, the eluate also contains a mixture of sodium, potassium, magnesium and calcium regardless of whether sea water or fresh water is passed through the column. The amounts of sodium, potassium, magnesium and calcium Anal. Chim. Acta, 58 (1972)
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M . I . ABDULLAH, L. G. ROYLE
in the eluate were found to vary between samples, particularly in the case of fresh water samples, and this adversely affected the subsequent polarographic determination, which is best carried out in a supporting electrolyte of known composition. The affinity of the alkali and alkali earth metals for the iminodiacetate type of chelating resins (such as the Chelex-100) is Ca > Mg > Ba > Sr > Li > Na > K 11, whereas Mn, Fe, Cu, Pb, Cd, Ni, Zn and Co have greater affinity than has calcium. Therefore, when the resin in its calcium form was used, complete recoveries of copper, lead, cadmium, nickel, zinc, cobalt, iron and manganese were obtained from sea water, fresh water, and synthetic mixes buffered to pH 4 8 ; and the eluate contained only very minor amounts of sodium, potassium, and magnesium. The amount of calcium present in the eluate depended on the volume of the chelating resin used, the amount of calcium eluted from a 5 × 1 cm column of resin being ca. 0.2 M when made up to 25 ml. This concentration proved to be sufficient to act as a supporting electrolyte for the polarographic determination of the trace metals under investigation. Polarographic determination of copper, lead, cadmium, nickel, zinc and cobalt Copper, lead and cadmium have all been determined in both acidic and neutral electrolytes. Acetate buffer (0.1 M) has also been used for the determination of copper, lead, cadmium and zinc in natural waters by anodic stripping techniques 12. Copper and zinc, concentrated by evaporating river water, have been determined simultaneously in ammonia-ammonium chloride electrolyte 9. Copper and lead have also been determined by pulse polarography in cadmium chloride supporting electrolyte 13. Since the eluate from the chelating resin contains a high concentration of calcium, the possibility of using the calcium (as calcium chloride) as a supporting electrolyte was investigated. Although calcium chloride is not commonly used as a supporting electrolyte, the polarographic behaviour of a number of metals in a 5 M solution of it has been studied 14. Numerous problems are encountered in the use of such a strong and viscous electrolyte, but most of these can be overcome by using a lower concentration of calcium chloride solution as electrolyte. In 0.5 M solution, copper, lead, and cadmium gave good reduction waves at the dropping mercury electrode. The half-wave potentials for copper(II), lead(II) and cadmium(II) are -0.45, - 0 . 7 0 and - 0 . 9 2 V vs. the mercury pool, respectively, compared with - 0.25, - 0.45 and - 0.65 V vs. the mercury pool in 0.5 M potassium chloride solution. In derivative polarography (50-m V pulse height) the calibration curves for copper, lead and cadmium are linear in the 0.05-10 p.p.m, range in both 1 M and 0.2 M solutions of calcium chloride as supporting electrolytes. No interference was observed from Fe, Mn, Sn, Bi, Cr, V, or Mo which are also retained by the resin. The copper(II) reduction wave occurs immediately after the mercury wave at 0.0 to - 0 . 1 V which is very strong in calcium chloride solution. Nitrate reduction takes place between - 0 . 1 and - 0 . 2 V vs. the mercury pool, and this was found to affect the positive side of the copper(II) wave, which, however, can be resolved if the copper concentration is greater than 0.05 p.p.m. It was found desirable to acidify the sample solution with pure hydrochloric acid in order to maintain the metals in solution. The presence of hydrochloric acid (up to 0.5 M) was found to have no effect on the half-wave potential, nor on the sensitivity of the copper, lead and cadmium waves. Both zinc and nickel give polarographic waves in 0.2 M calcium chloride at Anal. Chim. Acta, 58 (1972)
TRACE METALSIN NATURAL WATER
285
- 1 . 4 0 and - 1 . 3 8 V vs. the mercury pool, respectively; cobalt is also reduced at - 1.36 V. These three waves could not be resolved. However, if the calcium chloride solution is made 0.5 M with respect to a m m o n i a - a m m o n i u m chloride, cadmium(II), nickel(II) and zinc(II) waves appear at -0.85, -1.15, and - 1 . 4 3 V vs. the mercury pool, respectively. The cobalt (II) wave is formed at - 1.40 V coinciding with the zinc (II) wave. However, this cobalt reduction process is irreversible in this medium and the wave is ill-defined and weak. Since the zinc-cobalt ratio in natural waters is about 100 : 1, cobalt is not likely to interfere seriously with the zinc determination. However, when the cobalt content is higher than usual, it was found that by employing reverse potential sweep, the effect of cobalt on the zinc determination can be reduced to less than 1 ~ of the zinc value. No other interference was found on the determination of nickel and zinc, and the calibration in 0.5 M ammonia-ammonium chloride-0.2 M calcium chloride supporting electrolyte is linear in the range of 0.01-5 p.p.m, nickel and 0.02-30 p.p.m, zinc. The most sensitive polarographic method for the determination of cobalt depends on the measurement of the adsorption wave of cobalt~timethylglyoxime complex in alkaline medium 15. It was found that in 0.5 M ammonia-ammonium chloride4).2 M calcium chloride supporting electrolyte, this adsorption wave appears at a half-wave potential of - 1.44 V; the wave is well defined and adequately resolved from both the zinc and nickel waves, and from the wave arising from reduction of the reagent. Variation in the calcium chloride (0.2-1 M), and the ammonia-ammonium chloride (0.2~).5 M) concentrations had little effect on the sensitivity of the cobalt adsorption wave or its half-wave potential. The amount of alcohol used as solvent for dimethylglyoxime was found to be critical, making it necessary to control accurately the strength of the reagent and the amount added to the cell. Since a considerable excess of dimethylglyoxime is added, any consumption of the reagent by an unusually high concentration of nickel, which also forms a complex, has no effect on the cobalt adsorption wave, and the peak height of the c o b a l t - D M G adsorption wave shows a linear relationship with cobalt concentration over the range of 0.02-1 p.p.m. In order to ensure the highest accuracy, a standard addition technique was employed for the determination of the six metals because of short-term instrumental variation. This technique also takes into account possible effects caused by the variation in the trace metal comp6sition of the samples, and the slightly differing amounts of calcium eluted from the columns. EXPERIMENTAL Apparatus
A Southern-Harwell A 3100 pulse polarograph was used. The glass polarographic cells were maintained at 25° in a thermostatically controlled water bath. All polarograms were obtained in the derivative mode, with a 50-mV pulse height applied before the end of the 1-sec life of the mercury drop. The faradaic current was measured 20 msec after the pulse application. Polarographic-grade triple-distilled mercury was used. Silica ware was used when possible, and before use was cleaned by soaking in a (1 + 1) mixture of concentrated nitric and sulphuric acids and then rinsing thoroughly with metal-free redistilled water. Anal. Chim. Acta,
58 (1972)
286
M.I. ABDULLAH,L. G. ROYLE
Reagents Ammonia solution (7 M). Prepare by isothermal distillation from concentrated ammonia liquor (s.g. 0.88). Hydrochloric and nitric acids. Redistil in all-silica distillation apparatus. Calcium chloride (2 M). Prepare this by passing 2 1 of 0.5 M calcium chloride solution (pH 6-7) through a 10 × 1 cm column containing clean Chelex-100 resin in the ammonium form, to remove all impurities. Collect the effluent in a silica flask and reduce its volume to 400 ml by evaporation. Destroy organic matter by irradiation under a 1-kW mercury vapour lamp and finally dilute the solution to 500 ml. Tests made on samples of this solution showed that none of the metals under in(estigation was present above the detection limit. Standard solutions. Prepare solutions of the six metals from their nitrate or chloride salts. Evaporate aliquots to dryness on a water bath in silica beakers with 5 ml of 2 M calcium chloride solution and 5 ml of 6 M hydrochloric acid. Dissolve the residues in water and make up to the appropriate volume to give solutions containing, respectively, 10 p.p.m, copper, 10 p.p.m, lead, 10 p.p.m, cadmium, 10 p.p.m. nickel, 100 p.p.m, zinc, and 1 p.p.m, cobalt. Pre-concentration column Pre-concentration of metals from the sample was carried out with columns of chelating resin similar to those used by Riley and Taylor 1°. Clean the Chelex-100 column with 2 M ifitric acid, wash acid-free with metal-free water and then wash with 30 ml of 2 M ammonia solution. Convert to the calcium form by passing ca. 25 ml of purified 0.3 M calcium chloride solution, and finally wash with 100~200 ml of metal-free water. The column of resin in its calcium form should be approximately 5 x l cm. Procedure Pass the samples of fresh water or sea water (10 1), filtered through 0.45 #m membrane filters, through Chelex-100 columns in the calcium form at the rate of 4 ml m i n - 1. Wash the columns with 100 ml of water and elute the trace elements with 70 ml of 2 M nitric acid. Irradiate the eluate, in silica beakers, under a 1-kW mercury vapour lamp for 1 h, to destroy any organic matter present, and then evaporate to dryness on a water-bath. Redissolve the residue in 10 ml of 2 M hydrochloric acid and again evaporate the solution to dryness. Repeat this step to ensure the complete removal of all nitrate from the sample. Then dissolve the residue in water, acidify with 1 ml of 6 M hydrochloric acid, and make up to 25 ml. Regenerate the columns as described earlier, taking care to avoid leaving them in the hydrogen form because of the risk of decomposition. Place a 2.5-ml aliquot of the solution in the polarographic cell, deoxygenate for 3 rain with oxygen-free nitrogen, and then record a pulse polarogram over the range of - 0 . 2 to - 1.1 V, for the determination of copper, lead and cadmium. Accurately add 0.2 ml of a standard containing 10 #g ml 1 each of copper, lead and cadmium, and record a second polarogram. Care must be taken to ensure that none of the instrumental settings are altered during the recording of these polarograms. To the same solution in the cell, add 0.6 ml of 2 M ammonia solution. This makes the solution in the cell 0.2 M with respect to a m m o n i a - a m m o n i u m chloride. Record a pulse polarogram between - 0.8 and - 1.7 V for the nickel and zinc measureAnal. Chim. Acta, 58 (1972)
TRACE METALS IN NATURAL
287
WATER
ments. Obtain a second polarogram after the addition of 0.2 ml of nickel and zinc standard. Again to the same solution in the cell, add 0.2 ml of a 1 ~ alcoholic solution of dimethylglyoxime and record the polarogram between - 1.1 and - 1.5 V for the determination of cobalt. Obtain a second polarogram after the addition of 0.2 ml of cobalt standard. Calculate the concentrations of copper, lead, cadmium, nickel, zinc and cobalt by measuring the peak heights and peak height increments after the addition of standard. It is necessary to make corrections for the dilution of the sample in the cell caused by the addition of standards, ammonia and dimethylglyoxime. RESULTS The accuracy of the method was evaluated by spiking 5-I samples of sea water which had been stripped of trace metals by passing it through chelating resin column, with known amounts of copper, lead, cadmium, nickel, zinc and cobalt and analysed according to the procedure outlined above. The results (Table I) show that the determination of these metals at the levels occurring in natural waters is satisfactory. The precision of the polarographic method was evaluated by seven replicate analyses of sea water samples collected from Liverpool Bay. The amounts of metal detected and standard deviations are listed in Table I, and these appear to be satisfactory for routine trace metal determination in natural waters. TABLE I RESULTS ON SPIKED AND NATURAL SEA WATERS
(All amounts are given in]~g 1 1)
Amount added Amount found Amount added Amount found Amount added Amount found Amount added Amount found Sea water sample Standard deviation"
Cu
Pb
Cd
Ni
Zn
1.00 1.04 --
1.00 1.04 2.00 1.92 4.00 4.09 6.00 5.78 0.90 0.05
1.00 0.98 2.00 1.99 4.00 3.93 6.00 6.09 1.38 0.03
1.00 0.97 2.00 1.78 4.00 4.13 6.00 5.90 0.78 0.02
10.0 0.20 9.7 0.19 20.0 0.40 19.9 0.41 40.0 0.40 41.0 0.42 60.0 0.60 60.7 0.61 11.8 < 0.01 0.14 0.15 b
4.00 3.89 6.00 5.83 2.03 0.12
Co
" 7 determinations were made. bA spiked sample was used. This work was supported in part by the Natural Environment Research Council. SUMMARY A method is described for the pulse-polarographic determination of copper, lead, cadmium, nickel, zinc and cobalt in natural waters, after their preconcentration Anal. Chim. Acta,
58 (1972)
288
M . I . ABDULLAH, L. G. ROYLE
on chelating resin in its calcium form. The eluate from the resin contains sufficient calcium to act as a supporting electrolyte for the subsequent polarographic determination. The determination of the six metals is made on a single aliquot of the eluate after suitable adjustment of conditions. RI~SUMI~
Une m6thode est d6crite pour le dosage polarographique du cuivre, du plomb, du cadmium, du nickel, du zinc et du cobalt dans les eaux naturelles, apr6s pr6concentration sur r6sine sous forme de calcium. L'61uat contient suffisamment de calcium pour constituer l'61ectrolyte de base pour le dosage polarographique. Le dosage des six mdtaux se fait sur une simple aliquote de l'61uat, apr6s traitement appropri6. ZUSAMMENFASSUNG
Es wird eine Methode beschrieben ffir die pulse-polarographische Bestimmung von Kupfer, Blei, Cadmium, Nickel, Zink und Kobalt in in der Natur vorkommendem Wasser. Die Elemente werden mittels eines Chelataustauscher-Harzes in der Calciumform vorkonzentriert. Das Eluat aus dem Harz enthSlt genfigend Calcium, so dass dieses als Tr~gerelektrolyt ffir die anschliessende polarographische Bestimmung verwendet werden kann. Die Bestimmung der sechs Metalle erfolgt in einem einzigen aliquoten Anteil des Eluats nach geeigneter Einstellung der Bedingungen. REFERENCES 1 2 3 4 5 6 7 8 9 l0 ll 12 13 14 15
G. W. C. MILNER,J. D. WILSON,G. A. BARNETTAND A. A. SMALLS,J. Electroanal. Chem., 2 (1961) 25. M. ARIEL AND U. EISNER, J. Electroanal. Chem., 5 (1963) 362. M. ARIEL, ~U. EISNER AND S. GOTTESEIELD, J. Electroanal. Chem., 7 (1964) 307. G. MACCHI, J. Electroanal. Chem., 9 (1965) 290. G. C. WHITNACK AND R. SASSELLI,Anal. Chim. Acta, 47 (1969) 267. J. P. RILEY AND G. TOPPING, Anal. Chim. Acta, 44 (1969) 234. R. R. BROOKS, B. J. PRESLEY AND I. R. KAPLAN, Talanta, 14 (1967) 809. D. W. SPENCER AND P. G. BREWER, Geochim. Cosmochim. Acta, 33 (1969) 325. E. J. MAIENTHAL AND J. K. TAYLOR, in R. F. GOULD, Trace Inoroanics in Water, Am. Chem. Soc., Washington, D.C., 1963, p. 172. J. P. RILEY AND D. TAYLOR, Anal. Chim. Acta, 40 (1968) 479. R. ROSSET, Bull. Inf. Sci. Tech. Comm. Ener9. At., 85 (1964) 13. I. SINKO AND J. DOLEZAL,J. Electroanal. Chem., 25 (1970) 299. E. TEMMERMANAND F. VERBEEK,dr. Electroanal. Chem., 12 (1966) 158. G. F. REYNOLDS,H. L SHALGOSKY AND T. J. WEBBER,Anal. Chim. Acta, 8 (1953) 558 and 564. P. NANGNIOT, J. Electroanal. Chem., 7 (1964) 50.
Anal. Chim. Acta, 58 (1972)
283
THE DETERMINATION OF COPPER, LEAD, CADMIUM, NICKEL, ZINC AND COBALT IN NATURAL WATERS BY PULSE POLAROGRAPHY
M. I. A B D U L L A H aND L. G. ROYLE
Department of Oceanography , Liverpool University, Liverpool L69 3BX (England) (Received 20th September 1971 )
The application of electrochemical methods to the estimation of trace elements in natural waters has received little attention, principally because instruments with sensitivities in the 10 -4 10 -s g 1-1 range have not been widely available. Even the pulse polarograph, with its much greater sensitivity, is unsuitable for the direct determination of trace metals in sea waters. Indeed only uranium has been determined by this technique after pre-concentration 1. However, the pulse polarographic technique has the advantage not only of sensitivity but also of selectivity, so that only minimal chemical separation is required for the simultaneous determination of a number of components. The only method to have been used successfully for the direct determination of trace metals in sea water 2 5 is anodic stripping voltammetry. However, this method is time-consuming, so that it is unsuitable as a routine analytical method, and its range of application is rather limited. Despite the higher sensitivity of modem polarographic instruments, pre-concentration is usually necessary before trace elements can be determined in natural waters. Several different techniques have been used for this preconcentration; these include co-crystallisation6, solvent extraction of the metal complex with an organic reagent such as pyrollidine dithiocarbamate 7,s, and for fresh water, evaporation 9. The use of chelating resins 1° offers a simple procedure for the concentration of trace metals from natural water samples with a minimal use of reagents. By appropriate choice of the counter-ion of the resin, the concentrate can be obtained along with sufficient supporting electrolyte for the subsequent polarographic analysis. The present paper describes a procedure for the determination of copper, lead, cadmium, nickel, zinc and cobalt in natural waters on a single aliquot of the sample; pulse polarography is applied after concentration of the metals by means of chelating resins. Concentration of trace metals from natural water Chelating resins in the ammonium form have been used to concentrate several trace metals from sea water 1°. Experiments have shown that these resins also take up copper, lead, cadmium, nickel, zinc and cobalt quantitatively from fresh waters provided that the pH of the water sample is between 4 and 8. When the resins are eluted with 2 M nitric acid, the eluate also contains a mixture of sodium, potassium, magnesium and calcium regardless of whether sea water or fresh water is passed through the column. The amounts of sodium, potassium, magnesium and calcium Anal. Chim. Acta, 58 (1972)
284
M . I . ABDULLAH, L. G. ROYLE
in the eluate were found to vary between samples, particularly in the case of fresh water samples, and this adversely affected the subsequent polarographic determination, which is best carried out in a supporting electrolyte of known composition. The affinity of the alkali and alkali earth metals for the iminodiacetate type of chelating resins (such as the Chelex-100) is Ca > Mg > Ba > Sr > Li > Na > K 11, whereas Mn, Fe, Cu, Pb, Cd, Ni, Zn and Co have greater affinity than has calcium. Therefore, when the resin in its calcium form was used, complete recoveries of copper, lead, cadmium, nickel, zinc, cobalt, iron and manganese were obtained from sea water, fresh water, and synthetic mixes buffered to pH 4 8 ; and the eluate contained only very minor amounts of sodium, potassium, and magnesium. The amount of calcium present in the eluate depended on the volume of the chelating resin used, the amount of calcium eluted from a 5 × 1 cm column of resin being ca. 0.2 M when made up to 25 ml. This concentration proved to be sufficient to act as a supporting electrolyte for the polarographic determination of the trace metals under investigation. Polarographic determination of copper, lead, cadmium, nickel, zinc and cobalt Copper, lead and cadmium have all been determined in both acidic and neutral electrolytes. Acetate buffer (0.1 M) has also been used for the determination of copper, lead, cadmium and zinc in natural waters by anodic stripping techniques 12. Copper and zinc, concentrated by evaporating river water, have been determined simultaneously in ammonia-ammonium chloride electrolyte 9. Copper and lead have also been determined by pulse polarography in cadmium chloride supporting electrolyte 13. Since the eluate from the chelating resin contains a high concentration of calcium, the possibility of using the calcium (as calcium chloride) as a supporting electrolyte was investigated. Although calcium chloride is not commonly used as a supporting electrolyte, the polarographic behaviour of a number of metals in a 5 M solution of it has been studied 14. Numerous problems are encountered in the use of such a strong and viscous electrolyte, but most of these can be overcome by using a lower concentration of calcium chloride solution as electrolyte. In 0.5 M solution, copper, lead, and cadmium gave good reduction waves at the dropping mercury electrode. The half-wave potentials for copper(II), lead(II) and cadmium(II) are -0.45, - 0 . 7 0 and - 0 . 9 2 V vs. the mercury pool, respectively, compared with - 0.25, - 0.45 and - 0.65 V vs. the mercury pool in 0.5 M potassium chloride solution. In derivative polarography (50-m V pulse height) the calibration curves for copper, lead and cadmium are linear in the 0.05-10 p.p.m, range in both 1 M and 0.2 M solutions of calcium chloride as supporting electrolytes. No interference was observed from Fe, Mn, Sn, Bi, Cr, V, or Mo which are also retained by the resin. The copper(II) reduction wave occurs immediately after the mercury wave at 0.0 to - 0 . 1 V which is very strong in calcium chloride solution. Nitrate reduction takes place between - 0 . 1 and - 0 . 2 V vs. the mercury pool, and this was found to affect the positive side of the copper(II) wave, which, however, can be resolved if the copper concentration is greater than 0.05 p.p.m. It was found desirable to acidify the sample solution with pure hydrochloric acid in order to maintain the metals in solution. The presence of hydrochloric acid (up to 0.5 M) was found to have no effect on the half-wave potential, nor on the sensitivity of the copper, lead and cadmium waves. Both zinc and nickel give polarographic waves in 0.2 M calcium chloride at Anal. Chim. Acta, 58 (1972)
TRACE METALSIN NATURAL WATER
285
- 1 . 4 0 and - 1 . 3 8 V vs. the mercury pool, respectively; cobalt is also reduced at - 1.36 V. These three waves could not be resolved. However, if the calcium chloride solution is made 0.5 M with respect to a m m o n i a - a m m o n i u m chloride, cadmium(II), nickel(II) and zinc(II) waves appear at -0.85, -1.15, and - 1 . 4 3 V vs. the mercury pool, respectively. The cobalt (II) wave is formed at - 1.40 V coinciding with the zinc (II) wave. However, this cobalt reduction process is irreversible in this medium and the wave is ill-defined and weak. Since the zinc-cobalt ratio in natural waters is about 100 : 1, cobalt is not likely to interfere seriously with the zinc determination. However, when the cobalt content is higher than usual, it was found that by employing reverse potential sweep, the effect of cobalt on the zinc determination can be reduced to less than 1 ~ of the zinc value. No other interference was found on the determination of nickel and zinc, and the calibration in 0.5 M ammonia-ammonium chloride-0.2 M calcium chloride supporting electrolyte is linear in the range of 0.01-5 p.p.m, nickel and 0.02-30 p.p.m, zinc. The most sensitive polarographic method for the determination of cobalt depends on the measurement of the adsorption wave of cobalt~timethylglyoxime complex in alkaline medium 15. It was found that in 0.5 M ammonia-ammonium chloride4).2 M calcium chloride supporting electrolyte, this adsorption wave appears at a half-wave potential of - 1.44 V; the wave is well defined and adequately resolved from both the zinc and nickel waves, and from the wave arising from reduction of the reagent. Variation in the calcium chloride (0.2-1 M), and the ammonia-ammonium chloride (0.2~).5 M) concentrations had little effect on the sensitivity of the cobalt adsorption wave or its half-wave potential. The amount of alcohol used as solvent for dimethylglyoxime was found to be critical, making it necessary to control accurately the strength of the reagent and the amount added to the cell. Since a considerable excess of dimethylglyoxime is added, any consumption of the reagent by an unusually high concentration of nickel, which also forms a complex, has no effect on the cobalt adsorption wave, and the peak height of the c o b a l t - D M G adsorption wave shows a linear relationship with cobalt concentration over the range of 0.02-1 p.p.m. In order to ensure the highest accuracy, a standard addition technique was employed for the determination of the six metals because of short-term instrumental variation. This technique also takes into account possible effects caused by the variation in the trace metal comp6sition of the samples, and the slightly differing amounts of calcium eluted from the columns. EXPERIMENTAL Apparatus
A Southern-Harwell A 3100 pulse polarograph was used. The glass polarographic cells were maintained at 25° in a thermostatically controlled water bath. All polarograms were obtained in the derivative mode, with a 50-mV pulse height applied before the end of the 1-sec life of the mercury drop. The faradaic current was measured 20 msec after the pulse application. Polarographic-grade triple-distilled mercury was used. Silica ware was used when possible, and before use was cleaned by soaking in a (1 + 1) mixture of concentrated nitric and sulphuric acids and then rinsing thoroughly with metal-free redistilled water. Anal. Chim. Acta,
58 (1972)
286
M.I. ABDULLAH,L. G. ROYLE
Reagents Ammonia solution (7 M). Prepare by isothermal distillation from concentrated ammonia liquor (s.g. 0.88). Hydrochloric and nitric acids. Redistil in all-silica distillation apparatus. Calcium chloride (2 M). Prepare this by passing 2 1 of 0.5 M calcium chloride solution (pH 6-7) through a 10 × 1 cm column containing clean Chelex-100 resin in the ammonium form, to remove all impurities. Collect the effluent in a silica flask and reduce its volume to 400 ml by evaporation. Destroy organic matter by irradiation under a 1-kW mercury vapour lamp and finally dilute the solution to 500 ml. Tests made on samples of this solution showed that none of the metals under in(estigation was present above the detection limit. Standard solutions. Prepare solutions of the six metals from their nitrate or chloride salts. Evaporate aliquots to dryness on a water bath in silica beakers with 5 ml of 2 M calcium chloride solution and 5 ml of 6 M hydrochloric acid. Dissolve the residues in water and make up to the appropriate volume to give solutions containing, respectively, 10 p.p.m, copper, 10 p.p.m, lead, 10 p.p.m, cadmium, 10 p.p.m. nickel, 100 p.p.m, zinc, and 1 p.p.m, cobalt. Pre-concentration column Pre-concentration of metals from the sample was carried out with columns of chelating resin similar to those used by Riley and Taylor 1°. Clean the Chelex-100 column with 2 M ifitric acid, wash acid-free with metal-free water and then wash with 30 ml of 2 M ammonia solution. Convert to the calcium form by passing ca. 25 ml of purified 0.3 M calcium chloride solution, and finally wash with 100~200 ml of metal-free water. The column of resin in its calcium form should be approximately 5 x l cm. Procedure Pass the samples of fresh water or sea water (10 1), filtered through 0.45 #m membrane filters, through Chelex-100 columns in the calcium form at the rate of 4 ml m i n - 1. Wash the columns with 100 ml of water and elute the trace elements with 70 ml of 2 M nitric acid. Irradiate the eluate, in silica beakers, under a 1-kW mercury vapour lamp for 1 h, to destroy any organic matter present, and then evaporate to dryness on a water-bath. Redissolve the residue in 10 ml of 2 M hydrochloric acid and again evaporate the solution to dryness. Repeat this step to ensure the complete removal of all nitrate from the sample. Then dissolve the residue in water, acidify with 1 ml of 6 M hydrochloric acid, and make up to 25 ml. Regenerate the columns as described earlier, taking care to avoid leaving them in the hydrogen form because of the risk of decomposition. Place a 2.5-ml aliquot of the solution in the polarographic cell, deoxygenate for 3 rain with oxygen-free nitrogen, and then record a pulse polarogram over the range of - 0 . 2 to - 1.1 V, for the determination of copper, lead and cadmium. Accurately add 0.2 ml of a standard containing 10 #g ml 1 each of copper, lead and cadmium, and record a second polarogram. Care must be taken to ensure that none of the instrumental settings are altered during the recording of these polarograms. To the same solution in the cell, add 0.6 ml of 2 M ammonia solution. This makes the solution in the cell 0.2 M with respect to a m m o n i a - a m m o n i u m chloride. Record a pulse polarogram between - 0.8 and - 1.7 V for the nickel and zinc measureAnal. Chim. Acta, 58 (1972)
TRACE METALS IN NATURAL
287
WATER
ments. Obtain a second polarogram after the addition of 0.2 ml of nickel and zinc standard. Again to the same solution in the cell, add 0.2 ml of a 1 ~ alcoholic solution of dimethylglyoxime and record the polarogram between - 1.1 and - 1.5 V for the determination of cobalt. Obtain a second polarogram after the addition of 0.2 ml of cobalt standard. Calculate the concentrations of copper, lead, cadmium, nickel, zinc and cobalt by measuring the peak heights and peak height increments after the addition of standard. It is necessary to make corrections for the dilution of the sample in the cell caused by the addition of standards, ammonia and dimethylglyoxime. RESULTS The accuracy of the method was evaluated by spiking 5-I samples of sea water which had been stripped of trace metals by passing it through chelating resin column, with known amounts of copper, lead, cadmium, nickel, zinc and cobalt and analysed according to the procedure outlined above. The results (Table I) show that the determination of these metals at the levels occurring in natural waters is satisfactory. The precision of the polarographic method was evaluated by seven replicate analyses of sea water samples collected from Liverpool Bay. The amounts of metal detected and standard deviations are listed in Table I, and these appear to be satisfactory for routine trace metal determination in natural waters. TABLE I RESULTS ON SPIKED AND NATURAL SEA WATERS
(All amounts are given in]~g 1 1)
Amount added Amount found Amount added Amount found Amount added Amount found Amount added Amount found Sea water sample Standard deviation"
Cu
Pb
Cd
Ni
Zn
1.00 1.04 --
1.00 1.04 2.00 1.92 4.00 4.09 6.00 5.78 0.90 0.05
1.00 0.98 2.00 1.99 4.00 3.93 6.00 6.09 1.38 0.03
1.00 0.97 2.00 1.78 4.00 4.13 6.00 5.90 0.78 0.02
10.0 0.20 9.7 0.19 20.0 0.40 19.9 0.41 40.0 0.40 41.0 0.42 60.0 0.60 60.7 0.61 11.8 < 0.01 0.14 0.15 b
4.00 3.89 6.00 5.83 2.03 0.12
Co
" 7 determinations were made. bA spiked sample was used. This work was supported in part by the Natural Environment Research Council. SUMMARY A method is described for the pulse-polarographic determination of copper, lead, cadmium, nickel, zinc and cobalt in natural waters, after their preconcentration Anal. Chim. Acta,
58 (1972)
288
M . I . ABDULLAH, L. G. ROYLE
on chelating resin in its calcium form. The eluate from the resin contains sufficient calcium to act as a supporting electrolyte for the subsequent polarographic determination. The determination of the six metals is made on a single aliquot of the eluate after suitable adjustment of conditions. RI~SUMI~
Une m6thode est d6crite pour le dosage polarographique du cuivre, du plomb, du cadmium, du nickel, du zinc et du cobalt dans les eaux naturelles, apr6s pr6concentration sur r6sine sous forme de calcium. L'61uat contient suffisamment de calcium pour constituer l'61ectrolyte de base pour le dosage polarographique. Le dosage des six mdtaux se fait sur une simple aliquote de l'61uat, apr6s traitement appropri6. ZUSAMMENFASSUNG
Es wird eine Methode beschrieben ffir die pulse-polarographische Bestimmung von Kupfer, Blei, Cadmium, Nickel, Zink und Kobalt in in der Natur vorkommendem Wasser. Die Elemente werden mittels eines Chelataustauscher-Harzes in der Calciumform vorkonzentriert. Das Eluat aus dem Harz enthSlt genfigend Calcium, so dass dieses als Tr~gerelektrolyt ffir die anschliessende polarographische Bestimmung verwendet werden kann. Die Bestimmung der sechs Metalle erfolgt in einem einzigen aliquoten Anteil des Eluats nach geeigneter Einstellung der Bedingungen. REFERENCES 1 2 3 4 5 6 7 8 9 l0 ll 12 13 14 15
G. W. C. MILNER,J. D. WILSON,G. A. BARNETTAND A. A. SMALLS,J. Electroanal. Chem., 2 (1961) 25. M. ARIEL AND U. EISNER, J. Electroanal. Chem., 5 (1963) 362. M. ARIEL, ~U. EISNER AND S. GOTTESEIELD, J. Electroanal. Chem., 7 (1964) 307. G. MACCHI, J. Electroanal. Chem., 9 (1965) 290. G. C. WHITNACK AND R. SASSELLI,Anal. Chim. Acta, 47 (1969) 267. J. P. RILEY AND G. TOPPING, Anal. Chim. Acta, 44 (1969) 234. R. R. BROOKS, B. J. PRESLEY AND I. R. KAPLAN, Talanta, 14 (1967) 809. D. W. SPENCER AND P. G. BREWER, Geochim. Cosmochim. Acta, 33 (1969) 325. E. J. MAIENTHAL AND J. K. TAYLOR, in R. F. GOULD, Trace Inoroanics in Water, Am. Chem. Soc., Washington, D.C., 1963, p. 172. J. P. RILEY AND D. TAYLOR, Anal. Chim. Acta, 40 (1968) 479. R. ROSSET, Bull. Inf. Sci. Tech. Comm. Ener9. At., 85 (1964) 13. I. SINKO AND J. DOLEZAL,J. Electroanal. Chem., 25 (1970) 299. E. TEMMERMANAND F. VERBEEK,dr. Electroanal. Chem., 12 (1966) 158. G. F. REYNOLDS,H. L SHALGOSKY AND T. J. WEBBER,Anal. Chim. Acta, 8 (1953) 558 and 564. P. NANGNIOT, J. Electroanal. Chem., 7 (1964) 50.
Anal. Chim. Acta, 58 (1972)