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Simultaneous determination of several trace metals by asv after preconcentration by adsorption as padap complexes on C18-bonded glass beads
0039-9140/83/030169-04$03.00/O CopyrIght 0 1983 Pergamon Press Ltd
Trrkmu, Vol. 30. No 3. pp 169-172, 1983 Prmted in Great Bntam All rights reserved
SIMULTANEOUS DETERMINATION OF SEVERAL TRACE METALS BY ASV AFTER PRECONCENTRATION BY ADSORPTION AS PADAP COMPLEXES ON Cl *-BONDED GLASS BEADS SHICERU
Department
TAGLJCH~, TAKAYUKI YAI, YASUKO SHIMADA, KATSUMI GOTO
of Chemistry,
Faculty
of Science. Toyama
University,
Toyama,
Japan
930
and MINORU HARA Department
of Chemistry,
Faculty
of Education,
Toyama
University,
Toyama.
Japan
930
(Received 30 June 1982. Accepted 24 September 1982) Summary-Traces of zmc, lead, copper and cadmmm are determined simultaneously by anodic-strtpping voltammetry (ASV) combined with a preconcentratton technique uttlizing Cl,-bonded glass beads. The metals are collected as their 2-(2-pyridylazoJ-5-dtethylaminophenol (PADAP) complexes on a column of the beads and the complexes are eluted with a small volume of ethanol-hydrochlorrc acidchloroform mixture. The eluate is evaporated to dryness m the presence of hydrogen peroxide and the residue dtssolved in a small volume of acettc acid-sodium acetate buffer. The concentratrons of the metals are measured by ASV. Quantitative recoveries are obtained for O.Ol-ng/ml levels of the metals. Many ions which interfere in the direct ASV procedure do not interfere m the present method. preconcentration techniques for trace-element analysis of aqueous samples have been proposed and reviewed by many authors. The surface adsorption technique is one of the most versatile methods for the concentration and/or separation of trace elements from a variety of matrices.1p’4 In this technique, the trace element is adsorbed as a complex on an adsorbent such as a hydrophobic resin2~5,8-‘4 or activated carbon.‘,’ We have proposed C,s-bonded glass beads and polypropylene wool’ ’ as the adsorbent, on which traces of cobalt8 iron” and phosphorus9-‘i were collected as coloured complexes and eluted with a small volume of eluent, then determined spectronhotometricallv. Since these adsorbents have no inner surfaces, adsorption and desorption of the complexes are very fast. and more than 100-fold concentration can easily be achieved. Similar applications have been reported by Watanabe et aLi and Sturgeon et al.,14 in which Cis-bonded silica gel was used for the preconcentration of several heavy metals in sea-water samples, and the concentrates were analysed by inductivelyatomic-emission spectrometry coupled plasma (ICP-AES) or graphite-furnace atomic-absorption spectrophotometry. In the present study, zinc, lead, copper and cadmium were collected as their 2-(2-pyridylazo)-5-diethylaminophenol (PADAP) complexes on a column of C1 a-bonded glass beads and determined by anodicstripping voltammetry (ASV). ASV is one of the most sensitive methods for trace analysis, and the instrumentation is generally inexpensive and often home-
made, as in the present work. Simultaneous determinations can be done with a small volume of sample. PADAP was chosen as the complexing reagent since it forms highly coloured water-soluble complexes with the metals concerned under conditions in which calcium and magnesium do not form complexes.’ 5
Numerous
1
EXPERIMENTAL Reagents C,,-bonded glass beads. Prepared by treating 10&120 mesh non-porous glass beads as reported prevtously.” PADAP. Synthesized as reported previously.rh and dissolved in 0.05M hydrochloric acid to give a 0.05% solution. Buffer sohrtion, pH Y.5. Prepared by mixing equal volumes of 0.05M sodium tetraborate and 0.05M sodmm carbonate. The reagents were purified by recrystalhzatron. Eluent. A 7:2:1 v/v mixture of ethanol. O.lM hydrochlortc acid and chloroform. Supporting electrolyte. Prepared by mixing equal volumes of 0.1 M sodium acetate and 0.1 M acetic acid. The sodium acetate was purified by recrystallizatron. Hydroxylammonium chloride solution, 30%. Prepared with recrystallized hydroxylammonium chloride. Standard metal solutions. Prepared by dissolving the nitrates or chlorides in hydrochloric acid, and standardized by complexometry. Other chemicals used were reagent-grade. The water was doubly dtstilled.
,
Apparatus ASV mstrument. Constructed as described by Possley and Higgins.” Electrochemical cell. This was essentially of the same type as that described by Barendrecht.” A hanging mercury drop electrode and a saturated calomel electrode were used as the working and reference electrodes. respectively. The volume of the cell was 1 ml. 169
170
SHIGERU TAGUCHI
et al.
Pump. A Taiyo Kagaku model 150-N tubing pump was used The flow-rate could be controlled over the range from 0.5 to 10 ml/min. Column. The column was prepared in a glass tube. 1Omm m diameter and 300mm in length, by addmg a slurry of the beads m ethanol until the height of the bed was 250 mm. Before use. it was washed with water to remove the ethanol.
Procedure To a sample solution containing less than 0.1 pg of zmc, lead. copper and cadmium, add 1.0 ml of the PADAP solutlon and adjust the pH to about 9.5 with the buffer solutlon. Set aside for more than 30 but less than 120 min, then pump the solution through the column at a flow-rate of about 10 ml/min. Pump the eluent slowly through the column (< 1 ml/mm) and collect the coloured portion of the eluate in a beaker. Add 0.25 ml of 3036 hydrogen peroxide. and heat slowly. When the solution has become colourless, add 3 ml of the hydroxylammonmm chloride solution. and evaporate to dryness. Cool to room temperature and dissolve the residue with 2 ml of the supporting electrolyte. Transfer I ml of the solution into the electrochemical cell and measure the concentration by ASV under the followmg conditions: deposition period 5-20 mm; deposition potential - 1.2V 11s.SCE ; sweep-rate IO mV/sec
RESULTS AND DISCUSSION
Complesing reagent PADAP is proposed as the complexing reagent, because it forms highly coloured complexes with many heavy metals under conditions in which calcium and magnesium do not form complexes. and because the reagent and its complexes are rather water-soluble.‘5 4-(2-Pyridylazo)-resorcinol, a wellknown water-soluble complexing reagent, was also examined, but the adsorbed complexes were not easily eluted. A large excess of PADAP does not affect the results. In this study, 1.0 ml of 0.05% PADAP solution (more than 600-fold excess for 0.1 pg of copper) is added. Effect of pH Figure 1 shows that pH values higher than 9.2 give quantitative recovery for the individual metals. A pH of 9.5 was therefore chosen for complex formation. For pH adjustment, a sodium tetraboratesodium carbonate buffer was used, since these reagents can be easily purified by recrystallization. Standing time before adsorption Less than 30 min gives low recovery, since the complex formation is not complete. More than 2 hr also gives low recovery, owing to adsorption of the complexes on the beaker wall. EfSrct qfj7owrate Recoveries flow-rate
of the metals
in the range
lOml/min
(the
used)
selected
was
were eluted
did not
of 0.5-10
maximum
attainable
for use.
at flow-rates
The
lower
vary
ml/min.
with
adsorbed than
with
sample
A flow-rate the
pump
complexes
1 ml/min.
of
PH
Fig. I. Effect of pH on recovery (lOO-fold concentration factor). Concentration of each metal in the sample solution IS 1 0 pgg/l.
Adsorption capacity qf the adsorbent The adsorption capacity of the C,,-bonded glass beads used was found to be 0.2pmole/g, by the column “breakthrough” technique,” with the CdPADAP complex as the indicator species. The small capacity is due to the small surface area of the nonporous glass beads used, but causes no problem, since the heavy metals with which PADAP reacts under the conditions used are present only at trace level in natural waters. Ehent Hydrochloric acid and several organic solvents were tested. The complexes are decomposed by O.lM hydrochloric acid, but not completely eluted by it. Dimethylsulphoxide and dimethylformamide are effective eluents, but their low volatility makes it difficult to remove them by evaporation before the ASV measurement. Chloroform is one of the most effective solvents, but its high density causes mixing at the water-chloroform boundary during the elution. Various mixtures of ethanol, O.lM hydrochloric acid and chloroform were studied. A 7:2: 1 v/v mixture proved excellent, the adsorbed complexes being eluted with less than 10 ml of the eluent, and the eluate easily evaporated to dryness. Treatment of the eluate before ASV measurement Since ASV measurement could not be done directly in the eluate medium, the solvent was removed by evaporation before the ASV measurement, but recovery was poor if the PADAP was not decomposed. because of re-formation of the complexes in the supporting electrolyte medium. Addition of hydrogen peroxide to the eluate before the evaporation gave excellent recoveries. Optimum conditions for ASV Several media, such as acetlc acid, sodium acetate, ammonium acetate, potassium nitrate. sodium potassium tartrate and sodium citrate were tested as SUPporting electrolytes. A mixture of equal volumes of
171
Simultaneous determination of several trace metals Table
1. Recoveries
of the metals at various concentrations
in various volumes
Recovery, Metal concn., W/l.
Volume, ml
Deposition period, min
1.0
100
5
0.50
250
5
0.10
250
10
0.050
250
20
0.010
500
20
%
Zn2+
Cd2+
Pb’+
CU*+
99 4* 99 4* 97 7* 97 6* 94 8*
99 3* 99 4* 98 6* 95 6* 94 I*
99 3* 99 4* 97 6* 95 6* 92 7*
105 5* 104 5* 103 8* 97 7* 95 9*
Recoveries are mean values of 10 determinations. *Coefficient of variation, %.
sodium acetate and 0.M acetic acid is proposed, because reproducible stripping potentials and peak currents were obtained with this medium. The stripping potentials for zinc, cadmium, lead and copper are -0.96, -0.59, -0.42 and -0.05 V vs. SCE, respectively, in this supporting electrolyte. A potential of - 1.2OV DS. SCE was chosen for deposition. The peak current increased in proportion to the deposition period over the range tX40min. Sweeprates ranging from 1.6 to 53.2 mV/sec were tested. Higher sweep-rates do not give reproducible results, and 10 mV/sec is recommended.
O.lM
Recovery and precision Table 1 shows the recoveries of the metals at various concentrations from various volumes of doubly distilled water. Spiking with standard solution was used. Better than 95% recovery was obtained at
the 0.05-ng/ml level, with satisfactory precision. Better than 90% recovery was obtained with 500-ml sample volumes containing 5 ng of the metal. EfSect I_$diverse species Table 2 shows the effect of various species on the determination of the metals by the present method, as compared with that on direct ASV. It is clear that many ions which interfere with the direct ASV method do not interfere in the proposed method. Large amounts of potassium, sodium and chloride do not affect the determination, nor do 1OOOppm of magnesium or 500 ppm of calcium (because they do not form complexes with PADAP under the conditions used). Separation of the heavy metals from the matrices and from interfering ions in natural waters is a great this preconcentration technique. advantage in Cobalt(II), nickel, manganese(I1) and iron(W) form
Table 2. Effect of diverse ions Recovery,
%
Concn., Ion A?+ Fe3+ Sn2 + BrISCNGelatine
Method direct ASV present method present method* present method direct ASV present method direct ASV present method direct ASV present method direct ASV present method direct ASV present method
@l/l.
Zn2+
Cd2 +
Pb*+
cu2+
1.0 10 1.o 5.0 1.0
29 98 50 97 110 96 93 99 91 98 91 97 89 100
97 98 65 97 98 98 96 99 93 98 83 98 85 99
95 98 67 99
71 101 70 96
10
10 10 10 10 1.0 10 1.0 10
*In the absence of hydroxylammonium chloride. tCannot be measured. lOO-fold concentration was performed in the present method. Concentration of each metal was 0.1 mg/l. in the electrochemical
cell.
t
t
98 97 99 91 99 89 99 83 99
98 70 100 156 100 61 101 55 102
172
SHIGERUTAGUCHI et al.
reddish complexes with PADAP under the conditions used and are adsorbed on the glass beads. Of these, only iron(II1) interferes with the determination of the four metals of interest, and this interference can be eliminated by addition of hydroxylammonium chloride, as shown in Table 2. Chromium(II1) slowly forms a PADAP complex. which is adsorbed on the glass beads, but large amounts of chromium(II1) do not affect the ASV measurement. In the presence of very high concentrations of these metals, large amounts of reagent and adsorbent should be used so that enough will be available for the complexation and adsorption of the metals to be determined. CONCLUSIONS
Very high concentration factors are obtained by this simple preconcentration technique. A combination of the preconcentration technique with ASV is very promising for simultaneous determination of traces of heavy metals. PADAP is especially advantageous as the complexing reagent, forming watersoluble complexes with the four metals, but not with calcium and magnesium which are matrix species in natural waters. Arsenic and tin, if present. seriously interfere with the determination of the four metals by direct ASV, but can be removed in the preconcentration step. Since PADAP reacts with many heavy metals. this preconcentration technique will be applicable to the determination of many other metals in natural waters, if this separation method is combined with other
instrumental methods such as ICP-AES, graphitefurnace atomic-absorption spectrophotometry and differential-pulse anodic-stripping voltammetry.
REFERENCES
1 D. E. Leyden and W. Wegscheider. A&.
Chem., 1981,
53, 1059A. 2. Y. Sugimura, Bunseki, 1981, 148. 3 A. Warshawsky. Talanta. 1974, 21, 962. 4. R. B. Willis and D. Sangster,
Anal. Chem., 1976.48.
59.
5. Y. Sakai, Talanta, 1980. 27, 1073. 6. M. Kimura and K. Kawanaml. ibid., 1979. 26, 901 7. M. Kimura. Bunsekr. 1981. 297. 8. S Taguchi and K. Goto, Talanta. 1980, 27. 819. 9. S. Taguchi. K. Goto and H. Watanabe. ibid., 1981. 28, 613. 10. Idem, Water Purification and Llqurd Waste Treatment. 1981, 22, 13. I I. S. Taguchl. S. Amano and K. Goto, Enoironmmtal Consertutron Engmeermg Association, 1981, 10, 628. C. Yoshikura and K. Goto. Bunsekr 12. S. Taguchi. Kagaku, 1982, 31. 32. 13. H. Watanabe, K. Goto, S. Taguchl, .I. W. McLaren, S. S. Berman and D. S. Russell, Anal. Chem., 1981. 53, 738. 14. R. E. Sturgeon, S. S. Berman and S. N. Wdhe, Talanta. 1982, 29, 169. IS. S. Shibata, in Chelates rn Analytrcal Cheml.btr_v, A. J. Barnard. Jr. and H. Flaschka (eds.), Vol 4, p. 194. Dekker, New York. 1972. 16 T. M. Florence. D. A. Johnson and Y. J. Farrar. Anal. Chem., 1969. 41. 1652. 17. N. L. Possley and G W Higgms. J. Chem. Educ., 1974, 51, 351 m Electroana/ytica/ Chumstry. A. J. 18. E. Barendrecht, Bard (ed.), Vol. 2, p 89. Dekker. New York, 1967. 19. J. R. Jezorek and H Freiser, AnuI. C’hem., 1979. 51, 366.
Trrkmu, Vol. 30. No 3. pp 169-172, 1983 Prmted in Great Bntam All rights reserved
SIMULTANEOUS DETERMINATION OF SEVERAL TRACE METALS BY ASV AFTER PRECONCENTRATION BY ADSORPTION AS PADAP COMPLEXES ON Cl *-BONDED GLASS BEADS SHICERU
Department
TAGLJCH~, TAKAYUKI YAI, YASUKO SHIMADA, KATSUMI GOTO
of Chemistry,
Faculty
of Science. Toyama
University,
Toyama,
Japan
930
and MINORU HARA Department
of Chemistry,
Faculty
of Education,
Toyama
University,
Toyama.
Japan
930
(Received 30 June 1982. Accepted 24 September 1982) Summary-Traces of zmc, lead, copper and cadmmm are determined simultaneously by anodic-strtpping voltammetry (ASV) combined with a preconcentratton technique uttlizing Cl,-bonded glass beads. The metals are collected as their 2-(2-pyridylazoJ-5-dtethylaminophenol (PADAP) complexes on a column of the beads and the complexes are eluted with a small volume of ethanol-hydrochlorrc acidchloroform mixture. The eluate is evaporated to dryness m the presence of hydrogen peroxide and the residue dtssolved in a small volume of acettc acid-sodium acetate buffer. The concentratrons of the metals are measured by ASV. Quantitative recoveries are obtained for O.Ol-ng/ml levels of the metals. Many ions which interfere in the direct ASV procedure do not interfere m the present method. preconcentration techniques for trace-element analysis of aqueous samples have been proposed and reviewed by many authors. The surface adsorption technique is one of the most versatile methods for the concentration and/or separation of trace elements from a variety of matrices.1p’4 In this technique, the trace element is adsorbed as a complex on an adsorbent such as a hydrophobic resin2~5,8-‘4 or activated carbon.‘,’ We have proposed C,s-bonded glass beads and polypropylene wool’ ’ as the adsorbent, on which traces of cobalt8 iron” and phosphorus9-‘i were collected as coloured complexes and eluted with a small volume of eluent, then determined spectronhotometricallv. Since these adsorbents have no inner surfaces, adsorption and desorption of the complexes are very fast. and more than 100-fold concentration can easily be achieved. Similar applications have been reported by Watanabe et aLi and Sturgeon et al.,14 in which Cis-bonded silica gel was used for the preconcentration of several heavy metals in sea-water samples, and the concentrates were analysed by inductivelyatomic-emission spectrometry coupled plasma (ICP-AES) or graphite-furnace atomic-absorption spectrophotometry. In the present study, zinc, lead, copper and cadmium were collected as their 2-(2-pyridylazo)-5-diethylaminophenol (PADAP) complexes on a column of C1 a-bonded glass beads and determined by anodicstripping voltammetry (ASV). ASV is one of the most sensitive methods for trace analysis, and the instrumentation is generally inexpensive and often home-
made, as in the present work. Simultaneous determinations can be done with a small volume of sample. PADAP was chosen as the complexing reagent since it forms highly coloured water-soluble complexes with the metals concerned under conditions in which calcium and magnesium do not form complexes.’ 5
Numerous
1
EXPERIMENTAL Reagents C,,-bonded glass beads. Prepared by treating 10&120 mesh non-porous glass beads as reported prevtously.” PADAP. Synthesized as reported previously.rh and dissolved in 0.05M hydrochloric acid to give a 0.05% solution. Buffer sohrtion, pH Y.5. Prepared by mixing equal volumes of 0.05M sodium tetraborate and 0.05M sodmm carbonate. The reagents were purified by recrystalhzatron. Eluent. A 7:2:1 v/v mixture of ethanol. O.lM hydrochlortc acid and chloroform. Supporting electrolyte. Prepared by mixing equal volumes of 0.1 M sodium acetate and 0.1 M acetic acid. The sodium acetate was purified by recrystallizatron. Hydroxylammonium chloride solution, 30%. Prepared with recrystallized hydroxylammonium chloride. Standard metal solutions. Prepared by dissolving the nitrates or chlorides in hydrochloric acid, and standardized by complexometry. Other chemicals used were reagent-grade. The water was doubly dtstilled.
,
Apparatus ASV mstrument. Constructed as described by Possley and Higgins.” Electrochemical cell. This was essentially of the same type as that described by Barendrecht.” A hanging mercury drop electrode and a saturated calomel electrode were used as the working and reference electrodes. respectively. The volume of the cell was 1 ml. 169
170
SHIGERU TAGUCHI
et al.
Pump. A Taiyo Kagaku model 150-N tubing pump was used The flow-rate could be controlled over the range from 0.5 to 10 ml/min. Column. The column was prepared in a glass tube. 1Omm m diameter and 300mm in length, by addmg a slurry of the beads m ethanol until the height of the bed was 250 mm. Before use. it was washed with water to remove the ethanol.
Procedure To a sample solution containing less than 0.1 pg of zmc, lead. copper and cadmium, add 1.0 ml of the PADAP solutlon and adjust the pH to about 9.5 with the buffer solutlon. Set aside for more than 30 but less than 120 min, then pump the solution through the column at a flow-rate of about 10 ml/min. Pump the eluent slowly through the column (< 1 ml/mm) and collect the coloured portion of the eluate in a beaker. Add 0.25 ml of 3036 hydrogen peroxide. and heat slowly. When the solution has become colourless, add 3 ml of the hydroxylammonmm chloride solution. and evaporate to dryness. Cool to room temperature and dissolve the residue with 2 ml of the supporting electrolyte. Transfer I ml of the solution into the electrochemical cell and measure the concentration by ASV under the followmg conditions: deposition period 5-20 mm; deposition potential - 1.2V 11s.SCE ; sweep-rate IO mV/sec
RESULTS AND DISCUSSION
Complesing reagent PADAP is proposed as the complexing reagent, because it forms highly coloured complexes with many heavy metals under conditions in which calcium and magnesium do not form complexes. and because the reagent and its complexes are rather water-soluble.‘5 4-(2-Pyridylazo)-resorcinol, a wellknown water-soluble complexing reagent, was also examined, but the adsorbed complexes were not easily eluted. A large excess of PADAP does not affect the results. In this study, 1.0 ml of 0.05% PADAP solution (more than 600-fold excess for 0.1 pg of copper) is added. Effect of pH Figure 1 shows that pH values higher than 9.2 give quantitative recovery for the individual metals. A pH of 9.5 was therefore chosen for complex formation. For pH adjustment, a sodium tetraboratesodium carbonate buffer was used, since these reagents can be easily purified by recrystallization. Standing time before adsorption Less than 30 min gives low recovery, since the complex formation is not complete. More than 2 hr also gives low recovery, owing to adsorption of the complexes on the beaker wall. EfSrct qfj7owrate Recoveries flow-rate
of the metals
in the range
lOml/min
(the
used)
selected
was
were eluted
did not
of 0.5-10
maximum
attainable
for use.
at flow-rates
The
lower
vary
ml/min.
with
adsorbed than
with
sample
A flow-rate the
pump
complexes
1 ml/min.
of
PH
Fig. I. Effect of pH on recovery (lOO-fold concentration factor). Concentration of each metal in the sample solution IS 1 0 pgg/l.
Adsorption capacity qf the adsorbent The adsorption capacity of the C,,-bonded glass beads used was found to be 0.2pmole/g, by the column “breakthrough” technique,” with the CdPADAP complex as the indicator species. The small capacity is due to the small surface area of the nonporous glass beads used, but causes no problem, since the heavy metals with which PADAP reacts under the conditions used are present only at trace level in natural waters. Ehent Hydrochloric acid and several organic solvents were tested. The complexes are decomposed by O.lM hydrochloric acid, but not completely eluted by it. Dimethylsulphoxide and dimethylformamide are effective eluents, but their low volatility makes it difficult to remove them by evaporation before the ASV measurement. Chloroform is one of the most effective solvents, but its high density causes mixing at the water-chloroform boundary during the elution. Various mixtures of ethanol, O.lM hydrochloric acid and chloroform were studied. A 7:2: 1 v/v mixture proved excellent, the adsorbed complexes being eluted with less than 10 ml of the eluent, and the eluate easily evaporated to dryness. Treatment of the eluate before ASV measurement Since ASV measurement could not be done directly in the eluate medium, the solvent was removed by evaporation before the ASV measurement, but recovery was poor if the PADAP was not decomposed. because of re-formation of the complexes in the supporting electrolyte medium. Addition of hydrogen peroxide to the eluate before the evaporation gave excellent recoveries. Optimum conditions for ASV Several media, such as acetlc acid, sodium acetate, ammonium acetate, potassium nitrate. sodium potassium tartrate and sodium citrate were tested as SUPporting electrolytes. A mixture of equal volumes of
171
Simultaneous determination of several trace metals Table
1. Recoveries
of the metals at various concentrations
in various volumes
Recovery, Metal concn., W/l.
Volume, ml
Deposition period, min
1.0
100
5
0.50
250
5
0.10
250
10
0.050
250
20
0.010
500
20
%
Zn2+
Cd2+
Pb’+
CU*+
99 4* 99 4* 97 7* 97 6* 94 8*
99 3* 99 4* 98 6* 95 6* 94 I*
99 3* 99 4* 97 6* 95 6* 92 7*
105 5* 104 5* 103 8* 97 7* 95 9*
Recoveries are mean values of 10 determinations. *Coefficient of variation, %.
sodium acetate and 0.M acetic acid is proposed, because reproducible stripping potentials and peak currents were obtained with this medium. The stripping potentials for zinc, cadmium, lead and copper are -0.96, -0.59, -0.42 and -0.05 V vs. SCE, respectively, in this supporting electrolyte. A potential of - 1.2OV DS. SCE was chosen for deposition. The peak current increased in proportion to the deposition period over the range tX40min. Sweeprates ranging from 1.6 to 53.2 mV/sec were tested. Higher sweep-rates do not give reproducible results, and 10 mV/sec is recommended.
O.lM
Recovery and precision Table 1 shows the recoveries of the metals at various concentrations from various volumes of doubly distilled water. Spiking with standard solution was used. Better than 95% recovery was obtained at
the 0.05-ng/ml level, with satisfactory precision. Better than 90% recovery was obtained with 500-ml sample volumes containing 5 ng of the metal. EfSect I_$diverse species Table 2 shows the effect of various species on the determination of the metals by the present method, as compared with that on direct ASV. It is clear that many ions which interfere with the direct ASV method do not interfere in the proposed method. Large amounts of potassium, sodium and chloride do not affect the determination, nor do 1OOOppm of magnesium or 500 ppm of calcium (because they do not form complexes with PADAP under the conditions used). Separation of the heavy metals from the matrices and from interfering ions in natural waters is a great this preconcentration technique. advantage in Cobalt(II), nickel, manganese(I1) and iron(W) form
Table 2. Effect of diverse ions Recovery,
%
Concn., Ion A?+ Fe3+ Sn2 + BrISCNGelatine
Method direct ASV present method present method* present method direct ASV present method direct ASV present method direct ASV present method direct ASV present method direct ASV present method
@l/l.
Zn2+
Cd2 +
Pb*+
cu2+
1.0 10 1.o 5.0 1.0
29 98 50 97 110 96 93 99 91 98 91 97 89 100
97 98 65 97 98 98 96 99 93 98 83 98 85 99
95 98 67 99
71 101 70 96
10
10 10 10 10 1.0 10 1.0 10
*In the absence of hydroxylammonium chloride. tCannot be measured. lOO-fold concentration was performed in the present method. Concentration of each metal was 0.1 mg/l. in the electrochemical
cell.
t
t
98 97 99 91 99 89 99 83 99
98 70 100 156 100 61 101 55 102
172
SHIGERUTAGUCHI et al.
reddish complexes with PADAP under the conditions used and are adsorbed on the glass beads. Of these, only iron(II1) interferes with the determination of the four metals of interest, and this interference can be eliminated by addition of hydroxylammonium chloride, as shown in Table 2. Chromium(II1) slowly forms a PADAP complex. which is adsorbed on the glass beads, but large amounts of chromium(II1) do not affect the ASV measurement. In the presence of very high concentrations of these metals, large amounts of reagent and adsorbent should be used so that enough will be available for the complexation and adsorption of the metals to be determined. CONCLUSIONS
Very high concentration factors are obtained by this simple preconcentration technique. A combination of the preconcentration technique with ASV is very promising for simultaneous determination of traces of heavy metals. PADAP is especially advantageous as the complexing reagent, forming watersoluble complexes with the four metals, but not with calcium and magnesium which are matrix species in natural waters. Arsenic and tin, if present. seriously interfere with the determination of the four metals by direct ASV, but can be removed in the preconcentration step. Since PADAP reacts with many heavy metals. this preconcentration technique will be applicable to the determination of many other metals in natural waters, if this separation method is combined with other
instrumental methods such as ICP-AES, graphitefurnace atomic-absorption spectrophotometry and differential-pulse anodic-stripping voltammetry.
REFERENCES
1 D. E. Leyden and W. Wegscheider. A&.
Chem., 1981,
53, 1059A. 2. Y. Sugimura, Bunseki, 1981, 148. 3 A. Warshawsky. Talanta. 1974, 21, 962. 4. R. B. Willis and D. Sangster,
Anal. Chem., 1976.48.
59.
5. Y. Sakai, Talanta, 1980. 27, 1073. 6. M. Kimura and K. Kawanaml. ibid., 1979. 26, 901 7. M. Kimura. Bunsekr. 1981. 297. 8. S Taguchi and K. Goto, Talanta. 1980, 27. 819. 9. S. Taguchi. K. Goto and H. Watanabe. ibid., 1981. 28, 613. 10. Idem, Water Purification and Llqurd Waste Treatment. 1981, 22, 13. I I. S. Taguchl. S. Amano and K. Goto, Enoironmmtal Consertutron Engmeermg Association, 1981, 10, 628. C. Yoshikura and K. Goto. Bunsekr 12. S. Taguchi. Kagaku, 1982, 31. 32. 13. H. Watanabe, K. Goto, S. Taguchl, .I. W. McLaren, S. S. Berman and D. S. Russell, Anal. Chem., 1981. 53, 738. 14. R. E. Sturgeon, S. S. Berman and S. N. Wdhe, Talanta. 1982, 29, 169. IS. S. Shibata, in Chelates rn Analytrcal Cheml.btr_v, A. J. Barnard. Jr. and H. Flaschka (eds.), Vol 4, p. 194. Dekker, New York. 1972. 16 T. M. Florence. D. A. Johnson and Y. J. Farrar. Anal. Chem., 1969. 41. 1652. 17. N. L. Possley and G W Higgms. J. Chem. Educ., 1974, 51, 351 m Electroana/ytica/ Chumstry. A. J. 18. E. Barendrecht, Bard (ed.), Vol. 2, p 89. Dekker. New York, 1967. 19. J. R. Jezorek and H Freiser, AnuI. C’hem., 1979. 51, 366.