- Email: [email protected]
Simultaneous determination of lead and thallium by potentiometric stripping
0039-9140/U $3.00 + 0.00 Pergamon Press Ltd
SIMULTANEOUS DETERMINATION OF LEAD THALLIUM BY POTENTIOMETRIC STRIPPING S. JAYA,T. Central
AND
PRASADA RAO and G. PRABHAKARA RAO
Electrochemical
Research
Institute,
Karaikudi-623006,
India
(Received 15 March 1985. Accepted 14 June 1985) Summary-A potentiometric stripping analysis procedure has been developed for the simultaneous determination of lead and thallium in chloroacetate-chloride medium. This procedure allows the determination of lead or thallium concentrations as low as IO ng/ml. The method is precise, and applicable to the determination of lead and thallium in sea-water.
The toxicity of lead and its compounds’ has resulted in a voluminous literature on its electroanalytical determination in a variety of environmental samples, either by itself or in combination with other heavy metals.* The determination of lead by anodic stripping voltammetry (ASV)3,4or differential pulse ASV’ with the hanging mercury drop electrode (HMDE) suffers no problems in presence of copper, zinc and cadmium but encounters serious interference from thallium. ASV procedures have also been reported by other authors.6.7 For determination of lead and thallium in admixtures most authors have resorted to addition of EDTA.’ ’ Potentiometric stripping analysis, developed by Jagner,” offers a simple, rapid and reliable way of determining lead in sea-water, in presence of copper, zinc and cadmium, but so far no attempt has been made to use it to determine thallium or thallium and lead in mixtures9,” This paper reports the determination of lead and thallium in concentration ratios ranging from 1: 100 to 100: 1. EXPERIMENTAL Reagents The following solutions were prepared with BDH analytical-grade reagents and conductivity water: lead, O.IM: thallium(I). O.OlM, mercury(Il), O.OlM; sodium chloride. SM; chloroacetic acid. 2.OM.
Appurafus A Wenking Model 75 M potentiostat and potential scan generator were used, with a three-electrode cell assembly: a normal calomel reference electrode (NCE), platinum foil counter-electrode and a glassy-carbon (Tokai & Co., Japan, 3 mm diameter) workmg electrode. The recordings were made on a Digllog XY-2000 recorder. Procedure Transfer a suitable known volume (up to 45 ml) of the sample solution contaming lead and/or thallium (concentration of each < 2 pg,‘ml) mto I 50-ml standard flask. Add 2.5 ml of 2.OM chloroacetic acid. and 0.5 ml each of 5M sodium chloride and 10 ‘M mercury(l1) and dilute to volume with conductivity water. Transfer the solution into the electrochemical cell. Plate for 4 min at - I. I V 1’s NCE with stirring. At the end of the deposition period. switch ofl
the applied potential as a function
and record of time. Prepare
the open-circuit a calibration
potential graph for
O.Ol-2pg/ml lead or thallium by the same procedure. Determination of lead and thallium in sea-water To aliquots of sea-water (945 ml) add 2.5ml of 2.OM chloroacetic acid and make up to 50 ml. Determine lead and thallium by the procedure described above and/or by the standard-addition technique. RESULTS AND
DECUSSION
Preliminary studies on the chemical stripping profiles of 5 x lo-‘M lead and thallium in a variety of supporting electrolytes showed that the lead and thallium signals are not resolved in acetate, nitrate or carbonate medium. On the other hand, these signals are well resolved in neutral or weakly acidic complexing media. Effect of supporting electrolyte
The determination of 5 x lo-‘M lead and thallium with mercury(H) as the chemical stripping agent was studied in different supporting electrolytes. The results are represented in Table 1, from which it is clear that the stripping signal for thallium is maximal in sodium chloride medium, but the resolution of the thallium signal from the background or the lead signal was not good. Of all the supporting electrolytes studied, 0.1 M chloroacetic acid-O.5M sodium chloride gave the maximum signal for lead and thallium with sufficient resolution. Hence this mixture was chosen as supporting electrolyte for further studies. Oxidant concentration
As mercury(II) is used for both in situ formation of the mercury film and oxidation of the deposited lead and thallium in the chemical stripping step, the effect of varying its concentration between lob6 and 10m4M in the determination of lOOng/ml concentations of thallium and lead was examined. The presence of at least 2.5 x 10m6Mmercury(U) was found essential for obtaining maximum sensitivity, and mercury(U) con1061
1062
SHORT
Table
COMMUNICATIONS
I. Effect of supportmg electrolyte [concentration of Pb(I1) or T](l) = 5 x IO-‘M, Ed = - 1.1 V vs. NCE, fd = 4 min] Stripping time, set Supportmg electrolyte
Pb
Tl
Acetate buffer Na,CO, KNO, NaCl
(0.25M) (O.lM)’ (O.lM) (0.25M)
Not resolved Not resolved Not resolved 8.0 (Not resolved)
Not resolved Not resolved Not resolved 5.50 (Not resolved)
Chloroacetic acid buffer Acetate + chloride
(O.lM)
9.4
2.50
4.1 7.0 3.6
2.35 2.35 2.50
56
1.80
14.2
3.20
(0.025M) + (0.25M) HCl (O.lM) Cttrate (O.lM) Tetramethyl ammonium bromide (O.lM) Chioroacetic acid + sodium chloride (O.IM)+(O5M)
centrations up to 2 x 10m5M had no effect on the stripping signals thus obtained. Higher concentrations decreased the stripping signals, however, presumably because the stripping was faster. Hence 10m5M mercury(H) was used in further studies. Deposition potential The potential of deposition was varied from - I. 1 to -0.6 V t’s. NCE with plating for 4min for the preconcentration of lead and thallium from a solution containing each at 100 ng/ml concentration. The stripping signal was maximum for both lead and thallium at a deposition potential of - 1.05 V. Sodium chloride concentration Figure 1 shows that under the other conditions selected, increase in the sodium chloride concentration has no effect on the stripping signal of thallium, but the stripping signal of lead increases with
sodium chloride concentration up to 0.4M but remains unaltered on further increase in sodium chloride concentration up to l.OM. Deposition time The deposition times at - 1.1 V were varied from 2 to 16 min. The results are shown in Table 2, from which it is clear that the magnitude of the stripping signal is directly proportional to the plating time. This suggests that the detection limit could be lowered to < 10 ng/ml by increasing the time of deposition. Calibration data The calibration graphs obtained by the recommended procedure are linear over the range 5 x 10~“-10~5M for lead or thallium and pass through the origin. The coefficient of variation for IO replicate determinations of 0.1 pg/ml lead and thallium were found to be 2 and 1.2% respectively. Analysis of synthetic mixtures ,Table 3 presents the results of analysis of various mixtures of lead and thallium in ratios ranging from 100: I to 1: 100. From the recoveries it is clear that the procedure is suitable for the simultaneous determination of lead and thallium in widely differing concentration ratios.
0
I 5
I 10
I 15
I 20
Time tsec) Fig I. Elfect of sodium chloride concentration on E-I profiles of 5 x lO_‘M lead and thallium m O.IM chloroacetate buffer (pH 2.5) and 0-0.5M sodtum chloride, concentration of Hg(l1) = 10~mSM,total volume = 50 ml. deposition potential (Ed) = - I. I V rs. NCE: time of deposition (fd) = 4 min.
Table 2. Variation of deposition time (concentration of Pb’+ or Tl+ = 5 x IO-‘M, Ed = - I .l V L’S,NCE, 0. I M chloroacetic acid + 0.5M NaCI) Stripping time. set Platmg time. mm 2 4 8 16
Pb
Tl
7. I 14.2 28.4 56.0
1.60 3.2 6.4 12.7
Table
3. Analysis
Aliquot taken, ml
Amount
Synthetic 50 50 50
rntxtures 20 2000 20
of
added,
Pb
Sea-water 20 45
n.glml
mixtures Amount
and
sea-water
found,
ng/ml
Tl
Pb
Tl
20 20 2000
20 1980 20
20.4 19.8 2020
-
20 40 40
Analysis
synthetic samples
20 100 1000
20 10 10
20 99 995
1063
COMMtJNlCATtONS
SHORT
19.8 10 10
qf sea-water
Sea-water samples collected from the Bay of Bengal were analysed by the procedure described. The results, and the recoveries obtained on addition of known amounts of lead and thallium to the sea-water, are given in Table 3. The recoveries are quantitative. Conclusions
Unlike other stripping voltammetric techniques, the procedure described here allows the simultaneous
determination of lead and thallium, without mutual interference, over a wide range of ratios. The chloroacetate-chloride medium gives a fourfold enhancement in sensitivity for lead, relative to the PSA procedures which use acetatexhloride’ or O.OlM hydrochloric acid” as supporting electrolytes. The method is suitable for sea-water analysis. Acknowledgements-The authors wish to express their gratitude to Prof. K. I. Vasu, Director, Central Electrochemical Research Institute, Karaikudi, for his keen interest in the work and kind permission to publish these results. REFERENCES 1. E. Berman, Toxic Metals and Their Analysis, Heyden, London, 1980. 2. F. Vydra, K. Stulik and E. Julakovb, Electrochemical Stripping Analysis, Horwood. Chichester, 1976. 3. R. G. Dhaneswar and L. R. Zarparkar, Analyst, 1980, 105,386. 4. Cr. E. Bately and T. M. Florence, J. Electroanal. Chem., 1975, 61, 205. 5. L. K. Hoeflich, R. J. Gale and M. L. Good, Anal. Chem., 1983, 55, 1591. 6. D. K. Roe and J. E. A. Toni, ibid., 1965, 37, 1503. 7. W. R. Matson and D. K. Roe, ibid., 1965, 37, 1594. 8. D. Jagner and A. Granete, Anal. Chim. Acta, 1976, 83, 19. 9. D. Jagner, Analyst, 1982, 107, 593. IO. S. Jaya and T. P. Rao, Rev. Anal. Chem., 1982.6, 343. 11. D. Jagner, Anal. Chem., 1978, SO, 1924.
SIMULTANEOUS DETERMINATION OF LEAD THALLIUM BY POTENTIOMETRIC STRIPPING S. JAYA,T. Central
AND
PRASADA RAO and G. PRABHAKARA RAO
Electrochemical
Research
Institute,
Karaikudi-623006,
India
(Received 15 March 1985. Accepted 14 June 1985) Summary-A potentiometric stripping analysis procedure has been developed for the simultaneous determination of lead and thallium in chloroacetate-chloride medium. This procedure allows the determination of lead or thallium concentrations as low as IO ng/ml. The method is precise, and applicable to the determination of lead and thallium in sea-water.
The toxicity of lead and its compounds’ has resulted in a voluminous literature on its electroanalytical determination in a variety of environmental samples, either by itself or in combination with other heavy metals.* The determination of lead by anodic stripping voltammetry (ASV)3,4or differential pulse ASV’ with the hanging mercury drop electrode (HMDE) suffers no problems in presence of copper, zinc and cadmium but encounters serious interference from thallium. ASV procedures have also been reported by other authors.6.7 For determination of lead and thallium in admixtures most authors have resorted to addition of EDTA.’ ’ Potentiometric stripping analysis, developed by Jagner,” offers a simple, rapid and reliable way of determining lead in sea-water, in presence of copper, zinc and cadmium, but so far no attempt has been made to use it to determine thallium or thallium and lead in mixtures9,” This paper reports the determination of lead and thallium in concentration ratios ranging from 1: 100 to 100: 1. EXPERIMENTAL Reagents The following solutions were prepared with BDH analytical-grade reagents and conductivity water: lead, O.IM: thallium(I). O.OlM, mercury(Il), O.OlM; sodium chloride. SM; chloroacetic acid. 2.OM.
Appurafus A Wenking Model 75 M potentiostat and potential scan generator were used, with a three-electrode cell assembly: a normal calomel reference electrode (NCE), platinum foil counter-electrode and a glassy-carbon (Tokai & Co., Japan, 3 mm diameter) workmg electrode. The recordings were made on a Digllog XY-2000 recorder. Procedure Transfer a suitable known volume (up to 45 ml) of the sample solution contaming lead and/or thallium (concentration of each < 2 pg,‘ml) mto I 50-ml standard flask. Add 2.5 ml of 2.OM chloroacetic acid. and 0.5 ml each of 5M sodium chloride and 10 ‘M mercury(l1) and dilute to volume with conductivity water. Transfer the solution into the electrochemical cell. Plate for 4 min at - I. I V 1’s NCE with stirring. At the end of the deposition period. switch ofl
the applied potential as a function
and record of time. Prepare
the open-circuit a calibration
potential graph for
O.Ol-2pg/ml lead or thallium by the same procedure. Determination of lead and thallium in sea-water To aliquots of sea-water (945 ml) add 2.5ml of 2.OM chloroacetic acid and make up to 50 ml. Determine lead and thallium by the procedure described above and/or by the standard-addition technique. RESULTS AND
DECUSSION
Preliminary studies on the chemical stripping profiles of 5 x lo-‘M lead and thallium in a variety of supporting electrolytes showed that the lead and thallium signals are not resolved in acetate, nitrate or carbonate medium. On the other hand, these signals are well resolved in neutral or weakly acidic complexing media. Effect of supporting electrolyte
The determination of 5 x lo-‘M lead and thallium with mercury(H) as the chemical stripping agent was studied in different supporting electrolytes. The results are represented in Table 1, from which it is clear that the stripping signal for thallium is maximal in sodium chloride medium, but the resolution of the thallium signal from the background or the lead signal was not good. Of all the supporting electrolytes studied, 0.1 M chloroacetic acid-O.5M sodium chloride gave the maximum signal for lead and thallium with sufficient resolution. Hence this mixture was chosen as supporting electrolyte for further studies. Oxidant concentration
As mercury(II) is used for both in situ formation of the mercury film and oxidation of the deposited lead and thallium in the chemical stripping step, the effect of varying its concentration between lob6 and 10m4M in the determination of lOOng/ml concentations of thallium and lead was examined. The presence of at least 2.5 x 10m6Mmercury(U) was found essential for obtaining maximum sensitivity, and mercury(U) con1061
1062
SHORT
Table
COMMUNICATIONS
I. Effect of supportmg electrolyte [concentration of Pb(I1) or T](l) = 5 x IO-‘M, Ed = - 1.1 V vs. NCE, fd = 4 min] Stripping time, set Supportmg electrolyte
Pb
Tl
Acetate buffer Na,CO, KNO, NaCl
(0.25M) (O.lM)’ (O.lM) (0.25M)
Not resolved Not resolved Not resolved 8.0 (Not resolved)
Not resolved Not resolved Not resolved 5.50 (Not resolved)
Chloroacetic acid buffer Acetate + chloride
(O.lM)
9.4
2.50
4.1 7.0 3.6
2.35 2.35 2.50
56
1.80
14.2
3.20
(0.025M) + (0.25M) HCl (O.lM) Cttrate (O.lM) Tetramethyl ammonium bromide (O.lM) Chioroacetic acid + sodium chloride (O.IM)+(O5M)
centrations up to 2 x 10m5M had no effect on the stripping signals thus obtained. Higher concentrations decreased the stripping signals, however, presumably because the stripping was faster. Hence 10m5M mercury(H) was used in further studies. Deposition potential The potential of deposition was varied from - I. 1 to -0.6 V t’s. NCE with plating for 4min for the preconcentration of lead and thallium from a solution containing each at 100 ng/ml concentration. The stripping signal was maximum for both lead and thallium at a deposition potential of - 1.05 V. Sodium chloride concentration Figure 1 shows that under the other conditions selected, increase in the sodium chloride concentration has no effect on the stripping signal of thallium, but the stripping signal of lead increases with
sodium chloride concentration up to 0.4M but remains unaltered on further increase in sodium chloride concentration up to l.OM. Deposition time The deposition times at - 1.1 V were varied from 2 to 16 min. The results are shown in Table 2, from which it is clear that the magnitude of the stripping signal is directly proportional to the plating time. This suggests that the detection limit could be lowered to < 10 ng/ml by increasing the time of deposition. Calibration data The calibration graphs obtained by the recommended procedure are linear over the range 5 x 10~“-10~5M for lead or thallium and pass through the origin. The coefficient of variation for IO replicate determinations of 0.1 pg/ml lead and thallium were found to be 2 and 1.2% respectively. Analysis of synthetic mixtures ,Table 3 presents the results of analysis of various mixtures of lead and thallium in ratios ranging from 100: I to 1: 100. From the recoveries it is clear that the procedure is suitable for the simultaneous determination of lead and thallium in widely differing concentration ratios.
0
I 5
I 10
I 15
I 20
Time tsec) Fig I. Elfect of sodium chloride concentration on E-I profiles of 5 x lO_‘M lead and thallium m O.IM chloroacetate buffer (pH 2.5) and 0-0.5M sodtum chloride, concentration of Hg(l1) = 10~mSM,total volume = 50 ml. deposition potential (Ed) = - I. I V rs. NCE: time of deposition (fd) = 4 min.
Table 2. Variation of deposition time (concentration of Pb’+ or Tl+ = 5 x IO-‘M, Ed = - I .l V L’S,NCE, 0. I M chloroacetic acid + 0.5M NaCI) Stripping time. set Platmg time. mm 2 4 8 16
Pb
Tl
7. I 14.2 28.4 56.0
1.60 3.2 6.4 12.7
Table
3. Analysis
Aliquot taken, ml
Amount
Synthetic 50 50 50
rntxtures 20 2000 20
of
added,
Pb
Sea-water 20 45
n.glml
mixtures Amount
and
sea-water
found,
ng/ml
Tl
Pb
Tl
20 20 2000
20 1980 20
20.4 19.8 2020
-
20 40 40
Analysis
synthetic samples
20 100 1000
20 10 10
20 99 995
1063
COMMtJNlCATtONS
SHORT
19.8 10 10
qf sea-water
Sea-water samples collected from the Bay of Bengal were analysed by the procedure described. The results, and the recoveries obtained on addition of known amounts of lead and thallium to the sea-water, are given in Table 3. The recoveries are quantitative. Conclusions
Unlike other stripping voltammetric techniques, the procedure described here allows the simultaneous
determination of lead and thallium, without mutual interference, over a wide range of ratios. The chloroacetate-chloride medium gives a fourfold enhancement in sensitivity for lead, relative to the PSA procedures which use acetatexhloride’ or O.OlM hydrochloric acid” as supporting electrolytes. The method is suitable for sea-water analysis. Acknowledgements-The authors wish to express their gratitude to Prof. K. I. Vasu, Director, Central Electrochemical Research Institute, Karaikudi, for his keen interest in the work and kind permission to publish these results. REFERENCES 1. E. Berman, Toxic Metals and Their Analysis, Heyden, London, 1980. 2. F. Vydra, K. Stulik and E. Julakovb, Electrochemical Stripping Analysis, Horwood. Chichester, 1976. 3. R. G. Dhaneswar and L. R. Zarparkar, Analyst, 1980, 105,386. 4. Cr. E. Bately and T. M. Florence, J. Electroanal. Chem., 1975, 61, 205. 5. L. K. Hoeflich, R. J. Gale and M. L. Good, Anal. Chem., 1983, 55, 1591. 6. D. K. Roe and J. E. A. Toni, ibid., 1965, 37, 1503. 7. W. R. Matson and D. K. Roe, ibid., 1965, 37, 1594. 8. D. Jagner and A. Granete, Anal. Chim. Acta, 1976, 83, 19. 9. D. Jagner, Analyst, 1982, 107, 593. IO. S. Jaya and T. P. Rao, Rev. Anal. Chem., 1982.6, 343. 11. D. Jagner, Anal. Chem., 1978, SO, 1924.