- Email: [email protected]
Determination of lead in igneous rocks by differential pulse anodic stripping voltammetry at an HMDE
i-*hIto Vol. 20, pp. 65 to 67 0 Pergamon Press Ltd 1981. Prin:ed m Great Bntain
DETERMINATION OF LEAD IN IGNEOUS ROCKS DIFFERENTIAL PULSE ANODIC STRIPPING VOLTAMMETRY AT AN HMDE
BY
G. CALDERONI Istituto di Geochimica dell’Universit8, 00100 Rome, Italy (Received 7 April 1980. Accepted 10 July 1980) Summary-A versatile and sensitive voltammetric method has been used for the determination of lead in seven USGS standard rocks and in two CRPG (Centre des Recherches Pktrografiques et GCochimiques, Nancy) standard rocks. The results, showing satisfactory precision and accuracy, are discussed with respect to the sample treatment and the voltammetric method.
Anodic stripping techniques offer a very sensitive method for trace-element analysis and have been applied for the determination of lead in a variety of materials by several workers.‘-” The present note reports the use of differential pulse anodic stripping voltammetry (dpasv) at a hanging mercury drop electrode (HMDE) for the determination of lead in igneous rocks, with no preliminary extraction. Hydroxylammonium chloride has been selected as supporting electrolyte on account of its reducing power in the acid medium which is needed to avoid precipitation (with its related co-precipitation and sorption phenomena) of several metals as hydroxides, and because these conditions make it possible to achieve a fairly good separation of the peaks of some elements of interest. Hydroxylammonium chloride reduces Cu(II) to Cu(I), so only one peak is found for copper, and Fe(H) in solution is not reduced at the HMDE.
The investigation was concerned first with finding conditions for Cd(II), Pb(I1) and Cu(1): conditions and E, values for the three elements are reported in the caption of Fig. 1. Then attention was devoted to the determination of lead and copper, though the copper determination has not been fully explored. For the lead determination better results were obtained with a 1.25M hydroxylammonium chloride solution, and with a plating potential of -0.70 V us. SCE: under these conditions the E, value for Pb(I1) was -0.40 V vs. SCE. EXPERIMENTAL Reagents Standard lead solution. Made from Merck RG Pb(N03)2 dried at 110”. by dissolving it in doubly distilled water and acidifying with Aristar hydrochloric acid.
Fig. 1. Dpasv voltammogram showing peaks of Cd(II), Pb(I1) and Cu(I) respectively at -0.640, -0.450, -0.225 V 0s. SCE. HMDE area 0.024 cm’; cathodic deposition potential -0.850 V; deposition time 3 min; scan-rate 20 mV/sec; pulse height 40 mV; 1.25M hydroxylammonium chloride at pH 2.0. 65
66
SHORT COMMUNICATIONS
Hydroxylammonium chloride stock solution. Made from twice recrystallized Merck RG material and purified by controlled-potential electrolysis at a mercury-pool cathode at -0.8 V vs. SCE. Merck Suprapur perchloric acid. Merck RG hydrojfuoric acid. Purified by sub-boiling point distillation as described by Lancet and Huey.‘* Apparatus All samples were analysed by dpasv at an HMDE, with a 472 WR Amel multipolarograph equipped with a 7040 A X-Y Hewlett-Packard recorder. A three-electrode system was employed: a Beckman 39016 electrode and/or a Metrohom E410 electrode as working electrode(s), a platinum electrode as auxiliary and an SCE as reference. Procedure Dissolution of the samples follows a procedure similar to acid is not that described by Jeffery I3 but the perchloric totally eliminated. This is in order to avoid both loss of the more volatile components and the formation of insoluble
oxides. Thus the final solution is slightly acid and the metal perchlorates dissolve easily in the water added. Transfer 100-350 mg of sample into a Teflon beaker, add 10 ml of hydrofluoric acid and 2 ml of perchloric acid, and evaporate to incipient dryness on a hot-plate at 200”. Add 2 ml of perchloric acid and evaporate to incipient dryness again, then repeat this step. Dissolve the residue in doubly distilled water and dilute to 50 ml in a standard flask. Pipette a volume of between 1 and 3 ml of this solution into a 25-ml standard flask, and add enough hydroxylammonium chloride stock solution to give a final concentration of 1.25M and dilute to volume with doubly distilled water. Transfer 20 ml of this solution into a polarographic cell and deoxygenate for 10 min by passing high-purity nitrogen through the sample. Continue to pass nitrogen over the solution during the electrolysis and stripping steps. Electrolyse at -0.70 V vs. SCE for l-3 min, with magnetic stirring. Record the voltammograms with a sweep-rate of 10 mV/sec and a pulse-amplitude of 40 mV.
Table
Standard
1. Results for determination
rock
DTS-1 Dunite PCC-1
RESULTS
Peridotite
AND DISCUSSION
To test its applicability and the possibility of interference from other elements present in rocks, the method was used to analyse rock samples ranging from granite through mafic to ultramafic types. Two samples of each standard were dissolved, and each solution was analysed in triplicate; each measurement was run 4 or 5 times. Instrumental reproducibility, tested up to 10 times in some cases, was always better than 5%. As the peak current was found to vary linearly with the lead concentration the method of standard additions was used for quantitative work, and the measurements were corrected for the dilution caused by each increment added. Table 1 shows the mean concentration of lead (ppm), the standard deviation (a) and the relative standard deviation (:A) for the standard rocks. As reported by other authors, **14 the reproducibility was less satisfactory for dunites and basalts: this fact may indicate some dependence on the type of matrix and a possible lack of homogeneity in the samples. In fact, although the possibility of inhomogeneity of the standards from different stocks has been excluded by Aruscavage and Campbell,14 Jaffrezic” claims that if trace elements are determined in small weights of sample it is possible that the solid subsample taken is not statistically representative of the mean composition of the standard. Diehl16 has published a photograph showing the inhomogeneity in a standard material. It would certainly be interesting to have more data on which considerations of the statistical distribution of trace-elements in the standards could be based.
of lead in standard
rocks
Lead,
x * 0,
ppm
ppm
11.74; 10.7; 11.2 11.3; 11.8; 9.0 11.5; 12.0; 11.4 12.6; 12.4; 13.0
10.9 f 1.0
9.1
12.1 + 0.6
4.9
5.9 f. 0.7
11.8
17.1 * 0.7
4.0
8.7 k 1.3
14.9
W-l Basalt
6.6; 6.0; 6.7 4.8; 5.2; 6.6
BCR-1 Basalt
17.0; 16.9; 16.5 17.7; 18.4; 16.4
BR Basalt
8.2; 10.0; 9.1 9.7; 9.2; 6.4
AGV-1 Andesite
40.0; 39.3; 36.1 38.7; 35.2; 39.8
38.1 + 2.0
5.2
GSP-1
58.1; 58.6; 54.7 57.4; 56.9; 56.8
57.0 f 1.3
2.2
G-2 Granite
25.0; 24.2; 28.9 26.0; 25.1; 28.3
26.2 +_ 1.8
6.8
GH Granite
67.1; 60.5; 66.6 64.5; 67.6; 64.8
65.1 f 2.6
3.9
Granodiorite
61
SHORT COMMUNICATIONS
Table 2. Lead values for international
Standard rock
Data from this work
DTS-1 PCC-1 W-l BCR-1 BR AGV-1 GSP-1 G-2 GH
10.9 12.1 5.9 17.1 8.7 38.1 57.0 26.2 65.1
f + * * + + + k +
1.0 0.6 0.7 0.7 1.3 2.0 1.3 1.8 2.6
Polarographic data
Data by flameless
11.0 + 4.4* 9.8 + 5.9*
by Wedepohl’s
14.1 + 0.8
14.2 13.3 7.8 17.6
36.5 k 2.1 56.2 k 1.3 30.2 ) 1
35.1 51.3 31.2
8.3-16 7.9-13 6.0 + 9.4 10.9 & 29.5 8-12.5 33.1-46 51.9-67 29.5-30.8 15-58
absorption“’ 7.6 k 0.6 8.2 + 0.3
43’ 36’
In Table 2 comparison is made between the results of this work and those of other authors. It is worth noting that polarographic results for the analysis of standard rocks are rather scarce, but those results which are available tend to show a fairly good agreement. Concerning the accuracy we can observe that the results of this paper for DTS-1, PCC-1 and BCR-1 agree with those of Flanagan” better than those of Aruscavage and Campbell,14 but the reverse is true for AGV-1, GSP-1 and G-2. For standards GH and BR only a few results are available for comparison. Thallium, which has a peak potential very close to that of lead, interferes, as previously pointed out by other authors,’ and as it has been ascertained in this study testing the analytical procedure proposed by De Capitani et al.* We found that a given concentration of thallium produces a peak with only one-fifth of the height of a corresponding lead peak for the same concentration. We conclude that interference from thallium is virtually negligible since the thallium concentrations in the USGS standard rocks are very low (maximum 1.3 ppm, according to Flanagan’s recommended data). ”
by No.
Flanagan’s Ranges as reported values”
atomic
by other authors
Acknowledgement-This work was partially supported CNR (National Research Council of Italy) grant 75.00028.05.
standard rocks, from the literature, ppm
REFERENCES
1. J. Dieker and W. E. Van der Linden, Z. Anal. Chem., 1975, 214, 97. 2. A. A. Cernik, Proc. Anal. Dia. Chem. Sot., 1976, 13, 227. 3. A. H. I. Ben-Bassat, J. M. Blinderman and A. Salomon, Anal. Chem., 1975, 47, 534. 4. M. J. Pinchin and J. Newham, Anal. Chim. Acta. 1977, 90,91. 5. V. H. Levin and M. J. Rowell, Ejj’luent Water Treat. J., 1973, 13, 273. 6. M. Kopanica and V. Stara, J. Electroanal. Chem., 1977, 11, 57. R. Parthasarathy and M. S. Das, 7. K. A. Khasgiwale, Anal. Chim. Acta. 1972, 59, 485. 8. L. De Capitani and A. Maccagni, Rend. Sot. Iral. Min. Petrol., 1978, 34, 569. 9. G. Piccardi and P. L. Cellini, Ann. Chim. (Roma), 1974, 64, 1. 10. R. Benvenuti, M. Martini and G. Piccardi, Rend. Sot. Ital. Min. Petrol., 1977, 33, 199. 11. W. Lund and M. Salberg, Talanta, 1975, 22, 1013. 12. M. S. Lancet and J. M. Huey, Anal. Chem., 1974, 46, 1360. 13. P. G. Jeffery, Chemical Methods of Rock Analysis, p. 156, Pergamon, Oxford, 1970. and E. Y. Campbell, Talanta, 1979, 14. P. J. Aruscavage 26, 1052. 15. H. JaIlrezic, ibid., 1976, 23, 497. 16. H. Diehl, Quantitative Analysis, 1st Ed., p. 310. Oakland Street Press, Ames, Iowa, 1970. 17. P. J. Flanagan, Geochim. Cosmochim. Acta, 1973, 33, 81. 18. K. H. Wedepohl, (ed.), Handbook of Geochemistry, Springer, Berlin, 1974.
DETERMINATION OF LEAD IN IGNEOUS ROCKS DIFFERENTIAL PULSE ANODIC STRIPPING VOLTAMMETRY AT AN HMDE
BY
G. CALDERONI Istituto di Geochimica dell’Universit8, 00100 Rome, Italy (Received 7 April 1980. Accepted 10 July 1980) Summary-A versatile and sensitive voltammetric method has been used for the determination of lead in seven USGS standard rocks and in two CRPG (Centre des Recherches Pktrografiques et GCochimiques, Nancy) standard rocks. The results, showing satisfactory precision and accuracy, are discussed with respect to the sample treatment and the voltammetric method.
Anodic stripping techniques offer a very sensitive method for trace-element analysis and have been applied for the determination of lead in a variety of materials by several workers.‘-” The present note reports the use of differential pulse anodic stripping voltammetry (dpasv) at a hanging mercury drop electrode (HMDE) for the determination of lead in igneous rocks, with no preliminary extraction. Hydroxylammonium chloride has been selected as supporting electrolyte on account of its reducing power in the acid medium which is needed to avoid precipitation (with its related co-precipitation and sorption phenomena) of several metals as hydroxides, and because these conditions make it possible to achieve a fairly good separation of the peaks of some elements of interest. Hydroxylammonium chloride reduces Cu(II) to Cu(I), so only one peak is found for copper, and Fe(H) in solution is not reduced at the HMDE.
The investigation was concerned first with finding conditions for Cd(II), Pb(I1) and Cu(1): conditions and E, values for the three elements are reported in the caption of Fig. 1. Then attention was devoted to the determination of lead and copper, though the copper determination has not been fully explored. For the lead determination better results were obtained with a 1.25M hydroxylammonium chloride solution, and with a plating potential of -0.70 V us. SCE: under these conditions the E, value for Pb(I1) was -0.40 V vs. SCE. EXPERIMENTAL Reagents Standard lead solution. Made from Merck RG Pb(N03)2 dried at 110”. by dissolving it in doubly distilled water and acidifying with Aristar hydrochloric acid.
Fig. 1. Dpasv voltammogram showing peaks of Cd(II), Pb(I1) and Cu(I) respectively at -0.640, -0.450, -0.225 V 0s. SCE. HMDE area 0.024 cm’; cathodic deposition potential -0.850 V; deposition time 3 min; scan-rate 20 mV/sec; pulse height 40 mV; 1.25M hydroxylammonium chloride at pH 2.0. 65
66
SHORT COMMUNICATIONS
Hydroxylammonium chloride stock solution. Made from twice recrystallized Merck RG material and purified by controlled-potential electrolysis at a mercury-pool cathode at -0.8 V vs. SCE. Merck Suprapur perchloric acid. Merck RG hydrojfuoric acid. Purified by sub-boiling point distillation as described by Lancet and Huey.‘* Apparatus All samples were analysed by dpasv at an HMDE, with a 472 WR Amel multipolarograph equipped with a 7040 A X-Y Hewlett-Packard recorder. A three-electrode system was employed: a Beckman 39016 electrode and/or a Metrohom E410 electrode as working electrode(s), a platinum electrode as auxiliary and an SCE as reference. Procedure Dissolution of the samples follows a procedure similar to acid is not that described by Jeffery I3 but the perchloric totally eliminated. This is in order to avoid both loss of the more volatile components and the formation of insoluble
oxides. Thus the final solution is slightly acid and the metal perchlorates dissolve easily in the water added. Transfer 100-350 mg of sample into a Teflon beaker, add 10 ml of hydrofluoric acid and 2 ml of perchloric acid, and evaporate to incipient dryness on a hot-plate at 200”. Add 2 ml of perchloric acid and evaporate to incipient dryness again, then repeat this step. Dissolve the residue in doubly distilled water and dilute to 50 ml in a standard flask. Pipette a volume of between 1 and 3 ml of this solution into a 25-ml standard flask, and add enough hydroxylammonium chloride stock solution to give a final concentration of 1.25M and dilute to volume with doubly distilled water. Transfer 20 ml of this solution into a polarographic cell and deoxygenate for 10 min by passing high-purity nitrogen through the sample. Continue to pass nitrogen over the solution during the electrolysis and stripping steps. Electrolyse at -0.70 V vs. SCE for l-3 min, with magnetic stirring. Record the voltammograms with a sweep-rate of 10 mV/sec and a pulse-amplitude of 40 mV.
Table
Standard
1. Results for determination
rock
DTS-1 Dunite PCC-1
RESULTS
Peridotite
AND DISCUSSION
To test its applicability and the possibility of interference from other elements present in rocks, the method was used to analyse rock samples ranging from granite through mafic to ultramafic types. Two samples of each standard were dissolved, and each solution was analysed in triplicate; each measurement was run 4 or 5 times. Instrumental reproducibility, tested up to 10 times in some cases, was always better than 5%. As the peak current was found to vary linearly with the lead concentration the method of standard additions was used for quantitative work, and the measurements were corrected for the dilution caused by each increment added. Table 1 shows the mean concentration of lead (ppm), the standard deviation (a) and the relative standard deviation (:A) for the standard rocks. As reported by other authors, **14 the reproducibility was less satisfactory for dunites and basalts: this fact may indicate some dependence on the type of matrix and a possible lack of homogeneity in the samples. In fact, although the possibility of inhomogeneity of the standards from different stocks has been excluded by Aruscavage and Campbell,14 Jaffrezic” claims that if trace elements are determined in small weights of sample it is possible that the solid subsample taken is not statistically representative of the mean composition of the standard. Diehl16 has published a photograph showing the inhomogeneity in a standard material. It would certainly be interesting to have more data on which considerations of the statistical distribution of trace-elements in the standards could be based.
of lead in standard
rocks
Lead,
x * 0,
ppm
ppm
11.74; 10.7; 11.2 11.3; 11.8; 9.0 11.5; 12.0; 11.4 12.6; 12.4; 13.0
10.9 f 1.0
9.1
12.1 + 0.6
4.9
5.9 f. 0.7
11.8
17.1 * 0.7
4.0
8.7 k 1.3
14.9
W-l Basalt
6.6; 6.0; 6.7 4.8; 5.2; 6.6
BCR-1 Basalt
17.0; 16.9; 16.5 17.7; 18.4; 16.4
BR Basalt
8.2; 10.0; 9.1 9.7; 9.2; 6.4
AGV-1 Andesite
40.0; 39.3; 36.1 38.7; 35.2; 39.8
38.1 + 2.0
5.2
GSP-1
58.1; 58.6; 54.7 57.4; 56.9; 56.8
57.0 f 1.3
2.2
G-2 Granite
25.0; 24.2; 28.9 26.0; 25.1; 28.3
26.2 +_ 1.8
6.8
GH Granite
67.1; 60.5; 66.6 64.5; 67.6; 64.8
65.1 f 2.6
3.9
Granodiorite
61
SHORT COMMUNICATIONS
Table 2. Lead values for international
Standard rock
Data from this work
DTS-1 PCC-1 W-l BCR-1 BR AGV-1 GSP-1 G-2 GH
10.9 12.1 5.9 17.1 8.7 38.1 57.0 26.2 65.1
f + * * + + + k +
1.0 0.6 0.7 0.7 1.3 2.0 1.3 1.8 2.6
Polarographic data
Data by flameless
11.0 + 4.4* 9.8 + 5.9*
by Wedepohl’s
14.1 + 0.8
14.2 13.3 7.8 17.6
36.5 k 2.1 56.2 k 1.3 30.2 ) 1
35.1 51.3 31.2
8.3-16 7.9-13 6.0 + 9.4 10.9 & 29.5 8-12.5 33.1-46 51.9-67 29.5-30.8 15-58
absorption“’ 7.6 k 0.6 8.2 + 0.3
43’ 36’
In Table 2 comparison is made between the results of this work and those of other authors. It is worth noting that polarographic results for the analysis of standard rocks are rather scarce, but those results which are available tend to show a fairly good agreement. Concerning the accuracy we can observe that the results of this paper for DTS-1, PCC-1 and BCR-1 agree with those of Flanagan” better than those of Aruscavage and Campbell,14 but the reverse is true for AGV-1, GSP-1 and G-2. For standards GH and BR only a few results are available for comparison. Thallium, which has a peak potential very close to that of lead, interferes, as previously pointed out by other authors,’ and as it has been ascertained in this study testing the analytical procedure proposed by De Capitani et al.* We found that a given concentration of thallium produces a peak with only one-fifth of the height of a corresponding lead peak for the same concentration. We conclude that interference from thallium is virtually negligible since the thallium concentrations in the USGS standard rocks are very low (maximum 1.3 ppm, according to Flanagan’s recommended data). ”
by No.
Flanagan’s Ranges as reported values”
atomic
by other authors
Acknowledgement-This work was partially supported CNR (National Research Council of Italy) grant 75.00028.05.
standard rocks, from the literature, ppm
REFERENCES
1. J. Dieker and W. E. Van der Linden, Z. Anal. Chem., 1975, 214, 97. 2. A. A. Cernik, Proc. Anal. Dia. Chem. Sot., 1976, 13, 227. 3. A. H. I. Ben-Bassat, J. M. Blinderman and A. Salomon, Anal. Chem., 1975, 47, 534. 4. M. J. Pinchin and J. Newham, Anal. Chim. Acta. 1977, 90,91. 5. V. H. Levin and M. J. Rowell, Ejj’luent Water Treat. J., 1973, 13, 273. 6. M. Kopanica and V. Stara, J. Electroanal. Chem., 1977, 11, 57. R. Parthasarathy and M. S. Das, 7. K. A. Khasgiwale, Anal. Chim. Acta. 1972, 59, 485. 8. L. De Capitani and A. Maccagni, Rend. Sot. Iral. Min. Petrol., 1978, 34, 569. 9. G. Piccardi and P. L. Cellini, Ann. Chim. (Roma), 1974, 64, 1. 10. R. Benvenuti, M. Martini and G. Piccardi, Rend. Sot. Ital. Min. Petrol., 1977, 33, 199. 11. W. Lund and M. Salberg, Talanta, 1975, 22, 1013. 12. M. S. Lancet and J. M. Huey, Anal. Chem., 1974, 46, 1360. 13. P. G. Jeffery, Chemical Methods of Rock Analysis, p. 156, Pergamon, Oxford, 1970. and E. Y. Campbell, Talanta, 1979, 14. P. J. Aruscavage 26, 1052. 15. H. JaIlrezic, ibid., 1976, 23, 497. 16. H. Diehl, Quantitative Analysis, 1st Ed., p. 310. Oakland Street Press, Ames, Iowa, 1970. 17. P. J. Flanagan, Geochim. Cosmochim. Acta, 1973, 33, 81. 18. K. H. Wedepohl, (ed.), Handbook of Geochemistry, Springer, Berlin, 1974.