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Use of potentiometric stripping analysis in atmogeochemical prospection of deposits
MICROCHEMICAL
28, 82-86 (1983)
JOIJRNAI
Use of Potentiometric Stripping Analysis in Atmogeochemical Prospection of Deposits J. ADAM Andytical Laboratory, InstitutcJ of Geological Scirnc,e, Char1e.c University, Prague 2. Alhrrtov 6, Czechoslorakia Received
October 30, 1981
INTRODUCTION In a former contribution K&m&r rt al. (2) described a new method for finding deposits, called the atmogeochemical method of prospection. This method is based on analysis of soil air over the ore seams and deposits. This air contains an increased concentration of metals which are principal components of dikes or deposits. The concentration of these metals is very low (0.1. 100 pg Me/l m8 of air) but these small contents are detectable using sensitive analytical methods. Analytical results are depicted graphically: the peaks show the positions of the deposits (this situation is depicted schematically in Fig. 1). We used atomic absorption spectrophotometry for the determination of heavy metals (2). This method is not sufficiently sensitive. Color reactions and subsequent photometric determination are not sufficiently selective. A variation of chronopotentiometry-potentiometric stripping analysis (ISA)-introduced into analytical chemistry by Jagner and Graneli (I ), is the most convenient method for this purpose. Potentiometric stripping analysis is based on two periods. During the first period metal ions are reduced and form an amalgam on the surface of a glassy carbon electrode at a selected electrolysis potential (E,,,). This E,,, must be (about) 300 mV more negative than the half wave polarographical potential. During this period the surface of the electrode is plated with mercury, which is present in the solution as HgCl,. Metal ions are also reduced on the electrode and form an amalgam: Hg(II) + 2e- + Hg, Me”+ + ne- + Me(Hg). During the second period, after a sufficient preconcentration time, the electrolysis is stopped automatically. The metal ions are then stripped from the mercury film and oxidated by Hg(I1) ions or oxygen which is dissolved in the solution: 82 0026-265X18310 10082-05$0 1.5010 Copyripht
“) 1983 by Academic
Prrsr.
Inc
STRIPPING
ANALYSIS
OF
83
DEPOSITS
ofmfL’ d FIG. I. (b&veins:
Geological position of veins (c)-overlying rocks.
and graphical
evaluation
of analytical
results.
(a) and
MeWg) + ; Hg(II) + Me(n) + % Hg, MeWg) + $02
+ uH+ + Me(n) + ;
H,o,
These reactions are accompanied by potential changes at the working electrode. These changes are registrated as a potentiautime curve. The time values are a measure of the quantity; the potential values represent the quality. MATERIALS
AND METHODS
Appararus und reugrnts. All the potentiometric stripping analyses were carried out using the ion scanning system ISS 820 (Radiometer, Copenhagen) which consists of recorder REA 120, modul REC 61, and titration assembly TTA 60 with platinum (P lOl), calomel (K 4040), and glassy carbon (F 3500) electrodes. The PHM 64 Research pH meter (Radiometer, Copenhagen) was used for the measurement. The apparatus for the sampling of soil air was described by Krcmar et al. (2) in detail. A schema of this apparatus is given in Fig. 2. Stock solutions (1000 kg 1-l) of Hg(I1) and 5.10~” M solutions of the metals were prepared from analytical-grade metal nitrates dissolved in doubly distilled water. Similar conditions were observed in preparing the other solutions (0.2 M acetate buffer, etc.). All the mineral acids used were of suprapure grade. Sampling. All samples were collected removing 80 1 of soil air from a
J. ADAM
84
FIG. 2. Apparatus for the sampling of the soil air. (a)-sound: absorption flask; (d)-electric pump with flow-meter: (e)-battery.
(b)-dust
filter;
(c)-
depth of 80 cm and this gas was bubbled through 10 ml of 1. 1OPM HNO,,, where the metals were absorbed. These samples were stored in polyethylene flasks of 10 ml volume. Preparation oj’the \rorking electrode. The working electrode must be prepared before the measurement by polishing with 3 pm diamond paste for I min on a piece of felt. Then the electrode was rinsed in acetone. The electrode was connected to the apparatus and all three electrodes were immersed in 0.1 M HCl containing 25 mg Hg(II)/liter. The electrolysis was carried out for 1 min at -0.5 V (SCE) followed by the stripping period. The same process was repeated at -0.7, -0.8, and -0.9 V (SCE). The electrode prepared thus could be used for 3 days. When not in use, the electrode must be stored in distilled water. Recommended
Analytical
Procedure
A. The deterrninution oj’lead und zinc. To 10 ml of sample in a 25ml volumetric flask was added 1 ml of HgCI, solution, 10 ml 0.1 M sodium acetate, and 0.1 M acetic acid (1:l) and 500 pg Ga(II1) solution. The volume was adjusted to 25 ml with doubly distilled water. The whole
FIG. 3. Example of analysis. A. I. 10 z M HNO,, + HgCI, + 0.X pg Pb, Cd, and Cu. Final volume 25 ml. E,,, = ~0.9 V (SCE): T,,, = 8 min. B. Acetate buffer pH 4.5 + 500 pg Ga(lI1) + HgCI, + 1 pg Zn(I1) + I.5 pg Pb(Il). The final volume 25 ml.E,, = ~ 1.3 V (SCE): T,,, = 8 min.
STRIPPING ANAl.YSIS
OF DEPOSITS
85
TABLE I
1 2 3
IO IO 10
1 8 16
0.42 0.39 0.38
sample volume was transferred to the electrolytic cell and deaerated by passage of nitrogen for 5 min. Then the nitrogen was passed over the surface of the solution to protect it from oxidation. The electrolysis was carried out at E,,, = - 1.25 V (SCE) for times of 8 to 60 min (depending on the metal content), yielding the results for the determination of Zn and Pb. The amount of metals can then be calculated using the standard addition technique or a calibration curve. B. The determinution of lead and copper. To 10 ml of sample containing 1. lo-’ M HNO,, was added 1 ml of HgCl, solution and doubly distilled water to a volume of 25 ml in a volumetric flask. The sample was transferred to an electrolytic cell and deaerated 5 min. The electroanalytical determination is carried out at I?,,, = -0.9 V (SCE) for times from 8 to 60 min (after assumed content of metals). The calculation of results is similar to that of subsection A (Fig. 3). The results of analysis of three samples are summarized in Table 1. The determination of Zn and Cu was compared with the results of AAS determination and found to be in good agreement. (See Table 2.) SUMMARY
The proposed method for the PSA determination of Pb, Zn, and Cu in the samples of soil air for atmogeochemical prospecting of ore deposits was found to be very useful. In comparison with the AAS method the PSA method consumed more time but the sensitivity of determination is higher, especially for Pb. The method can be used in the field and it is expected that it will find wide use in geological research. TABLE 2 PSA
AAS
No.
Zn pgil
cu pgll
I 2 3
0.8 18.0 43.5
0.9 6.0 20.0
zn l-d 0.8 19.6 40.0
cu CL@ 1.1 5.6 22.0
86
J.
ADAM
REFERENCES I.
Jagner,
D., and Graneli,
A.,
Potentiometric
stripping
analysis.
Atul.
Chim. Ac,trr 83,
19-26 (1976). 2.
KrCm&f, B.. Adam, J., Jirovec, J., and Plibil. the research of ores and fluorits. Sh. rliitri
R., New atmogeochemical methods ~cc!f~~:iXrr, in press. [Czechoslovakia/
by
28, 82-86 (1983)
JOIJRNAI
Use of Potentiometric Stripping Analysis in Atmogeochemical Prospection of Deposits J. ADAM Andytical Laboratory, InstitutcJ of Geological Scirnc,e, Char1e.c University, Prague 2. Alhrrtov 6, Czechoslorakia Received
October 30, 1981
INTRODUCTION In a former contribution K&m&r rt al. (2) described a new method for finding deposits, called the atmogeochemical method of prospection. This method is based on analysis of soil air over the ore seams and deposits. This air contains an increased concentration of metals which are principal components of dikes or deposits. The concentration of these metals is very low (0.1. 100 pg Me/l m8 of air) but these small contents are detectable using sensitive analytical methods. Analytical results are depicted graphically: the peaks show the positions of the deposits (this situation is depicted schematically in Fig. 1). We used atomic absorption spectrophotometry for the determination of heavy metals (2). This method is not sufficiently sensitive. Color reactions and subsequent photometric determination are not sufficiently selective. A variation of chronopotentiometry-potentiometric stripping analysis (ISA)-introduced into analytical chemistry by Jagner and Graneli (I ), is the most convenient method for this purpose. Potentiometric stripping analysis is based on two periods. During the first period metal ions are reduced and form an amalgam on the surface of a glassy carbon electrode at a selected electrolysis potential (E,,,). This E,,, must be (about) 300 mV more negative than the half wave polarographical potential. During this period the surface of the electrode is plated with mercury, which is present in the solution as HgCl,. Metal ions are also reduced on the electrode and form an amalgam: Hg(II) + 2e- + Hg, Me”+ + ne- + Me(Hg). During the second period, after a sufficient preconcentration time, the electrolysis is stopped automatically. The metal ions are then stripped from the mercury film and oxidated by Hg(I1) ions or oxygen which is dissolved in the solution: 82 0026-265X18310 10082-05$0 1.5010 Copyripht
“) 1983 by Academic
Prrsr.
Inc
STRIPPING
ANALYSIS
OF
83
DEPOSITS
ofmfL’ d FIG. I. (b&veins:
Geological position of veins (c)-overlying rocks.
and graphical
evaluation
of analytical
results.
(a) and
MeWg) + ; Hg(II) + Me(n) + % Hg, MeWg) + $02
+ uH+ + Me(n) + ;
H,o,
These reactions are accompanied by potential changes at the working electrode. These changes are registrated as a potentiautime curve. The time values are a measure of the quantity; the potential values represent the quality. MATERIALS
AND METHODS
Appararus und reugrnts. All the potentiometric stripping analyses were carried out using the ion scanning system ISS 820 (Radiometer, Copenhagen) which consists of recorder REA 120, modul REC 61, and titration assembly TTA 60 with platinum (P lOl), calomel (K 4040), and glassy carbon (F 3500) electrodes. The PHM 64 Research pH meter (Radiometer, Copenhagen) was used for the measurement. The apparatus for the sampling of soil air was described by Krcmar et al. (2) in detail. A schema of this apparatus is given in Fig. 2. Stock solutions (1000 kg 1-l) of Hg(I1) and 5.10~” M solutions of the metals were prepared from analytical-grade metal nitrates dissolved in doubly distilled water. Similar conditions were observed in preparing the other solutions (0.2 M acetate buffer, etc.). All the mineral acids used were of suprapure grade. Sampling. All samples were collected removing 80 1 of soil air from a
J. ADAM
84
FIG. 2. Apparatus for the sampling of the soil air. (a)-sound: absorption flask; (d)-electric pump with flow-meter: (e)-battery.
(b)-dust
filter;
(c)-
depth of 80 cm and this gas was bubbled through 10 ml of 1. 1OPM HNO,,, where the metals were absorbed. These samples were stored in polyethylene flasks of 10 ml volume. Preparation oj’the \rorking electrode. The working electrode must be prepared before the measurement by polishing with 3 pm diamond paste for I min on a piece of felt. Then the electrode was rinsed in acetone. The electrode was connected to the apparatus and all three electrodes were immersed in 0.1 M HCl containing 25 mg Hg(II)/liter. The electrolysis was carried out for 1 min at -0.5 V (SCE) followed by the stripping period. The same process was repeated at -0.7, -0.8, and -0.9 V (SCE). The electrode prepared thus could be used for 3 days. When not in use, the electrode must be stored in distilled water. Recommended
Analytical
Procedure
A. The deterrninution oj’lead und zinc. To 10 ml of sample in a 25ml volumetric flask was added 1 ml of HgCI, solution, 10 ml 0.1 M sodium acetate, and 0.1 M acetic acid (1:l) and 500 pg Ga(II1) solution. The volume was adjusted to 25 ml with doubly distilled water. The whole
FIG. 3. Example of analysis. A. I. 10 z M HNO,, + HgCI, + 0.X pg Pb, Cd, and Cu. Final volume 25 ml. E,,, = ~0.9 V (SCE): T,,, = 8 min. B. Acetate buffer pH 4.5 + 500 pg Ga(lI1) + HgCI, + 1 pg Zn(I1) + I.5 pg Pb(Il). The final volume 25 ml.E,, = ~ 1.3 V (SCE): T,,, = 8 min.
STRIPPING ANAl.YSIS
OF DEPOSITS
85
TABLE I
1 2 3
IO IO 10
1 8 16
0.42 0.39 0.38
sample volume was transferred to the electrolytic cell and deaerated by passage of nitrogen for 5 min. Then the nitrogen was passed over the surface of the solution to protect it from oxidation. The electrolysis was carried out at E,,, = - 1.25 V (SCE) for times of 8 to 60 min (depending on the metal content), yielding the results for the determination of Zn and Pb. The amount of metals can then be calculated using the standard addition technique or a calibration curve. B. The determinution of lead and copper. To 10 ml of sample containing 1. lo-’ M HNO,, was added 1 ml of HgCl, solution and doubly distilled water to a volume of 25 ml in a volumetric flask. The sample was transferred to an electrolytic cell and deaerated 5 min. The electroanalytical determination is carried out at I?,,, = -0.9 V (SCE) for times from 8 to 60 min (after assumed content of metals). The calculation of results is similar to that of subsection A (Fig. 3). The results of analysis of three samples are summarized in Table 1. The determination of Zn and Cu was compared with the results of AAS determination and found to be in good agreement. (See Table 2.) SUMMARY
The proposed method for the PSA determination of Pb, Zn, and Cu in the samples of soil air for atmogeochemical prospecting of ore deposits was found to be very useful. In comparison with the AAS method the PSA method consumed more time but the sensitivity of determination is higher, especially for Pb. The method can be used in the field and it is expected that it will find wide use in geological research. TABLE 2 PSA
AAS
No.
Zn pgil
cu pgll
I 2 3
0.8 18.0 43.5
0.9 6.0 20.0
zn l-d 0.8 19.6 40.0
cu CL@ 1.1 5.6 22.0
86
J.
ADAM
REFERENCES I.
Jagner,
D., and Graneli,
A.,
Potentiometric
stripping
analysis.
Atul.
Chim. Ac,trr 83,
19-26 (1976). 2.
KrCm&f, B.. Adam, J., Jirovec, J., and Plibil. the research of ores and fluorits. Sh. rliitri
R., New atmogeochemical methods ~cc!f~~:iXrr, in press. [Czechoslovakia/
by