- Email: [email protected]
Determination of trace impurities in certain salts
MICROCHEMICAL
JOURNAL
Determination
11,
62-72 (1966)
of Trace W. KEMULA
Department
of Inorganic
Impurities
Salts
AND S. SACHA
Chemistry,
Received
in Certain
University,
Warsaw,
Poland
August 30, 1965
The preceding paper (1) described the apparatus for the determination of trace impurities in substances. The determination involved pre-electrolysis and subsequent registration of stripping currents produced by metal ions accumulated within the HMDE, and the use of the increment method, which consists of adding standard solutions. Electrochemical determination of impurities is known to involve dissolution of samples in an acid or salt solution. The latter should be as pure as possible, otherwise the substance investigated could become contaminated with impurities contained in the reagents used for preparation of the supporting electrolyte. Purification of reagents constitutes a very complicated problem that has been the subject of numerous investigations and is of great importance in many fields of research and technology. The present investigations are concerned with the increment method used in determining the purity of high-purity reagents, viz., potassium nitrate and potassium sulfate, as well as of naturally occurring rock salt, halite. EXPERIMENTAL
A Radiometer PO4 poiarograph was used. The electrolytical cell with a 0.14 cc sluice has been described (1). A synchronous motor was used to keep the revolutions per minute of the stirrer constant. The temperature of the electrolytic cell was maintained at 25” 2 0.1 “C. One-tenth normal solutions of the salts investigated were prepared by dissolving appropriate salt amounts, weighed on an analytical balance, in triple-distilled water in measuring flasks. The method employed involved addition of the standard which was prepared from the same salt and contained an increment of only one type of ions, whereby the cementation effects (2) could be eliminated. The salts were analyzed for copper, lead, and zinc only. 62
DETERMINATION
OF
TRACE
IMPURITIES
IN
SALTS
63
Determination of impurities in KNOB and K,S04. The pertinent 0.1 N solutions were deaerated for some time in the sluice-equipped cell. The cell contained the solution of the salt with zinc, lead, or copper added in concentrations 100 times as large as that of the impurity concerned. Since the impurity contents were of the order of 1O-5s the solution was pre-electrolyzed for 5 minutes. The voltage sweep rate was always constant. Stripping peaks were recorded before and after double addition of the solution from the sluice. The plots show the stripping peak data (in microamperes) found before and after the first and second additions from the sluice had been made. Figure 1 shows the copper stripping peaks for 0.1 N potassium nitrate. Each copper increment was 10W7 mole per liter; the copper content in the nitrate, evaluated by linear extrapolation, was found to be 1.2 X 10-r mole per liter, as can be seen from the graph. A similar determination of lead in the nitrate (Fig. 2) gave 1.2 X 10-r mole per liter. For zinc (Fig. 3), the content was found to be as small as 3 X 10Ws mole per liter. In potassium sulfate, only copper and zinc could be determined, because lead sulfate is slightly soluble. Its solubility product is 1.08 X lo-*; therefore, a 0.1 N sulfate solution is saturated with respect to Pb++. The copper (Fig. 4) and zinc (Fig. 5) contents were 1.6 X lo-? mole per liter, respectively. Since the concentration of zinc was large, the standard
FIG. 1. Copper stripping was 10-T mole per liter.
peaks for
0.1
N potassium nitrate.
Each copper increment
64
W.
KEMULA
AND
S. SACHA
solution was so prepared that each sluice added 1.6 X 10Y6 g-ion Zn++ per liter to the solution investigated. Determination of impurities in rock salt. Two rock salt specimens, one colorless and the other pale rose, were examined. A small lump was split from the bulk of a large colorless salt crystal and a 0.1 N solution was prepared. The copper (Fig. 6), lead (Fig. 7)) and zinc (Fig. 8) contents were found to be 2 X 10m7, 7.5 X lo-“, and 5 X 10ms mole per liter, respectively.
FIG.
2. Determination
FIG. 3.
Determination
of lead in potassium
nitrate.
of zinc in potassium
nitrate.
DETERMINATION
FIG. 4.
FIG. 5.
OF
TRACE
IMPURITIES
Copper content in potassium
Zinc content in potassium
IN
nitrate.
nitrate.
SALTS
63
66
W. KEMULA
AND S. SACHA
The pale-rose salt was more pure, the respective impurity contents (Figs. 9-11) being 1.75 X lo-“, 4 X lo-*, and 6 X 1O-s mole per liter. The data obtained show that the copper, lead, and zinc contents in the rock salts withdrawn from contiguous strata differ considerably. The
FIG. 6.
Copper content of a 0.1 N colorless salt solution.
pale-rose crystal from the two natural halites contains less copper and zinc (Table 1). Our earlier investigations (2) have shown that the amalgam produced by depositions of various ions on mercury undergoes cementation in a given solution; this is confirmed by the following experiment. Cadmium salt was added from the sluice to the pale-rose rock salt and to the potassium nitrate solutions so that the Cd++ concentration rose each time by lo-* mole per liter. Figures 12 and 13 show the successive TABLE COPPER, LEAD, AND ZINC CONTENTS
Salt KNO, K2S04
NaCl (colorless) NaCl (pale rose)
1 (lo-5%)
IN VARIOUS SALTS
cu
Pb
Zn
7.6 6 21.8 8.2
24.6 26.6 15
1.9 26 5.6 6.7
DETERMINATION
OF
I FIG.
TRACE
0
7. Lead content of a
IMPURITIES
IN
SALTS
67
I 0.1
S colorless salt solution
additions of Cd++ from the sluice, and reveal that, on extrapolation, the cadmium peak height vs. concentration line misses the origin. This fact indicates that a portion of cadmium must have been replaced by the Cuf + and Pb++ present in the solution according to the following reactions : Cd(Hg) + Pb++ -----+ Cd(Hg) + Cu++ w
Pb(Hg) + Cd++ Cu(Hg) + Cd+ +
The cadmium concentration deficits evaluated from Figs. 12 and 13 are 2 X 10Wg and 4 ?( IO-” mole per liter, respectively. The copper and lead contents were 1.75 X 10-s and 4 X 10es mole per liter and in the rock salt 1.2 X 1O-7 and 1.2 X IO-? mole per liter in the potassium nitrate, respectively. The nitrate therefore contained twice as much Cu+ + and Pb+ f as did the rock salt solution.
68
W.
FIG. 9.
AND
S. SACHA
-AL++I 1
FIG. 8.
KEMULA
0
Zinc content
i
2w2n
of a 0.1 N colorless salt solution.
Copper content of a pale-rose,
0.1 N salt solution.
DETERMINATION
OF TRACE
IMPURITIES
IN
SALTS
69
Actually, the cadmium cementation effect was found to be larger, and the above-presented consideration is thereby borne out. Cementation, like that described above, is always likely to occur in solutions and may affect the analytical results. To minimize this effect, the solution must be vigorously stirred for a fairly long period of time (2)
FIG. 10. Lead content of a 0.1 2%’pale-rose salt solution.
during deposition, and the resulting amalgam must be dissolved immediately and in a stagnant solution. The data obtained show that reliable results are obtained only if the supporting electrolyte forms neither insoluble precipitates nor permanently nonreducing complexes with the sluice-added standard. Formation of fresh precipitates with transiently increased solubilities may result in only spuriously correct determinations, as was the case with the determination of lead in potassium sulfate. However, since the present method is particularly suited for determination of traces, the above-mentioned
70
W.
FIG.
11.
KEMULA
Zinc content
AND
S. SACHA
of a 0.1 N pink salt solution.
phenomena of precipitation and formation of nonreducing complexes will be only rarely encountered, although their occurrence can never be ruled out as a possibility. SUMMARY Electrochemical determination of trace impurities is described that involves preelectrolysis on the hanging mercury drop electrode and subsequent chronovoltamperometric recording of the stripping currents. The sluice renders a precise determination of minute amounts of impurities that is possible without prior ascertaining of the calibration curve for a given analysis. Cementation with Cu++ and Pb++ was confirmed to take place on addition of Cd++ to the solution examined. Conditions were established in which the effect of cementation on stripping of the resulting amalgam is reduced.
DETERMINATION
OF
TRACE
IMPURITIES
IN
SALTS
FIG. 12.
Cadmium
stripping
from 0.1 N NaCl
solution.
FIG. 13.
Cadmium
stripping
from 0.1 A’ KNO,
solution.
71
72
W.
KEMULA
AND
S. SACHA
REFERENCES 1.
W. A simple apparatus for the determination of trace amounts of different ions using the hanging mercury drop electrode. Micro&m. J. 11, 54-61 (1966). W., AND STROJEK, J. W. Controlled chromopotentio metric stripping -7. KEMULA, of metals reported on the hanging mercury drop electrode. J. Electround. Chem., in press. KEMULA,
JOURNAL
Determination
11,
62-72 (1966)
of Trace W. KEMULA
Department
of Inorganic
Impurities
Salts
AND S. SACHA
Chemistry,
Received
in Certain
University,
Warsaw,
Poland
August 30, 1965
The preceding paper (1) described the apparatus for the determination of trace impurities in substances. The determination involved pre-electrolysis and subsequent registration of stripping currents produced by metal ions accumulated within the HMDE, and the use of the increment method, which consists of adding standard solutions. Electrochemical determination of impurities is known to involve dissolution of samples in an acid or salt solution. The latter should be as pure as possible, otherwise the substance investigated could become contaminated with impurities contained in the reagents used for preparation of the supporting electrolyte. Purification of reagents constitutes a very complicated problem that has been the subject of numerous investigations and is of great importance in many fields of research and technology. The present investigations are concerned with the increment method used in determining the purity of high-purity reagents, viz., potassium nitrate and potassium sulfate, as well as of naturally occurring rock salt, halite. EXPERIMENTAL
A Radiometer PO4 poiarograph was used. The electrolytical cell with a 0.14 cc sluice has been described (1). A synchronous motor was used to keep the revolutions per minute of the stirrer constant. The temperature of the electrolytic cell was maintained at 25” 2 0.1 “C. One-tenth normal solutions of the salts investigated were prepared by dissolving appropriate salt amounts, weighed on an analytical balance, in triple-distilled water in measuring flasks. The method employed involved addition of the standard which was prepared from the same salt and contained an increment of only one type of ions, whereby the cementation effects (2) could be eliminated. The salts were analyzed for copper, lead, and zinc only. 62
DETERMINATION
OF
TRACE
IMPURITIES
IN
SALTS
63
Determination of impurities in KNOB and K,S04. The pertinent 0.1 N solutions were deaerated for some time in the sluice-equipped cell. The cell contained the solution of the salt with zinc, lead, or copper added in concentrations 100 times as large as that of the impurity concerned. Since the impurity contents were of the order of 1O-5s the solution was pre-electrolyzed for 5 minutes. The voltage sweep rate was always constant. Stripping peaks were recorded before and after double addition of the solution from the sluice. The plots show the stripping peak data (in microamperes) found before and after the first and second additions from the sluice had been made. Figure 1 shows the copper stripping peaks for 0.1 N potassium nitrate. Each copper increment was 10W7 mole per liter; the copper content in the nitrate, evaluated by linear extrapolation, was found to be 1.2 X 10-r mole per liter, as can be seen from the graph. A similar determination of lead in the nitrate (Fig. 2) gave 1.2 X 10-r mole per liter. For zinc (Fig. 3), the content was found to be as small as 3 X 10Ws mole per liter. In potassium sulfate, only copper and zinc could be determined, because lead sulfate is slightly soluble. Its solubility product is 1.08 X lo-*; therefore, a 0.1 N sulfate solution is saturated with respect to Pb++. The copper (Fig. 4) and zinc (Fig. 5) contents were 1.6 X lo-? mole per liter, respectively. Since the concentration of zinc was large, the standard
FIG. 1. Copper stripping was 10-T mole per liter.
peaks for
0.1
N potassium nitrate.
Each copper increment
64
W.
KEMULA
AND
S. SACHA
solution was so prepared that each sluice added 1.6 X 10Y6 g-ion Zn++ per liter to the solution investigated. Determination of impurities in rock salt. Two rock salt specimens, one colorless and the other pale rose, were examined. A small lump was split from the bulk of a large colorless salt crystal and a 0.1 N solution was prepared. The copper (Fig. 6), lead (Fig. 7)) and zinc (Fig. 8) contents were found to be 2 X 10m7, 7.5 X lo-“, and 5 X 10ms mole per liter, respectively.
FIG.
2. Determination
FIG. 3.
Determination
of lead in potassium
nitrate.
of zinc in potassium
nitrate.
DETERMINATION
FIG. 4.
FIG. 5.
OF
TRACE
IMPURITIES
Copper content in potassium
Zinc content in potassium
IN
nitrate.
nitrate.
SALTS
63
66
W. KEMULA
AND S. SACHA
The pale-rose salt was more pure, the respective impurity contents (Figs. 9-11) being 1.75 X lo-“, 4 X lo-*, and 6 X 1O-s mole per liter. The data obtained show that the copper, lead, and zinc contents in the rock salts withdrawn from contiguous strata differ considerably. The
FIG. 6.
Copper content of a 0.1 N colorless salt solution.
pale-rose crystal from the two natural halites contains less copper and zinc (Table 1). Our earlier investigations (2) have shown that the amalgam produced by depositions of various ions on mercury undergoes cementation in a given solution; this is confirmed by the following experiment. Cadmium salt was added from the sluice to the pale-rose rock salt and to the potassium nitrate solutions so that the Cd++ concentration rose each time by lo-* mole per liter. Figures 12 and 13 show the successive TABLE COPPER, LEAD, AND ZINC CONTENTS
Salt KNO, K2S04
NaCl (colorless) NaCl (pale rose)
1 (lo-5%)
IN VARIOUS SALTS
cu
Pb
Zn
7.6 6 21.8 8.2
24.6 26.6 15
1.9 26 5.6 6.7
DETERMINATION
OF
I FIG.
TRACE
0
7. Lead content of a
IMPURITIES
IN
SALTS
67
I 0.1
S colorless salt solution
additions of Cd++ from the sluice, and reveal that, on extrapolation, the cadmium peak height vs. concentration line misses the origin. This fact indicates that a portion of cadmium must have been replaced by the Cuf + and Pb++ present in the solution according to the following reactions : Cd(Hg) + Pb++ -----+ Cd(Hg) + Cu++ w
Pb(Hg) + Cd++ Cu(Hg) + Cd+ +
The cadmium concentration deficits evaluated from Figs. 12 and 13 are 2 X 10Wg and 4 ?( IO-” mole per liter, respectively. The copper and lead contents were 1.75 X 10-s and 4 X 10es mole per liter and in the rock salt 1.2 X 1O-7 and 1.2 X IO-? mole per liter in the potassium nitrate, respectively. The nitrate therefore contained twice as much Cu+ + and Pb+ f as did the rock salt solution.
68
W.
FIG. 9.
AND
S. SACHA
-AL++I 1
FIG. 8.
KEMULA
0
Zinc content
i
2w2n
of a 0.1 N colorless salt solution.
Copper content of a pale-rose,
0.1 N salt solution.
DETERMINATION
OF TRACE
IMPURITIES
IN
SALTS
69
Actually, the cadmium cementation effect was found to be larger, and the above-presented consideration is thereby borne out. Cementation, like that described above, is always likely to occur in solutions and may affect the analytical results. To minimize this effect, the solution must be vigorously stirred for a fairly long period of time (2)
FIG. 10. Lead content of a 0.1 2%’pale-rose salt solution.
during deposition, and the resulting amalgam must be dissolved immediately and in a stagnant solution. The data obtained show that reliable results are obtained only if the supporting electrolyte forms neither insoluble precipitates nor permanently nonreducing complexes with the sluice-added standard. Formation of fresh precipitates with transiently increased solubilities may result in only spuriously correct determinations, as was the case with the determination of lead in potassium sulfate. However, since the present method is particularly suited for determination of traces, the above-mentioned
70
W.
FIG.
11.
KEMULA
Zinc content
AND
S. SACHA
of a 0.1 N pink salt solution.
phenomena of precipitation and formation of nonreducing complexes will be only rarely encountered, although their occurrence can never be ruled out as a possibility. SUMMARY Electrochemical determination of trace impurities is described that involves preelectrolysis on the hanging mercury drop electrode and subsequent chronovoltamperometric recording of the stripping currents. The sluice renders a precise determination of minute amounts of impurities that is possible without prior ascertaining of the calibration curve for a given analysis. Cementation with Cu++ and Pb++ was confirmed to take place on addition of Cd++ to the solution examined. Conditions were established in which the effect of cementation on stripping of the resulting amalgam is reduced.
DETERMINATION
OF
TRACE
IMPURITIES
IN
SALTS
FIG. 12.
Cadmium
stripping
from 0.1 N NaCl
solution.
FIG. 13.
Cadmium
stripping
from 0.1 A’ KNO,
solution.
71
72
W.
KEMULA
AND
S. SACHA
REFERENCES 1.
W. A simple apparatus for the determination of trace amounts of different ions using the hanging mercury drop electrode. Micro&m. J. 11, 54-61 (1966). W., AND STROJEK, J. W. Controlled chromopotentio metric stripping -7. KEMULA, of metals reported on the hanging mercury drop electrode. J. Electround. Chem., in press. KEMULA,