Radiometric Assessment of the Degree of Coprecipitation of a Series of Trace Impurities with Lead Sulfate
Abstract Generated abstract
This study evaluates the separation of trace impurities during precipitation of lead as lead sulfate, a preliminary concentration step used when direct analysis of high-purity lead is insufficiently sensitive. Using radiometric measurements of labeled cobalt, antimony, cadmium, zinc, sodium, thallium, and indium, the authors assess coprecipitation with lead sulfate, recovery in wash solutions and mother liquor, and the dependence of residual lead on settling time. Double washing with 6 N nitric acid largely removes coprecipitated impurities, except for thallium, whose extraction remains about 55 percent, while lead separation is incomplete because suspended lead sulfate passes into the mother liquor. The work concludes that impurity coprecipitation can generally be avoided, but amalgam polarographic determination with preconcentration requires further reduction of residual lead to approximately 10^-6 to 10^-7 g.
Full Text
RADIOMETRIC ASSESSMENT OF THE DEGREE OF COPRECIPITATION OF A SERIES OF TRACE IMPURITIES WITH LEAD SULFATE
N. A. KHIMAI, P. I. ARTYUKHIN, A. A. KAPLIN
(Presented by the scientific seminar of KhTF)
In the analysis of high-purity lead, the sensitivity of direct analytical methods is insufficient. Therefore, preliminary concentration of impurities is usually carried out. It is achieved by separating the bulk of the lead in the form of sparingly soluble salts [1, 2, 3] or electrolytically [4]. We have carried out studies aimed at quantitatively assessing the completeness of impurity extraction after precipitation of lead in the form of sulfate, the completeness of lead separation, and the possibility of accelerated separation of the bulk of lead from impurities.
Experimental Part
Lead precipitation was carried out according to the procedure described in [5], the only modification being double washing of the precipitate instead of a single washing. The completeness of impurity determination was studied radiometrically. β-radiation was recorded on the working setup. Coprecipitation with lead sulfate was studied for the following impurities: cobalt, antimony, cadmium, zinc, sodium, thallium, and indium. The results obtained are given in Table 1. It follows from Table 1 that, for all the elements considered, coprecipitation of trace impurities with the precipitate is observed; however, after double washing of the precipitate with 6N HNO₃, the coprecipitated elements are completely extracted. The exception is thallium, whose degree of extraction averages 55%. After separation of the bulk of lead in the form of sulfate, an average of 60% to 80% passes into the flask with the mother liquor. This value depends on the thoroughness of phase separation. Although lead precipitation is carried out with an excess of H₂SO₄, lead separation is not quantitative. Part of it passes with the mother liquor in the form of suspended particles of lead sulfate. Separation of lead by the procedure takes 7–8 hours.
Separation of the matrix in the form of sulfate in the method of amalgam polarography with accumulation is justified if the time required to separate the matrix can be substantially reduced without increasing losses of the impurities being determined as a result of coprecipitation. We evaluated the completeness of lead separation in the form of sulfate as a function of the holding time of the solution over the precipitate. The lead content in the mother liquor together with the wash solutions was estimated by the APA method. For this purpose, 5 ml of the solution under study was evaporated to dryness in a quartz beaker that had been preliminarily checked for purity. A supporting electrolyte—2.2 N tiron—was added to the beaker, and accumulation was carried out
Table 1
Results of the radiometric assessment of the behavior of microimpurities during the separation of lead in the form of sulfate
| Element determined | Amount of isotope introduced, % | Content of isotopes in the wash solutions, %, mother liquor | Content of isotopes in the wash solutions, %, 1st wash | Content of isotopes in the wash solutions, %, 2nd wash | Results after complete separation of lead, % |
|---|---|---|---|---|---|
| Zn-65 | $1 \cdot 10^{-3}$ | 63 | 30 | 14 | 107 |
| Na-22 | $1 \cdot 10^{-10}$—$1 \cdot 10^{-12}$ | 52 | 25 | 10 | 87 |
| Na-22 | (without carrier) | 48 | 30 | 10 | 88 |
| Co-60 | $1 \cdot 10^{-3}$ | 75 | 22 | 5 | 102 |
| Co-60 | $1 \cdot 10^{-3}$ | 66 | 22 | 10 | 98 |
| Cd-115M | $5 \cdot 10^{-3}$ | 78 | 30 | 0 | 108 |
| Cd-115M | $5 \cdot 10^{-3}$ | 72 | 26 | 4 | 102 |
| Sb-124 | $2 \cdot 10^{-4}$ | 70 | 30 | 3 | 103 |
| Sb-124 | $2 \cdot 10^{-4}$ | 63 | 34 | 5 | 102 |
| In-114 M | $2 \cdot 10^{-4}$ | 70 | 28 | 6 | 104 |
| In-114 M | $2 \cdot 10^{-4}$ | 68 | 34 | 7 | 105 |
| Tl-204 | $1 \cdot 10^{-3}$ | 3 | 28 | 20 | 51 |
| Tl-204 | — | 5 | 37 | 14 | 56 |
| Tl-204 | $1 \cdot 10^{-4}$ | 4 | 35 | 18 | 57 |
| Tl-204 | — | — | 37 | 21 | 58 |
of lead and the anodic peak was recorded. The lead concentration was determined by the method of additions. The results are given in Table 2.
It follows from Table 2 that the lead content after precipitation of the base in the form of sulfate and 20–30 min settling of the solution with double washing of the precipitate is $1.5 \cdot 10^{-5}$ g.
Table 2
Completeness of separation of lead in the form of sulfate
as a function of the holding time of the solution over the precipitate
| No. | Initial weighed portion of lead, g | Settling time, min | Lead detected, g | $C_{\text{init}}/C_{\text{final}}$ |
|---|---|---|---|---|
| 1 | 1 | 20 | $1.3 \cdot 10^{-4}$ | $8 \cdot 10^{3}$ |
| 2 | 1 | 40 | $1.5 \cdot 10^{-5}$ | $6 \cdot 10^{4}$ |
| 3 | 1 | 150 | $1.5 \cdot 10^{-5}$ | $6 \cdot 10^{4}$ |
| 4 | 0.625 | 1200 | $1.4 \cdot 10^{-8}$ | $4 \cdot 10^{7}$ |
Conclusions
On the basis of the investigations carried out, it has been established that coprecipitation of microimpurities with lead sulfate can be excluded; however, for determination of a number of microimpurities by the method of amalgam polarography with preconcentration, more complete separation of the remaining lead, down to concentrations of $10^{-6}$—$10^{-7}$ g, is necessary.
References
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