DETERMINATION OF Cu MICROIMPURITIES IN MATERIALS USED IN PHOSPHOR PRODUCTION

Communication 1. Determination of Copper in Antimony Trioxide and in Demineralized Water*)

L. G. BALETSKAYA, M. S. ZAKHAROV, Kh. A. LELCHUK, V. M. PICHUGINA

(Presented by the scientific-methodological seminar of the Department of Chemical Technology)

In the production of certain phosphors, high-purity antimony trioxide and demineralized water are used. The content of copper impurities in them must not exceed \(10^{-6}\)—\(10^{-5}\%\). Determination of such amounts of copper has recently been successfully carried out by the method of amalgam polarography with accumulation [1, 2]. In the present work we present the results of studies on the development of amalgam-polarographic procedures for determining \(10^{-6}\)—\(10^{-5}\%\) copper in the indicated materials.

For determining copper in antimony trioxide by the method of amalgam polarography with accumulation, the excess antimony must be removed.

Table 1

Check of Cu losses during analysis of \(Sb_2O_3\)

Of the various methods we tested for separating the bulk of antimony from copper, we chose distillation in the form of \(SbCl_3\).

The evaporation of antimony was carried out by us with pure concentrated hydrochloric acid. Pure hydrochloric acid obtained by distillation cannot be used for these purposes, since its concentration after distillation reaches only 6N and hydrolysis of antimony occurs in it. We used pure hydrochloric acid obtained by the isopiestic method.

Electrolyte was added to the residue after removal of Sb, and the resulting solution was polarographed.

We established the optimum evaporation temperature. To check for the absence of volatilization of microamounts of copper with antimony chloride, a definite quantity of copper was added to it with [[unclear: continuation on next page]].

*) Demineralized water is pure water obtained by ion exchange.

…amount was added to the weighed portion of \(Sb_2O_3\) before its dissolution, and in a parallel sample this addition was made directly into the electrolyzer. In this case, the blank analysis was the analysis of a weighed portion of \(Sb_2O_3\) without the addition of copper. The results of these studies are presented in Table 1.

Experimental conditions: electrolysis time 10 min, radius of the dropping mercury electrode 0.04 cm, solution volume 2 ml, copper concentration \(1 \cdot 10^{-7}\) M, instrument sensitivity \(8 \cdot 10^{-5}\) a/mm.

From Table 1 it is evident that at temperatures above 250°C splashing of the solution and losses of the impurity are observed. Therefore, in subsequent work, evaporation of antimony was carried out at 250°C. Bismuth and lead may be present in antimony oxide; their anodic waves lie close to the anodic wave of copper, and the latter, at certain concentrations, may interfere with polarographic determination of copper. In this connection, we studied the amalgam-polarographic behavior of copper, bismuth, lead, and antimony in a number of electrolytes. The results of the studies are presented in Table 2.

Table 2

Peak potentials of anodic elements in certain electrolytes

From Table 2 it is evident that the best supporting electrolytes for polarographing copper are 0.25 M \(NH_4\) tartrate and 0.2 M \(NH_4OH + 0.1\) M potassium tartrate: in the first supporting electrolyte, bismuth and antimony give no anodic waves at all, and the peak potential of the anodic wave of lead lies far from the peak of the anodic wave of copper.

As is known [3], the depth of the anodic wave depends on the electrolysis potential. To achieve the greatest sensitivity of the analytical procedure, accumulation of the substance in the drop must be carried out at such a potential at which an anodic wave of the greatest depth is obtained. For this purpose, we studied the dependence of the depth of the anodic wave of copper on the electrolysis potential in solutions of 0.2 M \(NH_4OH + 0.1\) M potassium tartrate and 0.25 M \(NH_4\) tartrate. The results of the studies are presented in Figs. 1 and 2.

In developing a procedure for determining copper in antimony oxide, on the basis of the above considerations, we selected a 0.25 M \(NH_4\) tartrate solution as the supporting electrolyte for polarography.

Preliminary analyses of a factory sample of \(Sb_2O_3\) showed that the latter contains a significant amount of lead (on the order of \(10^{-3}\)—\(10^{-2}\)%), which, if the electrolysis potential is chosen incorrectly, interferes with the determination of copper. For these reasons, the potential

the electrolysis potential in subsequent studies was chosen to be equal to −0.4 V (sat. cal. el.). At this potential the diffusion current of copper is attained, while lead is deposited on the electrode in an insignificant amount.

Fig. 1. Dependence of the depth of the anodic notch of copper and lead on the electrolysis potential against a background of 0.25 M NH₄ tartrate: curve 1—for copper, curve 2—for lead. Experimental conditions: 1. \(C = 1.5 \cdot 10^{-6}\) g-ion/l, \(V = 2\) ml; \(S = 1 \cdot 10^{-8}\) a/mm; \(\tau = 10\) min. 2. \(C = 4 \cdot 10^{-6}\) g-ion/l; \(V = 3\) ml; \(S = 1 \cdot 10^{-8}\) a/mm; \(\tau = 7\) min.

The calibration graph for copper in a pure 0.25 M NH₄ tartrate solution and in the solution being analyzed has the form of a straight line (Fig. 3); consequently, the determination of the copper impurity can be carried out by the method of additions.

On the basis of the studies carried out, the following procedure is proposed for determining copper in \(\mathrm{Sb_2O_3}\). Required reagents: pure hydrochloric acid, obtained by the isopiestic method; a pure 0.25 M NH₄ tartrate solution. This solution is obtained by mixing calculated amounts of tridistillate, ammonia purified by the isopiestic method, and tartaric acid.

Analysis. 0.2 g of antimony oxide is placed in a quartz beaker previously checked for cleanliness¹ and dissolved in 1.5 ml of pure concentrated hydrochloric acid. The resulting solution is evaporated at 250°C; another 1 ml of HCl is added and the evaporation is repeated. After evaporation, 2 ml of 0.25 M NH₄ tartrate (background) is poured into the beaker; for 10 min at −0.4 V (sat. cal. el.) electrolysis is carried out with simultaneous passage through the solution of nitrogen purified of oxygen and polarography in the potential range −0.4 to 0.05 V (sat. cal. el.). At a potential of −0.25 V the potentiometric drum is stopped for the time necessary for complete dissolution of the lead accumulated in the drop. After dissolution of the lead, polarography is continued; the copper content in the antimony oxide is evaluated by the method of additions.

Using the developed procedure, we analyzed one of the samples of antimony oxide. The results of the analyses and their statistical treatment are given in Table 3.

The sensitivity of the procedure, estimated from the threefold variance under the developed conditions, is \(1 \cdot 10^{-6}\)%. If it is necessary to increase the sensitivity of the copper-determination procedure, the weighed portion of antimony oxide should be increased or the electrolysis time should be increased.

For determination of copper in demineralized water, we propose two variants of the procedure. In this procedure, 0.25 M NH₄ tartrate was used as the background. According to the first variant, a solution is prepared

¹ The cleanliness of the beakers is checked by amalgam polarography with accumulation. For the check, the background (0.25 M NH₄ tartrate) is poured into the washed beaker, and electrolysis and polarography are carried out. If no notches are obtained on the anodic polarogram, the beaker is clean.

0.25 M NH₄ tartrate in tridistillate and in demineralized water.

For analysis, 2 ml of the background solution prepared in tridistillate (blank experiment) is poured into one beaker, and into the other beakers is poured a solution prepared in demineralized water. After this, all solutions are polarographed. Electrolysis is carried out for 10 min. During electrolysis the solutions are purged with nitrogen.

Fig. 2. Dependence of the depth of the anodic notch of copper and lead on a background of 0.2 M NH₄OH + 0.1 N K tartrate: curve 1—for copper, curve 2—for lead. Experimental conditions: 1. \(C=3\cdot10^{-6}\) g-ion/l; \(V=2\) ml; \(S=1\cdot10^{-8}\) A/mm; \(\tau=7\) min. 2. \(C=4\cdot10^{-6}\) g-ion/l; \(V=2\) ml; \(S=1\cdot10^{-8}\) A/mm.

The copper content in demineralized water is estimated by the method of additions. The sensitivity of the method, estimated from threefold dispersion with artificial additions of copper, under the developed conditions is \(4\cdot10^{-7}\%\).

According to the second variant of the procedure, 10 ml of demineralized water is poured into a quartz beaker, and at 120°C the water is evaporated in an evaporation chamber [4]. To avoid decomposition of copper salts, before evaporation one drop of chemically pure sulfuric acid of concentration 1:4 is added to all samples, including the blank. Evaporation is carried out to a moist residue. To the residue is added 2 ml of 0.25 M NH₄ tartrate solution, the beaker is placed in the electrolyzer, electrolysis is carried out for 10 minutes (simultaneously with electrolysis, the solution is purged with nitrogen), and polarographing is performed. A blank experiment is carried out in parallel. The copper content is estimated by the method of additions.

Fig. 3. Calibration graph for copper. 1 in a pure solution of 0.25 M NH₄ tartrate. 2 in the analyzed solution. Experimental conditions: \(S=1.5\cdot10^{-8}\) A/mm; \(\tau=7\) min; \(\varphi=-0.5\) V (sat. calomel electrode).

To check for the absence of losses of microquantities of copper during the analysis, a definite amount of it was added to demineralized water before evaporation, and in a parallel sample this addition was made

was introduced directly into the electrolyzer. The results of these studies showed that the completeness of Cu determination is 90–100%. Such accuracy of determination is quite satisfactory for determining microamounts of substances.

Table 3

Results of the analysis of an antimony oxide sample for copper content and their statistical treatment

Using the developed procedure, a number of analyses of demineralized water were carried out. Copper was not detected in the water. The sensitivity of the procedure, estimated from the threefold variance with artificial additions of copper, under the developed conditions is $4\cdot 10^{-7}\%$.

In conclusion, we express our gratitude to Professor A. G. Stromberg for his attention to the work.

Conclusions

1. An amalgam-polarographic procedure has been developed for determining microconcentrations of copper in antimony trioxide.

2. A procedure has been developed for determining microconcentrations of copper also in demineralized water. As a result of the studies carried out, a procedure is proposed for determining copper in $\mathrm{Sb_2O_3}$ with a sensitivity of $1\cdot 10^{-6}\%$ and in demineralized water with a sensitivity of $4\cdot 10^{-7}\%$.

Literature

1. A. G. Stromberg, E. A. Stromberg. Zavodskaya laboratoriya, 27, 3 (1962).

2. A. G. Stromberg, E. A. Zakharova. Zavodskaya laboratoriya, 31, 261 (1964).

3. M. S. Zakharov. Zavodskaya laboratoriya, 30, 15 (1964).

4. A. G. Stromberg, M. S. Zakharov, V. S. Smorodinov, V. M. Pichugina. Collection Methods for the Analysis of Chemical Reagents and Preparations, IREA, Moscow, 5–6, 72 (1963).