Determination of Trace Copper Impurities in L-34 Phosphor and High-Purity Perchloric Acid

L. G. Baletskaya, M. S. Zakharov, Kh. A. Lel’chuk, L. N. Dudenko

(Submitted by the scientific seminar of the Chemical-Technological Faculty)

To convert L-34 phosphor into solution, a number of mineral acids were tested: \(\mathrm{HNO_3}\), \(\mathrm{HNO_3 + HCl}\), \(\mathrm{H_3PO_4}\), hot \(\mathrm{H_2SO_4}\), \(\mathrm{HCl}\), and \(\mathrm{HClO_4}\).

The best solvent for the phosphor proved to be 7 N hydrochloric acid.

As an activator, the phosphor contains 2% \(\mathrm{Sb_2O_3}\). In the presence of such an amount of antimony, it is impossible to determine copper in dissolved phosphor, since antimony and copper are reduced at identical potentials. An attempt to determine copper by the anodic stripping polarography method [1] did not yield the desired results.

During electrolysis, antimony is reduced together with copper. Antimony is poorly soluble in mercury [2]; its content in the phosphor is several orders of magnitude greater than that of copper. Upon reduction, antimony ions cover the mercury electrode with a film that shields the electrode and prevents anodic dissolution of copper. For the reasons indicated, it became necessary to separate microconcentrations of copper from the bulk of the phosphor. According to literature data [3], complete separation of copper in a hydrochloric-acid medium can be achieved by extraction with acetylacetone. Studies were carried out to determine the completeness of extraction of copper by acetylacetone in chloroform at various pH values of the solution. The required pH value of the solution was obtained by adding appropriate amounts of 1 M \(\mathrm{NH_4OH}\). The phosphor solution and known amounts of 1 M \(\mathrm{NH_4OH}\) were added to a separatory funnel. The results are presented in Table 1.

It is seen from Table 1 that the most complete extraction of copper (up to 100%) with acetylacetone in chloroform is observed at pH 4–5. It should be noted that when the phosphor is dissolved, a small insoluble precipitate sometimes remains. Qualitative analysis of the precipitate showed the presence of antimony oxides. This precipitate constitutes no more than 5% of the initial sample. Studies showed that the determined copper content in the phosphor, when the analysis is carried out in the presence of the precipitate and without the precipitate, varies within the limits of experimental error (10–15%). Spectral analysis of the precipitate revealed no traces of copper.

Description of the Determination

In a quartz beaker (checked for purity) with a capacity of 10 ml, 0.35 g of phosphor is dissolved in 5 ml of 7 N HCl hydrochloric acid. The solution

is evaporated at a temperature of 180–200° in an evaporation chamber [4]. After evaporation, the precipitate is dissolved in 10 ml of 0.5 N HCl and the contents of the beaker are poured into a clean separatory funnel.

Table 1

Results of studies of the completeness of separation of microamounts of copper from L-34 phosphor by extraction with acetylacetone in chloroform at various pH values.
Experimental conditions: phosphor sample weight 0.35 g, galvanometer sensitivity \(1 \cdot 10^{-8}\%\) a/mm, electrolysis time 7 min, \(\varphi_{\mathrm{el.}} = -1\) V (sat. cal. el.), \(W = 333\) mV/min, \(V = 3\) ml.

The beaker is rinsed with 2 ml of tridistillate and the solution is transferred to the separatory funnel; the pH of the solution in the funnel is brought with 1 M \(\mathrm{NH_4OH}\) to 4.5. To the contents of the funnel, 4 ml of acetylacetone in chloroform (1:4) is added and extraction is carried out for 3 min; the lower phase is transferred to a beaker. To the funnel, 2 ml of chloroform is added and extraction is carried out for 3 min in order to collect the remaining acetylacetone. The organic phase is evaporated at 250°. To avoid decomposition of copper salts during evaporation, 1 ml of HBr is added to the residue and it is evaporated again. After evaporation, 2 ml of 0.25 M \(\mathrm{NH_4}\)-tartrate [5] is added to the cooled beaker; the beaker is placed in the electrolyzer, and electrolysis is carried out at a potential of \(-1\) V (sat. cal. el.), followed by polarography.

The impurity content is evaluated by the method of additions. The sensitivity of the procedure, estimated from three times the variance, is \([1 \cdot 10^{-5}]\%\).

Determination of Copper in Perchloric Acid

The possibility of determining microamounts of copper directly in perchloric acid was investigated. It does not appear possible to determine traces of copper using perchloric acid as the electrolyte. Perchloric acid is a strong oxidizing agent, and oxide films of mercury are formed on the surface of the mercury electrode; these strongly passivate the electrode. This leads to poor reproducibility of the results. For these reasons, in determining copper, perchloric acid must be removed by evaporation. Studies showed that, upon evaporation at temperatures above 250–270°, loss of microimpurities is observed. In subsequent studies, evaporation was carried out at temperatures of 250–270°. To convert the copper oxides that may form during evaporation of perchloric acid into salt, 1 ml of HBr is added to the residue and the evaporation is repeated.

After evaporation, 2 ml of 0.25 M NH₄ tartrate are added to the beaker, and the solution is polarographed.

The possibility of losses of microquantities of copper during evaporation of hydrochloric acid under the specified temperature conditions was studied. Before evaporation, copper was added to the beaker containing hydrochloric acid; a sample without added copper served as the blank experiment. The copper content was determined by the method of standard additions.

The results of the studies are presented in Table 2.

Table 2

Study of losses of microquantities of copper during evaporation of hydrochloric acid.
Experimental conditions: electrolysis time 10 min, electrolysis potential — −1.6 (vs. SCE), galvanometer sensitivity \(4 \cdot 10^{-9}\) A/mm, radius of the mercury drop — 0.04 cm, background volume — 2 ml.

Table 3

Results of analysis of hydrochloric acid for copper content.
Experimental conditions: \(\varphi_{\mathrm{e}}=-1.6\) (vs. SCE), electrolysis time — 10 min, \(S=4 \cdot 10^{-9}\) A/mm, \(R=0.04\) cm, \(V=2\) ml.

From Table 2 it is evident that the losses of microquantities of copper do not exceed 15%, which is permissible in the determination of microquantities.

A procedure is proposed for determining copper in high-purity perchloric acid. A 10 ml portion of perchloric acid is poured into a quartz beaker that has been checked for purity. At 250–270° the acid is evaporated in an evaporation chamber. Evaporation is carried to dryness over the course of an hour. To the residue after evaporation, 1 ml of HBr is added and the solution is again evaporated to dryness. To the residue after evaporation, 2 ml of 0.25 M NH₄-tartrate is added; the beaker is placed in the electrolyzer, electrolysis is carried out for 10 minutes at a potential of −1 V (saturated calomel electrode), with continuous passage of nitrogen, and polarography is performed.

Using the procedure developed, analyses were carried out on one of the plant samples of perchloric acid. The analytical results and their statistical treatment are given in Table 3.

Arithmetic mean:

\[ \frac{62.5}{12}=5.4. \]

Mean-square error of a single measurement:

\[ \frac{4.8}{12}=0.4. \]

Maximum possible error: \(\Delta = 3\sigma = 0.4 \cdot 3 = 1.2.\)

Mean-square error of the arithmetic mean:

\[ \sigma_0=\frac{\sigma}{\sqrt{n}}=\frac{0.4}{\sqrt{12}}=0.11, \]

\[ \chi_{\text{true}}=(5.4\pm 0.11)\cdot 10^{-7}\%. \]

The sensitivity of the procedure for determining copper in perchloric acid is \(1\cdot 10^{-7}\%\).

Conclusions

1. A procedure has been developed for determining copper in L-34 phosphor. The sensitivity of the procedure is \(1\cdot 10^{-5}\%\).

2. A procedure has been developed for determining copper in high-purity perchloric acid. The sensitivity of the procedure is \(1\cdot 10^{-7}\%\).

References

1. A. A. Kaplin, M. S. Zakharov, A. G. Stromberg. In: Methods of Analysis of Chemical Reagents and Preparations, 5–6, IREA Publ., Moscow, 1963, 92.

2. L. F. Zaichko, M. S. Zakharov. Determination of the solubility of antimony in mercury by the amalgam method. ZhAKh, 1965 (in press).

3. J. Morrison, G. Freiser. Extraction in Analytical Chemistry. Scientific-Technical Publishing House, L., 1960.

4. A. G. Stromberg, M. S. Zakharov, V. S. Smorodinov, V. M. Pichugina. In: Methods of Analysis of Chemical Reagents and Preparations, 5–6, IREA Publ., Moscow, 1963, 92.

5. L. G. Baletskaya, M. S. Zakharov, Kh. A. Lelchuk, V. M. Pichugina. Determination of microconcentrations of copper in materials used in the production of phosphors. Izv. TPI, 1965 (in press).