APPLICATION OF ANION EXCHANGE FOR THE SEPARATION AND CONCENTRATION OF TRACE COPPER

R. D. Gukhavskaya, Yu. L. Lel’chuk, E. D. Lukker

(Presented by the scientific seminar of the Department of Analytical Chemistry)

Traces of iron and copper adversely affect the properties of certain phosphors. Earlier [1–3] we studied the sorption and desorption of chloride complexes of divalent and trivalent iron. As a result of the investigation, optimum conditions were found for the separation and concentration of traces of iron in determining microamounts of this element in the phosphor L-34 and in materials used for its production. The sorption–photometric method developed on this basis for determining traces of iron in these materials [4] attains a sensitivity of \(10^{-6}\)—\(10^{-7}\%\).

In the present work, optimum conditions have been studied for the sorption and desorption of copper on the anion exchangers AN-2F and ceratit FF, with the aim of establishing the possibility of simultaneous separation, concentration, and determination of traces of copper and iron from a single weighed portion when determining microimpurities of these elements in certain materials used in the production of phosphors.

Experimental Part

In all our experiments, especially pure hydrochloric acid was used, practically completely purified from traces of iron and copper by isothermal distillation and subsequent chromatographic purification [4]. All other reagents were of chemically pure grade; the water was triply distilled. The final distillation of the water was carried out from a quartz apparatus.

The initial standard copper solution with a titer of 100 \(\mu\text{g}/\text{ml}\) was prepared from pure copper. A weighed portion of copper in a quartz beaker was dissolved in nitric acid, evaporated with sulfuric acid until fumes of sulfuric anhydride appeared, and the solution was brought to the required volume in a volumetric flask with water.

A working standard copper solution with a titer of 1 \(\mu\text{g}/\text{ml}\) was prepared each time on the day of its use by the appropriate dilution of the initial solution with hydrochloric acid of the normality required for the given series of experiments.

Copper in the eluates was determined by an extraction–photometric method in the form of a complex with sodium diethyldithiocarbamate. Chloroform was used as the extractant. Extraction was carried out at pH 4–5. The medium was maintained with an ammonia–acetate buffer mixture. This mixture was prepared by adding to 64.3 ml of concentrated acetic acid 19.7 ml of concentrated ammonia and water to 100 ml. The pH of the mixture was found to be 4.2. Sodium diethyldithiocarbamate was used as a 0.1% solution in water.

Study of the Sorption and Desorption of Copper and Iron

Chromatographic columns with the anion exchanger AN-2F and ceralite FF in the chloride form were used in the work [2].

The first experiments established the complete absence of copper sorption by the anion exchangers from 0.1 N HCl solution with various additions of standard copper solution, and demonstrated the possibility of using hydrochloric acid of this concentration for eluting copper from the columns.

In our subsequent experiments, to study the effect of HCl concentration on copper sorption through an anion-exchange column, each time 10 ml of HCl of a given normality with a specified addition of copper was passed at an effluent rate of 2–3 ml/min. Charging of the anion exchanger into the chloride form and washing of the sorbed complexes were carried out with hydrochloric acid of the same concentration. For eluting copper, 0.1 N HCl was passed through the column at the same rate. As this acid passed through, the anion-exchanger layer became lighter. When a dark-brown layer 2–3 cm thick remained at the bottom, the stopcock was closed, it was washed with water, and for the determination of copper 15 ml of eluate was collected in a separatory funnel.

A few drops of phenolphthalein were added to the eluate, then ammonia to a pale-pink coloration, 2 ml of ammonia-acetate buffer mixture with pH 4.2, and 2 ml of sodium diethyldithiocarbamate solution; the copper complex formed was extracted twice with chloroform, 5 ml each time. The extracts were brought to 25 ml with chloroform in a volumetric flask, and the optical density of the solution was measured on an FEKN-57 photoelectric colorimeter-nephelometer in a cuvette with a 5 cm layer thickness, using a blue light filter with maximum transmittance at 453 mµ. Chloroform served as the comparison solution.

To construct the calibration curve, the experiments were carried out under the same conditions, except that instead of eluate the corresponding amounts of standard copper solution were taken. The data obtained on copper sorption are given in Table 1. They show that the anion exchanger AN-2F quantitatively sorbs copper from 7–10 N HCl solutions, and ceralite FF from 5–9 N.

Table 1

Sorption of copper on the anion exchangers AN-2F and ceralite FF from hydrochloric acid solutions.
Through the column with AN-2F, a solution of 5 µg of copper in 10 ml of hydrochloric acid of the corresponding normality was passed; with ceralite FF, 4.77 µg of copper.

Data on the dynamics of the process of washing copper out of columns with AN-2F or ceralite FF by 0.1 N hydrochloric acid solution are given in Table 2. Similar data were obtained by us earlier for iron [1, 2, 3].

We also determined the exchange capacities of the anion exchangers for copper and iron and calculated the distribution coefficients of these elements [5].

Dynamics of elution of sorbed copper from anion-exchange columns with 0.1 N hydrochloric acid solution

Copper was determined by a colorimetric method in the form of the ammonia complex [6], and iron in the form of iron rhodanide [7]. The data obtained are given in Tables 3 and 4.

Table 3

Exchange capacity of anion exchangers for copper and the distribution coefficient of copper.
Solution volume 25 ml. Copper equivalent 31.77.

Table 4

Exchange capacity of anion exchangers for iron and the distribution coefficient of iron.
Solution volume 50 ml. Iron equivalent 18.62

In carrying out the experimental part of the work, students G. N. Dembovskaya and A. N. Muran took an active part.

Conclusions

The processes of sorption and desorption of copper on the anion exchangers AN-2F and ceralite FF have been studied. It has been established that the ranges of hydrochloric-acid concentration in which quantitative sorption and quantitative desorption of copper chloride complexes occur differ little from those for iron. This opens up the possibility of simultaneous separation and concentration of copper and iron when determining traces of these elements from a single sample in certain materials used in phosphor production.

Literature

1. R. D. Glukhovskaya, Yu. L. Lel’chuk. Proceedings of the 1st Scientific Conference of the Tomsk Branch of the All-Union Chemical Society named after D. I. Mendeleev. Tomsk, 1967.

2. Yu. L. Lel’chuk, R. D. Glukhovskaya. Izv. TPI, 148, 68, 1966.

3. R. D. Glukhovskaya, Yu. L. Lel’chuk, E. D. Lukker. Izv. TPI, 152, 40, 1967.

4. R. D. Glukhovskaya, Yu. L. Lel’chuk. Abstracts of reports of the Conference on the Analytical Chemistry of Semiconductor Substances. 19, 46, Kishinev, 1965.

5. I. P. Alimarin, G. A. Belyavskaya. Chromatography, Its Theory and Application, 372. Publishing House of the Academy of Sciences of the USSR, Moscow, 1960.

6. V. N. Alekseev. Quantitative Analysis, 478, Goskhimizdat, Moscow, 1958.

7. E. Sendel. Colorimetric Methods for Determining Traces of Metals. Mir Publishing House, Moscow, p. 471, 1964.