EXTRACTION–AMALGAM–POLAROGRAPHIC METHOD FOR DETERMINING TRACES OF ANTIMONY IN SOLUTIONS OF HIGH-PURITY NaCl AND HCl

Yu. A. Karbainov, S. N. Karbainova

(Presented by the scientific methodological seminar of the CTF)

Previously [1], a method of extraction amalgam polarography with accumulation (the EAPN method) was proposed, the essence of which consists in determining microconcentrations of elements directly in extracts. The EAPN method provides double concentration of the element being determined—extraction and electrochemical—eliminates the reextraction operation, simplifies the apparatus design of the analysis, and reduces reagent consumption in comparison with the usual variant involving reextraction. This method was applied to determine traces of antimony in model solutions of 0.8% NaCl and 0.5% HCl.

According to literature data, quantitative separation of antimony from the matrix can be achieved by extraction with ethyl acetate [2], diisopropyl ether [3], methyl isobutyl ketone, carbon tetrachloride, chloroform [4], and others. In our case, chloroform proved to be the most suitable. Studies in extraction APN usually begin with selection of an electrically conducting system, one component of which is the extract being analyzed.

We studied the electrical conductivity of the binary nonaqueous system chloroform—0.2 N NH₄NO₃ in ethyl alcohol (96%) at different volume ratios of these components in the mixture.

Electrical conductivity was measured by the Kohlrausch method. The isotherm of specific electrical conductivity is shown in Fig. 1. It can be seen from the figure that, at a component ratio of 1:1, the electrical conductivity of the solution is 0.003 ohm⁻¹ cm⁻¹, which is only one order of magnitude lower than the electrical conductivity of aqueous 0.1 N KCl (0.012 ohm⁻¹ cm⁻¹) at 25°C. As the proportion of ethyl alcohol increases, the electrical conductivity of the binary nonaqueous mixture continues to increase.

Fig. 1. Isotherm of the specific electrical conductivity of a mixture of chloroform—0.2 N \(NH_4NO_3\) in 96% ethyl alcohol

On the basis of the data obtained, in the amalgam-polarographic determination of traces of antimony in NaCl and HCl, the chloroform extract of antimony was mixed with 0.2 N \(NH_4NO_3\) in ethyl alcohol in a ratio of 1:2. We did not use a higher ethanol content in the binary nonaqueous mixture, since with an increase in the proportion of alcohol the concentration of the element being determined decreases and, consequently, the sensitivity of the determination also decreases.

From the literature data it is known [4] that, with the aid of chloroform, trivalent antimony is most often extracted in the form of diethyldithiocarbamate complexes. Potassium iodide and Trilon B are used as masking agents in this case. However, the literature contains no unambiguous data regarding the optimum pH of extraction under these conditions. A certain part of our investigations was devoted to clarifying this question. The experimental procedure was as follows: 15 ml of 0.8% NaCl was transferred to a 100-ml separatory funnel, mixed with 2.5 ml of 5% Trilon B, 2.5 ml of 5% KI¹, and 5 ml of 0.5% NaDDC. After this, a definite addition of a standard \(SbCl_3\) solution was added to the mixture being analyzed and, at different pH values of the solution, which were adjusted with HCl or NaOH, the antimony was extracted with 2.5 ml of chloroform for 1 min. The pH interval studied in the work was 1–9. The results obtained are presented in Fig. 2, from which it is seen that the height of the anodic peak of antimony and, consequently, the percentage extraction increase with increasing pH from 1 to 7. A further increase in pH promotes a decrease in the height of the anodic peak. This experimental fact can probably be explained by hydrolysis of the antimony salt in alkaline solution.

In the work, studies were also carried out to determine the optimum electrolysis potential in the chloroform extract of antimony with 0.2 N \(NH_4NO_3\) in ethanol (component ratio 1:2). With a shift of the electrolysis potential into the cathodic region, the height of the anodic peak of antimony increases. However, beginning at a potential of 0.9 V, with increasing electrolysis potential the height of the anodic peak decreases noticeably. This circumstance is apparently connected primarily with the evolution—

¹ It was established that the indicated ratios of potassium iodide and Trilon B in the determination of \(10^{-5}\%\) antimony are optimal.

by the evolution of gaseous hydrogen and its adsorption on the electrode surface; as a result, the working surface of the electrode decreases, which also leads to a decrease in the height of the anodic peak.

The results obtained made it possible to develop an extraction–amalgam–polarographic method for determining traces of antimony in model solutions of 0.8% NaCl and 0.5% HCl. The method is highly sensitive, sufficiently accurate, rapid, and simple to perform. The sensitivity of the determination depends on the volume of 0.8% NaCl or 0.5% HCl solution taken for analysis. With a volume of 50 ml it is \(5 \cdot 10^{-6}\%\). It is important to note that increasing the volume of NaCl or HCl does not cause any complications in carrying out the analysis; in particular, it does not increase the total duration of the analysis. The total duration of the analysis of two parallel samples and one blank is no more than 1 hour. The relative error of the determination does not exceed \(\pm 10 \div 15\%\).

Fig. 2. Percent extraction of antimony as a function of the pH of the solution

Procedure for the extraction–amalgam–polarographic determination of traces of antimony in model solutions of 0.8% NaCl and 0.5% HCl:

Fig. 3. Dependence of the height of the anodic peak on the concentration of antimony ions in the initial solution (extraction from 0.8% NaCl)

50 ml of 0.8% NaCl or 50 ml of 0.5% HCl, previously neutralized with NaOH, is transferred into a separatory funnel with a capacity of 150 ml. Add 6 ml of a 5% solution of Trilon B, 6 ml of 5% KI, and 8 ml of 0.5% NaDDC. The contents are thoroughly mixed and extracted—

Fig. 4. Dependence of the height of the anodic peak on the concentration of antimony ions in the initial solution (extraction from 0.5% HCl)

They are extracted with chloroform, in portions of 1.5 ml, twice for 1 minute each time. The combined extract is transferred to a polarographic cell, mixed with 6 ml of 0.2 N NH₄NO₃ in C₂H₅OH, and, after preliminary removal of oxygen from the solution with nitrogen, polarographed. The antimony content is determined either from a calibration curve or by the method of standard additions.

Conditions of analysis: electrolysis potential \(\varphi_{\mathrm{e}} = -0.8\) V. Electrolysis time \(t_{\mathrm{e}} = 3\) min. Instrument sensitivity \(1 \cdot 10^{-9}\) A/mm.

Figures 3 and 4 present calibration curves obtained in the range of variation of the antimony concentration in the initial solution \(2\text{–}12 \cdot 10^{-8}\) g-ion/l.

REFERENCES

1. Yu. A. Karbanov. Abstract of Candidate’s dissertation, Tomsk, 1965.
2. I. Khogasani, M. Khundkar. Anal. Chim. Acta, 21, 24, 1959.
3. C. Gallzi, S. Manschi. Metallyrgia, 58, 311, 1966.
4. I. Stary. Extraction of Chelates. “Mir,” 1966.