EXTRACTION–AMALGAM–POLAROGRAPHIC METHOD FOR THE DETERMINATION OF TRACE IMPURITIES DIRECTLY IN A CHLOROFORM SOLUTION OF OXYQUINOLINE

Yu. A. KARBAINOV, A. G. STROMBERG

At present, extraction is finding wide application in analytical practice as an effective method for isolating and separating ultramicroimpurities in the analysis of high-purity materials. In this procedure, after extraction has been carried out, re-extraction, destruction of the complex, removal of the solvent, and further treatment of the impurities contained in the residue are required, depending on the method of analysis. In particular, when the final determination is carried out by amalgam polarography, a suitable indifferent electrolyte, or background, is selected depending on the nature of the impurities to be determined. Considerable time is spent in selecting such an electrolyte. The idea of determining traces of impurities directly in the solvents used as extractants [1] by the method of amalgam polarography with accumulation (the extraction–amalgam–polarographic method—EAPM) is an attractive one. In our opinion, the use of this method in analytical practice is highly promising, since it eliminates re-extraction. The total duration of analysis by the EAPM method is 2–3 times shorter than by the usual method (with re-extraction). Reduced consumption of reagents and simplicity of apparatus design are also indisputable advantages of the proposed method.

One of the most universal complexing agents used in analytical practice is oxyquinoline. The present work is devoted to elucidating the possibility of determining the anodic waves of the elements Pb (2+), Tl (+), Cu (2+), In (3+), and Cd (2+) directly in a 1% solution of oxyquinoline in chloroform by the EAPM method, to establishing the maximum sensitivity of their determination in this system, and also to the practical application of 1% oxyquinoline in CHCl₃ for the analysis of lithium chloride (chemically pure) for copper content by the EAPM method. To create a conducting medium during polarography, the solution of oxyquinoline in CHCl₃ was mixed with a solution of NH₄NO₃ in CH₃OH. Figure 1 shows the isotherm of the specific electrical conductivity of chloroform–methanol mixtures of various composition, but at a constant NH₄NO₃ concentration equal to 0.1 N. From Fig. 1 it is seen that at a 50% chloroform content in the mixture, the electrical conductivity of the solution (0.006 ohm⁻¹ cm⁻¹) is only half the electrical conductivity of a 0.1 N aqueous KCl solution (0.012 cm⁻¹ cm⁻¹ at 23°C). This value of the specific electrical conductivity satisfies the requirements imposed on polarographic analysis. Proceeding from this, a 1% solution

oxyquinoline in chloroform was mixed with a 0.2 N methanolic solution in a ratio of 1 : 1. The experiments were carried out in a cell (Fig. 2) using a Hungarian polarograph “OH-101.” The anode was mercury at the bottom of the electrolyzer. A vessel for mercury was connected to the anode, from which, when necessary, a new portion of mercury was introduced into the electrolyzer. After completion of the work, the mercury was removed from the cell through a stopcock. All potential values in the text, tables, and figures are given relative to the mercury at the bottom of the electrolyzer. The chloroform used in the experiments was checked for the presence of side decomposition products in accordance

Fig. 1. Conductivity isotherm of a mixture of $\mathrm{CHCl_3} + \mathrm{CH_3OH}$ at different ratios of the mixture components and at a constant concentration of electrolyte (0.1 N $\mathrm{HN_4NO_3}$) in the mixture.

Fig. 2. Electrolyzer for investigations in nonaqueous solutions by the EAP method.

with the procedure described in the State Pharmacopoeia of the USSR [2] and was subjected to triple fractional distillation over calcined soda, as recommended by Walden [3].

The experiments showed that, with good purification of chloroform from side products (mainly from oxidation products of peroxide character) and with careful removal of oxygen from the mixture, the magnitude of the residual current can be considerably reduced. Thus, for example, in investigations with unpurified chloroform, the residual-current density for lead and cadmium at an instrument sensitivity of $1 \cdot 10^{-6}$ A/mm was, respectively, $2 \cdot 10^{-5}$ and $5 \cdot 10^{-5}$ A/cm$^2$. In investigations with purified chloroform, the residual-current density decreased, respectively, to $2 \cdot 10^{-6}$ and $2 \cdot 10^{-5}$ A/cm$^2$. Oxygen was removed from the solution by bubbling nitrogen, previously purified from traces of moisture by passing it through a furnace with copper turnings at $T = 150^\circ$. Before entering the solution, the nitrogen was first saturated with alcohol and chloroform.

Stirring of the solution during the preliminary electrolysis was also carried out with nitrogen. The volume of solution in the experiments was 5 ml. The radius of the stationary mercury drop was \(r_k = 0.048\) cm. Oxyquinoline was purified by double recrystallization from hot aqueous solution. After cooling it crystallized in the form of shiny needles.

As can be seen from Fig. 3, the greatest sensitivity of determination of Pb, Cu, Tl, In, and Cd in the indicated nonaqueous solution and under the conditions of our experiments is, respectively: \(4 \cdot 10^{-7}\); \(2 \cdot 10^{-7}\); \(5 \cdot 10^{-7}\); \(3 \cdot 10^{-6}\); \(2 \cdot 10^{-7}\) g-ion/l, if the maximum sensitivity is taken to be a depth of the anodic tooth of 10 mm (the experimental conditions are indicated in the caption to Fig. 3).

Fig. 3. Dependence of the depth of the anodic teeth of elements on the concentration of metal ions in a solution of 1% oxyquinoline in \(\mathrm{CHCl_3} + 0.2N\ \mathrm{NH_4NO_3}\) in \(\mathrm{CH_3OH}\) (1:1): 1—lead; 2—cadmium; 3—indium; 4—copper; 5—thallium. Experimental conditions: sensitivity \(1 \cdot 10^{-8}\) A/mm; electrolysis potential \(\varphi_e = -1.2\) V, electrolysis time \(\tau_e = 5\) min.

Previously [4, 5] we showed that the sensitivity of the method of amalgam polarography with accumulation can be increased by 1.5–2 orders of magnitude owing to the effect of increasing the surface of the ammonium amalgam at an elevated solution temperature. The essence of this procedure is as follows. When working with ammoniacal solutions and carrying out preliminary electrolysis at potentials more negative than \(-2.0\) V, ammonium ions are reduced on the stationary mercury drop. In this process ammonium amalgam is formed, and its surface increases considerably in comparison with the initial surface. If preliminary electrolysis is carried out on the increased surface of the drop at a temperature close to the boiling point of the solution, then, owing to the combined influence of the two indicated factors (the effect of increasing the surface of the ammonium amalgam and more intensive stirring at an elevated temperature of the solution), the sensitivity of the EAPN method can be increased by 1.5–2 orders of magnitude (Fig. 4). At the same time, as can be seen from Table 1, the electrolysis constant \(k_1\) increases by a factor of 45–58; moreover, owing to the increase in the surface of the mercury drop it increases by a factor of 5–7, owing to the increase with temperature of the diffusion coefficients of ions in solution by a factor of 3–4, and owing to the increase with temperature of convection of the solution by a factor of 2–3.

The effect of increasing the surface area of the mercury ammonium amalgam at an elevated solution temperature was used by us to increase the sensitivity of the determination of lead and copper. If, in the above-mentioned nonaqueous mixture containing a mixture of electrolytes,

Table 1

Increase in the electrolysis constant (number of times) under the influence of various factors in an experiment with lead, concentration \(1 \cdot 10^{-7}\) mole/liter, in a background of 0.2 N \( \mathrm{NH_4NO_3} \) in \( \mathrm{CH_3OH} \)

a) \(K_1^{\tau}\) and \(K_1^{25}\) are the electrolysis constants at \(\tau\) and \(25^\circ\mathrm{C}\);
b) \(S\) and \(S^*\) are the surface area of the mercury drop before and after formation of the ammonium amalgam;
c) \(D_{\tau}\) and \(D_{25}\) are the diffusion coefficients of the ions at \(T\) and \(25^\circ\mathrm{C}\).

The numerical value of the exponent 0.45 was established by us elsewhere.

0.10 N \( \mathrm{NH_4NO_3} + 0.025 \) N \( \mathrm{LiCl} \), preliminary electrolysis is carried out on a stationary mercury drop at \(T = 80^\circ\mathrm{C}\) and \(\varphi_{\mathrm{e}} = -2.8\) (V), the sensitivity of the determination of Pb and Cu increases respectively by 73 and 65 times; thus, the maximum sensitivity achieved in our experiments for Pb (2+) and Cu (2+) in the above-mentioned nonaqueous mixture became respectively equal to \(3 \cdot 10^{-9}\) and \(6 \cdot 10^{-9}\) g-ion/liter (Figs. 5 and 6). Obviously, in a similar manner the sensitivity of the determination of Tl (+), In (3+), and Cd (2+) can be increased by 1–1.5 orders of magnitude.

The investigations carried out to clarify the reversibility of the reduction of copper, lead, thallium, indium, and cadmium in the nonaqueous mixture of 1% oxyquinoline in \( \mathrm{CHCl_3} + 0.2 \) N \( \mathrm{NH_4NO_3} \) in \( \mathrm{CH_3OH} \) (1 : 1) showed that lead and cadmium are reduced reversibly, while thallium, copper, and indium are reduced irreversibly.

The results set out above on the study of the anodic waves of Pb (2+), Tl (+), Cu (2+), In (3+), and Cd (2+) show that there is a quite real possibility of determining them directly in a nonaqueous extractant by the EAPM method. To confirm this circumstance, we used 1% oxyquinoline in chloroform for the determination of copper in chemically pure \( \mathrm{LiCl} \). The procedure was based on the oxyquinoline method described by Morrison and Freiser [6]

Fig. 4. Dependence of the height of the anodic peaks of lead and copper on the electrolysis potential when using the ammonium-amalgam effect and elevated temperature to increase the sensitivity of the method. Background: 1% oxyquinoline in $\mathrm{CHCl_3} + (0.2N\ \mathrm{NH_4NO_3} + 0.05N\mathrm{LiCl})$ in $\mathrm{CH_3OH}$ (1 : 1). Curve 1—lead; curve 2—copper. Experimental conditions: $C = 2 \cdot 10^{-7}$ g-ion/l; sensitivity $3 \cdot 10^{-8}$ A/mm; $\tau_\mathrm{e} = 5$ min.

Fig. 5. Dependence of the height of the anodic peaks of lead on the concentration of metal ions in solution: 1—sensitivity $1 \cdot 10^{-8}$ A/mm, $\tau_\mathrm{e} = 5$ min; $\varphi_\mathrm{e} = -1.0$ V; $T = 25^\circ\mathrm{C}$; when using the ammonium-amalgam effect and elevated temperature; sensitivity $1 \cdot 10^{-8}$ A/mm; $\varphi_\mathrm{e} = -2.85$ V; $\tau_\mathrm{e} = 5$ min; $T = 80^\circ\mathrm{C}$.

Fig. 4. Dependence of the depth of the anodic copper peaks on the concentration of ions in solution. 1. Sensitivity \(1\cdot 10^{-8}\) A/mm; \(\varphi_{\mathrm{e}}=-1.0\) (V); \(\tau_{\mathrm{e}}=5\) min; \(T=25^\circ\mathrm{C}\); 2. With use of the ammonium amalgam effect and elevated temperature; sensitivity \(1\cdot 10^{-8}\) A/mm; \(\varphi_{\mathrm{e}}=-2.85\) (V); \(\tau_{\mathrm{e}}=5\) min; \(T=80^\circ\mathrm{C}\).

Table 2

Results of the determination of traces of copper in synthetic mixtures (wt. parts) by the extraction amalgam polarography method with accumulation.
\(\mathrm{pH}=4.7\)

determination of copper. The completeness of determination of Cu(2+) was studied on synthetic solutions. In this case the standard copper solution was introduced into the aqueous phase. From the data given in Table 2 it is seen that the completeness of extraction of copper by the method indicated above averages 65–70%. Determination of the copper content in LiCl salt (chemically pure) was carried out as follows: a weighed portion of the salt (10 g) was dissolved in 50 ml of tridistilled water. To this solution, after transfer to a separatory funnel, 2.5 ml of masking solution¹ and 5 ml of 1% oxyquinoline in CHCl₃ were added. After this, copper is extracted twice, for 2 min each time. After completion of the extraction, 2.5 ml is taken from the total volume of the chloroform phase, equal to 10 ml, and poured into an electrolyzer for polarography. To the electrolyzer 2.5 ml of a 0.2 N solution of NH₄NO₃ in CH₃OH is added, and polarography is carried out. The electrolysis potential, $\varphi_\mathrm{e} = -1.2$ (V), electrolysis time, $\tau_\mathrm{e} = 5$ min, sensitivity $1 : 10^{-8}$ A/mm. The copper content in the reagent LiCl (chemically pure) is equal to $(1 \div 1.5) \cdot 10^{-6}\%$. The total time for one analysis (two parallel and one blank determinations) is about one hour. The results of the analysis are given in Table 2.

Conclusions

1. A method of extraction amalgam polarography with accumulation (EAPN) has been proposed. The possibility has been shown of direct determination by the EAPN method of $10^{-6}$–$10^{-7}$ g-ion/l of ions of lead, copper, thallium, indium, and cadmium in the mixture 1% oxyquinoline in CHCl₃ + 0.2 N NH₄NO₃ in CH₃OH (1 : 1).

2. The possibility has been shown of increasing the sensitivity of determination of traces of lead and copper in the indicated nonaqueous solution to $(3 \div 5) \cdot 10^{-9}$ g-ion/l by the EAPN method when using the ammonium-amalgam effect and a temperature of 80°C.

3. The possibility has been shown of applying the EAPN method in the studied extractant, using as an example the determination of $10^{-5}$–$10^{-6}\%$ copper in lithium chloride (chemically pure).

LITERATURE

1. Yu. A. Karbainov, A. G. Stromberg. Abstracts of reports at the VII Conference of Workers of Plant and Production Laboratories of Kazakhstan and Central Asia, Alma-Ata, 1964, p. 98.

2. State Pharmacopoeia of the USSR, Medgiz, p. 117, 1952.

3. A. Weissberger, E. Proskauer, J. Riddick, E. Toops. Organic Solvents. IL Publishing House, p. 391, 1958.

4. Yu. A. Karbainov, A. G. Stromberg. Factory Laboratory (in press).

5. Yu. A. Karbainov, A. G. Stromberg. Journal of Analytical Chemistry (in press).

6. J. Morrison, G. Freiser. Extraction in Analytical Chemistry. Goskhimizdat, p. 223, 1960.

¹ Composition of the masking solution: 2 millimoles of disodium EDTA salt and 3 millimoles of calcium acetate are dissolved in such an amount of 0.4 M ammonium acetate that the final volume of the solution is 100 ml.