DETERMINATION OF ULTRAMICROIMPURITIES DIRECTLY IN A CHLOROFORM SOLUTION OF DITHIZONE BY THE METHOD OF EXTRACTION ASV

A. G. STROMBERG, Yu. A. KARBAINOV, V. F. GRITSAEV

(Presented by the Scientific-Methodological Seminar of the Department of Chemical Technology)

Earlier [1] we proposed a method of extraction amalgam polarography with accumulation (EAP) for the determination of ultramicroimpurities in high-purity materials. Its essence consists in determining traces directly in extracts by the EAP method. The use of such a method in analytical chemistry is, in our view, very promising, since in comparison with ordinary methods involving re-extraction, the analysis is considerably simplified, reagent consumption is reduced, the total analysis time is substantially shortened, and its reproducibility is improved. At the same time, in order to increase the sensitivity of determining elements, various procedures for concentrating microimpurities by means of extraction can be used, for example, extraction from a large volume of solution into a small volume of extractant, etc.

This work is devoted to studies on the determination of traces of impurities by the extraction EAP method directly in a chloroform solution of dithizone. To create a conducting medium, a \(1 \cdot 10^{-4}\%\) solution of dithizone in chloroform was mixed with a \(0.2\,N\) solution of \(\mathrm{NH_4NO_3}\) in methanol. Figure 1 shows the isotherm of the specific electrical conductivity of the chloroform–methanol mixture, recorded at different chloroform contents in the mixture but at a constant electrolyte concentration in it, equal to \(0.1\,N\ \mathrm{NH_4NO_3}\). As is seen from the figure, the electrical conductivity of the indicated mixture at a 50% chloroform content is approximately 5 times lower than the electrical conductivity of a \(0.1\,N\) solution of \(\mathrm{NH_4NO_3}\) in \(\mathrm{CH_3OH}\) and is equal to \(\chi = 0.006\ \Omega^{-1}\mathrm{cm}^{-1}\). This value of electrical conductivity corresponds in order of magnitude to the electrical conductivity of a \(0.1\,N\) solution of KCl in water \((0.012\ \Omega^{-1}\mathrm{cm}^{-1})\) and thus fully satisfies the requirements imposed on polarographic analysis. Proceeding from this, a \(1 \cdot 10^{-4}\%\) solution of dithizone in chloroform

Fig. 1. Conductivity isotherm of a chloroform–methanol mixture at a constant electrolyte concentration in the mixture equal to \(0.1\,N\ \mathrm{NH_4NO_3}\).

was mixed with 0.2 N NH₄NO₃ solution in methanol in a ratio of (1:1). Measurements were carried out in the cell described in [1], on an “OH-101” polarograph. The anode was mercury at the bottom of the electrolyzer. All potential values in the text, tables, and figures are given relative to this reference electrode in the same solution.

The chloroform used in the experiments, of chemically pure grade, was checked for the presence of side decomposition products and purified according to the procedure described in [2]. The degree of purification of the chloroform, as before, was monitored by the value of the residual current. As can be seen from Figs. 3, 4, 5, the maximum sensitivity for the determination of Pb, Cu, Tl, In, Cd, and Bi in a mixture of 1·10⁻⁴% dithizone in chloroform + 0.2 N NH₄NO₃ solution in CH₃OH (1:1) is respectively equal to: 1·10⁻⁷; 2·10⁻⁷; 2·10⁻⁷; 1·10⁻⁶; 1.5·10⁻⁷ and 5·10⁻⁸ g-ion/l at a tooth depth of 10 mm and an electrolysis duration τₑ = 5 min. In [3] we showed that the sensitivity of the ASP method can be increased by more than an order of magnitude by carrying out preliminary concentration of the metal in an Hg drop at a temperature near the boiling point of the solution. The procedure of carrying out preliminary electrolysis on a stationary mercury drop at a temperature near the boiling point of the solution, in combination with the effect of increasing the surface of the ammonium amalgam [4], makes it possible to increase the sensitivity

![Fig. 2 and Fig. 3 graphs]

Fig. 2. Dependence of the depth of the anodic teeth of thallium on the concentration of metal ions in solution. Curve 1—sensitivity 1·10⁻⁸ A/mm, electrolysis time τₑ = 5 min, electrolysis potential φₑ = −1.2 V; curve 2—effect of ammonium amalgam together with temperature, sensitivity 3·10⁻⁸ A/mm, τₑ = 5 min, φₑ = −2.85 V, T = 80°C.

Fig. 3. Dependence of the depth of the anodic teeth of lead on the concentration of metal ions in solution. Curve 1—sensitivity 2·10⁻⁸ A/mm, τₑ = 5 min, φₑ = −1.2 V, temperature 20°C; curve 2—under conditions of elevated temperature (80°C) and the effect of ammonium amalgam, φₑ = −2.85 V; other conditions the same.

of the ASP method by 1.5–2 orders of magnitude. This procedure was used by us to increase the sensitivity of determination of Pb(2+), Cu(2+), Tl(+), In(3+), Cd(2+), and Bi(3+) in the system 1·10⁻⁴% dithizone in chloroform + 0.2 N NH₄NO₃ solution in methanol (1:1). In this case the preliminary electrolysis on an Hg drop was carried out at a potential of −2.85 V and a temperature of 80°C. As can be seen from Figs. 2, 3, and 4, the maximum sensitivity for determination of the above-mentioned ele-

ments, attained in these experiments, became respectively equal to: \(2\cdot10^{-9}\); \(4.5\cdot10^{-9}\); \(5\cdot10^{-9}\); \(6.1\cdot10^{-9}\); \(4\cdot10^{-8}\) and \(2.6\cdot10^{-9}\) g-ion/l \(^{1)}\). Thus, the sensitivity of determination of the above-mentioned elements under conditions of the ammonium amalgam effect at \(80^\circ\)C increased on the average by more than 1.5 orders of magnitude. As was reported earlier, the main factor in increasing the sensitivity of the method of amalgam polarography with accumulation under these conditions is an increase in the electrolysis constant \(K_1\), characterizing the intensity of delivery of the substance to the electrode and depending mainly on the rate of stirring of the solution during preliminary electrolysis. The electrolysis constant \(K_1\) is determined from the relation:

\[ K_1=\frac{\Theta}{\tau_\mathrm{э}\cdot S\cdot C_1}, \tag{1} \]

where \(\Theta\) is the amount of electricity, in coulombs, expended for deposition of the metal on the Hg drop during an electrolysis time equal to \(\tau_\mathrm{э}\); \(\Theta\) is determined from the area under the anodic peak; \(S\) is the surface of the Hg drop, cm\(^2\). \(C_1\) is the concentration of metal ions in solution, g-ion/l.

It is of interest to estimate the change in the electrolysis constant \(K_1\) as a function of various factors: temperature, the ammonium amalgam effect separately, and their combined influence.

In Table 1, in the first column, are given values of the electrolysis constants calculated from the area under the anodic peak by formula (1) given above. These values of the electrolysis constant \(K_1\) were obtained by us under ordinary conditions (temperature \(25^\circ\)C, stirring of the solution during electrolysis with nitrogen in the absence of the ammonium amalgam effect). When preliminary electrolysis is carried out on a stationary Hg drop near the boiling temperature of the solution, the values of \(K_1\), and consequently the sensitivity of the APS method, as is seen from Table 1, increase by 10–15 times in comparison with ordinary conditions (\(T=25^\circ\)C). In this case the dependence of \(K_1\) on temperature is exponential [3], i.e., is expressed by the relation

\[ K_1=Ae^{-\frac{E}{RT}}. \tag{2} \]

The values of \(A\) and \(E\) for each element are also given in Table 1. Under the same conditions of the effect of increase in the surface of the mercury ammonium amalgam, the value \(K_1^{*}\) is determined from the relation:

\[ K_1^{*}=K_1\cdot\frac{S^{*}}{S}, \tag{3} \]

Fig. 4. Dependence of the depth of the anodic peaks of bismuth on the concentration of metal ions in solution. Curve 1—sensitivity \(6\cdot10^{-9}\) a/mm, \(\tau_\mathrm{э}=5\) min, \(\varphi_\mathrm{э}=-1.0\) V; curve 2—under conditions of elevated temperature (\(80^\circ\)C) and the ammonium amalgam effect, \(\varphi_\mathrm{э}=-2.85\) (V), other conditions the same.

\(^{1)}\) The minimum concentration of the element being determined in the cell in our experiments was equal to \(5\cdot10^{-9}\) g-ion/l. A concentration equal to \(2\cdot10^{-9}\) and \(2.6\cdot10^{-9}\) g-ion/l was obtained ...

where \(K_1^*\) may be called the effective electrolysis constant, since it takes into account the factor \(\dfrac{S^*}{S}\), equal to the ratio of the surface area of the Hg drop under the conditions of the ammonium amalgam effect to its initial surface area.

Table 1

Values of the electrolysis constant as a function of various factors

\(^{1)}\) \(A=0.52\cdot10^8;\ E=6.35\cdot10^3\ \mathrm{cal/mol};\)
\(^{2)}\) \(A=1.04\cdot10^8;\ E=6.35\cdot10^3;\)
\(^{3)}\) \(A=5.95\cdot10^8;\ E=7.4\cdot10^3;\)
\(^{4)}\) \(A=1.04\cdot10^8;\ E=6.35\cdot10^3;\)
\(^{5)}\) \(A=8.67\cdot10^9;\ E=9.75\cdot10^3;\)
\(^{6)}\) \(A=1.22\cdot10^{10};\ E=9.35\cdot10^3\ \mathrm{cal/mol}.\)

We have shown experimentally that the ratio \(\dfrac{S^*}{S}\) at 70 and 80°C is equal, respectively, to 7.3 and 5. The values of \(K_1^*\), calculated by us from formula (3), are taken as the \(K_1\) values of the electrolysis constants given in Table 1, column 1, and are also presented in Table 1 (see column 3). It is evident from the table that, solely on account of the ammonium amalgam effect, the value of \(K_1\), and consequently the sensitivity of the ASV method, can be increased by more than an order of magnitude. The change in the electrolysis constant under the combined influence on \(K_1\) of temperature and of the ammonium amalgam effect should evidently obey the relation:

\[ K_1^* = A \cdot e^{-\frac{E}{RT}} \cdot \frac{S^*}{S}. \tag{4} \]

The values of the electrolysis constants calculated from formula (4) are given in Table 1 (see column 4). In the same table (see column 5) are given the values of the electrolysis constants obtained under the conditions of the combined influence of temperature and the ammonium amalgam effect and calculated from the area under the anodic peaks. As is evident from the table, the values of \(K_1\) calculated from formula (4) and determined from the area under the anodic peaks agree satisfactorily with one another. The reproducibility of \(K_1\) under the conditions of the ammonium amalgam effect and temperature is on average \(\pm 10\%\).

The results presented above from the study of the anodic peaks of Pb(2+), Cu(2+), Tl(+), Cd(2+), In(3+) and Bi(3+) show that there is

determined by extrapolating the corresponding curves of the dependence of the anodic-peak depth on the concentration of metal ions in solution, taking the minimum peak depth to be 10 mm at the same instrumental sensitivity.

there is a quite real possibility of determining \(10^{-6}\)—\(10^{-7}\%\) impurities in high-purity materials by the method of extraction ASV. To confirm this conclusion we used a \(1 \cdot 10^{-4}\%\) solution of dithizone in chloroform for the determination of lead and cadmium in chemically pure LiCl. For this purpose, the completeness of extraction of Pb(2+) and Cd(2+) ions from aqueous solution as a function of its pH was studied on synthetic mixtures. The data are given in Fig. 5. It follows from the figure that the optimum pH value of the aqueous solution, at which the maximum recovery occurs, is \(pH = 6.7\) in the case of lead and \(pH = 9\) in the case of cadmium, with extraction percentages of 65 and 80%, respectively. The procedure for determining lead in LiCl is as follows: a 10 g weighed portion of the salt is dissolved in 50 ml of 0.2 N ammonia solution. To this mixture are added 5 ml of a 10% KCN solution and 0.1 g of \(\mathrm{Na_2SO_3}\). The pH of the solution is brought to 6.7. The mixture is transferred to a separatory funnel, 5 ml of a \(1 \cdot 10^{-4}\%\) dithizone solution in \(\mathrm{CHCl_3}\) is added, and extraction is carried out twice for 2 min. After extraction, 2.5 ml is taken from the total volume of the chloroform phase, equal to 10 ml, and poured into the electrolyzer for polarography. To this is added 2.5 ml of 0.2 N \(\mathrm{NH_4NO_3}\) solution in \(\mathrm{CH_3OH}\); the solution is stirred for 10 min with nitrogen (for the purpose of removing oxygen), and polarography is performed. The preliminary electrolysis potential is \(-1.5\) (V), the electrolysis time is 5 min, and the sensitivity is \(1 \cdot 10^{-8}\) A/mm. The amount of lead contained in the LiCl sample (chemically pure) is \((6 \div 8) \cdot 10^{-7}\%\). The duration of one analysis is 15–20 min.

Fig. 5. Percentage extraction as a function of the pH of the aqueous solution. 1—lead, 2—cadmium.

Table 2

Results of analysis of nanogram contents of lead and cadmium by extraction ASV

The procedure for determining cadmium in chemically pure LiCl is as follows: a 10 g weighed portion of the salt is dissolved in 50 ml of tridistilled water. To the solution are added 5 ml of 20% tartaric acid and 1 ml of 20% hydroxylamine hydrochloride. The pH of the solution is brought to \(pH = 9.0\). The solution is transferred to a separatory funnel; to it is added

5 ml of a \(1\cdot 10^{-4}\%\) solution of dithizone in \(\mathrm{CHCl_3}\) is added, and extraction is carried out twice (for 2 min each). From the total volume of extractant, equal to 10 ml, a 2.5-ml aliquot is taken and poured into the electrolyzer for polarography. To this is added 2.5 ml of 0.2 N \(\mathrm{NH_4NO_3}\) solution in \(\mathrm{CH_3OH}\); nitrogen is passed through for 10 min, after which polarography is performed. The electrolysis time is 5 min; the electrolysis potential is \(-1.5\) V. The cadmium content in LiCl (chemically pure) is equal to \((1\div 2.7)\cdot 10^{-7}\%\). The duration of one determination is 15–20 min. A similar result was obtained from a 1-g weighed portion of the salt and from carrying out preliminary electrolysis at a potential of \(2.85\) V and a temperature of \(80^\circ\mathrm{C}\), i.e., under conditions of the ammonium-amalgam effect and temperature. The results of the analysis of lead and cadmium in synthetic mixtures are presented in Table 2.

Conclusions

1. The possibility has been demonstrated of determining \(10^{-6}\div 10^{-7}\%\) impurities of copper, lead, thallium, bismuth, indium, and cadmium directly in a nonaqueous mixture of \(1\cdot 10^{-4}\%\) dithizone in \(\mathrm{CHCl_3}+0.2\) N \(\mathrm{NH_4NO_3}\) in \(\mathrm{CH_3OH}\) by the method of extraction ASV.

2. With the aid of the ammonium-amalgam effect at a solution temperature of \(80^\circ\mathrm{C}\), the sensitivity of determining the six elements indicated above has been increased by more than 1.5 orders of magnitude.

3. The practical application of the extraction-amalgam-polarographic method (the extraction ASV method) has been shown by the example of analyzing the reagent LiCl (chemically pure) for the content of lead and copper (of the order of \(10^{-6}\)—\(10^{-7}\%\)).

REFERENCES

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3. Yu. A. Karbainov, A. G. Stromberg. Zavodskaya laboratoriya (in press).

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