USE OF EXTRACTION IN THE APN METHOD FOR THE ANALYSIS OF HIGH-PURITY MATERIALS.

MAIN TYPES OF EXTRACTANTS

Yu. A. Karbainov

(Presented by the Department of Analytical Chemistry)

The further progress of modern science and technology requires the development and introduction of the newest methods for the investigation and analysis of materials. The methods must be accurate, rapid, selective, and amenable to automation. These requirements are met by extraction methods for the separation of elements.

Extraction methods are especially convenient for the analytical chemist, since they are suitable for both micro- and macroconcentrations; the apparatus used in this case is no more complicated than a separatory funnel, and only a few minutes are required to carry out the extraction process itself. Extraction techniques are conveniently combined with various methods of determination, in particular with photometric, spectral, and polarographic methods.

The combination of extraction with the method of amalgam polarography with accumulation on a stationary mercury electrode is one of the promising approaches to the analysis of high-purity materials, if one takes into account that the APN method makes it possible to determine \(10^{-7}\) percent or less of impurities. The most important area of application of extraction in trace analysis is the determination of microcomponents, which is usually completely impossible without separation from the macrocomponent—the matrix. For example, analysis of high-purity indium for the content of copper, lead, bismuth, and zinc by the APN method becomes practically impossible without preliminary separation of the excess indium by extraction.

According to literature data [1–3], good separation of indium (\(\sim 99\%\)) from copper, lead, zinc, and bismuth can be achieved by extracting it with diisopropyl or diethyl ether from \(5N\) HBr. This was confirmed in work [4] during the development of an amalgam-polarographic procedure for the simultaneous determination of traces of copper, lead, bismuth, and zinc in indium, with separation of the excess indium by extraction with diethyl ether and with subsequent determination of the above-mentioned impurities against a background of \(0.5\,M\) ethylenediamine \(+\) \(0.2\,M\) KOH. The maximum sensitivity of the procedure is: for copper—\(4 \cdot 10^{-7}\%\); for lead—\(2 \cdot 10^{-7}\%\); for bismuth—\(4 \cdot 10^{-7}\%\); for zinc—\(2 \cdot 10^{-7}\%\), with an electrolysis time of 30 minutes. The duration of analysis of an indium sample (with two parallel and one blank determinations) is 5–6 hours. Subsequently this procedure was improved [5]. In particular, the authors replaced the alkaline solution of ethylenediamine (the supporting electrolyte for polarography), since it proved unsuitable for systematic

analyses; ethylenediamine requires prolonged purification by electrolysis; moreover, it decomposes and, in this connection, creates considerable difficulties when working with it. A solution of 0.1 M tartaric acid + 0.25 M ammonium acetate was used here as the supporting electrolyte. So that indium would not interfere with the determination of lead and copper, before the In peak was reached the potentiometric drum was stopped in order to dissolve the indium completely, after which the peaks of lead and copper were recorded.

Preliminary separation of indium from accompanying impurities by extraction with diisopropyl ether from 5 M HBr was also used by Sinyakova et al. [6—7] in developing an amalgam-polarographic procedure for determining zinc, cadmium, lead, and copper in indium and its salts, with subsequent polarography in alkaline supporting electrolytes.

Separation of In from any amounts of various elements (zinc, copper, lead, cadmium, etc.), with which indium is often encountered in natural objects and industrial products, can also be carried out by extraction with a 20% solution of di-(2-ethyl)-hexylphosphoric acid in kerosene [8] from sulfuric, nitric, and perchloric acid solutions over a comparatively wide acidity range, while the listed elements remain in the aqueous phase. Quantitative separation of indium can also be achieved by extracting it from hydrochloric acid solutions (pH 4—5) with n-butanol and with a solution of di-n-butyl phosphate in CCl₄ [9].

In [3] a method was proposed for concentrating indium, based on its coprecipitation with CoS, followed by separation of indium from SO₄²⁻ by extraction with dithizone in the presence of sulfosalicylic acid (pH 4.8÷5.2). The method was used to separate indium from gallium.

S. I. Sinyakova [10] showed that extraction of microgram amounts of indium with diethyl ether from 4.5 M HBr solutions does not proceed quantitatively, and the presence of cadmium in the solution lowers the degree of extraction of indium, the more so the higher the cadmium content.

In a number of cases [11—16], on the contrary, separation of the microcomponent from the matrix is the most expedient. Good separation of thallium from indium is achieved by extracting it with diisopropyl or diethyl ethers from 6 N HBr [1—2].

In developing an amalgam-polarographic procedure for determining thallium in high-purity indium [12], the comparatively scarce diisopropyl ether was successfully replaced by the more readily available diethyl ether. In this case thallium was extracted in the trivalent state (oxidation of Tl(+) → Tl(3+) was carried out with a 3% solution of H₂O₂). To remove indium that had passed into the ether layer, a twofold re-extraction of the latter was carried out. Determination of thallium by the APN method is more conveniently performed as Tl(+). Its reduction to the monovalent state was carried out with a solution of hydrochloric hydrazine in 6 N HCl medium, followed by polarography in a supporting electrolyte of 0.1 N NH₄Cl + 0.05 M complexon III. The maximum sensitivity of the procedure for thallium is \(2 \cdot 10^{-6}\%\) from an indium sample of 0.2—0.5 g.

The method of amalgam polarography with accumulation was also used in developing a procedure for determining traces of germanium in high-purity indium [14], with preliminary separation of germanium by extraction with carbon tetrachloride from 9 N H₂SO₄ medium, followed by re-extraction of germanium with water. The maximum sensitivity of the procedure for germanium is \(1 \cdot 10^{-6}\%\), with a determination error of 6÷10% rel. The possibility of quantitative separation of germanium from the matrix by its extraction with carbon tetrachloride from 9 N H₂SO₄ medium was indicated in [17]; germanium is also extracted quantitatively by carbon tetrachloride from 6—8 N HCl [18—20].

The combination of extraction with the method of amalgam polarography with accumulation proved successful in the development of a number of highly sensitive procedures for determining ultramicroconcentrations of antimony in high-purity tin [13, 15—16], as well as gallium in high-purity aluminum [11]. According to the literature data [21—23], antimony is separated from a number of elements (Sn (4+), Cu, Pb, etc.) in the form of Sb(5+) by ethyl acetate from a 6 \(N\) HCl solution; this, in particular, was also used in work [13]. Here, after completion of the extraction, pentavalent antimony was reduced to the trivalent state (sodium hypophosphite was used for these purposes). The determination of antimony was carried out against a background of 0.2 \(N\) HCl. The maximum sensitivity of the procedure under the developed conditions is \(1 \cdot 10^{-5}\%\).

Amalgam-polarographic procedures have also been developed for determining antimony in high-purity tin with separation of antimony from the matrix by extraction with diisopropyl ether [15] and alkylphosphoric acids [16]. Since antimony is extracted in the pentavalent state, whereas it is more convenient to polarograph it in the form of Sb(3+), in these works studies were carried out to determine the completeness of reduction of Sb(+5) to Sb(3+) by a solution of hydrochloric-acid hydrazine in 1 \(N\) HCl. It was shown that the percentage reduction of antimony under these conditions is close to 99–100%. To accelerate the dissolution of metallic tin in concentrated H\(_2\)SO\(_4\), the authors [15—16] used CoCO\(_3\). The sensitivity of the procedures for antimony under the developed conditions is \(1 \cdot 10^{-5}\%\), and when a film electrode is used instead of a stationary mercury drop it is \(2 \cdot 10^{-6}\%\).

Vasil’eva and Vinogradova [11] applied the APN method to the determination of ultralow amounts of gallium, zinc, and cadmium in especially pure aluminum. To separate gallium from the matrix, extraction of it with ethyl ether from 6 \(N\) HCl was used. The determination of gallium in two weighed portions with two blank experiments requires 8 hours. The sensitivity of the procedure (from an aluminum weighed portion of 2 g) is \(1 \cdot 10^{-5}\%\).

Amalgam-polarographic procedures have also been developed for determining thallium in rocks [10] and copper in the phosphor L-34 [24], with preliminary separation of thallium and copper by extraction. Extraction of Tl(3+) was carried out from 3 \(N\) HBr with isopropyl ether (bromine was used for oxidation Tl(3+)→Tl(3+)). The determination of thallium (Tl(+)) was carried out against a background of 0.2 \(N\) HCl (a 1% solution of sulfuric-acid hydrazine was used for reduction Tl(3+)→Tl(+)). Extraction of copper was carried out from a hydrochloric-acid medium (pH 4.5) with a solution of acetylacetone in chloroform (1:3), with subsequent determination of copper against a background of 0.2 \(M\) ammonium tartrate solution. The sensitivity of the procedure is \(1 \cdot 10^{-6}\%\), and the duration (three parallel and one blank samples) is 6–7 hours.

In a number of cases the use of extraction in the APN method proved effective for purification of background solutions [11, 25].

The combination of extraction with polarography has the following advantages: high selective ability, the possibility of working both with macro- and microconcentrations of the substance, etc. An undesirable operation in this combined method of analysis is re-extraction, since this process complicates and lengthens the analysis and, in addition, leads to additional losses of substance and errors of analysis. Recently a fundamentally new and promising direction has begun to develop in the use of extraction methods in polarographic analysis, the essence of which consists in determining traces of impurities directly in the extractants. The promise of this direction lies in the elimination of operations

reextraction, which leads to a significant reduction in the total analysis time, a reduction in reagent consumption, etc. In addition, the elimination of such operations as reextraction, destruction of the complex, and removal of the organic phase, which are directly or indirectly associated with losses, should lead, besides speeding up the analysis, also to an improvement in its reproducibility.

Let us dwell on the use of the indicated method in classical polarography. It has been established [26] that a chloroform solution of copper oxinolate in a mixture with 0.1 M NaClO₄ solution in CH₃OH (in a ratio of 1:10) gives a clear polarographic wave with $\varphi_{1/2} = -0.43$ (volts).

In [27], instead of the usually employed reextraction and destruction of the complex, it was proposed to use, as the supporting background, a homogeneous ternary system, one of the components of which is the organic substance—the extract. The authors use extraction of diethyldithiocarbamates (DDC) of Tl, Pb, and Cu with chloroform from a citrate buffer solution at pH 11, after which the extract containing the DDC complexes of Tl, Pb, and Cu was mixed with methyl cellosolve and water in a ratio of 25:56:19 and analyzed. Values of $\varphi_{1/2}$ are given for the indicated elements.

In [28], the polarographic behavior of DDC complexes of Bi, Cu, and Pb was studied on a background containing 5 ml CHCl₃ + 1.7 ml conc. HCl (H₂SO₄, HClO₄), 11.3 ml methyl cellosolve, and 2 ml H₂O. It was shown that the wave height is proportional to the concentration of metal ions in the solution in the range $1 \cdot 10^{-3}$—$1 \cdot 10^{-5}$ g-ion/l. Values of $\varphi_{1/2}$ and diffusion-current constants for Bi, Cu, and Pb under the indicated conditions are presented. The practical application of the method for analytical purposes was demonstrated by the example of the determination of $5 \cdot 10^{-4} \div -5,10^{-5}$% Pb in LiCl reagent. More promising with respect to sensitivity is the combination of extraction with amalgam polarography with accumulation (APN) [29].

LITERATURE

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