On the Determination of Tellurium in Tellurates

I. P. Onufrienok, V. B. Sokolovich

Presented by Professor, Doctor of Chemical Sciences A. G. Stromberg

The most common methods for determining tellurium are based on its separation in elemental form by reduction of tellurites in hydrochloric-acid medium with stannous chloride, hydrazine salts, sodium sulfite, and others, followed by gravimetric completion of the determination [1, 2]. Some titrimetric methods are also known [2]. From solutions of tellurates (especially at low concentrations), most reducing agents do not precipitate elemental tellurium. It is necessary first, by some means, to reduce tellurates to tellurites [4]. By the method proposed by one of us, using milk sugar as the reducing agent in an alkaline medium [3], elemental tellurium can be separated both from tellurite solutions and from tellurate solutions. However, work under alkaline conditions is not always convenient, especially in the analysis of natural and technical materials.

For the separation of tellurium from tellurates we used stannous chloride in hydrochloric-acid medium, first reducing tellurates to tellurites with concentrated hydrochloric acid, potassium iodide, and sodium sulfite. In all cases, however, the precipitate of elemental tellurium proved to be contaminated with occluded tin, and it had to be reprecipitated with milk sugar in an alkaline medium.

The investigations carried out showed that in moderately acidic media, produced by certain buffer mixtures or organic acids, sodium sulfite can separate elemental tellurium of sufficient purity directly both from tellurite solutions and from tellurate solutions. In the presence of a mixture of ammonium chloride, nitrate, and citrate, the separation of tellurium from tellurate requires, with heating, about two hours. In this case, apparently, first the pH of the solution decreases as a result of hydrolysis, and then reduction by sulfite, and consequently reduction of tellurate, becomes possible.

The time required for tellurium separation is markedly shortened by replacing ammonium citrate with citric acid and certain other organic acids. The use of organic acids weaker than or not greatly exceeding sulfurous acid in strength \((K_1 = 1.3 \cdot 10^{-2})\) makes it possible to maintain the required acidity of the reaction mixture.

The following acids were tested: citric \((K_1 = 8.40 \cdot 10^{-4})\), malonic \((K_1 = 1.49 \cdot 10^{-3})\), monochloroacetic \((K_1 = 1.40 \cdot 10^{-3})\), \(d\)-tartaric \((K_1 = 1.04 \cdot 10^{-3})\), malic \((K_1 = 4.00 \cdot 10^{-4})\), oxalic \((K_1 = 3.8 \cdot 10^{-2})\).

Most rapidly (almost within 25–30 min), with good coagulation of the precipitate, the quantitative separation of tellurium occurred in the presence of oxalic acid. It proved possible to use oxalic acid for the separation of tellurium from solutions of alkali-metal tellurates both in the absence and in the presence of copper. In the latter case, upon addition of oxalic acid a precipitate of copper oxalate is formed, which may, however, be obtained sufficiently coarse, readily washable, and almost not occluding tellurium. Copper is separated as the oxalate, while tellurium is then reduced in the same way as in solutions of alkali-metal tellurates. In the presence of iron, oxalic acid cannot be used, since as iron(III) is reduced to iron(II), a precipitate of iron(II) oxalate is gradually formed, with which tellurium is coprecipitated.

In this case, and also when copper and iron are present together, the reduction is carried out with other organic acids. The best results (among the acids tested by us) were obtained using monochloroacetic acid. The separation of tellurium in the presence of citric and tartaric acids required more time (about one hour). Malonic and maleic acids proved inconvenient in practice.

Determination of tellurium in alkali-metal tellurates

To a tellurate solution containing about 0.01 g of tellurium, in a conical flask with a stopper, are added 2 g each of previously dissolved ammonium chloride, ammonium nitrate, and oxalic acid (or another of the acids indicated above). The volume of the solution is brought to 60–80 ml (from a smaller volume ammonium oxalate may precipitate). To the cold reaction mixture, with gradual shaking by circular motions, about four grams of crystalline sodium sulfite are added; the solution is kept at gentle boiling for 25–30 min (when tartaric or citric acid is used, about one hour) until the precipitate of elemental tellurium coagulates. Coagulation is accelerated by adding, at the end of heating, several crystals of dry sulfite.

After the reaction mixture has cooled, the tellurium precipitate is filtered through a plug of glass wool and asbestos and washed with hot 2% ammonium sulfate solution. The determination is completed titrimetrically by the method of Onufrienok and Aksenenko [3]. The results of the determinations are given in Table 1.

Table 1

Determination of tellurium in tellurates in the presence of copper and iron

To a tellurate solution in a volume of 60–80 ml, containing about 0.01 g of tellurium, 0.5 g of copper, and 0.5 g of iron, 2 g each of chloride …

and ammonium nitrate and monochloroacetic acid. Crystalline sodium sulfite is gradually added to the cold reaction mixture with shaking until the solution is decolorized. Each new portion is added after the preceding one has dissolved. Decolorization is preceded by the formation of a brownish-red coloration. Addition of sulfite is stopped after the formation of a cherry-red coloration that does not disappear for 1–2 minutes. The consumption of sulfite is 6.0–6.5 g. The colorless or cherry-red solution is carefully boiled for 30–40 minutes.

On heating, the reaction mixture first becomes decolorized, then turns yellow while remaining transparent, after which it becomes cloudy with the separation of a grayish-black precipitate of elemental tellurium. For better coagulation, several crystals of sulfite are added toward the end of heating. The precipitate is filtered off quickly from the hot solution (on cooling, oxidation of copper occurs); the filtrate should be colorless or faintly bluish. The precipitate is washed several times with a hot 2% solution of ammonium sulfate until the reaction for copper with \(K_4[Fe(CN)_6]\) ceases. The determination is completed titrimetrically [3].

By the described procedure, tellurium was determined in artificial mixtures prepared from copper and iron salts with additions of known amounts of telluric acid, as well as in solutions of tellurates of alkali metals (Table 2).

Table 2

The time required for a determination is about two hours. The relative error is within \(\pm 2.0\%\).

Conclusions

1. A method is proposed for determining tellurium in solutions containing tellurates of alkali metals, as well as in solutions containing tellurate ions together with copper and iron ions.

2. At low acidity of the solutions, the \(NO_3^-\) ion does not interfere with the precipitation of elemental tellurium by sodium sulfite.

Literature

1. Analysis of Mineral Raw Materials. Collection. Goskhimizdat, 1956.
2. Hildebrand V. F., Lendel G. E., Bright G. A., Hoffman D. I. Practical Manual of Inorganic Analysis. Goskhimizdat, 1957.
3. Onufrienok I. P., Aksenenko V. M. Journal of Analytical Chemistry, 13, issue 1, pp. 119–122, 1958.
4. Dnaria M., Mariko J. Chem. Soc. Pure Chem. Sec., 79, 696, 1958.