Reduction of Selenium (IV) and Tellurium (IV) by Pyrogallol in a Sulfuric Acid Medium
Abstract Generated abstract
This paper addresses the quantitative determination of selenium and tellurium in technical alloys containing lead and antimony, where earlier alkaline reduction procedures are hindered by lead precipitation or incomplete reduction. It evaluates reducing agents for acidic conditions and proposes pyrogallol in sulfuric acid as a reagent for reducing selenium(IV) and tellurium(IV) to the elemental state, with procedural conditions designed to avoid colloidal tellurium and obtain filterable precipitates. The authors describe preparation of standard solutions, reduction, filtration, oxidative dissolution, and back titration with dichromate and Mohr’s salt, and recommend prior removal of lead as sulfate for alloy analysis. Antimony is reported not to interfere under the sulfuric acid conditions tested.
Full Text
REDUCTION OF SELENIUM (IV) AND TELLURIUM (IV) BY PYROGALLOL IN SULFURIC ACID MEDIUM
E. S. Novikova, A. N. Moskalchuk
(Presented by the seminar of the Department of Inorganic Chemistry, CTF)
Technical alloys of selenium and tellurium contain large amounts of lead and antimony; therefore, in order to apply to technical alloys the method of quantitative determination of selenium and tellurium by reducing them to the metals, it was necessary to determine the influence of impurities of the above-mentioned metals.
As studies have shown, when the lead content in the solution is high, quantitative reduction of tellurium and selenium by the method proposed by I. P. Onufrienok [1] is impossible. This is explained by the fact that, to keep large amounts of lead in solution, a large amount of alkali is required, and this leads to incomplete precipitation of selenium and tellurium. If the alkali is taken in a slight excess, lead forms a gelatinous precipitate that hinders the separation of the reduced selenium and tellurium. These circumstances made it necessary: first, to separate lead beforehand by the action of sulfuric acid [2]; second, in order to reduce the consumption of reagents and the time required for carrying out the analysis, to search for a reducing agent in an acidic medium; and third, to refine the optimal conditions for the action of this reducing agent.
Investigation of a large number of different reducing agents showed that one representative of the phenol series—pyrogallol—when reducing selenium (IV) and tellurium (IV) in sulfuric acid solution gives a quantitative yield of selenium and tellurium. The mechanism of reduction by pyrogallol in sulfuric acid medium has not been clarified. This question may serve as the subject of an independent investigation. Specially designed experiments showed that the presence of antimony in the sulfuric acid solution does not distort the results of reduction of selenium and tellurium. The elucidation of the optimal conditions for reduction of selenium (IV) and tellurium (IV) by pyrogallol in sulfuric acid medium was carried out on standard solutions.
Preparation of Standard Solutions
A weighed portion of metallic tellurium, 0.1000 g, is placed in a 25–50 ml beaker covered with a watch glass, moistened with several drops of water, 15 drops of concentrated nitric acid and the same amount of concentrated hydrochloric acid are added, and the mixture is gently heated until the metal dissolves completely. The watch glass is then rinsed with water, washing drops of splashed acid into the beaker in which the dissolution was carried out; 2.5 ml of sulfuric acid (1:1) is poured into the same beaker, and the mixture is heated on a water bath until the nitric and hydrochloric acids are removed. During evaporation, $\mathrm{TeO_2}$ and some amount of tellurous acid are precipitated. To bring them into solution, 2–3 ml of water are added and the mixture is again heated until the precipitate dissolves. The contents of the beaker are quantitatively transferred to a 500 ml volumetric flask, diluting—
add another 2.5 ml of sulfuric acid (1:1), dilute with water to the mark, and mix. The standard selenium solution is prepared analogously.
Reduction of Selenium and Tellurium by Pyrogallol
10 ml of the standard solution are placed in a 150-ml conical flask; approximately one gram of pyrogallol and 4 ml of concentrated sulfuric acid are added to the same flask.
During reduction, tellurium shows a strong tendency to form colloidal solutions. This can significantly distort the results of the determination. To prevent the formation of a colloidal solution of metallic tellurium, its reduction must be carried out with moderate heating. The prepared mixture is carefully heated to a temperature close to the boiling point of the solution and kept in this state for 6–10 min, after which the solution is allowed to boil. During heating, the contents of the flask are mixed by a rotary motion. Under these conditions, the precipitate of elemental selenium and tellurium forms, changing into a coarse-grained form more convenient for filtration. The solution with the precipitate, without cooling, is filtered under vacuum through a funnel with an asbestos pad, washed 3–4 times by decantation and 3–4 times on the filter with a hot 2% sodium sulfite solution. The washed precipitate, together with the asbestos pad, the glass-wool column remaining in the stem of the funnel, is removed from the funnel with a glass rod and placed in the flask in which the reduction was carried out.
Into the same flask, through the funnel, in order to dissolve traces of reduced selenium or tellurium on its surface, add 2 ml of concentrated sulfuric acid and 10 ml of titrated potassium dichromate solution. When the last drops of the mixture of sulfuric acid and potassium dichromate have drained from the funnel, the funnel is rinsed several times with small portions of water, collecting the wash liquid in the same flask. The solution with the precipitate is heated until the precipitate is completely dissolved, cooled, 20 ml of 0.1 N Mohr’s salt solution are added, and the excess reducing agent is determined by titration with 0.1 N potassium dichromate solution. The amounts of selenium and tellurium are calculated by the formulas
\[ \mathrm{Te}_2 = \frac{21.27\left(H_{\mathrm{K_2Cr_2O_7}}\cdot V_{\mathrm{K_2Cr_2O_7}} - H_{\mathrm{Mohr\,salt}}\cdot V_{\mathrm{Mohr\,salt}}\right)}{1000}, \]
\[ \mathrm{Se}_2 = \frac{19.74\left(H_{\mathrm{K_2Cr_2O_7}}\cdot V_{\mathrm{K_2Cr_2O_7}} - H_{\mathrm{Mohr\,salt}}\cdot V_{\mathrm{Mohr\,salt}}\right)}{1000}. \]
When carrying out quantitative determinations of Se and Te in lead alloy, it is recommended, after decomposition of the alloy in tartaric and nitric acids, to separate lead from the solution by precipitation with sulfuric acid (1:3) while boiling and stirring until visibly complete precipitation [2]. The precipitate of lead sulfate is filtered through a glass-wool pad with asbestos and washed with a 2% sulfuric acid solution. The resulting solution is evaporated to a volume of 10–15 ml, after which the reduction of selenium and tellurium is carried out according to the procedure described above.
Conclusions
A method has been proposed for the quantitative reduction of Se and Te by pyrogallol in a sulfuric-acid medium.
References
- I. P. Onufrienok, V. M. Aksenenko. Journal of Analytical Chemistry, XIII, 1, 1958.
- I. P. Onufrienok, V. B. Sokolovich. Izv. TPI, vol. III, TGU Publishing House, Tomsk, 1961.