GRAVIMETRIC DETERMINATION OF MERCURY AS SULFIDE

I. P. ONUFRIENOK, Z. N. TERESHCHOVA

(Presented by Prof. A. G. Stromberg, Doctor of Chemical Sciences)

The gravimetric method for determining mercury as sulfide has long been known [1]. According to this method, mercury is precipitated with hydrogen sulfide, filtered through a weighed Gooch crucible, the free sulfur is washed from the precipitate with carbon disulfide, ether, and alcohol, and then it is dried and weighed. In addition, the work of Taimni and Salaria [2] should be noted; they free the mercury sulfide precipitate from sulfur by washing with ammonium sulfide freshly prepared under special conditions.

We found that, when mercury(II) thiosulfate is treated with hydrogen peroxide in alkaline solution, all the sulfur is oxidized to sulfate ion, with the exception of the sulfur required to form mercury sulfide. Mercury is so firmly bound to sulfur in mercuric sulfide that even treatment with a large amount of perhydrol in alkaline medium does not cause oxidation of the sulfur. On this basis we developed a method for the gravimetric determination of mercury as sulfide.

To test the proposed method, a 0.01 N solution of mercury(II) nitrate was prepared, and determinations of mercury were compared by three methods.

Aliquot portions of this solution, 10 ml each, were titrated with ammonium thiocyanate [3]; identical portions were taken for gravimetric determinations of mercury as sulfide. In one series of experiments, the mercuric sulfide precipitate was freed from sulfur with carbon disulfide, ether, and alcohol, and in the other by means of hydrogen peroxide according to the method described below. The data from these experiments are given in Table 1.

Table 1

The proposed method of desulfurizing a precipitate containing mercury sulfide and sulfur was also tested in the analysis of natural ore samples containing mercury. The results of the comparison are given in Table 2.

Table 2

Comparing the results of the experiments carried out, it may be said that the gravimetric method for determining mercury as sulfide, with ashing of the precipitate by means of hydrogen peroxide in an alkaline medium, is entirely reliable. The use of hydrogen peroxide considerably simplifies the analytical procedure, especially in comparison with the method of extracting sulfur from the precipitate by means of carbon disulfide.

The proposed method for the gravimetric determination of mercury in natural samples is as follows: a weighed portion of ore, in an amount of 0.5000–1.0000 g, is placed in an even layer in a porcelain boat, the bottom of which has previously been covered with a 1-mm layer of calcium carbonate. The weighed portion is covered on top with iron powder, and then with additional powdered calcium carbonate. The boat is placed in a tube of refractory glass about 30 cm long. The end of the tube is drawn out and bent in the form of a long beak, which is lowered into a test tube containing 2 ml of nitric acid, sp. gr. 1.40. Carbon dioxide is passed through the tube. When the tube is heated, the mercury is driven off from the sample and then, with the aid of a gas burner, the volatilized mercury is condensed in the drawn-out end of the tube. Heating is then stopped, while carbon dioxide is passed through the tube for another 10 minutes. After this, the nitric acid is carefully drawn from the test tube into the tube in order to dissolve the mercury deposit in the tube. In the same way, the end of the tube is washed with water acidified with a small amount of nitric acid. The resulting nitric-acid solution of mercury is diluted with water to 150 ml, and the mercury is precipitated with hydrogen sulfide.

The precipitate of mercuric sulfide is filtered through a dense filter, washed with hydrogen-sulfide water acidified with sulfuric acid, then washed on the same filter with a freshly prepared 2.5% solution of ammonium sulfide and, finally, with a 2% solution of sodium sulfate. The washed precipitate of mercuric sulfide on the filter is dissolved in the smallest possible amount of warm 2.5% potassium sulfide solution; the filter is washed with the same solution, after which the filter is washed again with a 2% solution of sodium sulfate. To the filtrate, collected in a 100–150 ml conical flask, 5 ml of 25% ammonium hydroxide hydrate is added and perhydrol is added dropwise until bubbles appear throughout the whole mass of liquid. After boiling until the small bubbles disappear, the solution is neutralized to methyl red with sulfuric acid to an acid reaction. The precipitate of mercuric sulfide is allowed to settle for 15–20 minutes and is filtered with the aid of a vacuum through a weighed No. 4 glass filter. The precipitate on the filter is washed with hot water, then with alcohol, dried at 105–110° and weighed as mercuric sulfide.

In addition to the above, experiments were carried out on the extraction of mercury from natural compounds not by the distillation method, but by a wet method with the aid of potassium polysulfide. It is known that mercuric sulfide is soluble in potassium sulfide and polysulfide, whereas metallic mercury is insoluble in sul-

potassium sulfide, but is soluble in potassium polysulfide [4]. In natural samples, in addition to mercury sulfide (cinnabar), free mercury may also be present; therefore, in the extraction experiments we used potassium polysulfide. A 1.0000 g weighed portion of ore was treated with hydrochloric acid (1:1) with heating; then the precipitate, filtered off and washed with water, was washed back with water into the flask, where dissolution took place, and was treated with 20 ml of 2.5% potassium polysulfide while boiling for 40 minutes. Under these conditions, cinnabar and metallic mercury were supposed to form mercury thiosalt with potassium polysulfide. The thiosalt was then filtered through the same filter; in the filtrate, after the addition of ammonium hydroxide hydrate, the sulfur was oxidized with perhydrol and mercuric sulfide was precipitated; after acidifying the solution to pH 4 with sulfuric acid using methyl red as indicator, it was filtered through a weighed glass crucible, washed, dried, and weighed. Quantitative experiments showed that with such treatment mercury is not completely extracted from natural samples: when the mercury content in sample No. 1 was 1.34–1.32%, 1.14–1.17% was extracted. This simple method of extracting mercury from natural samples may be recommended only for qualitative tests, especially under field conditions.

Conclusions

1. The work has shown the full possibility of simplifying the method for determining mercury in the form of sulfide by replacing carbon disulfide with the more readily available perhydrol.

2. The possibility has been shown of using potassium sulfide and polysulfide for qualitative tests for mercury in natural samples.

Literature

1. Stukalov M. M. Mercury. In: Analysis of Mineral Raw Materials, ONTI, Leningrad, III, 1936.

2. Taimni I. K., Salaria G. B. S. Analyt. chim. acta, 12, 519, 1955.

3. Kolthoff I. M. and Sandell E. B. Quantitative Analysis, GONTI, Moscow, 509, 1938.

4. Treadwell F. P., Hall W. T. Qualitative Analysis, Goskhimizdat, Moscow–Leningrad, 125 and 138, 1946.