Accurate Separation of Precipitated Mercuric Sulfide and Sulfur in the Gravimetric Determination of Mercury ~
EARLE R. CALEY AND iM.GILBERT BURFORD‘ Frick Chemical Laboratory, Princeton University, Princeton, N. J.
T
at 110’ C., and weigh. Replace the crucible in the holder without
HE common method for the determination of mercury
as the sulfide suffers from the disadvantage that the precipitate is often contaminated with sulfur. The usual practice is to attempt to remove the free sulfur from the precipitate by various methods before weighing. Hot extraction with carbon disulfide is considered one of the best methods (3). Another is the successive washing of the precipitate with hydrogen sulfide water, hot water, alcohol, carbon disulfide, alcohol, and ether (1). All such procedures involve an element of uncertainty because of the difficulty of knowing when the removal of sulfur is complete, particularly since some forms of sulfur are but slightly soluble in certain of these solvents. The method of final treatment presented here avoids this possible source of error. It consists, in brief, of drying and weighing the precipitate without attempting to remove the free sulfur, treating it with cold concentrated hydriodic acid to dissolve out the mercuric sulfide without affecting the sulfur, and weighing this residual sulfur. The difference between these two weights gives the weight of pure mercuric sulfide present. That hydriodic acid dissolves mercuric sulfide even in the cold was first observed by Kekule (9). With acid as concentrated as the constant-boiling mixture, hydrogen sulfide is vigorously evolved, and only a comparatively small volume is required to dissolve a given amount of the sulfide, the mercury going into solution in the form of a very soluble complex. The reaction can be expressed by the equation HgS 4HI + HZHgId HiS
+
turning on the suction pump, and add cold stabilized constantboiling hydriodic acid in the proportion of about 5 ml. for each gram of precipitate. Stir the mixture with a glass rod until all black articles of mercuric sulfide have disappeared, then turn on a gentye suction and draw the solution from the crucible. Wash the residual sulfur first with three or four successive 5-ml. portions of dilute (5 to 10 per cent) hydriodic acid and then with cold water. Water must not be used for the initial washings because of the danger of decomposing the soluble mercury complex, thus precipitating mercuric iodide in the pores of the filtering disk. Dry the crucible and its contents for about 2 hours in a vacuum desiccator and reweigh. The difference in the two weighings gives the amount of pure mercuric sulfide present. Results from experiments on the quantitative separation of synthetic mixtures of mercuric sulfide and sulfur by this method are shown in Table I. The sharpness of the separation is evident. TABLEI. SEPARATIONS OF MERCURIC SULFIDE FROM SULFUR Sulfur Taken
Sulfur Found
Gram 0.0865 0.1717 0.2099 0.3895
Gram 0.4220 0.0822 0.0802 0.0052
Gram 0.4221 0.0821 0.0801 0.0056
Error Gram
+O. OQOl -0.0001 -0 0001 +0.0003
,
Apparently this method can be applied also to the rapid estimation of the mercuric sulfide content of technical grades of this sulfide prepared in the dry way, since, when genuine, these are mixtures of mercuric sulfide with more or less free sulfur. Results of test analyses of actual specimens are shown in Table 11.
+
Though this reaction proceeds even more vigorously with hot concentrated acid, the latter cannot be applied in this method because free sulfur reacts to a slight extent with it forming hydrogen sulfide and iodine. The cold acid apparently does not react with sulfur, a t least during the short period of contact required in the practical procedure. For this purpose hydriodic acid stabilized with hypophosphorous acid is the most convenient to use, since the rapid formation of troublesome free iodine from air oxidation is thus avoided. The hypophosphorous acid content in no way interferes with the separation. Merck’s ordinary grade of specific gravity 1.70 was used in all the experiments recorded in this paper. The mercuric sulfide was prepared by precipitation of a solution of recrystallized mercuric chloride with hydrogen sulfide under conditions that led to the formation of no free sulfur. Samples treated with hydriodic acid gave no weighable residues. The specimen of pure finely divided sulfur was found to have no appreciable solubility in cold concentrated hydriodic acid. In testing this point the weighed samples of sulfur were placed in weighed glass crucibles, treated with about 5 ml. of the acid for several minutes with stirring, then washed with water and dried for 2 hours in a vacuum desiccator. In a typical run a 0.4225-gram sample left a 0.4222gram residue after treatment, a difference of 0.0003 gram, which was probably in large part a weighing error. Collect the precipitated mercuric sulfide in a weighed glass or porcelain filtering crucible, wash with cold water, dry thoroughly 1
Mercuric Sulfide Taken
TABLE11. ANALYSESOF COMMERCIAL MERCURIC SULFIDES Variety Black
Red
Sample Taken
Residue Found
Grams 0.5860 1.1210 0 3855 0 6103 0.4807 0 3748 0.7638 0 5385 0 4529 0,4768
Grana 0.0275 0.0531 0.0182 0,0289 0.0226 0.0087 0 0178 0 0124 0.0104 0.0110
Mercuric Sulfide
% 95.31 95.26 95.28 95.26 95.30 97.68 97.67 97.70 97.70 97.69
In these determinations the samples were weighed directly into weighed glass crucibles and treated by the above procedure. The very satisfactory agreement of duplicate trials on the same sample is evident. Since mercuric sulfide should be the only component in unadulterated commercial sulfides that is soluble in hydriodic acid, this method should also be highly accurate. Certainly it has the advantages of being much more rapid and much simpler than the usual procedures for the estimation of the mercuric sulfide content of such products.
Literature Cited (1) Hillebrand, W. F.,and Lundell, G . E. F., “Applied Inorganic Analysis,” pp. 174-5, New York, John Wiley & Sons, 1929. (2) Kekule, A.,Ann. Suppl., 2, 101 (1862).
(3) Treadwell, F. P., and Hall, W. T., “Analytical Chemistry,” 7th ed., p. 173, New York, John Wiley & Sons, 1930. RECEIVED July 27, 1935. Constructed from part of a dissertation submitted by M. Gilbert Burford in partial fulfillment of the requirements for the degree of doctor of philosophy, Princeton University, 1935.
Present address, Department of Chemistry, Cornel1 University, Ithaca,
N. T.
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