T H E DECOMPOSITION OF MERCUROUS CHLORIDE I N CONCENTRATED SOLUTIONS O F OTHER CHLORIDES* BY T.
w.RICHARDS
AND MARCEL FRANCON**
I. Introduction I n 1885, Gockell published a paper when he was studying cells for which there was a difference between the total chemical heat and the part of it which is transformable into electrical energy. Gibbs’ work was not well known then and often it was asked what relation, if any, there was between heat of reaction and electrical energy. They spoke of “free” and “bound” energy. Today it seems clearer to us to use the functions of “heat content” and “free energy,” which are well defined and which enable us to express the correct relations in a simple and accurate way, by the following formulae: (AF)P,T = - E dq E dq
+ (AH),
=
dq
dq = S A C p dlnT
+K
(g)p
where F, E, H, dq, P, ACp, T represent respectively the free energy, the electromotive force of the cell where the reaction considered takes place, the “heat content,” the amount of electricity passing through the cell, the atmospheric pressure, the change of heat capacity a t constant pressure, the temperature. Gockel studied cells, the two ends of which were at different temperatures, so that he might get information on the temperature coefficient of the electrode potential. He found that for cells of the general type: Hg, Hg?Cl*/RICl/HgzCl,/Hg the electromotive force depended on the salt solution, and explained that phenomena by the decomposition of mercurous chloride into mercury and mercuric chloride,-decomposition which depended on the salt solution used. Richards? afterwards studied the same cells and found that the cation of the electrolyte influences the temperature coefficient of the potential of the calomel electrode, the disturbing influence being decreased very much by dilution. Attention was thus drawn upon the reaction: HgZeHg
+ HgCL.
* Contribution from the Chemical Laboratory of Harvard University. * * The research reported in this paper was undertaken under the direction of Professor
T . IT. Richards, a t the Wolcott Gihbs Memorial Laboratory, and finished after Professor Richards’ death, in 1928. The junior author is responsible for the form of the following article. 1 Gockel: W e d . Ann., 24, 618 (188j). Richards: Proc. Am. Acad. Arts. Sei., 33, 3 (1897).
DECOMPOSITION O F MERCUROUS CHLORIDE
93 7
The action of common salt upon calomel seems to have been noticed by J. Capellina;‘ Proust,2 then Miahlea studied it. A. Larocque4 claimed that solutions of alkali chlorides dissolve calomel without changing it simultaneously into mercuric chloride and mercury, for, when the solution is agitated with ether, no mercuric chloride is extracted by that solvent. Ruyssen and Varennes measured the solubility of mercurous chloride in different amounts of hydrochloric acid and found that the solubility increases as the proportion of acid increases. According to Ditte6 calomel is feebly dissociated by cold water, but much more by hot water; in the presence of an alkali halide, a double salt is formed which disturbs the equilibrium of dissociation and more calomel is dissolved. This continues until the double salt is also in equilibrium with its components. For S. Hada,7 dissociation fully accounts for the decomposition of mercurous chloride into mercuric chloride in aqueous solution, without any, oxidizing effect of air. He made some quantitative measurements and found that potassium chloride has a much greater action than hydrochloric acid which, already, had a great action on the decomposition of mercurous chloride. Neither light nor oxygen are important causes in effecting the decomposition, but the reaction is much furthered by increase of temperature. Richards found also that light has no considerable effect on the reaction and that dissolved air is not the essential factor. Later, Richards and Archibald* found that the decomposition 3f mercurous chloride into mercuric chloride is more marked with sodium chloride than with hydrochloric acid, more marked with hydrochloric acid than with barium chloride, with barium chloride more than with calcium chloride, the action being scarcely perceptible with cadmium chloride. Geweckeg also studied the phenomena which he explained by the formation of a double salt between mercuric chloride and the alkali chloride. Richards and Singerlo found the action of strontium chloride less marked than the action of barium chloride, but more than the action of calcium chloride. The action of magnesium chloride a t first more marked than the action of calcium chloride, became, when the concentration was increased, less important than the action of calcium chloride, and passed by a maximum a t a concentration of four times normal. One determination showed that the action of zinc chloride is less marked than the action of magnesium chloride. It was pointed out that the great difference between the action of zinc chloride for instance, and that of potassium chloride could be found in the differences in tendency of the chlorides “to form a soluble double salt.” J. Capellina: Manuscript 58 de la Bibliotheca Madrid (1576); cf. J. TV. Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry. * L . J. Proust: J. Phys., 81, 321 (1815). a M. bliahle: Ann. Chim. Phyn., (3) 5, 177 (1842). ‘ A . Larocque: J. Pharm. Chim. (3) 4, 9 (1843). E. Ruyssen and E. Varenne: Compt. rend., 92, 1161 (1881). 6 h .Ditte: 4 n n . Chim. Phys., (5) 22, 558 (1881). ‘S.Hada: J. Chem. SOC.,69, 1667 (189j). T. W.Richards and E. H. Archibald: Proc. Am. .%cad.,37, 13, 347 (1902). J. Gewecke: Z. physik. Chem., 45, 684-696 (1903). lo This investigation has not been published, but w e is made here of the qualitative results.
93 8
T. W. RICHARDS AND .MARCEL FRANCON
2. Object of the Present Investigation Determinations of the action of concentrated solutions of potassium chloride, lithium chloride, caesium chloride, a t 2 j°C. on mercurous chloride.
3. Method and Apparatus Calomel was treated with solutions of alkali chlovides of various concentrations for six hours in a thermostat. The extent of the reaction was obtained by determining the amount of mercury dissolved.
The same details of procedure as those described by Richards and Archibald were made me of: large test tubes of 60 cc. capacity, corked with rubber stoppers (boiled with dilute alkali, rubbed and washed); into each tube, a large excess of calomel, about a decigram of mercury and about jo cc. of one of the chloride solutions used. The test tubes were placed in a big thermostat, and continually rotated for six hours at 2 5 O C i 0.02. It was found that after six hours, the equilibrium had been reached. The mercury in solution was determined by precipitation with hydrogen sulfide, collected on a hlunroe-Gooch crucible, washed with alcohol, carbon disulphide, again with alcohol, and finally dried at 100’. Analyses were always made in duplicate. I n all cases, except one with caesium chloride, the concentrations of the chloride solutions were determined by analysis; the silver chloride formed by precipitation was weighed in hlunroe-Gooch crucibles. The densities of the solutions of lithium chloride and caesium chloride were determined with a pycnometer; ordinary corrections for the buoyancy of air were made. The densities of potassium chloride were obtained from a diagram constructed by means of the figures of Wagner.’
It ought to be rememberd, as Richards had stated about his work with Archibald, that : “In this paper, no evidence is given concerning the size of the grains of calomel. Ostwald has shown that this may be an important factor in determining the concentration of a saturated solution, and hence in fixing the basis of the present equilibrium. Concerning this point, it need only be said that while the absolute extent of solubility may vary with the size of the grains, the relatzoe results upon which alone the conclusions of this paper are founded, are not affected. This is the case because the same preparation of calomel was used in every instance.” The chloride solutions, before and after reaction with mercurous chloride, were tested with phenol-phthalein and methyl red; the PH was between 6 and 9. That the mercury in solution was, in part a t least, in the mercuric state was made sure of: a solution of permanganate never suffered an appreciable trace of reduction; stannous chloride gave a definite white precipitate of calomel (the last test could not be used with caesium chloride because of the insoluble double salt formed with stannous chloride). Kagner: Z. phgsik. Chem., 5 , 36 (1890).
DECOMPOSITIOX O F MERCCROUS CHLORIDE
93 9
4. Preparation of Material The mercurous chloride used was Baker’s mercurous chloride; its analysis was N.V.11. : o . 0 0 2 ~Potassium ~ . chloride waspurifiedbytworecrystallizations of Baker’s analysed potassium chloride, C. P. Strontium chloride was purified by two recrystallizations of Baker’s chemicals, C. P. Lithium chloride was obtained from lithium nitrate prepared in connection with the atomic weight of lithium, and very pure. The nitrate was t’ransformed into chloride by several evaporations of its solution with hydrochloric acid (purified by distillation) unt,il no vapours of chlorine appeared, then crystallized. Caesium chloride was prepared from pollucite which was attacked by hydrochloric acid as shoT7n for the determination of atomic weight of caesium,‘ then the chloride solution was precipitated by perchloric acid, the perchlorates were recrystallized three times; the perchlorate was transformed into chloride by heating, and the chloride crystallized twice. One solution of caesium chloride was made with pure and dry caesium chloride prepared by Rlr. Root. The dry salt was weighed and the solution weighed, and the density was determined with a pycnometer. I n the experiments with lithium chloride the solution containing mercury was analyzed by taking a known volunie of the solution (measured with a calibrated burette). The results are given in the following tables and represented in the accompanying diagram, as well as the data of Richards and hrchibald.
TABLE I Mercuric Chloride found in Solutions of Potassium Chloride Keight of Solution
Keight of HgP
8 1 36 80 i 7 8 2 63
o.ooz+
82 69 84 41
0.0111
46 39 82 0 2
0.0024 0.0023
0.0110
0.0208 0.0375
IIgClz per of Solution
1000cc.
c’
Density of the Chloride Solution
Sormality of the Chloride Solution
0.035 0.034 0.035
0.035
53.94
1.033
0.75
0.16; 0.167
0.166
11.23
1.074
1.62
0.j90
18.03
1.131
2.73
1.256
23 67
1.167
3.70
I.jj0
25.50
1.181
4.05
1.666
26.2
1.188
4.17
0.592
0.589 .2j8
0.0449
I
0.0120
I .2j2
44 87
0.0504
I ,j48
57 7 5
0.0666
1.592
55 9 2 * 56 2 0 *
0.0669 0.0678
1.673
48 843 45 463
Average
IiCl in weight
1.659
* These last data were found 17y 11r. R. I
