SULPHATES OF MERCURY AND STANDARD CELLS BY R. B. ELLIOTT AND G. A. HULETT
A most important stage in the development of our standards of electromotive force was the introduction of mercurous sulphate as a depolarizer. The unique properties of this salt have made our standard cells possible. It is a mercurous salt with a rather small solubility (about a gram in a liter) so that diffusion from the cathode system is not excessive, while the solubility is sufficient to give a rather large potential a t a mercury electrode. Some 90% of the E.M.F. of the cadmium cell is at the cathode.' Now this cathode potential depends upon the ability of the mercurous sulphate depolarizer to establish and maintain a very precise concentration of mercury ions a t the mercury electrode, so that the reproducibility and constancy of standard cells depends on this property of mercurous sulphate, and obviously the chemical and physical properties of mercurous sulphate are the important factors. Any factor that has a bearing on the mercury concentration resulting from a given preparation of mercurous sulphate is important from the standard cell standpoint. Any impurities are to be feared, especially any other mercury salts present as a result of adsorption or isomorphism. Also it is well known that solubility varies with the size of the particles of the solid.* When the electrolytic method of preparing mercurous sulphate was developed it was found that when the product was stirred with electrolyte to become sufficiently coarse-grained so that it settled rapidly, that this product showed no measurable effect due to surface tension. Later von Steinweh9 attempted to explain the variations of standard cells as due to size of particle of the depolarizer. Now, a product that is obtained as well-formed crystals with reflecting surfaces is less subject to these chemical and physical defects. Consequently, considerable attention has been given to obtaining a well crystallized preparation of mercurous sulphate. Both the chemical and electrochemical method of preparing mercurous sulphate give products of good particle size which appear to be crystalline, but a microscopic examination reveals no reflecting crystalline surface but a rather rough crust-like aspect that by no means excludes adsorption or surface tension effects. Gardiner and Summers4 took advantage of the difference in solubility of mercurous sulphate in 1.5 molar sulphuric at 100' and zoo and by a hot-cold circulation method, succeeded in obtaining some sizable water-clear crystals with perfectly reflecting surfaces. The yield was not satisfactory so we have pursued the problem further. Mercuric sulphate is readily reduced by SOz Int. Electrical Congress, St. Louis, 2, 113 (1904). Z. physik. Chem., 37, 385 (1901). 8 Z. Instrumentenkunde, 20, 205 (1905). 1 Trans. Am. Electrochem. SOC.,56, 143 (1929). 1
* Surface tension effect, Hulett:
2084
R. B. ELLIOTT AND G . A. HULETT
or mercury and the products give perfectly normal values to cells in which they are used as depolarizers. It was thought that by bubbling air-diluted SOz through an acid solution of mercuric sulphate that it might be slowly reduced and give a crystalline product but no matter how much the SO2 was diluted the mercurous sulphate appeared as a fine-grained precipitate. Now experience shows that most satisfactory crystals appear in quiet solutions where slow diffusion brings the reagents together, so it seemed possible that by allowing SO2to slowly diffuse into an acid solution of mercuric sulphate our objective might be attained. A solution of mercuric sulphate was placed in a deep crystallizing dish and in this was placed a bottle partly filled with dilute SO2 solution and all covered with a glass plate with the idea that the SO2 would diffuse out of the bottle slowly and reduce the mercuric sulphate. The diffusion was too rapid and the undesired precipitate appeared. Therefore, the bottle was removed from the crystallizing dish and connected to the upper part of the dish through a long tube with a stopcock. By this means it was possible to so restrict the flow of SO2 that definite crystals appeared and grew on the bottom of the dish. The solution was about I cm. deep. Now it was desirable to use the SO2 direct from a cylinder of the liquid with a pressure of some 4 to 5 atms. By using a very fine capillary, the flow was sufficiently retarded. A piece of 6 mm. thermometer tubing was softened in the flame and drawn down to a capillary about .3 mm. in diameter with a calculated internal diameter of only .013 mm. Some I/Z meter lengths of this capillary joined to the liquid SO2 cylinder restricted the flow of the SO2 satisfactorily so that brilliant crystals formed on the bottom of the crystallizing dish. By adjusting the length of the capillary a desirable variation in the rate was attained and by leading the SO2 into 0.1 normal iodine solution the rate of flow was determined. With a 30 cm. Crystallizing dish satisfactory crystals were obtained when the capillary delivered SOz into the upper part of the dish at a rate of 2 0 0 cu. mm. (0.2 0111.3) a second, while with a rate of . 7 7 ~ mper . second, ~ a precipitate was formed in the mercuric sulphate solution. Clear crystals with dimensions of I / Z to I mm. were readily grown. These were very brilliant crystals indicating a high refractive index. Some of this product turned slightly yellowish in contact with a neutral cadmium sulphate solution. On carefully examining the crystals they were seen to be made up of two types, little plates and cubes. On separating them it was found that the plates were pure mercurous sulphate, while the cubes turned yellow in neutral electrolyte and proved to be mercuric sulphate. The product had been obtained from a I to 6 sulphuric acid saturated with mercuric sulphate and it seemed that some mercuric salt crystallized from this solution as the mercuric was reduced to the less soluble mercurous sulphate and crystallized. With a solution only 7 5 % saturated with mercuric in the crystallizing dish, diffusion of SO2 yielded only plates and it appears that a 50% saturated solution is quite satisfactory. 150 grams of mercuric sulphate in a liter of I to 6 sulphuric acid is quite safe, as only plates of pure mercurous sulphate crystallize from such a solution and these plates may be
SULPHATES OF MERCURY AND STANDARD CELLS
2085
thoroughly washed with dilute sulphuric acid to remove every trace of the electrolyte and crystallized mercuric sulphate. These crystals may be washed with alcohol and ether and dried and may then be used directly in cells. Some of these crystals have been analyzed by a method developed in this laboratory.’ A platinum crucible was amalgamated internally and the crystals weighed in, then covered with distilled water and with a platinum spiral anode above the crystal. A current of .OI amperes deposited the mercury as fast as the crystals dissolved, liberating the acid. When the mercury was all deposited the acid was syphoned off, water being added simultaneously, so that when milliammeter showed zero reading the deposit was completely washed. The last wash water was removed and the crucible and deposit dried in a vacuum desiccator, and the mercury weighed directly, while the acid was titrated with 0.I N NaOH solution. The sum of the weights of mercury and mass of SOa titrated gave a check on the weight of crystals used. Two analyses gave as the ratio of mercury to Sod, 2.009 t o I and 2.010 to I, so that the plates are HgZSOA and an exceptionally pure product and the behavior of this product as a depolarizer in cells is of interest. A typical cadmium and zinc cell gave the following record at 25.00:Time after construction
Saturated cadmiurn cell
Saturated zinc cell
I
I .or8419v.
I .420244 V.
I
I
day week 2 weeks 7 weekg
,018281v.
I ,420200v.
1.018261v.
I .420197V.
I .018234v.
I .420201 v.
Gardiner and Summers* were successful in preparing some water-clear sizable crystals of mercurous sulphate by crystallization from 1.5 molar sulphuric acid, by a hot-cold circulation method. These were exceptionally well formed crystals with reflecting surfaces, but since these crystals gave a higher value to the cadmium cell than did the gray electrolytic mercurous sulphate, this gray electrolytic has been taken as the standard on account of its reproducibility and excellent re,cord in standard cells. There is certainly some difference between the electromotive properties of the two products, and one would be inclined to regard the well crystallized product as giving the “normal value.” Certain it is that the crystals are quite free of surface tension effects on the solubility. If this were a factor we would find the electrolytic giving the greater concentration of mercury in the electrolyte and so the greater E.M.F. to the cell but just the opposite is observed. Therefore such an explanation is untenable. Aside from crystalline differences there is much finely divided mercury on the surfaces of the electrolytic product and this may well be a factor. 1 8
Perdue and Hulett: J. Phys. Chem., 15, 147 (1911). Trans. Am. Electroohem. Soo., 56, 143 (1929).
R. B. ELLIOTT AND G. A. HULETT
2086
Now if the crystals are brought into a dilute sulphuric acid solution of SO2 they are at once covered with a grey coat of finely divided mercury globules, so it is a simple matter t o add this factor of finely divided mercury to crystals of mercurous sulphate. It is only necessary to wash the crystals on the filter with a dilute acid solution of SO2. They turn grey and with continued washing the product may be made to appear quite black. Crystals of mercurous sulphate treated as just indicated have been used for the depolarizer of standard cells with the following results: Time after construction I
Saturated cadmium cell I ,016840
I
1.017924
day week 2 weeks 7 weeks '
Saturated zinc cell
1.41975 I ,420108
-
1.420174
I . 018105
I . 420203
Obviously finely divided mercury plays an important r61e in the depolarizer of standard cells and calls for a careful study from the practical and theoretical standpoints.
