628 ON THE CLARK CELL AS A STANDARD OF ELECTROMOTIVE FORCE. 
Notes to the Memorandum on Clark’s Cell. 
The Zinc Sulphate Solution .—The object to be attained is the preparation of a neutral solution of pure 
zinc sulphate saturated with ZnS0 4 .7 H 2 0. 
At temperatures above 30° C. the zinc sulphate may crystallise out in another form; to avoid this, 
30° C. should be the upper limit of temperature. At this temperature water will dissolve about 
1'9 times its weight of the crystals. If any of the crystals put in remain undissolved they will be 
removed by the filtration. 
The amount of zinc oxide required depends on the acidity of the solution, but 2 per cent, will, in all 
cases which will arise in practice with reasonably good zinc sulphate, be ample. Another rule would be 
to add the zinc oxide gradually until the solution became slightly milky. The solution, when put into 
the cell, should not contain any free zinc oxide; if it does then, when mixed with the mercurous 
sulphate, zinc sulphate and mercurous oxide are formed; the latter may be deposited on the zinc and 
affect the E.M.F. of the cell. The difficulty is avoided by adding as described about 12 per cent, of 
mercurous sulphate before filtration; this is more than sufficient to combine with the whole of the zinc 
oxide originally put in, if it all remains free ; the mercurous oxide formed together with any undissolved 
mercurous sulphate is removed by the filtration. 
The treatment of the mercurous sulphate has for its object the removal of any mercuric sulphate 
which is often present as an impurity. 
Mercuric sulphate decomposes in the pi’esence of water into an acid and a basic sulphate. The latter 
is a yellow substance—turpeth mineral—practically insoluble in water; its presence at any rate in 
moderate quantities has no effect on the cell. If, however, it is formed, the acid sulphate is formed also. 
This is soluble in water and the acid produced affects the E.M.F. The object of the washings is to 
dissolve and remove this acid sulphate, and for this purpose the three washings described in the 
memorandum will in nearly all cases suffice. If, however, a great deal of the turpeth mineral is formed, 
it shows that there is a great deal of the acid sulphate present, and it will then be wiser to obtain a fresh 
sample of mercurous sulphate rather than to try by repeated washing to get rid of all the acid. 
The free mercury helps in the process of removing the acid, for the acid mercuric sulphate attacks it, 
forming mercurous sulphate and acid which is washed away. 
$ 35, Conclusion. 
We conclude, then, as the result of these experiments, that Clark cells put up at 
very different times and in very different manners have very approximately the same 
E.M.F. 
1-434 volts at 15°, 
and that they may very conveniently be used as standards of electromotive force. 
