PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS, 101 
lising dish and place in a draught chamber until the action is over. If 
any mercury remains add more acid and continue to do so until the 
mercury has completely disappeared and a strongly acid solution assured, 
Prepare dilute H,SO, (1 to 4 by volume), allow to cool, and then add the 
acid nitrate solution drop by drop, keeping the mixture violently agitated. 
Mercurous sulphate is precipitated, which should be filtered and treated 
as per note A. No more nitrate solution must be added to the dilute 
H,SO, than will suftice to neutralise 30 per cent. of the H,SO, present. The 
maximum amount it is permissible to add may be estimated by taking a 
small portion of the dilute H,SO, and adding the nitrate solution until 
no further precipitation results. The proportion of nitrate solution to 
dilute H,SO, in such circumstances must be reduced to one-third its 
value for the preparation of mercurous sulphate by method 0. 
(c) Precipitation method, strong and dilute sulphuric acid being em- 
ployed, 
Purchased mercurous sulphate is warmed with pure strong H,SO, and 
a little mercury to a temperature of about 150° C. for about ten minutes, 
the operation being conducted in an evaporating dish covered with a clock 
glass and the mixture kept well stirred. The suspended matter is then 
allowed to settle, the hot liquid cooling sufficiently meanwhile for the 
vessel to be handled with comfort. The clear acid is then poured into 
dilute H,SO, (1 to 6), when crystalline mercurous sulphate separates out. 
About ten times the bulk of dilute acid should be employed, and to avoid 
spitting the hot liquid should be poured through a funnel, having its stem 
immersed in the dilute acid. The mixture is well stirred, cooled, and 
filtered, and the salt treated as per note a. As the operation yields but a 
small quantity of the salt, it is advisable to repeat several times. 
(d@) By means of Nordhausen sulphuric acid. 
Place distilled mercury in the bottom of a beaker or bottle to the 
depth of about 3mm. Add about four times its volume of Nordhausen 
sulphuric acid and stir well, keeping the mouth of the bottle closed as 
much as possible, as the acid fumes are very unpleasant. Mercurous 
sulphate is formed in the cold and appears in the crystalline form after 
a few minutes. Allow the operation to continue until the acid strength 
has been considerably diminished ; warm the product to expel SO, and 
add to dilute H,SO, (1 to 6). Considerable spitting always occurs, so 
that caution is necessary. Proceed with the product as per note A. 
Nore A.—The mercurous sulphate obtained by the foregoing methods 
is first agitated with dilute H,SO, (1 to 6) and distilled mercury. It is 
then filtered (a small Gooch crucible and filter flask are convenient), and 
the greater part of the mercury removed as it interferes with the filtering. 
The salt is next washed with small quantities of saturated cadmium 
sulphate solution until free from acid. For the first few washings some 
of the first acid solution may be employed, but the final washings must 
be made with a little of the neutral solution. Trouble is often experienced 
in ridding the salt prepared with Nordhausen sulphuric acid from all 
trace of acidity, and it is preferable to wash five or six times with the 
cadmium sulphate solution, and then place in a bottle together with a 
little of the solution, shaking from time to time and filtering again in a 
few days. The acidity of the washing liquid should be tested with congo 
red. Instead of washing with cadmium sulphate solution, sulphuric 
ether (water free) may be employed. 
