190 Proceedings of Indiana Academy of Science 
the solution of the amalgam and the subsequent separation of the two 
elements by the precipitation of one of them. The first of these is 
based upon the fact that mercuric sulfide is insoluble in dilute, boiling 
nitric acid (Sp. G. 1.2-1.3), while lead sulfide is soluble. For this 
determination Treadwell,’ and also Scott,” recommended the precipitation 
of the elements from their solution (the mercury being present entirely 
in the mercuric form) by hydrogen sulfide. The precipitate is filtered 
off, washed with hydrogen sulfide water, transferred to a dish and 
boiled for a considerable time with the dilute nitric acid. The solution 
is then diluted, the mercuric sulfide filtered off and washed with water 
containing nitric acid. Certain precautions are necessary in the final 
determination of the two elements. 
As a second gravimetric method Crookes’® suggests the addition of 
sulfuric acid to the solution containing the lead and mercury, followed 
with alcohol to form about one-sixth the volume of the solution. The 
precipitated lead sulfate requires washing with dilute alcohol contain- 
ing a little sulfuric acid. The separation of the lead by this means is 
based upon the insolubility of its sulfate and the solubility of the mer- 
curic sulfate. Horsford used this method.’ 
Physico-Chemical Methods. Richards and Wilson’ have measured 
the densities of lead amalgams as a means for obtaining the concen- 
tration of the lead. -From these results they were then able to obtain 
this concentration by reference to a curve showing the variation in 
density of the amalgam with change in the amount of lead present. 
Such a method is based upon the fact that the physical property being 
measured varies with, or is a function of, the concentration of the 
constituent being determined. 
Meyer® and Richards and Forbes’ have investigated a second, in- 
teresting physico-chemical method. Their work included the elements 
zinc, cadmium, lead, tin, copper, and sodium. The principle of the 
method has as its basis the familiar equation for electrode potentials 
as developed by Nernst. When applied to concentration cells in which 
the two electrodes are amalgams or alloys it has the form— 
in which the various terms all have their usual significance. In such 
a combination as 
Zn. Hg (C1)—ZnSO, (Sat. Sol.)—Zn.Hg (C2) 
one will find a given potential whose magnitude will depend upon the 
concentrations of the two amalgams and the temperature, as the chief 
factors. To apply the above equation in calculating the concentration 
of a given element in an amalgam, one would set up a combination such 
as that mentioned for zinc amalgams, using an amalgam of accurately 
1 Treadwell-Hall—Analytical Chemistry II, p. 194 (1915). 
2 Standard Methods of Chemical Analysis, p. 271 (1917). 
% Select Methods in Analytical Chemistry, p. 324 (1894). 
#Am: J; Sei [21 13, 305 (1852). 
> Carnegie Inst. Pub. 118, 1 (1909). 
67. phys. Ch. 7, 477 (1891). 
* Carnegie Inst. Pub. 56, 1 (1906). 
