Lead in Lead Amalgam 191 
known concentration for one of the electrodes and the one of unknown 
concentration for the other. The potential E could then be measured 
for the combination. This leaves as the one unknown quantity in the 
equation the concentration of the amalgam being measured, and its 
value may readily be calculated. 
Hulett and Minchin* have made use of this scheme in their study 
on the distillation of amalgams and the purification of mercury. They 
state that one part of zinc can be detected with certainty in ten billion 
parts of mercury, and that the method is probably the most delicate 
analytical procedure known. 
Criticism of Methods. None of the preceding methods seemed to 
meet the requirements, as to ease and accuracy, for certain work that 
is being conducted in this laboratory. As already noted, the first 
electrolytic method was unsuitable because of oxidation of the amalgam 
during washing and drying. 
For the method involving the electrolytic separation of the lead 
and mercury, or the gravimetric separation, either of the mercury as 
mercuric sulfide, or of the lead as lead sulfate, the amalgams under 
investigation contained entirely too much mercury. There was gen- 
erally present from 30 to 50 grams of this element and only about 0.5 
gram as the maximum amount of lead. For an electrolytic separation 
this amount of mercury would require altogether too much time, even 
if there were involved no other undesirable features. Likewise, in the 
gravimetric methods which involve a separation by precipitating one 
of the constituents, the mechanical difficulties of handling a solution 
containing such a large proportion of mercury would be too great to 
insure a high degree of accuracy in the determinations. 
Two distinct difficulties are evident in the method involving the de- 
termination of the density of the amalgams. In the first place, it is 
not easy to handle lead amalgams without oxidation of the surface. 
The difficulty is increased if the amalgam is wet and must be dried 
during the procedure. In the second place, the densities of lead and 
mercury are so near each other that a considerable change in density 
of the amalgam does not result from a small change in the concentra- 
tion of the lead in the amalgam. This means that the accuracy in 
determining the concentration of the lead by this method would not 
be so great as in the case of such elements as cadmium and zinc, whose 
densities are much less than that of lead. 
Although the method based upon the measurement of the poten- 
tials of amalgams apparently may be very accurate, distinct precau- 
tions must be observed in making such determinations. Rather elab- 
orate electrical apparatus is required along with an accurately con- 
trolled thermostat for holding the temperature factor constant. No 
attempt was made to use the method in the present work, although 
it is hoped a later study may be made on concentration cells with lead 
amalgams. Previous’ work has indicated that, under certain condi- 
tions, a very constant and reproducible potential is obtained for a sat- 
urated lead amalgam. 
1Phys. Rev. 21, 388 (1905). 
2Mellon and Henderson—J. Am. Chem. Soc. 42, 676 (1920). 
