190 M. C. Lea on Estimation of Nitrogen, etc. 
of the exact saturation of the acid liquid by the joint action of 
the ammonia and copper. Dr. Mohr observes :* 
“Donec le moindre excés de la liqueur de dosage formera un 
précipité bien net, tout 4 fait insoluble dans la dissolution des 
deux sels complétement neutres.” 
Here is an important error, this precipitate is by no means 
insoluble in such solutions, it is soluble with considerable facility 
in solution of sulphate of ammonia (in the above passage Dr. 
Mohr refers more particularly to the dosing of sulphuric acid) 
and also, though to perhaps not to so great an extent, in solu- 
tions of sal-ammoniac and of nitrate of ammonia. Consequently 
no doubt sufficiently great to have awakened suspicion. But 
the error is just sufficiently great to make the process dangerous. 
sulphate of ammonia, tested in this way, would give quite a dif- 
ferent result from one in which 5 grammes of free acid was con- 
tained with 20-or 50 of sulphate of ammonia. Likewise its re- 
free sulphuric acid was over-estimated, the ammonio-cupri¢ $0- 
lution having been added till a erp was obtained, which 
ill after the saturation point 
has been reached. y 2 
The precipitate which appears in all these cases is spoken of 
by Dr. Mohr as hydrated oxyd of copper. It appears to be 
basic (probably quadrobasic) sulphate, 
* Not having a German copy at hand I quote from the French translation made 
by Forthomme under the superintendence of Dr. Mohr. 
+ Forthomme’s translation, p. 409. 
it.,-p. 412. 
