ON THE ASSAY OF ALKALOIDS IN EXTRACTS. 
271 
It is a question, however, whether or not the rectification of equivalents 
may not be extended, after M. Valser’s method, to other alkaloids less known 
than those of the cinchona series. Certainly no compounds can well exceed 
the iodohydrargyrates in adaptibility for that purpose. 
Professor Mayer has, in a more recent paper in continuation of the subject, 
recommended the use of an excess of iodohydrargyrate, and the application of 
Mohr’s method for the estimation of that excess ; intending thus to arrive at 
the amount actually combined with the alkaloid. 
Mohr’s volumetric method for estimating iodine and chlorine consists in 
adding to the liquor to be tested, an alkaline carbonate to remove free mineral 
acid if present, and then a little chromate of potash. A volumetric solution of 
nitrate of silver, dropped in until the red colour of chromate of silver remains 
permanent, indicates the amount of chlorine and iodine present. It requires 
a little practice to detect the passage from yellow to the first tint of orange, 
but, that difficulty surmounted, the method is easy, and gives pretty accurate 
results. 
In applying it to the assay of the alkaloids, Mr. Mayer directs that, after 
treatment with excess of iodohydrargyrate, “ the beaker be placed on white 
paper, and carbonate of soda and a few drops of a solution of chromate of 
potash added, and the tenth solution of silver dropped in until the red colour 
of chromate of silver remains permanent after stirring. The number of cubic 
centimetres of silver, divided by 4, and subtracted from the number of cubic 
centimetres of iodohydrargyrate, leaves the quantity of the latter that has 
been consumed for precipitation. The last cubic centimetre of silver is not 
counted.” Of the application of this method several examples are given 
using pure solutions of sulphate of atropia, and the results have every appear 
ance of nicety and exactness. But they must have been mere coincidences 
if the ‘professor followed his own directions , as the chlorine and excess of iodine 
belonging to that portion of the iodohydrargyrate that had combined 
with the alkaloid have been entirely overlooked. Even with the formula 
(AtrHI,HgI), adopted by Mr. Mayer, notwithstanding the results of his vo¬ 
lumetric assay, the chlorine would have to be accounted for, and one equiva¬ 
lent of iodine. But on this supposition the numbers will not come right. 
In fact, I cannot see how the results are arrived at with the numbers given. 
I quote one of the examples. 10 cubic centimetres of solution of sulphate of 
atropia are taken—assumed to be equal to 2’95 cubic centimetres of iodohy¬ 
drargyrate. But 4 cubic centimetres are used, being T05 in excess. This 
should have required 4'20 cubic centimetres of silver. But no ; 12'2 cubic cen¬ 
timetres are found, equal to 3’05 of iodohydrargyrate, which, being accepted as 
excess, leaves but '95 cubic centimetres combined with the alkaloid, instead of 
the 2’95 cubic centimetres required by the theory. Mr. Mayer, however, gives 
the result as 3'05 cubic centimetres. This is exactly one-fourth of the 12 2 
cubic centimetres, but surely the result could not have been so arrived at. 
The really serious objection to the process is that the iodohydrargyrates 
with few exceptions, are attacked by the nitrate of silver. The more soluble, 
morphia for instance, are completely decomposed by it, so that no indication 
at all of an alkaloid is obtained. Mr. Mayer has now perceived the necessity 
of meeting both this and the disturbance caused by solubility, for in his sub¬ 
sequent paper on the “ Assay of Opium” he points out that the morphia li¬ 
quors must be concentrated, and the excess of test liquor estimated away 
from the precipitate. But when strong solutions are used, the precipitate is 
either clotty or gelatinous, and so not to be washed without much water, thus 
reproducing the difficulty sought to be avoided. In the “ Assay of Opium,” 
the residuary chlorine and iodine is not overlooked, but the assumption of an 
incorrect formula for the morphia salt, entirely vitiates the figures. 
