Some Beviarhahlc Oxidation-ino ducts of Benzidine. 209 
to remove anything soluble. After this an extract made with warm 
benzene yielded on evaporation at a low temperature about ^ gram of 
deep red-orange crystals with violet lustre melting at 255° to 265°, and 
apparently identical with Willstatter's anhydrous diimine. This specimen 
was not reducible with SO2 or sulphite (which blackens it), but easily 
by SnCla to benzidine. 
The diimine base can apparently also be got direct from benzidine 
by heating its solution in acetic acid with H2O2, but a satisfactory 
specimen for analysis was not obtainable by this method. The solution 
of its hydrochloride made by the permanganate method when diluted or 
nearly neutralised turns deep indigo-blue, but it is impossible, on account 
of its extraordinary instability, to say whether this is due to formation of a 
basic salt or to reduction to a quinhydronic compound. Similar fugitive 
blue basic compounds are obtained (1) by insolation of an ethereal solu- 
tion of benzidine (2) in the H2O2 + benzidine tests for blood and peroxy- 
dases, (3) in the author's test for HON with benzidine and cupric acetate, 
and generally in the oxidation of nearly neutral solutions of benzidine and 
its derivatives. 
In the course of the author's earlier work five years ago on this 
subject, he obtained in the sulphite-reduction of these compounds are 
insoluble, infusible, glistening white substance which he supposed to be 
benzidine sulphonic acid from its solubility in dilute ammonia and its 
quantitative behaviour on titration. This is now found to be a basic 
benzidine sulphate G^Jl^^^^ + 4112804. Found N = 12-3 per cent., 
S from 7 to 10 per cent, in different specimens ; mol. w^t. by back 
titration found 256 (assumed monobasic). 
Normal benzidine-sulphate crystallised from boiling dil. HOI is as 
lustrous as silver, forming a network of pseudo-hexagonal plates with 
extinction 20|° to right. Benzidine picrate forms long densely-felted 
yellow needles which blacken at 190°. 
True benzidine chromate cannot be obtained in stable form. KJ^vO^ 
has no action on aqueous benzidine, but in presence of a trace of acid 
in very cold solution a pale green precipitate, rapidly turning blue, results. 
Normal benzidine dihydrochloride gives the blue substance with K20r04 
at once. 
"Benzidine chromate" is a basic type of salt and is very sensitive 
to mineral acids which in small quantities darken it, whilst it dissolves 
easily in excess of cold dilute acid to the normal orange diphenoquinone- 
diimme salt. The nitrate can be easily isolated in crystalline form either 
by suitable oxidation of a cold saturated benzidine nitrate solution, or by 
dissolving the blue " chromate " in dil. HOI at 40° and adding 30 per cent. 
HNO3. fairly easily soluble in pure water to an intense orange 
solution, which turns green and eventually violet-brown on great dilution. 
