541 
ON THE DETECTION OE ARSENIC IN CASES OE POISONING. 
L. A. Buchne r* has pointed out some details of manipula- 
tion relating to the detection of arsenic. The method to 
which he gives preference is that based upon the direct 
conversion of arsenious acid, by reaction with chloride of 
sodium and sulphuric acid, into the volatile chloride of arsenic, 
a state of combination in which arsenic may be very readily 
recognised by Marsh’s test. The use of chloride of sodium 
and sulphuric acid is also advantageous, because the exclusion 
of adventitious arsenic is more certain than when specially 
prepared hydrochloric acid is used. 
In adopting this method of separating arsenic, it is requisite 
that an excess of chloride of sodium should be used, so that 
the sulphuric acid may not react with organic substances, and 
give rise to the production of sulphurous acid, which would 
interfere with the tests to be applied subsequently. 
However, Buchner found that, in despite of this precaution, 
the distillate, though without any smell of sulphurous acid, 
always gave, when subjected to Marsh’s test, instead of the 
true metallic deposit, a yellow ring of sulphide ; and as the 
zinc used was free from sulphur, he inferred that the sulphur 
must originate from sulphurous acid, which, reacting with 
nascent hydrogen, is converted into sulphide of hydrogen. 
The same result would be produced when the zinc used in 
Marsh’s test contains sulphur. 
Before applying Marsh’s test to the acid distillate, which 
would contain the arsenic as chloride, the absence of 
sulphurous acid must be provided for. This may be easily 
done by adding a few grains of chlorate of potash, until, 
when the liquid is gently warmed, chlorine is evolved ; — or 
the formation of sulphurous acid may be prevented in the 
first instance by mixing with the chloride of sodium a small 
quantity of chlorate of potash. 
The latter method is the better of the two, because decom- 
position of organic substances is effected in a greater degree 
by the chlorine generated, and in consequence of that the 
conversion of the arsenic into its chloride is rendered more 
certain. 
Even the sulphuric acid used in Marsh’s test may contain 
sulphurous acid ; in which case it should, when diluted, 
be mixed with a little chlorine water, and boiled until the 
excess of chlorine is separated. 
When for testing the metallic deposit obtained by Marsh’s 
* ‘Neues Repertorium fur Pliarmacie,’ 1855. 
