540 
RESEARCH EG 11 ARSENIC AND ANTIMONY. 
This arrangement was, it appears, first proposed and em¬ 
ployed by M. Lassaigne, in 1842.* Care should be taken to 
employ glass free from lead. The tube should resist a red 
heat without blackening’. 
A piece of pure zinc is now dropped into the bottle con¬ 
taining the distillate. If the action of the acid is too violent, 
a little water may be added, and the cork then fitted. Two 
test glasses or tubes, the one containing about a drachm of 
moderately strong solution of nitrate of silver, the other about 
the same quantity of the strongest nitric acid containing 
nitrous acid (sp. gr. 1*522), should be at hand to receive the 
gas from the bent tube as it escapes. 
The solution of nitrate of silver furnishes an excellent 
medium for detecting minute traces of arsenic. So soon as 
the air from the flask is removed, and the hydride of arsenic 
passes over, the solution of silver is blackened. This blacken¬ 
ing* is owing to the production of metallic silver which is 
precipitated with a trace of arsenic. The arsenic takes the 
oxygen of the silver salt and passes to the state of arsenious 
acid —the nitric acid being set free in the liquid. The solu¬ 
tion of silver may not show any blackening or darkening 
from the passage of the gas even after half an hour. Under 
these circumstances, no arsenic is present. We have in this 
method, therefore, the best means for detecting any arsenical 
impurity in the hydrochloric and sulphuric acid or zinc that 
we employ; and before using these in any analysis, it would 
be desirable thus to test them by this process. Unless we 
obtain negative results by passing the hydrogen produced 
into nitrate of silver, we cannot safely employ them.f 
Third stage— -production of-Arsenic and its oxygen compounds . 
—The silver solution completely arrests the gas and oxidizes 
the arsenic. When the current has passed through it for 
some time, a strong heat may be applied to the horizontal 
portion of the conducting-tube about one quarter of an inch 
before each contracted space, beginning with that which is 
nearest to the flask evolving the gas. The hydride is decom¬ 
posed at a full red heat, and metallic arsenic is deposited in 
a dark ring or crust on the interior of the glass, at a short 
distance from the spot which is heated. The appearance of 
* ‘Manuel Pratiquede PAppareil deMarsh/ par Chevallier, 1843, p. 119. 
t It not unfrequently happens, even in testing pure zinc, as well as pure 
acids (hydrochloric and sulphuric), by this method, that although, after a 
quarter of an hour, the solution of silver may show no darkening, a black 
ring will be formed on the inside of the extremity of the tube which dips 
into the solution. This is precipitated silver, probably caused by the 
presence of minute traces of arsenic or phosphorus. 
