ARSENIC. 
6ll 
§ 777-J 
The precipitate is now dried, and transferred as completely as 
possible from the filter into a small porcelain crucible, included in 
a larger one made of platinum, moistened with nitric acid, covered, 
and heated at first gently, lastly to a bright redness ; the filter is then 
treated similarly, and the crucible with its contents weighed. Pyro- 
arseniate of magnesia (Mg 2 As 2 0 7 ) contains 48-29 per cent, of metallic 
arsenic. 
(b) Conversion of the Trisulphide of Arsenic into the Arseno- 
molybdate of Ammonia. —The purified sulphide is oxidised by nitric acid ; 
the acid solution is rendered alkaline by ammonia, and then precipitated 
by a molybdenum solution, made as follows :—100 grms. of molybdic 
acid are dissolved in 150 c.c. of ordinary ammonia and 80 of water ; this 
solution is poured drop by drop into 500 c.c. of pure nitric acid and 300 c.c. 
of water ; it is allowed to settle, and, if necessary, filtered. The molybdic 
solution must be mixed in excess with the liquid under treatment, the 
temperature raised to 70° or 80°, and nitric acid added in excess until a 
yellow coloration appears ; the liquid is then passed through a tared 
filter, and dried at 100°. It contains 5-1 per cent, of arsenic acid 
[3-3 As]. 1 
(c) Conversion of the Sulphide into Metallic Arsenic. —If there 
should be any doubt as to the nature of the precipitated substances, the 
very best way of resolving this doubt is to reduce the sulphide to metal. 
The easiest method of proving this is to dissolve in potash and obtain 
arsine by the action of aluminium ; or if it is desired to evolve arsine 
from an acid solution with zinc in the usual way, then by dissolving a 
slight excess of zinc oxide in potash or soda, and dissolving in this the 
arsenic sulphide ; the zinc combines with all the sulphur, and converts 
the sulpharsenite into arsenite ; the zinc sulphide is filtered off, and the 
filtrate acidified and introduced into Marsh’s apparatus. The original 
process of Fresenius was to mix the sulphide with carbonate of soda 
and cyanide of potassium, and place the mixture in the wide part of a 
tube of hard German glass, drawn out at one end bo a capillary fineness. 
Carbonic anhydride, properly dried, was passed through the tube, and 
the portion containing the mixture heated to redness ; in this way the 
arsenical sulphide was reduced, and the metal condensed in the capil¬ 
lary portion, where the smallest quantity could be recognised. A more 
elaborate and accurate process, based on the same principles, has been 
advocated by Mohr. 2 
A convenient quantity of carbonate of soda is added to the sulphide, 
and the whole mixed with a very little water and gently warmed. The 
yellow precipitate is very soon dissolved, and then the whole is 
evaporated carefully, until it is in a granular, somewhat moist, adhesive 
1 Champion and Pellet, Bull. Soc. Chim., Jan. 7, 1877. 
2 Mohr’s Toxicologic, p. 57. 
