ARSENIC. 
§ 777-] 
Arsenic trisulphide usually, when first precipitated, contains organic 
matter, and nearly always in toxicological cases organic sulphides, and it 
is essential to purify it. Morner ( Zeit.f . anal. Chem., 1902) has found the 
* >v 
following the best process of purifying small quantities of arsenic tri¬ 
sulphide (up to 0-5 mgrm.) when dissolved off the filter by ammonia 
or other solvent and evaporated to dryness in a porcelain dish :— 
Two c.c. of 0*5 per cent, potash solution are added and warmed for 
about one minute. Next, 2 c.c. of 5 per cent, potassic permanganate, and 
warmed for three minutes ; followed by 2 c.c. of 5 per cent, sulphuric acid, 
and warmed for about three minutes. The dark fluid is now decolourised 
by 1 c.c. of 20 per cent, tartaric acid solution. When colourless, he adds 
1 c.c. of 5 per cent, thioacetic acid 1 while on the water-bath. The 
precipitate is collected on a very small filter, and washed five times with 
2 c.c. of 05 per cent, sulphuric acid, and then three times with 2 c.c. of 
distilled water. The precipitate is next dissolved off the filter by 
three washings of 2 c.c. of 0'5 per cent, potash, directly into a known 
quantity (25 c.c.) of N/100 solution of potassic permanganate and 5 c.c. 
of 5 per cent, sulphuric acid. The liquid is now decolourised by adding 
the necessary quantity of N/100 oxalic acid solution, and warming. The 
centinormal potassic permanganate is now added from a burette until 
there is a faint pink colour. Each c.c. of centinormal permanganate 
multiplied by 0 0536 gives the arsenic as metalloid present. The method 
as described is only applicable to small quantities, such as from 0‘25 
to 0‘5 mgrm. of arsenic ; but the principle can be applied, with the 
necessary increase in the amount of the reagents, to larger quantities. 
The test analyses are remarkably good. 
Should the quantity of sulphide be anything approaching half a 
gramme, the best process, according to the writers’ experience, is to purify 
the sulphide from sulphur and organic matter by applying the principles 
of the previous process. The sulphide on representation is ultimately 
dissolved in normal alkali solution, and the clear yellow liquid made up 
to 50 or 100 c.c., fractional parts of which are added to known quantities 
of decinormal permanganate, acidified by sulphuric acid, and kept at 
about 100° C. for half an hour. Should the liquid be decolourised, more 
permanganate is run in. Ultimately the liquid is cooled, a few crystals 
of KI added, and the free iodine destroyed in the usual way by d.n. 
thiosulphate and starch. The value of the thiosulphate in relation to 
the permanganate is ascertained by running a blank at the same 
temperature and in the same time with the permanganate and distilled 
water. 
1 This acetic acid is not in commerce at present ; it is a convenient substitute for 
sulphuretted hydrogen, and is made by the action of phosphorus pentasulphide on 
glacial acetic acid. Strong sulphuretted hydrogen water may be used instead of thio¬ 
acetic acid. 
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