6 lO POISONS : THEIR EFFECTS AND DETECTION. [§ 777 . 
A second fractional quantity of the original alkaline solution of 
sulphide is taken, and, if there should be any wide deviation from the 
first, a thiid ; in this way quite accurate estimations of trisulphide may 
be obtained, 1 c.c. of d.n. permanganate equalling 0-536 As or A 2 0 3 . 
If, instead of d.n. permanganate, it is preferred to titrate with d.n. 
iodine solution, the purified sulphide had better be dissolved in sodic 
bicarbonate. 
(a) Oxidation of the Sulphide and Precipitation as Ammonia 
Magnesian Arseniate. The tersulphide is dissolved in ammonia (not 
omitting the filter-paper, which should be soaked in this reagent), 
the solution filtered, and evaporated to dryness. The dry residue 
is now oxidised by fuming nitric acid, taking care to protect the dish 
with a large watch-glass (or other cover) during the first violent action ; 
the dish is then heated in the water-bath until all the sulphur has dis¬ 
appeared, and only a small bulk of the liquid remains ; it is then diluted 
and precipitated by magnesia mixture.” 1 The fluid must stand 
for several hours, and, if the arsenic is to be determined as the 
usual ammoniacal salt, it must be passed through a weighed filter, and 
washed with a little ammoniacal water (1 : 3). The solubility of the 
precipitate is considerable, and for every 16 c.c. of the filtrate (not the 
washings) 1 mgrm. must be allowed. The precipitate, dried at 100°, 
2(NH 4 MgAs0 4 )H 2 0, represents 39-47 per cent, metallic arsenic. 
The solubility of the magnesium arseniate itself, and the general dis¬ 
like which chemists have to weighing in such hygroscopic material as a 
filter, are, perhaps, the main reasons for the variation of this old method, 
which has lately come into notice. Rose proposed some time ago the 
conversion of the double salt into the pyro-arseniate—a method con¬ 
demned by Fresenius and Parnell, but examined and pronounced a 
practicable and accurate process by Remol, Rammelsberg, Thorpe, 
Fuller, Wittstein, Emerson, Macivor, Wood, and Brauner. The 
modification of Rose’s process, recommended by Wood, 2 and still further 
improved by Brauner, 3 may be accepted. 
The precipitation is effected by magnesia mixture, with the addition 
of half its bulk of alcohol. The solution is allowed to stand for several 
hours, until it is possible to decant the clear liquid from the precipitate ; 
the latter is now'dissolved in C1H, reprecipitated as before, thiown on a 
small filter, and washed with a mixture of one volume of ammonia, two 
volumes of alcohol, and three of water. 
1 Magnesia mixture :— 
Sulphate of magnesia 
Chloride of ammonium 
Solution of ammonia . 
Water . 
Dissolve ; then allow to stand for several days ; 
2 Zeitschr. fur anal. Chem., xiv. 356. 
.1 
1 
. 4 
• • . . 8 
finally filter, and keep for use. 
3 Ibid., xvi. 57, 58. 
