604 poisons : their effects and detection. [§ 775. 
that the solution must take place in the cold, without the application 
of heat. The principle of the tritration is, that arsenious acid, in the 
presence of water and free alkali, is converted into arsenic acid— 
-A-S 2 O 3 + 41 + 2Na 2 0 = As 2 0 6 + 4NaI. 
The end of the reaction is known by adding a little starch-paste to the 
solution ; as soon as a blue colour appears, the process is finished. 
Another convenient way by which (in very dilute solutions of 
arsenious acid) the arsenic may be determined, is a colorimetric method, 
which depends on the fact that sulphuretted hydrogen, when arsenious 
acid is present in small quantity, produces no precipitate at first, but a 
yellow colour, proportionate to the amount of arsenic present. The 
silver solution containing arsenious acid is freed from silver by hydro¬ 
chloric acid ; a measured quantity of saturated SH 2 water is added to a 
fi actional and, if necessary, diluted portion, in a Nessler cylinder or 
colorimetric apparatus, and the colour produced exactly imitated, by 
the aid of a dilute solution of arsenious acid, added from a burette to a 
similar quantity of SH 2 water in another cylinder, the fluid being acidi¬ 
fied with HC1. 
§ 775. Electrolytic Methods. —The method used in the Govern¬ 
ment laboratory, as arranged by J. E. Thorpe , 1 requires the following 
apparatus :— 
A glass vessel of the shape shown in the figure is open at the bottom, 
and at the top fitted with a ground-glass stopper. Through this stopper 
is passed the stem of the tap funnel ; it also carries the gas exit tube, 
on which there is a bulb. This tube is connected by means of a ground- 
glass joint with a drying tube. Through the glass cap is fused a stout 
platinum wire for making the connection outside with the current and 
within the vessel to the electrode. 
1 Journ. Chem. Soc., T., 1903, p. 974. 
