VALUATION OP BLEACHING- POWDER. 
281 
sary when great precision is required; but for commercial 
purposes, when the determination of a bleaching liquor to 
half or one per cent, of its real value, is quite sufficient, a 
standard liquor can be obtained by dissolving seventy grains 
of pure crystallized arsenious acid in one ounce of hydro- 
chloric acid, and then diluting the whole with a sufficient 
quantity of water to give a total of 10,000 grains, conse- 
quently 1000 grains of distilled water will contain seven 
grains of arsenious acid, which will require 5.1 of chlorine 
to transform it into arsenic acid. This method of operating 
is very simple, and easy, a fair sample may be taken from a 
cask from which fifty grains are weighed, to be well mixed 
with water in a mortar, and then poured into a 1000 grain 
measure, divided into one hundred divisions, or in other 
terms into an alkalimeter ; the measure is then filled with 
the washings of the mortar up to the of the instrument, 
so that each division will contain half a grain of chloride. 
If great accuracy is required, a given quantity of the liquid 
must be poured into a chlorimeter tube, which may indicate 
in tenths each division of the alkalimeter. On the other 
hand, there must be put into a vessel, with rather a narrow 
mouth, 1000 grains of the arsenious acid liquor, which 
contains seven grains of this acid, and which will indicate 
5.1 of chlorine. This solution is coloured by a drop of sul- 
phate of indigo, and whilst giving the liquor a revolving 
movement in the left hand, the chlorine mixture must be 
ponred with the right. 
When the blue colour becomes weak, a fresh quantity of 
blue must be added, and then proceed to add with great 
care the bleaching liquor, as the end of the operation is 
near, which point is indicated by the immediate disappear- 
ance of the blue colour. 
To prevent the error of about from 1 to 2 thousandth, 
which the two drops of indigo may have introduced in the 
essay, a second operation is begun by pouring into a new 
portion of arsenious liquor, the quantity of chloride solution 
25* 
