328 WATER HYGIENE 
EVALUATION OF BLEACHING POWDER 
Avaiiabie Chlorine. Weigh 7.09 gms. of the sample from a weighing 
bottle into a mortar. Add water and rub into a smooth cream. Add 
more water, and after settling, decant the clear liquid into a liter flask. 
Add more water, grind and decant until all of the powder has been 
transferred to the flask. Dilute to the mark. Aliquot portions of this 
may be removed for analysis but in each case the flask should be thor- 
oughly shaken. 
Great care must be exercised in the sampling of bleach. It rapidly 
decomposes in the presence of air. Very often samples taken from the 
center of a carboy will contain a larger amount of available chlorine 
than a sample which is removed from the surface. 
A. Thiosulphate Method. 
Solution required: 
1. N/10 sodium thiosulphate (Naz2S2Qs3). 
Dissolve 24.83 gms. of sodium thiosulphate in 1 liter of boiled, cooled 
distilled water. Boiling the distilled water decreases the error caused 
by the presence of COz. The solution may be checked against standard 
iodine solution. It keeps very well on standing. 
2. Starch solution. 
Rub a gram of starch to a smooth paste and pour into boiling water 
with constant stirring. Decant after settling into test tubes putting 
about 5 ¢.c. into each tube. Plug and sterilize in the autoclave. 
3. Potassium iodide. 
Dissolve 25 gms. potassium iodide in 500 c.c. of distilled water. 
Procedure. Transfer 50 c.c. of the bleach solution (either that pre- 
pared from the powder or that from the orifice box) to an Erlenmeyer 
flask by means of a pipette. Dilute with 50 ce. distilled water, add 
10 c.c. of the potassium iodide solution, a little acetic acid and titrate 
with the N/10 NaegS2O3 until a faint yellow color remains. Add 1 c.c. 
of the sterile starch solution and continue addition of N/10 Na2Se2O3 
until the blue color is discharged. The number of cubic centimeters of 
N/10 sodium thiosulphate used is equivalent to percentage of available 
chlorine in the sample. 
1 c.c. N/10 NazS203 = .003545 gm. chlorine. 
1 c.c. N/10 NaeS203—1 per cent available chlorine. 
B. Arsenious Acid Method. 
Solutions required: 
1. N/10 arsenious acid. Transfer 4.95 gms. of pure arsenious acid 
to a beaker, add water and 107 gms. of NazHPO.,—12H20. Stir 
