— 



206 Water Analysts [344-346 



each determination, 10 c.c. of the stock solution is diluted to 

 ioo c.c. 



Sulphuric Acid. Mix 50 c.c. strong H 2 S0 4 with 150 c.c. 

 water. When cool, add a drop or two of the standard perman- 

 ganate solution. If, on standing for a few minutes, the acid 

 does not remain pink, add a few more drops until the acid has 

 a faint permanent colour. 



344. The Estimation. — Carefully clean two 16-oz. flasks 

 and measure into one of them 250 c.c. of the water undergoing 

 analysis. Into the other measure 250 c.c. distilled water. In 

 each place 10 c.c. permanganate solution and 10 c.c. sulphuric 

 acid (prepared as in paragraph 343). Allow them to stand for 

 four hours at 8o° F. 



After this time add to each a few drops of a saturated solu- 

 tion of potassium iodide, and a drop of starch solution ; run in 

 the dilute sodium thiosulphate solution from a burette until the 

 blue colour entirely disappears. 



345. Calculation. — The volume of permanganate solution 

 used must be calculated from the difference between the volume 

 of thiosulphate used by the distilled water, and that used by the 



sample — i.e., = volume of permanganate used, where x 



is the number of c.c. of thiosulphate used by the distilled water 

 and y the number used by the sample. 



The weight of oxygen for 70,000 parts is therefore 



x —y *ooi x 70,000 

 x 250 



(x — v)o*28 . i, 



= v =^ grains per gallon. 



x 



346. Estimation of Hardness. — The hardness of 

 water may be denned as its soap-destroying power, and is due 

 to the salts of calcium and magnesium which it contains in 



