228 ANALYSIS OF WATER 



acidity of 250 c.c. of the water is then determined as 



before, the residual acidity being thus obtained. 



The acidity may be conveniently recorded in terms 



of the acid equivalent to the number of parts of KOH 



per 100,000 of water. This is obtained by multiplying 



N 

 the number of c.c. of — KOH required for i litre by 



0-56. 



Example : — 



N 

 250 Q.c. required 3-5 c.c. — KOH 



I litre requires 3-5 X4= 14 c.c. = 14x0-0056 grams KOH, 



.". 100 poo parts require 14x0-56== 7-84 grams KOH. 

 The acidity is consequently equivalent to 

 7-84 parts per 100,000. 



Estimation of Lead. 

 221. Under certain conditions lead may be absorbed 

 from the water-pipes in quantity sufficient to be 

 extremely objectionable to health, although small in 

 actual amount. 



Qualitatively, lead may be detected by making i 

 litre of the water distinctly acid with hydrochloric acid, 

 concentrating to 200 c.c. and saturating the liquid with 

 hydrogen sulphide. If a precipitate is formed it is 

 dissolved by warming with dilute nitric acid, the excess 

 of the latter removed by evaporation, and the residue 

 dissolved in a little water. The aqueous solution on 

 treatment with sulphuric acid and alcohol gives a white 

 precipitate of lead sulphate, which is again turned black 

 by ammonium sulphide solution. 



The quantitative estimation of lead is carried out 

 colorimetrically, in absence of iron salts, as follows : — A 



