ACIDITY 



227 



of such acids renders the water liable to dissolve the lead 

 from pipes, its estimation in potable waters is a matter of 

 considerable importance. " It is necessary to discriminate between 

 the acidity due to carbon dioxide and that due to other organic 

 or inorganic acids, termed "residual acidity" 



220. The total acidity is estimated in 250 c.c. of the 



N 

 water by titration with — KOH, using phenolphthalein 



as indicator. The potash must be obtained from 

 alcoholic solution, and therefore containing no 

 carbonate. 



Residual Acidity. — A current of air which has been 

 first freed from carbon dioxide by passing through a 



Cotton. 



i£oi 



ujateir 





Fig. 32. — Apparatus for determining the residual acidity of water. 



tower containing solid potash is drawn through 500 c.c 

 of the water for half an hour by means of an aspirator. 

 The apparatus is arranged as shown in Fig 32. The 



