230 ACIDITY. [CH. IX 



changes in some of the constituents (e.g. cane-sugar may 

 be inverted by citric acid). Dilute soda is most convenient 

 for this purpose. 



When lead acetate, normal or basic, mercuric chloride, 

 etc., are used in excess for precipitating, and the metal 

 is removed from the filtrate by H^S, free acid (acetic, 

 hydrochloric, etc.) is produced in the solution. This should 

 be neutralised as soon as the HaS has been expelled by 

 warming [acetic and hydrochloric acids are not removed 

 by heating dilute solutions]. 



