248 ACIDITY. [CH. IX 



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 HgS, free acid (acetic, 

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

 be neutralised as soon as the HoS has been expelled by 

 warming [acetic and hydrochloric acids are not removed 

 by heating dilute solutions]. 



