296 ORGANIC [CH. XV 



in the free state or as acid salts. If it is required also to 

 determine the combined acids in neutral salts, this may be 

 done by precipitating with lead acetate, suspending the 

 precipitate in water and decomposing with HgS ; the free 

 acid can be estimated in the solution (after filtering and 

 warming to expel HoS) by titration with standard baryta- 

 water, using phenolphthalein as the indicator. 



Acids of which the calcium salts are insoluble in water 

 will not be included in this estimation of the total acid, but 

 if present they can be determined in the same way as for 

 calcium oxalate. 



Qualitative examination for Organic Acids. 



For this purpose it is better to make a special extract 

 of fresh tissues, by pressing out the juice as completely as 

 possible, washing the residue with cold water, and diluting 

 up to a definite volume. 



Organic acids may occur free, or as acid salts, or as 

 neutral salts. Ethereal salts (esters) of organic acids may 

 also occur, but as these are insoluble in water and soluble 

 in petroleum ether, benzene, etc., they are extracted and 

 examined with oils and fats. 



The identification of organic acids in a mixture is 

 rather a difficult problem : full instructions will be found 

 in Fresenius, Qualitative Analysis. 



It must be remembered that salts of sulphuric, 

 phosphoric and hydrochloric acids are nearly always 

 present. 



Students who have not had some practical experience 

 in testing for organic acids in mixtures, should practise 



