268 § 138. FODDER AND FOOD. 



d. Salt. — By evaporating the aqueous extract obtained 

 in the preceding operation, or an aliquot part of it, to dry- 

 ness, and igniting and weighing the residue, the amount 

 of salt in the butter or cheese may be estimated. 



VINEGAR. 



138. The only constituent of vinegar which it is usually 

 desired to estimate quantitatively is the acetic acid. 



a. For this estimation, proceed as directed in § 70. 

 Good vinegar should contain about 5° | ^ of this acid. 



The vinegar should give no reaction for sulphuric or 

 hydrochloric acid, after acidification with nitric acid. If 

 it has been adulterated with these acids, they will of 

 course saturate a portion of the standard sodic solution. 



h. Free sulphuric acid in vinegar may be detected and 

 determined by adding baric carbonate to it, filtering, 

 washing the contents of the filter, and then treating the 

 residue with hydrochloric acid ; the excess of baric car- 

 bonate that was used will be dissolved by the acid, while, 

 if ^\rj free sulphuric acid was present, baric sulphate will 

 remain undissolved ; it may be washed and weighed in 

 the usual mamier (§ 59). 



