280 § 142. BEVERAGES. 



by digestion with several portions of alcohol, as long as 

 a fresh portion is colored, and estimate the gum in the 

 insoluble residue as directed in § 80. 



Half of the alcoholic extract may be evaporated to 

 dryness, the residue weighed, and then incinerated, and 

 the ash weighed; thus the total volatile and non-volatile 

 (organic and inorganic) matter, soluble in alcohol^ may be 

 estimated. 



Sugar may be determined in the other half of the alco- 

 holic extract, after adding water and heating the liquid 

 on the water-bath until all the alcohol is expelled (§ 81). , 



i. Tannic acid, — This acts upon the cupric solution, 

 used in determining sugar, precisely as sugar does, 3.7 

 parts reducing as much cupric oxide as 5 parts of sugar. 

 Tannic acid is absorbed when the wine is decolorized by 

 contact with bone-black ; the difference, then, between the 

 amount of cupric solution required Avith and without 

 treatment with bone-black, will give, approximately, the 

 amount of tannic acid. But in many cases the wine would 

 be too dark-colored to admit of a determination of sugar 

 by the cupric solution without treatment with the decol- 

 orizing agent, and there are, moreover, other substances 

 in the wine that are removed by the charcoal, and that, 

 at the same time, act on the cupric solution. When, how- 

 ever, the determination can be made, it answers very 

 well for the comparison of different wines with each 

 other, since the proportion of the other reducing agents 

 does not seem to vary much. 



7c, Free acids.— Titrate 100 grms. of the wine with the 

 standard sodic solution. 



Then mix another portion of 100 grms. with clean sand, 

 and evaporate it to dryness on the water-bath, with con- 

 stant stirring, and heat it as long as any odor of acetic 

 acid is evolved ; dissolve the residue in water, and titrate 

 the solution with the standard sodic solution. The dif- 



