BEVERAGES 121 



of sulphuric used, gives the amount of N/i sulphuric 

 required to decompose the carbonates formed from the 

 citrates by ignition. These citrates are partly present as 

 such, and partly from neutralization of the free acid by 

 soda, some of which is not citric. The result, therefore, 

 gives the total organic acid, free and combined, in the 

 sample, calculated as citric acid. The combined acid 

 expressed as citric acid, is got by subtracting from the 

 total acid the amount of free acid in sample. I c.c. N/i 

 sulphuric acid =* 0-07 grm. crystallized citric acid. 



Free Mineral Acid. Place several drops of methylene- 

 violet solution on a white slab. Add a drop of juice ; 

 if free mineral acid is present, a greenish coloration is 

 got. With logwood solution dried on a slab, a drop of 

 juice, if it contains free mineral acid, gives a red coloration. 



Sulphites. Add Zn and HC1, and heat ; if sulphites 

 are present, H 2 S is given off, and will blacken lead acetate 

 paper (moist). Quantitatively : take 50 c.c. of sample 

 and add 25 c.c. N/i KOH, and shake at intervals for 

 fifteen minutes. Add 10 c.c. 25 per cent sulphuric and 

 starch solution, and titrate rapidly with N/10 iodine 

 until a permanent blue colour, lasting two minutes, is 

 produced. 1 c.c. N/10 iodine = 0-0032 grm. S0 2 . When 

 the sulphurous acid is present in the free state, a known 

 excess of N/10 iodine may be added to a measured or 

 weighed quantity of the sample, the whole allowed to 

 stand, with occasional shaking, for one hour, and then 

 titrated with standard thiosulphate solution to find 

 excess of iodine. Another process for sulphites, is to take 

 a quantity of the sample in a flask, add a large excess of 

 water, and then HC1. Distil over the liberated SO 2 into 

 a known quantity of N/10 iodine, and find excess of 

 iodine solution used by titration with standard thiosulphate. 

 The difference is the amount of N /io iodine used in oxidiz- 

 ing sulphurous acid to sulphuric. 



H 2 S0 3 + I + H 2 = H 2 S0 4 + 2HI. 



Alcohol. By distillation (see under "Beer," page 128). 



Salicylic Acid. Precipitate albuminous matters with 

 lead acetate solution ; filter, extract filtrate several times 

 with ether, evaporate ether, and dissolve residue in distilled 



