116 PUBLIC HEALTH CHEMISTRY 



soluble is low, the inference is an admixture with exhausted 

 leaves. 



Alkalinity of the Soluble Ash is determined by titrating the 

 washings of the total ash (taking 50 c.c.) with N /io, HC1 or 

 H 2 SO 4 , using methyl-orange as indicator and returning 

 the answer in terms of K 2 O. Varies from 1-3 to 2 per 

 cent, i c.c. N/io acid = 0-0047 grm. K 2 0. 



Extract. Dry some of the leaves at 100 C. ; then 

 weigh out 2 grm. and exhaust thoroughly by boiling under 

 a reflux condenser for one hour. Filter off water, and repeat 

 with more water until no more colour is imparted to the 

 water. Collect the exhausted leaves, dry, and weigh. The 

 difference from original weight gives the amount of extract, 

 which should be from 35 to 40 per cent of the dried tea. 



Caffeine. Extracted as indicated under " Coffee." 

 Varies from 1-8 to 3-5 per cent ; average 2-6 per cent. 



Tannin. Treat 2 grm. as described under " Extract," 

 but keep the nitrates and make up to a known bulk (i litre). 

 Take an aliquot part (say 100 c.c.) in a beaker, add excess 

 of 5 per cent copper-acetate solution, and boil. The 

 precipitate contains all the tannin. Filter and wash 

 precipitate until the washings are free of copper. Dry, 

 ignite, weigh, and deduct filter ash. The weight of ash x 

 1-305 = tannin. The ash is CuO. The amount of tannin 

 varies from 13 to 18 per cent. 



Caffeine - Tannate. Experiments in the "Lancet" 

 laboratory (Lancet, 1911, vol. i, page 46 ; and vol. ii, page 

 1573) led to the conclusion " that an infusion of tea is a 

 solution of caffeine-tannate in an alkaline medium." On 

 neutralization (with HC1) the caffeine-tannate is gradually 

 thrown out of solution, it being only slightly soluble in 

 cold water but readily soluble in hot water. Caffeine- 

 tannate is a compound of one part of caffeine with three 

 parts of tannic acid. When these substances are present 

 in these proportions in a tea infusion, they neutralize each 

 other's effects by combination, and are precipitated by 

 the gastric juice. Excess of either is detected by the 

 palate as bitterness or astringency in the infusion. 



Aroma is mainly due to a volatile oil, which is present 

 to the extent of 0-5 per cent. 



