§§ 81, 82. ORGANIG AND MINEBAL AGIDS. 71 



observed by Russow in the stems of species of Marattia which 

 had been kept in alcohol. 



If the presence of an oxalate has been indicated by the action 

 of acetic acid, the acid solution should be filtered off and super- 

 saturated with lime-water, which would re-precipitate tartaric, 

 citric, racemic acid, etc. Citric acid may be detected by boiling 

 the filtrate. Citric and tartaric acids may be separated quantita- 

 tively by Allen's method.^ The acids are dissolved in 20 volumes 

 of spirit, and a concentrated solution of acetate of potassium 

 added. After standing twelve hours the acid tartrate of potassium 

 is collected and estimated either gravimetrically, or by titration 

 with normal solution of soda. (See also §§ 214 e^ se^ ; 217 e/ seq.) 



% 82. Vol. Estimation ; Mineral Acids ; Free and Gomhined Acid. 

 — If only one of the non-volatile acids mentioned can be de- 

 tected, the estimation made in § 80 may be checked by decom- 

 posing the lead precipitate from a known fraction of the aqueous 

 extract Avith sulphuretted hydrogen, evaporating the filtrate and 

 titrating the residue dissolved in water. But in this case the 

 phosphoric and sulphuric acids previously estimated in § 7 must 

 be deducted (§ 214). 



Mineral acids may be tested for qualitatively by adding a drop 

 of an alcoholic solution of methyl-violet. Mineral acids change 

 the colour to bluish-green. 



The amount of free acid present in fruits, etc., may be estimated 

 in the aqueous extract by titration Avith normal alkali. A similar 

 determination may be made in the alcoholic extract (§ 47). Any 

 excess of acid found in the first estimation over that in the second 

 may generally be ascribed to the acid salts present. 



If an extract from a vegetable substance is to be specially 

 examined for free tartaric acid in the presence of acid tartrates 

 (of calcium and potassium), the liquid may be evaporated to a 

 syrupy consistence and the tartaric acid extracted with ether or 

 absolute alcohol.^ The alcoholic or ethereal solution is evapo- 

 rated, the residue dissolved in a little spirit and the tartaric acid 

 separated by the addition of alcoholic solution of acetate of 

 potassium.^ 



1 Zeitschr. f. anal. Chemie, xvi. 251 (Pharm. Journ. Trans. [3], vi. 6). 

 - Cf. Claus, Zeitschr. f. anal. Chemie, xvii. 311. 



^ See also Xessler, Zeitschr. f. anal. Chemie, xviii. 230 (Journ. Chem. Soc. 

 xxxvi. 9S1). 



