§ 218. TARTARIC AND CITRIC ACID. 229 



If the solution contains oxalic and sulphuric, in addition to 

 tartaric and citric acids, all of them must be precipitated with 

 acetate of lead (which would also throw down part of any chlorine 

 that may be present) ; the precipitate should be washed with 

 dilute spirit and treated with ammonia (free from carbonate), 

 which dissolves citrate and tartrate of lead and part of the 

 chloride. The tartaric acid may be estimated in the solution by 

 decomposing the lead-salts with sulphide of ammonium and acetic 

 acid, filtering, and precipitating with acetate of potassium as above 

 described. If chloride is present, the citric acid must be subse- 

 quently precipitated from a hot solution, either as lead-salt by 

 excess of acetate (washing with dilute spirit) or as calcium-salt 

 in the presence of spirit ; the latter must be redissolved in acetic 

 acid, and precipitated with acetate of lead. 



If lime and phosphates are also present (in hydrochloric acid 

 solution), they may be precipitated by acetate of ammonia as 

 oxalate of calcium and phosphate of iron, and Avashed with solu- 

 tion of chloride of ammonium to avoid loss of tartaric acid as 

 acid tartrate of calcium. The phosphoric acid may be subse- 

 quently precipitated as phosphate of lead, which is insoluble in 

 ammonia. The residue of the lead precipitate, after treatment 

 with ammonia, can be decomposed by sulphide of ammonium, and 

 in the filtrate from the sulphide of lead, phosphoric, sulphuric, and 

 oxalic acids determined by the usual processes. 



Malic acid if present would be precipitated together with the 

 citric acid as calcium salt ; the former, however, is soluble in 

 boiling lime-water. 



Racemic acid would be thrown down with the tartaric acid, but 

 could be separated by dissolving the acid tartrate and racemate 

 in hydrochloric acid, supersaturating with ammonia and adding 

 chloride of calcium, which, under these conditions, precipitates 

 racemate of calcium only. ^ 



Eacemic acid is optically inactive, whilst tartaric acid is dextro- 

 rotatory (a)^ = -f 14-18°(^j=:5), or(a)D= -^ 12-80° (i) = 15). It is 

 well-known, however, that racemic acid can be made to yield 

 Iffivoracemic acid by suitable treatment. 



For detection of tartaric acid in presence of malic, formic acids, etc., see 

 Braun, Zeitschr. f. anal. Chem. vii. 349, 1868 ; Kämmerer, viii. 300, 1869. 

 See also Cailletet, xvii. 499, 1878 (Year-book Pharm. 150, 1S78). 



1 Compare Fleischer, Archiv d. Pharm. [3], v. 97, 1874 (Journ. Chem. Soc. 

 xxvii. 1181). 



