§ 72. CITRIC ACID. 103 



but does not i^revent it. The precipitate is soluble in 

 boiling soclic hydrate, but re-precipitation follows on 

 cooling. 



With lime-water in large excess, so as to turn red lit- 

 mus-paper blue, tartaric acid gives the same white precipi- 

 tate. 



With calcic sulphate, tartaric acid in tartrates gives no 

 precipitate, thus distinguishing it from oxalic acid. 



With argentic nitrate, neutral tartrates give a precipi- 

 tate that is turned black when the mixture is boiled. 



Quantitatiyc estimation. — An approximate determina- 

 tion of tartaric acid may be made by adding potassic 

 acetate to its moderately concentrated solution, and con- 

 siderable alcohol, collecting the precipitate on a weighed 

 filter, washing it with alcohol, and drying it at 100° C, 

 and weighing. 



The residue of potassic tartrate, KHC^H^Og, contains 

 70.18' I, of tartaric anhydride, C,H,0,. 



CITRIC ACID. H3C6H5O7. 



72. Citrates of barium, calcium, and aluminium, are 

 sparingly soluble in water. 



Reactions. — When citric acid is heated, it fuses at first, 

 and then carbon is separated with the evolution of pun- 

 gent acid fumes. Citrates are blackened when heated. 



Citrates give no precipitate with potassic salts. 



With lime-water in excess, at ordinary temperatures, 

 they give a very slight precipitate, which, on boiling, be- 

 comes quite abundant, but is mostly dissolved when the 

 mixture is cooled. 



Calcic chloride gives a precipitate Ca^{Cflfi^)r,i m 

 solutions of neutral citrates, which, if obtained without 

 heat, is soluble in ammonic chloride ; it is re-precipitated 

 from this solution on boiling, and is not then soluble in 

 ammonic chloride ; it is insoluble in potassic hydrate. 



