REACTIONS OF CITRIC ACID 49 



The solution is then boiled and filtered from 

 calcium sulphate. To remove the sulphate remaining 

 in solution, the solution is concentrated by boiling 

 until calcium sulphate commences to separate out, 

 and then an equal volume of absolute alcohol is 

 added to complete the precipitation. The liquid is 

 again filtered, the clear solution obtained being 

 one of citric acid in dilute alcohol. The liquid is 

 evaporated on the water-bath until it attains the 

 consistency of a thick syrup, and is then set aside 

 to crystallise. 



As this syrupy solution of citric acid shows the 

 phenomenon of supersaturation to a marked degree, 

 it is advisable to hasten the crystallisation by seeding 

 with a small crystal of citric acid. The citric acid 

 crystals obtained after some time are freed from 

 mother liquor by pressing between filter papers. 



29. Qualitative Reactions of Citric Acid. 



Citric acid forms colourless crystals containing one molecule 

 of water of crystallisation. It readily dissolves in water and 

 alcohol, and forms three classes of metallic salts, being a tribasic 

 acid. 



The following tests should be applied to the citric acid obtained 

 in the above preparation : — 



(i.) On heating in a dry tube, citric acid or citrates 

 char, with evolution of acid vapours. 



(ii.) On heating with concentrated HgSO^, CO and 

 CO2 are evolved, but charring does not take place 

 immediately (distinction from tartrate). 



(iii.) Calcium chloride, if moderately dilute, gives 

 no precipitate with a neutral solution of a citrate in 

 the cold. On heating, however, calcium citrate is 

 precipitated as a white crystalline precipitate. 



(iv.) Silver nitrate produces with a neutral solution 



D 



