806 



CITRATES 



slightest excess of sulphuric acid, than to leave any citrate undecomposed. There 

 should not, however, be any great excess of sulphuric acid. If there bo, it is easily 

 detected by nitrate of baryta, but not by the acetate of lead as prescribed by some 

 chemical authors ; because the citrate of lead is not very soluble in the nitric a id, 

 and might thus be confounded with the sulphate, whereas citrate of baryta is per- 

 fectly soluble in that test acid. Sometimes a little nitric acid is added with advan- 

 tage to the solution of the coloured crystals, with the effect of whitening them. 



Twenty gallons of good lemon-juice will afford fully ten pounds of white crystals 

 of citric acid. 



Citric acid crystallises from a cold saturated solution in prisms belonging to the 

 rhombic system, and containing C 12 H"0 M .2HO (CH 8 O 7 .H 2 O). If, however, the 

 crystals be deposited from a hot solution, they present different forms, and contain 

 only one-half the normal proportion of water. 



The specific gravity of the crystals of citric acid is 1-617. They are unalterable 

 in the air. When heated, they melt in their -water of crystallisation ; and at a higher 

 heat, they are decomposed. They contain 18 per cent, of water, of which one-half 

 may be separated in a dry atmosphere, at about 100 F., when the crystals fall into 

 a white powder. Citric acid is soluble in 0*75 parts of cold, and in 0-5 parts of 

 boiling water. 



Citric acid in crystals is composed, by Dr. lire's analysis, of carbon, 33-0, oxygen, 

 62'37, and hydrogen, 4-63 ; results which differ very little from those of Dr. Prout, 

 subsequently obtained. Dr. Ure found its atomic weight to be 8'375, compared to 

 oxygen 1,000. The composition of crystallised citric acid has been thus represented : 



Attempts were made, both in the West Indies and Sicily, to convert the lime and 

 lemon-juice into citrate of lime, but they seem to have failed through the difficulty of 

 drying the citrate for shipment. 



Citric acid in somewhat crude crystals is employed with much advantage in 

 calico-printing ; for many of the finer colours it cannot be replaced by any other. 

 If adulterated with tartaric acid, the fraud may be detected by adding potash to 

 the solution of the acid, which will cause a precipitate of cream of tartar. 



The manufacture of citric acid so closely resembles that of tartaric acid, that the 

 makers of one commonly fabricate the other. The raw material in this case is pretty 

 generally a black fluid, like thin treacle, which comes from Sicily, and is obtained by 

 inspissating the expressed juice of the lemon, the rind having previously been 

 removed from the lemon for the sake of its essential oil. This black juice is impure 

 citric acid, and requires to be treated with chalk, as practised with respect to the first 

 operation on tartar ; by which means an insoluble citrate of lime is formed ; and this 

 after being well washed with cold water, is decomposed by sulphuric acid ; and the 

 solution, after undergoing the action of animal charcoal and proper evaporation, yields 

 brownish crystals on cooling. These are re-dissolved, decoloured, and crystallised 

 three or four times ere they can be sent into the market, for citric acid is more 

 tenacious of colouring matter than most of the other vegetable acids. At Nice, and 

 in the South of France, a portion of chloride of lime is digested upon the citrate of 

 lime, to bleach it prior to decomposition by sulphuric acid. For this purpose, the 

 washed citrate is exposed in shallow vessels to the action of the sun's rays covered by 

 a weak solution of chloride of lime. In a few hours decolouration ensues ; and it is 

 moreover stated that the mucilage which hangs about the citrate of lime, and impedes 

 the subsequent crystallisation of the acid, is in this way destroyed, and the number 

 of re-crystallisations requisite to give a saleable aspect to the citric acid thereby dimi- 

 nished. 



According to Perret's process, the' lemon-juice is clarified with excess of magnesia, 

 and the salt thus obtained after having been washed with cold water, in which it is 

 insoluble, is dissolved in hot lemon-juice. The solution, on evaporation, yields a 

 bibasic citrate of magnesia in a form convenient for exportation. 



The use of a vacuum-pan for the final evaporation of a solution of citric acid has 

 been introduced by Mr. E. A. Pontifex. 



CITRATES. Combinations of citric acid with alkalis, earths and metals 

 forming salts. See Watts's ' Dictionary of Chemistry.' 



