On Citric Jcid. 53 



add occasionally a small portion of diluted sulphuric acid*, 

 which will arrest the lime, and precipitate it. To this end, the 

 steaming should every novv and then be discontinued fur some 

 hours, to allow time for the sulpiiate of lime to separate, whicii 

 otherwise would prevent the crystallization of the pure citric 

 acid t; for Crell has shown, that a very small quantity of cal- 

 careous earth will have this effect J- Where the product is de- 

 signed for crystallization, it will not be necessary to attend much 

 to the temperature of the room ; for I have found that the cry- 

 stals shoot equallv well in a warm as in a cold situation. 



The crystals thus produced wi.l sometimes, though not gene- 

 rally, be needle-formed § ; and, as the process is usually con- 

 ducted, they will be nearly as dark as the brownest sugar. In- 

 deed, every writer on citric acid, whom I have consulted, tells us, 

 that the first crystals will be either dark brown, or black ; but 

 1 am confident, if the article has not been burnt during the eva- 

 poration, that this is chiefly owing to the uuu-ilage not having 

 been sufficiently washed out of the citrate of liuie before it is 

 treated with the sulphuric acid; for I have more than once seen 

 crystals of t!ie first crop, of a very bright light brown, and even 

 distinct in their form ; and such I conceive they always will be 

 when the proper precautions have been taken. However, though 

 the crystals in any of these states may very well answer most of 

 the purposes to which they are applied by the calico-printer, it 

 is sometimes desirable to have them quite pure ; and by dis- 

 solving and crystallizing the acid three or four times successively 

 in pure water ||, very white and beautiful solid crystals of citric 



• A single drop of this acid will he a sufTicient test, at any time, to show 

 whether tlie concentrated citric acid holds any citrate of linie in solution. 



\ A sinall excess of sulphuric acid is not injurious, hut will rather favour tiie 

 crystrtllizatiou of the citric acid, though, perhaps, only inasmuch as it tends to 

 destroy the tnucilage. 



\ In opposition to this, Westrumb says that most of the citric acid, even when 

 crystallized, contains a considerable quantity of lime. However, where this is 

 suspected, it may quickly be known bv dissolving some of the crystals in water, 

 and, when the solution has been saturated with ammonia, by treating- it with 

 an oxalate of that alkali, wiiich, if thert be lime, will immediately separate it in 

 a palpable f-jrm. 



§ It has been repeatedly said by the writers on citric acid, that the crystals 

 produced by the first evaporation are always needle-formed ; but I am inclined 

 to think that this will never be tlif- case if the citrate of lime has been properly 

 freed from the ni'.icil.ijje, and the solution of citric acid not evaporated beyond 

 the due point of concentration, before it be laid iside for crystalli/ation. In- 

 deed, by proper manat;;ement, 1 know that perfectly formed ihomboidal crystals 

 may always be procured from the liquor of the first boiling. 



II The most ceconomical way of condiictinj); this part of the proceis is to em- 

 ploy very little more water than is ab.solutclv necessary to dissolve the crystals, 

 and then allow sufTicieut time tor the impurities to subside betore the solution 

 is again set apart for crystallization. It would add to the perfection of the 

 crystals, in point of figure, were not only the first, hut every subscquiMit solu- 

 tion passed through a clean leather filter, previous to its being allowed lo shoot. 



D 3 acid 



