On Citric Add. 51 



that it would not be prudent for the operator to rely solely Oh 

 these appearances ; for, if he is to ensure the best possible ma- 

 nagement of the process, he must have recourse to the agency of 

 chemical tests, as thev will afiT)rd the only sure criterion by which 

 the real state of the saturation of the lime can be known. 



In many chemical operations, preparations of barytes, or a 

 simple solution of this earth in water, are employed to detect the 

 presence of sulphuric acid. But as citrate of barytes would nn- 

 mediatelv precipitate on the addition of barytic water to a solu- 

 tion of citric acid, this reagent cannot be conveniently employed 

 to show when the exact quantity of sulj)huric acid has been used. 

 It is necessary, therefore, to have some other test, and this may 

 be found in the joint employment of acetate of lead and nitric 

 acid: and although this niight have suggested itself to any 

 chemist who had thought on the subject, 1 confess I am indebted 

 for the idea to a person who has for several years done much m 

 the business, and wh'^ tells me he has long employed these two 

 articles for this purpose with perfect success. Whenever the sa- 

 turation of the citrate of lime is nearly completed, and the ope- 

 rator is fearful of supersaturating it, all that is necessary is to 

 take out a little of the clear liquor after the precipitate has sub- 

 sided, to filtrate it through paper, and then treat it with a few 

 drops of acetate of lead. In this instance a citrate of lead will 

 be formed and precipitate, which precipitate will be immediately 

 redissolved on the addition of a little of the nitric acid. On the 

 contrary, should too much sulphuric acid have been used, there 

 will be a mixed precipitate of citrate of lead and sulphate of lead; 

 and the latter of these salts not being soluble in nitric acid, leads 

 to the detection of an excess of the decomposing acid. 



But to return to the process. The consequence of this af- 

 fusion of the citrate of lime with sulphuric acid is, that the latter 

 having a more powerful affinity with the lime, it immediately 

 combines with that earth, and expels the citric acid, which will 

 then be found in the supernatant fluid. The insoluble part of 

 the mixture is now sulphate of lime ; and when sufficient time 

 has been allowed for the whole of this to precipitate, the fluid, 

 which is the pure citric acid in a state of solution, may be drawn 

 off for use. In order to procure the whole of the acid, more 

 water is to be poured on the sulphate of lime *, and repeated 

 time after time, till the last portion comes off clear and taste- 

 less. The whole of these washings are to be put together, and 



# In opposition to some directions which I have seen, 1 wpuld advise that «ZJ 

 water be employed for these washings, as it will separate the fluid c.tnc ac>d 

 7rZ the precipitate, equaUy well with hot water, and w.ll dissolve less of he 

 sulphate of lime, which i, a .ub.tance very unfavourable to the success of th« 

 process of crystallization. , 



