ESTIMA'rrON OF CITRIC- A<.'ID. 89 



precipitate of calcium phosphate is tilteied off, aiid the solution 

 boiled and concentrated to a small bulk ; the calcium citrate is 

 thus precipitated. This should be washed with boiling water, 

 <<)llected on a small tilter and ignited. To the ignited residue 

 an excess of standard hydrochloric acid is added and the excess 

 titrated back with standard alkali, methyl oran^ie being the best 



X 



indicator. Each cubic centimetre of ' - hydrochloric acid used 



represents ()-0064 gramme of citric acid. 



The result must be corrected for the ^'olume of the fat and 

 protein thrown down as directed under milk-sugar. 



Gowing - Scopes' Method. — L. Gowing-Scopes has imesti- 

 i^ated the method originated by Deniges, which consists in 

 oxidising citric acid tn acetone dicaibo.xylic acid, and the con- 

 version of this into an insoluble basic mercury salt. He tijuls 

 that, to obtain exact results, strict attention must be paid to 

 <letails of manipulation. 



For tlie estimation of citric acid in milk, the clear filtrate 

 obtained by adding acid mercuiic nitrate to milk as for the 

 estimation of milk sugar is used ; lU c.c. of this is e.vaitiy neutial- 

 i.sed with alkali, usinj; |)lien(jl-phthalein as indicator ; a preci])itate 

 will form, hut this will lie redissolved on the addition of H' c.c. 

 of a reagent |)repave(l by covering 01 grammes of mercuric nitrate 

 and •")1 nvamines of nuni^anese nitrate with (iS e.c. of strong 

 nitric acid, and after the addition of IHIt c.c. of \\;itei' to 

 dissoh'e the salts, making up the volume to '2.j() c.c. and 

 filteriiii;. 



The solution is diluted to 'JHl) e.c., and boiled under reflux for 

 tiiree liours, filtered through a weighed (iooch crucible, and the 

 precipitate well washed with cold water ; the deposit on the 

 sides of the Hask may l>e removed by adding 1 or "2 c.c. of 1 per 

 cent, nitric acid, and rubbing with a rod ; after drying for tive 

 hours the precipitate is weighed. The colour of the precipitate 

 should be nearly white ; if it is yellow, this is due to the presence 

 of basic .salts, and tlie result will be high. 



The weight of the precipitate multiplied by 046(37 will mve 

 the amount of citric acid, and in calculating the percentage in 

 the milk, the corrections for the volume of the precipitated 

 proteins and fat must be made as in Vieth's method of milk- 

 sui;ar estimation. 



Tlie method given above departs slightly from Gowing-Scopes' 

 original method, as. owing to the presence of mercury in the 

 filtrate, slightly more mercuric salts are present than he prescribes, 

 but his researches have shown that variarions of the amount 

 of mercury u-<ed have far less infiuence on the results than vari- 

 ations in the other iui;redients of his reagent, and the ^inall 



