ESTIMATION OF IRON, ALUMINA AND PHOSPHORIC ACID. 165 



modification and simplification in the Government Labora- 

 tory for the last two years with very satisfactory results, 

 and it is especially applicable to the estimation of phospliates 

 in the ashes of various kinds of foods, as milk, vinegars, 

 wines, etc., which do not contain iron or alumina. The 

 method in detail is the following: — The ash is dissolved in 

 hydrochloric acid, a drop of methyl orange solution added, 

 and caustic potash run in until the solution is just slightly 

 acid. Carbonic acid is removed by boiling, and excess of 

 calcium chloride and a few drops of phenol-phthalein solu- 

 tion added. The neutralisation of the mixture is carefully 

 adjusted by means of decinormal caustic potash, the point 

 at which the methyl orange just becomes yellow is read off, 

 and the addition of the decinormal potash continued until 

 a slight but permanent pink is obtained. The number of 

 cc. required between tjiese two points gives the amount of 

 phosphate present, 1 cc. being equivalent to one quarter 

 molecule P 2 5 = '00355 gm. or one half group P0 4 = *00475 

 gm. The great advantage of the method lies in its accuracy 

 and in the rapidity and facility with which it can be carried 

 out. Provided there is excess of calcium chloride present 

 magnesium does not interfere, but iron and aluminium 



must be absent. 



"vr 

 10 cc. of a — solution of a phosphate are found to require 



6*6 cc. of a decinormal solution of caustic potash; this is 

 the theoretical amount. Of course the standard potash 

 solution must be corrected for the small amount of carbonic 

 acid which it may contain. This is easily allowed for by 

 finding its equivalent against standard acid with phenol- 

 phthalein as indicator. 



The estimation of iron and aluminium in the absence of 

 phosphoric acid.— The methods already known for the 

 volumetric estimation of iron can hardly be improved upon, 

 and the one adopted in the following process of analysis 



