82 ANALYSIS OF MILK. 



If it be desired to examine the ash further, it is desirable to 

 keep the insoluble ash separate from the soluble portion. 



In the solution of the soluble ash the alkalinity may be deter- 

 mined by titration with - acid, methyl orange being used as 



an indicator ; -and the chlorine, by titration with standard 

 nitrate of silver, using potassium chromate as indicator. 



The insoluble ash is dissolved in a slight excess of dilute hydro- 

 chloric acid, and the solution (nearly neutralised with ammonia, 

 if necessary) heated to boiling ; a cold saturated solution of 

 ammonium oxalate is dropped in slowly till the addition of a 

 further drop gives no more precipitate. After standing at least 

 two hours the precipitate is filtered off, washed, and ignited at a 

 low temperature to convert the oxalate into carbonate ; it is 

 best to moisten the ignited precipitate with ammonium carbonate 

 solution and re-ignite at a very low temperature. The pre- 

 cipitate, after weighing, is dissolved in dilute hydrochloric acid, 

 keeping the bulk small ; ammonia is added to alkaline reaction, 

 and the small precipitate of calcium phosphate collected, ignited, 

 and weighed. Its weight is subtracted from the previous weight, 

 and the difference gives the ^veight of the calcium carbonate, 

 which, multiplied by 0'4, gives the calcium, or by 0'56 the lime, 

 contained in it ; the weight of the calcium phosphate multiplied 

 by 0-3871 gives the calcium, or by 0-5419 the lime, contained in 

 it. The total calcium or lime is the sum of the two. 



The filtrate is made strongly ammoniacal by the addition of 

 0'880 ammonia and allowed to stand twenty-four hours. The 

 precipitated magnesium-ammonium phosphate is filtered off, 

 washed with dilute ammonia, ignited, and the magnesium pyro- 

 phosphate weighed. Its weight multiplied by (J'21622 will give 

 the magnesium, and by 0'36036 the magnesia contained in it. 



To the filtrate from this, magnesia mixture is added. The 

 precipitate of magnesium-ammonium phosphate is filtered off 

 after twenty-four hours and treated as above. 



From the total weight of the two quantities of magnesium pyro- 

 phosphate the phosphoric anhydride is calculated by multiplying 

 by 0-63964 ; to this is added the phosphoric anhydride in the 

 calcium phosphate calculated by multiplying the weight by 0'4581. 



The above method has proved satisfactory in the author's 

 hands, though it takes no account of the traces of iron present, 

 which is precipitated with the calcium phosphate, or the mag- 

 nesium-ammonium phosphate. If desired, this may be estimated 

 by dissolving up the precipitate of calcium phosphate and the 

 first magnesium-ammonium phosphate precipitate in dilute hydro- 

 chloric acid, and determining the iron colorimetrically as sulphide, 

 ferrocyanide, or thio-cyanate. 



