ESTIMATION OF COMMON SALT 211 



The above differentiation between the several forms of com- 

 bined nitrogen should, for the sake of comparison, be carried out 

 with an immature or "green" cheese and also with an over-ripe 

 sample. 



205. Determination of Ash. 



About 15 grams of cheese are carefully ignited 

 in a platinum or porcelain basin of known weight in 

 a muffle furnace until all the carbon has been burnt 

 off. After cooling in a desiccator the basin and 

 contents are weighed, and the percentage of ash 

 constituents thus determined. 



206. Estimation of Sodium Chloride. 



The common salt is best determined in the ash 

 obtained above, by extracting the latter several times 

 with small quantities of hot water, and filtering the 

 aqueous solution obtained into a 100 c.c. flask, and 

 filling up to the mark. Portions of 25 c.c. are with- 



N 

 drawn, and titrated as usual with — AgNOg solution, 



using potassium chromate as indicator. 



207. Detection of Phosphoric Acid in Ash 



Constituents. 



The insoluble residue remaining on the filter after 

 the washing is examined for phosphoric acid as 

 follows '.— 



A portion of the residue is dissolved in fairly con- 

 centrated HNO3 in a test-tube, ammonium molybdate 

 added, and the solution warmed. The characteristic 

 canary - yellow precipitate of ammonium phospho- 

 molybdate shows the presence of phosphates in the 

 ash, these phosphates being derived from phospho- 

 proteins, etc, in the cheese. 



