APPLICATION OF ANALYSIS 53 



The percentage of added water may be calculated 

 from the aldehyde figure (A) by the formula : 



Added water = X 100. 



20 



The amount of cream abstracted may be calculated 

 by the formula : 



Tf 



Cream abstracted = x 100. (F fat.) 



3 



This gives the minimum percentage of cream abstracted, 

 and the more probable amount is obtained by sub- 

 stituting the average percentage for the month as 

 given in chap. i. (p. 3) for 3 in both places. 



Detection of Preservatives. Boric Add. To a 

 little milk add a few drops of phenolphthalein, and 

 caustic soda solution drop by drop till a faint pink 

 colour is produced ; place some of the milk in two test- 

 tubes, dilute one with an equal volume of water, and 

 the other with a neutral 50 per cent, solution of 

 glycerine ; in the absence of boric acid the two tubes 

 will have almost the same colour, in its presence the 

 glycerine tube will be the lighter, and usually white. 



As an alternative method the milk or its ash may be 

 made distinctly, but not strongly, acid with hydro- 

 chloric acid and a piece of turmeric paper dipped into 

 the solution ; on drying, the paper turns pink in the 

 presence of boric acid, and is turned a greenish black 

 by alkalies. 



ESTIMATION OF BORIC ACID. To 10 c.c. of milk 

 add at least 5 c.c. of phenolphthalein solution; raise to 



the boil, and neutralise with alkali while still boiling ; 



the pink colour produced when neutral is faint, though 

 quite distinct. Add 8 to 10 c.c. of glycerine, and run in 

 the alkali till a pink colour appears, boiling at this 



N 

 stage being unnecessary ; the quantity of alkali used, 



corrected for the acidity of the glycerine, multiplied 



