ACIDITY. 179 



the percentage of fat in the milk and in the whey will give the 

 curd actually obtained. 



Determination of Total Acidity Lactic Acid. The acidity 

 of milk is determined by titration with alkali, using phenol- 

 phthalein as indicator ; the author prefers placing 10 c.c. in 

 a small beaker, adding 1-0 c.c. of phenol- phthalein solution (0*5 

 gramme per 100 c.c. of 50 per cent, alcohol), and titrating with 



baryta or strontia till a faint pink colour equal to that pro- 

 duced by the addition of 1 drop of a 0-01 per cent, solution of 

 rosaniline acetate in 96 per cent, alcohol, is obtained. The 

 procedure should not be varied. The figures thus obtained are 

 termed by the author and Huish acidity (U.S.) or rosaniline 

 standard. 



Storch uses a solution of lime (lime-water), containing solid 

 lime as a standard alkali solution ; this remains constant in 

 composition, and is nearly twentieth normal. The strength 

 of the solution remains constant, as if any of the lime is removed 

 by carbon dioxide, more is dissolved ; its strength is little 

 affected by ordinary variations of temperature. 



It is to be recommended for dairy use, as no precaution, except 

 to have an excess of lime in the bottle, is necessary. 



N 

 Generally speaking, about 1*7 c.c. to 2'0 c.c. of ^ alkali is 



required ; each cubic centimetre of N alkali used per litre of milk 



N 

 is called 1 of acidity, hence a milk requiring 2 c.c. of solution 



for 10 c.c. will have*20 of acidity. 



Van Slyke and Bosworth point out that the acidity of milk 

 should be estimated by titration after the addition of 2 c.c. of 

 a saturated solution of neutral potassium oxalate to 100 c.c. 

 of milk to remove the calcium. The results are about 8 to 9 

 lower if this is done, and is due to the fact that while Na 2 HP0 4 

 is neutral to phenol- phthalein, Ca 3 (P0 4 ) 2 is the insoluble neutral 

 calcium salt ; the difference between the acidity before and 

 after the precipitation of the calcium is an index of the soluble 

 phosphoric acid in the milk. 



M'Creath has devised a convenient form of apparatus for the 

 estimation of acidity (Fig. 25) ; he employs a caustic soda solu- 

 tion of such strength that each c.c. = 0-01 gramme of lactic acid. 

 Ten c.c. of milk, after the addition of phenol- phthalein solution, 

 is titrated with this, and the number of c.c. used divided by 10 

 will give the acidity in terms of lactic acid ; this practice is, 

 however, to be deprecated, as freshly-drawn milk has a very 

 distinct acidity, though lactic acid is in all probability absent. 

 The acidity of milk to phenol- phthalein is due partly to the 



