182 H. B. TAYLOR. 



Fresh milk when titrated with *1 normal NaOH has an 

 apparent acidity of '2 per cent., calculated as lactic acid; 

 this is due to the presence in the milk of a salt, primary 

 sodium phosphate, which in the presence of NaOH is con- 

 verted into secondary sodium phosphate as follows, 

 NaH 2 P0 4 + NaOH = Na 2 HP0 4 + H 2 0. 



The amount of alkali that it is necessary to add to 5 cc» 

 of milk to give an alkaline reaction to phenolphthalein is 

 1*2 cc. Also when milk is titrated with an acid a similar 

 change takes place, 



Na 2 HP0 4 + HOI = NaH 2 P0 4 + NaOl, 

 with the production of a salt of greater H ion concentra- 

 tion. The amount of *1 normal hydrchloric acid necessary 

 to give to 5 cc. of fresh milk an acid reaction to methyl 

 orange is 5 cc. 



The reaction of fresh milk towards litmus is what is 

 known as an amphoteric reaction, i.e., it turns red litmus 

 blue and blue litmus red. Since neutral litmus is rendered 

 red by the addition of an acid, i.e., increasing its H ion 

 concentration and blue by decreasing its H ion content,, 

 red and blue litmus must have different H ion concentra- 

 tions. If a piece of red litmus'paper is moistened with an 

 H ion solution of concentration between red and blue 

 litmus it will change colour towards the blue; similarly 

 blue litmus dipped in the same solution will tend towards 

 the same colour. The amphoteric reaction of milk to 

 litmus probably depends therefore simply on the fact that 

 its H ion concentration lies between those of red and blue 

 litmus. It was thought that some new light might be 

 thrown on the quality of a milk by investigating the H ion 

 concentration of milk rather than the total acidity. 



Two methods of determining the H ion concentration of 

 milk suggest themselves: — the E.M.F. method, and a 

 method depending on the use of indicators. Both methods 



