BY II. G. CHAPMAN. 437 



reacts with monohydrogen phosphate to form dihydrogen phos- 

 phate. This change may be represented — 



Na 2 HP0 4 + HC1 = NaH 2 P0 4 + NaCl. 



Similarly, when soda is added to a solution of mixed phosphates 

 the reverse change occurs and dihydrogen phosphate is converted 

 into monohydrogen phosphate. Thus — 



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



In these cases no free acid or alkali will be present until all 

 the phosphate has been converted into the acid or basic phosphate 

 as the case may be. 



When this change is considered from the standpoint of the 

 concentrations of hydroxyl and hydrogen ions* certain important 

 applications become evident. 



In distilled water ionisation occurs to a slight extent and H 

 and Oil ions are both present in relatively equivalent concentra- 

 tions. If to this distilled water we add sufficient acid to make a 

 solution of 577^0 then we have a great increase in the concentra- 

 tion of H ions and a relative diminution of the concentration of 

 OH ions. In other words, we have a weak acid but a definitely 

 acid solution. Similarly the addition of alkali to make a t ^jq 

 solution will lead to an increase in the concentration of OH ions 

 and a relative diminution in the concentration of H ions. 



But if to a solution of mixed phosphates of similar concentra- 

 tion to the phosphates in milk, acid is added, then the amount of 

 acid must be 5- before the acid properties develop. Here the 

 addition of acid causes no rapid change in the relative concen- 

 trations of H and OH ions, but instead there is the change in 

 the phosphatic type shown above. If, on the other hand, to the 

 phosphatic solution base is added, then sufficient alkali must be 

 added to raise the alkaline content to ^y before the concentration 

 of OH ions becomes markedly increased and the concentration of 

 H ions relatively diminished. In distilled water the addition 

 therefore of a small quantity of acid or alkali is sufficient to 



* Compare Moore, and also Whitley, Biochem. Journ. Vol.i. 1906. 



