64 



ION I SAT ION 



Instead of writing 10'"'' one may write merely the positive index 7, 

 keeping the rest of the formula in mind. This is called the p^, 

 p denoting the index to the base 10, and H, of course, showing that 

 hydrogen ions are under consideration. That is, in neutral water 

 at about 23° C. 



or 



Pn 

 C 



H 



Poll = 7, 



In words, neutral water has a hydrogen ion concentration of 

 10"^ or Si pyj of 7. 



Appended is a list of values of ^% of water for various tempera- 

 tures. 



TABLE VIII 



Effect of Altebation of Temperature on the Dissociatton op Water 



Some people prefer a more cumbrous Ijut nevertheless a more 

 comprehensible method of recording ionic concentrations. In 

 S0rensen's method it is rather difficidt to see at a glance the 

 relative concentrations of ions at two temperatures. As the 

 temperature increases, dissociation increases, but the negative 

 exponent or pj^^^ decreases. At S°, for example, the p^ is 7-3, 

 and at 22° it is 7-0. Put in this way, one does not readily grasp 

 the fact that the p^^ at 22° is double the p^j^ at 8°. If, however, 

 the negative exponent be kept a whole number and the fraction 

 be put as a multiplier, the relation is seen at once, e.g., 

 8°Pjj = 7-3 = 0-5 X 10-7 = Ch, 

 22° Pjj = 7-0 = 1 X 10-7 = c^. 



The conversion of one expression into the other is simple. 

 For example : To convert p^ 7-6 to other notation 



p^^ 7-6 = 10-7*5 ^ 10-70 X 10-06 ^ 0-25 X lO"' 

 antilog of — 0-6 = 0-25. 

 Conversely we find the short expression for 



