HYDROGEN ION CONCENTRATION 



57 



Still further to shorten the symbols, S^rensen suggested the 

 use of the logarithm to denote the hydrogen ion concentration. 

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

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

 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 C H =C OH =10- 7 . 



In words, neutral water has a hydrogen ion concentration of 7. 

 Appended is a list of values of P H2 o for various temperatures, 

 calculated thermodynamically. 



TABLE VIII. 



Some people prefer a more cumbrous but nevertheless a more 

 comprehensible method of recording ionic concentrations. In 

 S^rensen's method it is rather difficult to see at a glance the 

 relative concentrations of ions at two temperatures. As the 

 temperature decreases, dissociation increases, but the negative 

 exponent or p^ o decreases. At 18, for example, the p H is 7-3, 

 and at 23 it is 7-0. Put in this way, one does not readily grasp 

 the fact that the p H at 23 is double the p H at 18. 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. 



18p H =7-3=0-5xlO- 7 =C H , 

 23 p H =7 -0-1 X10- 7 =C H . 



