CULTURAL METHODS 81 



The physical chemist has also discovered that the product 

 of the normality of hydrogen ions by the normality of 

 hydroxyl ions is a constant number. "What this number is 

 may be determined by multiplying ICh' (normality of 

 hydrogen ions in a neutral solution) by 10"^ (normality of 

 hydroxyl ions in a neutral solution) giving 10"^*. In other 

 words, the product of the normality of the hydrogen ion 

 concentration of a solution by the normality of the hydroxyl 

 ion concentration must always be approximately 10"^*- If 

 we know either the hydroxyl or hydrogen ion concentration 

 we can at once determine the concentration of the other. 

 For example, if we are dealing with a solution having an 

 hydrogen ion concentration of 10-" normal its hydroxyl 

 ion concentration must be 10-^ normal. It is thus possible 

 to arrange a scale to designate the acidity of any solution 

 in terms of its hydrogen ion concentration as follows : 



100 lO-i 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 

 10-12 10-13 10-1*. 



On this scale it will be noted that the larger the numerical 

 value of the exponent, the smaller the hydrogen ion con- 

 centration. It will be recalled that 10"^ represents 

 neutrality. Numbers to the right of ICh^ represent increas- 

 ing values of alkalinity or decreasing hydrogen ion concen- 

 tration. Numbers toward the left represent increasing 

 acidity. Inasmuch as this method of statement is somewhat 

 cumbersome, it has been suggested by Sorenson that the ex- 

 ponents be used to indicate the scale, using positive signs 

 instead of negative. This gives the scale : 



0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14. 

 Each of these numbers is termed the pH of a solution. A 

 solution having pH of 0, for example, would have a nor- 

 mality of hydrogen ion concentration of 10° normal or 1. 

 One having a pH value of 7 would have a hydrogen ion 

 concentration of lO-'' normal, that is, it would be neutral. 

 It is evident, therefore, that the smaller the number in 



