REACTIONS IN HOMOGENEOUS SYSTEMS 39 



As p is generally used to represent pressures a better symbol 

 would be the term log[H'] as that is the correct mathe- 

 matical symbol for the values expressed by S^rensen's p R . 

 The following table shows the acidity of some well-known 

 solutions expressed in the various ways : 



TABLE VIII 



Solution. Normality of hydrogen ions [H 1 ] log[H'] or pa. 



o-i n Hydrochloric Acid . . . 0-91 X 10 ' or 10 * 1-04 



o-i n Acetic Acid 0-13 X io~ 2 or 10 2-8 9 2-89 



Neutral solution 0-85 x 10-? or io-7 7-07 



o-i n Ammonia solution. . . 0-52 X 10 "or 10 * 12 9 11-29 



o-i n Sodium hydrate . . . o88 X 10 13 or 10 13 ' 06 13-06 



ELECTROMOTIVE FORCE 



One of the methods for measuring the concentration of 

 hydrogen ions is an electrical one. 



A plate of metal dipped into water gives off positively 

 charged ions of the metal used and the plate becomes 

 negatively charged. If a salt of the same metal is present in 

 the solution the positively charged metallic ions decrease 

 the ease with which the ions are given off by the plate, there- 

 fore the electrical potential at the plate is decreased. If 

 the concentration of the metallic ions is high enough the 

 potential may be reversed and the plate becomes positively 

 charged. 



Platinum electrodes saturated with hydrogen gas behave 

 as if they were a metallic form of hydrogen and the electrical 

 potential varies with the concentration of hydrogen ions in 

 the solution. The electrical potential at platinum electrodes 

 in an atmosphere of hydrogen can be used therefore as a 

 measure of the concentration of hydrogen ions in the solution.* 



In order to measure the electrical potential due to the ions 

 in a solution two electrodes are required, one the electrode 

 at which the potential is being measured and the other a 

 constant electrode the electrical potential of which is known. 

 Frequently a calomel electrode is used for the electrode with 

 known potential and the arrangement of an experiment is 

 as follows : 



* W. Oswald, Lehrbuch d. allgem. Chem, t 2nd edition, 1893, vol. 2, 

 Part I, p. 895. 



