May 4, 1921 clark: acidity 201 



under (1). There the point of view of a theorist seems to be confused 

 with the point of view of an experimentalist. Wherry and Adams 

 wish to divide the acidity -alkalinity scale into two distinct regions, 

 a region of "acidity" and a region of "alkalinity," divided from one 

 another by the theoretical neutral point. Now the experimental 

 indicator method of determining "reaction," while it need not neces- 

 sarily be so, is still based experimentally upon hydrogen electrode 

 measurements. The hydrogen electrode in turn gives us, according 

 to present conceptions, an indication of hydrogen-ion concentration 

 or activity and tells us nothing at all directly regarding the hydroxyl 

 ion. Our two chief methods therefore are concerned only with hy- 

 drogen-ion concentrations or activities. In view of the fact that our 

 experimental standards of concentration are somewhat indefinite 

 it might be wise were we to express our data in terms of electrode 

 potential differences, but we have chosen to employ pH values which 

 are a linear function of those potential difTerences. In so doing we 

 have introduced no consideration regarding hydroxyl ions. Hence 

 the student employing pH values for any region of the scale, including 

 that which other considerations lead us to designate as the "alkaline" 

 region, can easily be taught the physical and experimental significance 

 of the values. 



Furthermore, since the pH scale shows no change of sign or altera- 

 tion of its continuity at the "neutral point" it gives the student a 

 correct picture of the continuity in the actual conduct of the hydrogen 

 electrode as solutions pass this point, and a correct picture of the con- 

 tinuity in the acid-base system of equilibria at or on either side of a 

 point which we take into consideration only in calculations of certain 

 theoretical values. 



Wherry and Adams are of course clear in their minds about the 

 matter, but their remark that the pH method of statement involves 

 the data for pure water is made in such a way that it tends to confuse 

 experimental physical facts with theoretical considerations. When 

 we use the dissociation constant of a base (as determined for instance 

 by conductance methods) to fix the approximate position of its equi- 

 libria on the pH scale we introduce several theoretical implications 

 and should be conscious of the risk. When we use the dissociation 

 constant of a base, as determined by hydrogen electrode measure- 

 ments and expressed in pH values, we can correctly use the data to 

 fix the position of the equilibria on the pH scale but the constant so 

 found involves additional theory when we compare it with the con- 



