ACIDITY 313 



While considerable confusion has resulted, the situation is not 

 so bad as it seems, for there is a definite relation between con- 

 centration and what we are actually measuring; furthermore, the 

 numerical differences are not great. The relationship between 

 concentration and activity is shown in the following formula: 



a = Cf 



where a is the activity; C, the concentration; and/, the activity 

 coefficient, which varies for different ions and different concen- 

 trations. The expression a = activity, or effective concentration, 

 indicates the relationship further. Kilpatrick recommends that 

 the definition based upon concentration be maintained, for pH 

 represents the logarithm of a quantity, and in many cases a 

 knowledge of the concentration of the hydrogen ion is more 

 important than a knowledge of its activity. 



The Potentiometer. — If two solutions of different ionic con- 

 centrations, whether containing hydrogen or not, are brought 

 into contact conveniently by what is known as an agar bridge, 

 which is a glass U tube containing a salt solution held in jelly 

 (Fig. 152), then the difference in potential between them can 

 be measured. What we have is a galvanic chain, just as exists 

 in an electric (Daniell's) cell. A potential is set up at each 

 electrode because of the reaction between the metal and the 

 surrounding electrolytic solution. The difference in the 

 potentials of the two electrodes may be measured and expressed 

 in volts. If one of the electrodes is sensitive to hydrogen ions, 

 then the potential difference established is a measure of the 

 pressure or activity of the hydrogen ions. The potential can be 

 determined by opposing it with a known electromotive force 

 (e.m.f.) which is just sufficient to prevent a flow of current. 

 That a condition of no flow has been reached is ascertained by 

 the absence of deflection of a galvanometer. 



The German physicist Poggendorff introduced the compensa- 

 tion, or potentiometric, method for measuring electromotive force 

 with the aid of a potentiometer, so called because potentials are 

 measured. The potentiometer of today includes most of the 

 parts and a number of accessory ones that were formerly placed 

 out on the laboratory table. In Poggendorff's day, the poten- 

 tiometer was a "setup" rather than a single piece of 

 apparatus. 



