CHAPTER III 



MEASUREMENT OF pH 



A detailed account of the theory and practice of hydrogen ion determinations is 

 outside the scope of this book, but in view of the analogies and relationships with 

 oxidation-reduction potentials a brief discussion of pH determinations is desirable. 



HYDROGEN ELECTRODE 



The hydrogen electrode was once the main basis for pH determinations and is a 

 special example of an oxidation-reduction electrode. Instead of measuring the 

 oxidation-reduction condition of the system, this is held constant by maintaining an 

 atmosphere of hydrogen in presence of a platinised platinum electrode and then the 

 electrode potential depends upon the concentration of hydrogen ions. The reaction 

 studied is : — - 



H ^ H® + e 



Applying the general electrode equation (10) we obtain at 30°C : — 



E, = E„ -f 0-06 log^ 



Since pH = — log [H®] and [H] is held constant by bubbUng hydrogen at one 

 atmospheric pressure, the expression can be simplified to : — 



E^ = ki — 06 pH 



so that there is a linear relationship between the potential and pH, the higher the 

 pH the lower the potential. 



In practice pH measurements are not determined by absolute potential measure- 

 ments but by measuring the difference between the potentials of a standard buffer 

 solution and the unknown solution. As with oxidation-reduction potential measure- 

 ments a standard reference half-cell is needed to complete the cell and the half-cells are 

 joined by a suitable liquid junction. 



The experimental arrangement having been set up the pH of the test solution is 

 given by the expression : — 



'pH of standard solution — difference of voltage of cells of test solution and 

 standard buffer respectively -t-0-06. 



The factor 0-06 depends on temperature and values are given on page 40. 



The hydrogen electrode is not commonly used for biological pH measurements, 

 firstly because more convenient methods not necessitating the use of hydrogen are 

 available now, secondly because bubbling of hydrogen introduces difficulties due to 

 frothing and to the removal of carbon dioxide which alters pH values, and thirdly 

 because hydrogen may reduce components of the system 



