REACTIONS IN HOMOGENEOUS SYSTEMS 43 



substances such as carbon dioxide may be driven out of the 

 solution with the result that this concentration of hydrogen 

 ions in the solution will be altered.* 



The loss of carbon dioxide can be minimised by the use of 

 special forms of electrodes. f 



COLORIMETRIC MEASUREMENT OF HYDROGEN ION 

 CONCENTRATION 



A simpler method for measuring the hydrogen ion con- 

 centration is by means of indicators. These are coloured 

 acids or bases. The free acids or bases are very slightly 

 ionised but when neutralised to form salts they ionise strongly. 

 The ion and the undissociated substance have different colours, 

 hence a change of colour occurs when the weak acid or base is 

 neutralised. There is probably a tautomeric change in many 

 indicators when the reaction changes. 



The ionisation constant (K in tho equations given on p. 37) 

 varies in different indicators and according to the law of mass 

 action the amount of ion and unionised substance changes 

 over a certain range of concentration of hydrogen ions. 

 Therefore the colour change takes place over a restricted 

 range of hydrogen ion concentrations and the limit of this 

 range varies with different indicators. 



Friedenthal { and Salm classified indicators according to 

 the range of hydrogen ions at which the colour change occurred. 



S0f ensen has elaborated this indicator method for measuring 

 hydrogen ion concentrations. || He uses mixtures of solutions 

 which are so selected that any desired hydrogen ion con- 

 centration can be produced. These mixtures have the pro- 

 perty of neutralising a certain amount of acid or alkali without 

 an appreciable change in their hydrogen ion concentration. 

 These mixtures are called " buffer " or " stabilising " solutions. 



To equal quantities of an unknown solution and one of these 

 standard solutions the same amount of indicator is added. 

 The colours are compared and if the colours^ match the con- 

 centration of hydrogen ions in the unknown solution equals 

 that in the standard solution. If the colours do not match a 



* For a method of converting log [H-] into concentrations of 

 hydrogen ions see H. E. Roaf, Journ. Physiol., 1921, vol. 54, proc. 

 p. xvii. 



t For inaccuracy due to catalytic alteration of carbon dioxide at 

 the platinum surface see C. L. Evans, Journ. Physiol., 1921, vol. 54, 



P- 353- 



J H. Friedenthal, Zeitsch. f. Electrochem., 1904, vol. 10, p. 113. 



E. Salm, Zeitsch. f. Electrochem., 1904, vol. 10, p. 341. 



|| S. P. L. S0rensen, Ergeb. d. Physiol., 1912, vol. 12, p. 423 et seq. 



