CHAPTER V 



THE PRINCIPLES INVOLVED IN THE MEASUREMENT OF THE 

 HYDROGEN-ION CONCENTRATION (Cont'd) 



THE METHODS OF MEASUREMENT 



The Electrical Method 



In order to understand the principle of the standard method used for 

 measuring the H-ion concentration, it is necessary that a few words be 

 said concerning the factors governing the development of electric cur- 

 rents in chemical batteries. There may be a further application of this 

 knowledge in connection with the generation of the electric currents 

 which occurs during physiological activity, as in active glands and muscles. 



When a metal is immersed in a solution of one of its salts, it has a 

 tendency to give off ions into the solution. Similar ions are, however, 

 already present in this solution, and these, by their osmotic pressure, 

 tend to oppose the passage of the ions coming from the metal. The 

 force with which the metal sends out its ions into the solution is called 

 the electrolytic solution pressure. If this is equal to the osmotic pres- 

 sure of the metallic ions in the solution, there will be no electric current 

 generated, but if it is greater or less than the osmotic pressure of the 

 metallic ion, an electric current will be set up. When the solution pres- 

 sure is the greater, the metal will become negatively charged, because its 

 ions carry positive charges into the solution (cations) ; on the contrary, 

 when the osmotic pressure is greater than the solution pressure, the metal 

 will have a positive charge, owing to the receipt of the positive cations 

 from the solution. 



Because of a force called electrostatic attraction, the ions given off 

 from the metal can not travel any measurable distance from the oppositely 

 charged mass of metal, so that from one of the electrodes alone it is 

 impossible for us to lead off any electric current. For this purpose we 

 must form a circuit. This is done in the manner shown in Fig. 7 by 

 connecting side tubes coming from the electrode vessels with an inter- 

 mediate vessel containing a solution of high conductivity and by con- 

 necting the metals by wires. If the circuit is formed between the 

 same metals in solutions of the same concentration, no electric cur- 

 rent will be generated, because the two electrode potentials will be 



29 



