HYDROGEN lOX CONCENTRATION 



13 



on a potentiometer until no ciuTent flows, as 'determined 

 by a galvanometer (Fig. 2). Hence, loiowing the electrode 

 potential of the standard cell, that of the iinknowni may 

 be calculated, and from this the hydrogen ion concentra- 

 tion and the pH determined. A third simple and 

 convenient standard electrode is the quinhydrone electrode 



] 



<i^ 



Unknown 



in^iij 



TANDARD 



Fig. 



in which quinhydrone is dissolved in a standard birffer 

 solution of known 2^H, the potential of a platinmn 

 electrode immersed in such a solution depending on the 



A system very commonly used in the laboratory for 

 measurement of ^^H is the glass electrode in which a glass 

 membrane separates a solution of normal hydrochloric 

 acid, into which dips a calomel electrode, from the 

 solution of unknown pH which is in contact with another 

 calomel electrode immersed in 3*5 normal potassium 

 chloride solution. The potential which is set up across the 

 membrane depends on the jjH of the solution and is 

 measured by determining the null point on a potentio- 

 meter and electronic valve circuit. 



Colorimetric methods depend on the use of various 

 dyes which show colour changes over characteristic 



