ACIDITY 



317 



If one is known, and the difference measured, then the other can 

 be calculated (by the formula of Nernst). That electrode of 

 which the potential is known is termed the reference electrode. 

 In the case of hydrogen cells, the reference electrode may be made 

 by immersing a hydrogen electrode in a buffer solution of known 

 pH value. The other electrode is immersed in the solution of 

 unknown acidity. The two are connected through an instru- 

 ment for measuring potential (the potentiometer), the solutions 



n-n ^-6 /ass tube fo \ 



U" connect yvii-h KCZ 



-. — Pure mercury 

 -S 



-Ordinary mercury 

 Fig. 148. — Calomel cell. 



themselves being joined by an agar bridge. The original 

 standard hydrogen reference electrode was one of hydrogen gas 

 immersed in a solution that is normal in respect to hydrogen ions 

 at one atmosphere pressure. At present, the calomel electrode 

 is the standard reference electrode. It is a glass tube containing 

 a potassium chloride solution saturated with and resting on 

 mercurous chloride (calomel) and mercury (Fig. 148). It and 

 the hydrogen electrode are called halj cells. The two constitute 

 a complete cell. The potential difference between them is meas- 

 ured. The potential of the calomel cell must, obviously, be 

 known for reference. If it contains normal potassium chloride, 



