The Estimation of Acidity. 



247 



Some writers consider the potential of the calomel electrode 

 as zero, but it is perhaps more convenient to call the normal 

 hydrogen electrode zero, and then say that the potential of the 

 calomel electrode is 0.251 volt, or 'whatever the figure may be. 



A convenient form of calomel electrode suggested by Baren- 

 drecht is shown in fig. 6, where d is a glass jar, closed by a 

 double bored rubber stopper, containing a saturated solution of 

 potassium chloride. A test tube c perforated a short distance from 

 the bottom contains a small quantity of pure mercury. Dipping 

 below the surface of this a glass tube b provided with a sealed 

 platinum wire enables one to effect electrical connection through 

 a fine column of mercury contained in it. The glass tube e 

 serves to connect the cell with a vessel containing saturated KC1. 

 The stopcock g, provided it is not greased, may be kept closed 

 during measurements. At / a plug of cotton wool prevents dif- 

 fusion into c. 



Fig. 6. 



The connecting fluid. 



For obvious reasons it is impossible to connect the calomel 

 and hydrogen electrodes electrically by means of a metallic con- 

 ductor. The usual practice is therefore to allow both electrodes 

 to dip into some conducting solution. The proper selection of this 



