246 J. M. Leivis: 



In the concentration chain, and also in the gas chain previously 

 referred to, an e.m.f. was shown to develop when electrodes were 

 in contact with solutions which differed in the concentrations of 

 the particular ion concerned, the two solutions being made part 

 of an electric circuit. 



In practice it would be possible to measure the H ion concen- 

 tration of an unknown fluid, using for one side of the " chain " 

 a solution of unit H ion concentration, i.e., corresponding to one 

 gram ion per litre (12). 



In the case of hydrogen, however, there is " much uncertainty 

 in the reduction to normal ionic concentration," (10) and in 

 routine work other means of completing the " cell " must be 

 sought. 



It will have become obvious that the hydrogen electrode which 

 dips into the unknown fluid constitutes but one-half of the gal- 

 vanic cell which must be constructed in order to determine the 

 e.m.f. It is therefore necessary to complete the system by in- 

 troducing as the other half, an electrode which develops a con- 

 stant e.m.f. For this purpose the standard practice is to employ 

 a " calomel electrode." For a full description of its preparation 

 - f ext-book on the subject may be consulted. 



It will be sufficient here to remark that these electrodes are 

 named from the normality of the potassium chloride solutions 

 used in them. The decinormal electrode has been very generally 

 employed for this purpose, but the saturated calomel electrode 

 has the advantage of easy preparation, and the absence of any 

 need of a temperature correction. The latter has been almost 

 exclusively employed by the writer, and has been found to meet 

 all requirements. Its construction has followed the form sug- 

 gested by Barendrecht. (13) 



The materials used in making these electrodes have to be of 

 extreme purity, and in connection with this, reference should 

 be made to Findlay, (14) the classic article by Loomis and Acree 

 ( 15), and the recent text book by W. M. Clark. (23) 



As the e.m.f. of a calomel electrode varies with the concen- 

 tration of the KG solution employed, it is" necessary to record 

 on the electrode vessel the normality of the solution. For future 

 reference it may be noted that the e.m.f. of a hydrogen electrode 

 when the hydrogen ion concentration of the solution is normal, 

 and when a saturated KC1 electrode is employed, is 0.251 volt,, 

 whilst with the deci-normal KC1 it is 03377 volt. 



