The Estimation of Acidity- 257 



fluid into gas space. The electrode tube is now wiped with 

 filter paper in order to remove any fluid adhering to it, and the 

 vessel containing the connection solution is brought below it, the 

 tube making minimal contact with the solution. The tube con- 

 nected with the calomel electrode having also been brought below 

 the surface of the connecting fluid, the operator may now pro- 

 ceed to measure the resultant e.m.f. according to the directions- 

 supplied with the particular potentiometer used.' 



In recording these measurements, a note should be made of 

 the time elapsing between each test, a convenient interval being 

 five minutes. In general it will be noticed that during the first 

 few minutes there will be a gradual increase in the e.m.f. This- 

 effect is less noticeable with those electrodes which employ a 

 shaking device, and if such be used, it would appear to be de- 

 sirable to depend upon measurements taken a short time after 

 the preliminary shaking rather than upon the record of a single 

 long experiment. This matter of " potential drift " is treated 

 at length in the article by Clark, previously referred to. 



Between each experiment the electrode tube must be thor- 

 oughly washed with distilled water; it should be then saturated 

 with hydrogen, and immersed in a beaker of distilled water in. 

 which it is kept until required. 



The Calculation of the Result. 



Having obtained the e.m.f. on the potentiometer scale, the 

 H. ion concentration is calculated from it by means of Nernst's 

 formula as follows, to quote Milroy: — 



Let 7T — the e.m.f. of the system. 



7^,= the e.m.f. of the calomel electrode employed when the H' 

 concentration is normal, viz., 0.251 volt at 18 C, using the satur- 

 ated KG calomel electrode ; while C, and G are the H* ion con- 

 centrations of the two fluids. 



Then— 7r p =7r + . 0577.log. ~ 



Cmay therefore be calculated when tt is measured. 

 Or, if P be substituted for Cp (10 _ ^), the result may be made 

 to serve as a basis of measurement. 

 So, then— 



7T p -.251 i 



=log. — or — log. C. 



•0577 ° C 



The value "log C is, in accordance with the suggestion of 

 Sorensen, commonly employed to express the H ' ion concentra- 



