374 Scientific Proceediuc/s, Royal t)ubtin Bocieiy. 



measurements of the rates of certain reactions. (5) By the use of coloured 

 indicators, which give definite tints at Ivnown concentrations. 



All these methods agree in giving values which are not very far from 

 pH 7'1. Method (5) is used by the writer to test the purity of freshly boiled 

 distilled water and to test glass apparatus for solubility. As shown by its 

 behaviour to brom thymol blue and phenol red, and compared with Clark's 

 standard solutions, the purest, freshly boiled, thrice distilled water which has 

 been prepared here gave pH 7"10-7'05 at about 15° C. Boiling to remove 

 the carbon dioxide in certain types of " hard " glass tube may, however, give 

 any value from about pH 8 upwards, so every tube has to be tested and 

 re-tested after use with alkaline precipitates. If used in the cold, however, 

 such "hard" glass tubes are safe, whereas soft glass tubes are not. Once 

 the indicator has been added the liquid is no longer pure water, hence 

 method (5) is open to objection ; but if due precautions are taken in making 

 up the indicator, and if only a very minute amount, say two drops of a 0'02 

 per cent, solution, is added to 10 c.c. of liquid, the error from this source is 

 slight. Precautions are necessary on account of the fact that pure water has 

 a negligible buffer action. 



An entirely new method has been introduced by Beans and Oakes (1920). 

 In this the hydrogen electrode is set up with the purest water, and the cell 



Hg I HgCl . KCl I KCl I H^O | H. 



is used to charge a condenser of one microfarad capacity, which it does in 

 three to five minutes, whereas a cell of lower internal resistance in which 

 dilute acid is substituted for pure water only requires an instant. Now, the 

 quantity of electricity stored is equal to the product of the E.M.F. of the 

 cell and the capacity of the condenser, viz. Q - EG. When discharged 

 through a ballistic galvanometer a deflection d^ is obtained when the pure 

 water hydrogen electrode is used. Using a standard cell of known voltage 

 a deflection d^ is shown. Now, since d is proportional to Q, and Q = EC, 

 therefore 



E G 



diM'i = ^„ or Ei/Ei = djdi ; 



and, since E2 is known, Ei may be found, and the pH value calculated as in 

 the potentiometer method. The value arrived at for pure water at 25° C. is 

 pH 7'91, or [H*] 1-23 x 10-". This differs considerably from the results 

 previously obtained, but the method seems very simple and direct. From 

 the biological standpoint it is not of great importance to ascertain this 

 constant precisely, as pure water is never found in nature, and its reaction is 



