THE MEASUREMENT OF PH AND TITRATABLE ACIDITY 73 



The hydrogen electrode method. This is regarded as the basic experi- 

 mental method whereby the various other methods are standardized. 

 It consists in the measurement of the potential difference (emf) estab- 

 lished under conditions of maximum work between the ''hydrogen half- 

 cell/' or ''hydrogen electrode," and a calomel or other half-cell which is 

 employed as a working standard. The standard reference half-cell is 

 usually a calomel electrode. 



The hydrogen half-cell consists of a suitable vessel provided with (1) a platinum 

 foil electrode, coated with platinum-black, which is immersed or intermittently dipped 

 in the solution to be measured, and (2) an inlet and outlet for oxygen-free hydrogen to 

 saturate both solution and electrode at atmospheric pressure. 



A convenient reference half-cell is the ''saturated calomel electrode" which consists 

 of a vessel containing a layer of purified mercury covered with a paste of calomel 

 (HgoCh), mercury, and saturated KCl solution; the calomel paste is layered with crys- 

 tals of KCl, and the rest of the vessel is filled with saturated KCl solution which has 

 been saturated with calomel. A platinum wire provides the electrical lead to the 

 mercury of the calomel cell, and a siphon containing saturated KCl solution provides 

 liquid junction with the solution to be measured in the hydrogen half-cell. 



In the normal hydrogen half-cell, which provides the standard of potential for all 

 measurements of potential in electrochemistry, the hydrogen partial pressure is one 

 normal atmosphere and the hydrogen ions are at unit activity. The potential differ- 

 ence between electrode and solution in the normal hydrogen half-cell is assumed to be 

 zero at all temperatures. 



In standardizing the pH scale by means of measurements with a cell composed of a 

 hydrogen half-cell and a saturated KCl calomel half-cell, it is customary to ignore 

 the small and indeterminate liquid junction potential between the saturated solution 

 of KCl and the solution in the hydrogen half-cell. 



The combination of the two half-cells to make an electric cell is indicated schema- 

 tically as follows: 



(Pt)H2; H+ in solution X | sat. KCl | sat. KCl; HgsCh; Hg (Pt) 

 Hydrogen KCl sat. calomel (reference) 



electrode bridge electrode 



For a pH determination, purified hydrogen is bubbled, through the test 

 solution to saturate it and the platinized platinum electrode until equi- 

 hbrium is attained as indicated by constancy of the emf determined 

 potentiometrically between the metal terminals of the hydrogen and the 

 calomel half-cells. The observed emf, in volts, ^ is converted to pH by 

 the following equation, where T is the absolute temperature. 



TT _ Observed emf — emf of calomel cell _ Eh ,. 



^ " 0.000,198,3227^ ~ 0.000, 198,322^ ^ ' 



^ The electrical units employed herein are based on the "international" system 

 in which, according to the National Bureau of Standards, 1 international volt (U.S.) 

 equals 1.00033 absolute volts. The Bureau has announced that, as of January 1, 1948, 

 absolute electrical units will supersede international units. 



However, the effect of this new convention for potentiometry is to introduce change? 

 which may be regarded as negligibly small in ordinary measurements of pH and oxida- 

 tion-reduction potentials. For example, in Eq. (2) — A^^/AkH equals 0.05912 inter- 

 national volt and 0.05914 absolute volt, at 25°C (298.1° absolute). 



