48 



PHYSICAL CHEMISTRY 



pressure of blood (or alveolar air) is known or the PH of blood 

 at any known C0 2 pressure is determined. 



It should be noted that hydrogen electrodes are "poisoned" 

 by H 2 S, NH 3 , Cl 2 and arsene, therefore the hydrogen should be 

 freed from arsene by passing it through potassium perman- 

 ganate+KOH solution and through mercuric chloride solution. 

 If H 2 is produced electrolytically the 2 should be removed 

 (Clark and Lubs, 1916). Putrescent solutions are best studied 

 by Rowntree's method (see indicators). 



Since the potentiometer measures the emf in millivolts, this 

 reading must be converted into the PH by the following formula : 



emf — 337 



PH = , where T = absolute temperature. 



.1984 T 

 Or the conversion table, Fig. 19, may be used. 



For conversion tables for the saturated calomel electrode see Mc- 

 Clendon (1916 b). 



The formula and conversion table refer to the emf obtained 

 with the partial pressure of hydrogen in the electrode = 760 mm, 

 but since this is lowered by N 2 , C0 2 , water vapor, and by the 

 barometric deficiency (760 — bar.) the potentiometer reading 



760 

 must be increased by (30 log ) where q = partial pressure 



q 



of hydrogen in the electrode. According to Clark and Lubs 

 (1916) a correction should also be applied for the expansion of 

 H 2 by heat. This would make the number 338 apply to a wide 



