274 



THE PROPERTIES OF PROTOPLASM 



blown out by a continuous stream of gas, the electrod e must be enclosed 

 in a gas-tight vessel containing hydrogen and the fluid to be investi- 

 gated and the vessel must be shaken to secure continuous contact of 

 the electrode with hydrogen so as to maintain its saturation (Fig. 12) . 

 Certain special precautions must be taken, when potentiometrically 

 measuring the reaction of fluids containing proteins, especially that of 

 bringing the hydrogen electrode to equilibrium with neutral distilled 

 water before immersing it in the protein solution, for otherwise the 

 acid reaction of the platinum due to the great excess of hydrogen ions 

 which it contains will precipitate many proteins in a film upon its 

 surface. Foaming, which is often troubesome in protein solutions, 

 may be prevented by addition of a few drops of octyl alcohol, or of a 

 mixture of amyl alcohol and kerosine, or of isoamyl isovalerate. 



FIG. 12. "Shaking" hydrogen electrode. (After Clark.) 



The potentiometric method was first employed for the estimation 

 of the reaction of blood by Hoeber. The alkalinity of the blood which 

 was indicated by his earliest measurements was excessive, owing to the 

 fact that the stream of hydrogen employed to saturate the electrode 

 blew out the carbon dioxide which in circulating blood stands in 

 equilibrium with the bicarbonates, and contributes materially to the 

 maintenance of neutrality. Later and more accurate measurements 

 by Hoeber and many others are unanimous in establishing the fact that 

 the normal reaction of the blood is so faintly alkaline as to approximate 

 very closely to neutrality. Thus at absolute neutrality, as in neutral 

 distilled water, the hydrogen and hydroxyl ions are equal in concentra- 

 tion, namely 0.8 x 10~ 7 normal. The actual hydroxyl concentration 

 in the blood is only about double this, namely 1.6 x 10~ 7 or less than 

 one five-millionth normal at the CO 2 -pressures prevailing in the 

 circulating blood (0.028 to 0.054 atmosphere). 



