The Estimation of Acidity. 253 



use in routine investigations, the hydrogen electrode must be 

 subjected to a test in order to ascertain if it be in good working 

 order. To do this one must place in the electrode vessel a 

 solution of known hydrogen ion concentration, and for this 

 purpose Michaelis suggests a " buffer " mixture made as fol- 

 lows : — 



10 c.c. N Na OH. 



20 c.c. N HA. 



70 c.c. distilled water. 

 Using the N/10 KCL calomel electrode this mixture should, . 

 at 18°C. give an e.m.f . of 0.6045 v. At other temperatures the 

 e.m.f. of this chain is as follows: — 



20 c . . 0.6061 v. 



22 c. 0.6078 v. 



24 c 0.6093 v. 



That is to say, there is a rise of 0.8 millivolt for every degree 

 of temperature. (7) 



Examination of Fluids Containing Gases in Solution. 



The presence of such gases as oxygen, chlorine, ammonia, 

 carbon dioxide or nitrogen in solutions makes their examina- 

 tion by the electrometric method difficult, or in certain cases, im- 

 possible. 



Chlorine gas in such a fluid for instance, would partly saturate 

 the electrode, and also diffuse into the gas space above. In 

 this case, the e.m.f. would be in part due to its presence, thus 

 making the result worthless from the point of view of hydrogen 

 ion concentration estimation. 



Ammonia is a gas, the presence of which makes an examina- 

 tion by this method impossible. 



The occurrence of CO^ in solution is common enough in 

 physiological fluids, and it is of some importance that the effect 

 of its presence on the e.m.f. developed be noted. The diffusion 

 of C0 2 into the gas space above the fluid will lower the partial 

 pressure of the hydrogen, and as the electrolytic solution pres- 

 sure of hydrogen is a function of its partial pressure, the read- 

 ing on the potentiometer would not be an accurate measure of 

 the hydrogen ion concentration of the solution. As to whether 

 the error thus introduced is sufficient to be of importance, auth- 

 orities differ. Milroy, in carrying out some important investi- 



