46 PHYSICAL CHEMISTRY 



by it. The electrode is first filled with serum through a. The 

 apparatus is set up as in the figure, and the tonometer filled with 

 mercury by raising the mercury funnel. The connections with 

 the H 2 and C0 2 generators are provided with side tubes with 

 traps, o, to make sure that the gas issuing from them never varies 

 more than about a millimeter of water from atmospheric pres- 

 sure. The three way cock, a, is connected with the C0 2 generator, 

 its outlet washed through the opening, b, and the cock turned 

 so as to admit C0 2 into the tonometer while the funnel is being 

 carefully lowered. When the right per cent of C0 2 is read off 

 on the graduations the lower cock, e, and three way cock, a, 

 are closed. Hydrogen is admitted in the same way as was the 

 C0 2 , in order to fill the tonometer with gas at atmospheric pres- 

 sure, and the lower cock, e, closed. If it is desired to make gas 

 mixtures containing less than i per cent C0 2 , for example 0.03 

 per cent C0 2 , first make 1 per cent C0 2 , mix by allowing to 

 stand or inverting the apparatus, and expel all but 3 cc by raising 

 mercury funnel, then fill with hydrogen. The apparatus is re- 

 moved, the three way cock, a, is so turned as to connect the 

 tonometer with the electrode, the serum shaken down into the 

 tonometer and rotated fifteen minutes. One half cc of serum is 

 shaken back into the electrode, connections made and reading 

 taken. If it is desired to use another C0 2 pressure on the same 

 serum, all of the serum is shaken into the electrode and the 

 process repeated. By measuring the PH of the serum at C0 2 

 tension of the alveolar air, the PH of the arterial blood may be 

 obtained. It is not always possible to distinguish between the 

 PH of venous and of arterial blood, because the slight difference 

 may come within the limit of error of the method, but since the 

 PH of arterial blood is more constant than that of venous blood, 

 the significance of this procedure is obvious. 



Fig. 17. Hydrogen electrode for C0 2 -free fluids (from MRR). 



A simple hydrogen electrode for small quantities of fluid is 

 shown in Fig. 17. If used for titrations the rubber bulb may 



