42 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



sequent addition of acid or alkali). The blood in the first tube is used for the de- 

 termination of the normal H-ion. In each of the next three tubes are added respec- 

 tively 0.1, 0.2 and 0.3 c.c. N/50 HC1, and to the last three, similar quantities of N/50 

 NaOH. After inverting the tubes so as to mix the contents, the blood in each is trans- 

 ferred to celloidin sacs and the C H determined according to the method described else- 

 where (page 32). 



The tubes are noted in which a change in tint from that of the normal blood is 

 evident, and the results are expressed as the c.c. of N/50 HC1 or NaOH which must 

 be added to blood to change its O H . Thus, the alkali buffer is the c.c, of N/50 NaOH 

 which can be added to 2 c.c. of blood without change of C R of the dialysate, and 

 the acid buffer the c.c. of N/50 HC1. 



The method suffers from the following drawbacks: 



1. Very small quantities of acid and alkali are employed. 



2. It is often difficult to tell just exactly when a slight difference in tint has been 

 produced. 



3. Even with the precautions described above, it is impossible to be sure that the 

 amount of OX) in the different samples of blood is the same, which means, of course, 

 that some bloods will, on this account alone, be able to bind more alkali than others. 



The Method of Van Slyke. A method based on somewhat the same principle, but 

 which is more accurate because it meets the above objections, is that suggested by Van 

 Slyke, Stillman and Cullen. 14 Plasma is freed of CO 2 by placing it in a vacuum, 

 and is then mixed with an equal volume of N/50 HC1 (or NaOH) and the C H deter- 

 mined by the electric method (see page 29). In the case of normal blood, after such 

 an addition of acid, a practically normal C H will be found, whereas in the blood of 

 cases of acidosis it will be very distinctly increased (i.e., P H lower). 



C0 2 -combining Power 



The above objections to the titration of blood plasma or dialysate 

 with standard solutions of acids are removed if we measure the com- 

 bining power of the blood alkali towards carbonic acid itself at normal 

 blood reaction. This may be done either in blood immediately after its 

 removal from the animal or in blood that has been first of all saturated 

 outside the body with- carbonic acid at a partial pressure equal to that 

 existing in the body. Since for practical reasons venous blood must be 

 used in the clinic at least the former of these methods suffers from 

 the fault that varying amounts of carbonic acid^will be added to the 

 blood during its passage through the tissues, and the error thereby 

 incurred will become greatly aggravated if venous stasis has been pro- 

 duced in drawing the specimen for analysis. But the chief reason why 

 this method has not been extensively employed, as pointed out by Van 

 Slyke, is the technical difficulty of making the necessary analysis. 



It is most satisfactory to collect venous blood after a period (one hour at least) 

 of muscular rest (so that there is no excess of CO ) and without venous stasis, and 

 to centrifuge without permitting any considerable loss of carbonic acid. The latter 

 precaution is necessary because there is a migration of acid radicles, e. g., HC1, from 

 plasma into corpuscles when the CO,, of the former is increased, and in the reverse 



