42 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



layer of plasma to separate on the surface (this prevents laking of the 

 blood during the subsequent addition of acid or alkali). The blood in 

 the first tube is used for the determination of the normal H-ion. In 

 each of the next three tubes are added respectively 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 elsewhere (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 CH. 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 H 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 C0 2 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 objec- 

 tion, is that suggested by Van Slyke, Stillman and Cullen. 14 Plasma is 

 freed of C0 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 determined by the electric 

 method (see page 29). In the case of normal blood, after such an addi- 

 tion 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). 



2. 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 



