70 COLLOID CHEMISTRY OF THE PROTEINS 



The degree of combination of weak acids can be derived from 

 the values of C H on the basis of the law of mass action. The 

 method of calculation is shown by the following example, as 

 worked out by the above authors for the acetic acid acetate 

 mixtures which they used. 



When an acetate is added to acetic acid the dissociation 

 isotherm of the acid 



K . CCH.S.COOH C H 

 is transformed into the expression 



K . CCH S .COOH = C H . (C H + CCHSCOO) M 

 where K is the dissociation constant of acetic acid (1-8 X io- 5 ), 

 C H the concentration of free hydrions, and C CH3 coo the concen- 

 tration of the acetate ions which have their origin in the com- 

 bination with albumin. We can equate the latter value to the 

 hydrogen combined with the albumin, or to the acid combined 

 therewith, as the albumin salt is highly dissociated in the dilute 

 solution. The number of acetate ions arising from the free 

 acid must be equal to C H . Further, if n is the normality of 

 the total acetic acid employed in the reaction, the following 

 relation holds between it and the undissociated part of the 

 acid CcH 3 cooH 



CcHsCOOH n ~ CH CcH 8 COO 



Substituting in equation i 



K(n C H CCH S COO) = C H (C H + C CH3 coo), 

 so that the acid combined with the protein is 



r K(n CH) -- CH , , 



Woo = K-f C H 



In this way the combination of acetic acid and albumin can be 

 studied, and, indeed, the expression is capable of general 

 application provided the dissociation constant of the acid is 

 known, and the salts formed with albumin are considerably 

 dissociated (see Table 27, Fig. 15). 



The measurements of Pauli and Hirschf eld given below show 

 that the combination of albumin with acetic acid falls con- 

 siderably short of that which occurs with strong acids. This 



