The Titration Curve of Gelatine. 79 



Substituting for N', H, G(H)OH in equation (1), we get the following values 

 for Ki : — 



N 7 . 



[H.] 



[G(H) OH.] 



E,. 



-00679 



-00316 



-00521 



-00242 



-00812 



-00380 



-00388 



-00184 



-00898 



-00490 



-00302 



-00165 



-00939 



-00644 



-00261 



'00180 



Ki may therefore be approximated to O'OOIS, 



whence K. = = = -48x 10- (4) 



A comparison of this value with Procter's value of 5 - 2 x 10 -12 shows that 

 they are both of the same order. It is a workable hypothesis to suppose 

 that gelatine in the presence of hydrochloric acid, the concentration of 

 which lies between 0-00 and 0-04 1ST, behaves according to the law of mass 

 action, like a weak base with a reacting weight of 839 and ionisation constant 

 ' of 4-8 x 10 -12 , each reacting mass combining with one equivalent of acid. 

 Since we do not consider, on the chemical evidence at present available, that 

 the molecular weight of gelatine can be less than 10,000, then it follows that, 

 in its first stage of combination with hydrochloric acid, the gelatine molecule 

 has available thirteen points of attachment for acid, all with a chemical 

 potential very close to 048 x 10 -11 . Procter has considered this possibility, 

 and rejects it as improbable, but, if we consider that the acid is attached to 

 the free — NH 2 groups of the lysin and arginin. and possibly some other 

 di-amino-acid, then it does not seem so improbable that the ionisation 

 constants of these basic groupings might be of the same order. Kanitz (8) 

 gives the following values for histidin, arginin and lysin : — 





First ionisation 



Second ionisation 





constant. 



constant. 





5 -7 x 10" 9 



5 -0 x 10- 13 





1 -0 x 10-" 



2-2xl0"' 2 



Lysin 



1 -0 x 10"' 



1-lxlO" 12 



These values are for the amino-acids in the free state. With arginin and 

 lysin the first and second ionisation constants are of the same order of 

 magnitude, hence it might be expected that even when bound by one amino- 

 group into the protein molecule, the free amino-groups of both acids would 

 have ionisation constants of the same order of magnitude. Evidence for the 

 binding of the acid at these groups is given below. 



