69 



TRe Titration Curve of Gelatine. — Report to the Medical Research 



Council. 



By Dorothy Jordan Lloyd, Biochemical Laboratory, Cambridge, and 

 Charles Mayes, Physical Laboratory, Eton College. 



(Communicated by Prof. F. G. Hopkins, F.R.S. Received September 26, 1921.) 



I. Introduction. 



During the course of some work on the swelling of gelatine gels in acid 

 and alkaline solutions, it became increasingly evident that no fully satis- 

 factory theory of swelling could be postulated until further information had 

 been obtained as to the quantitative relations, both general and particular, 

 holding between the gelatine base and its combined acid in acid systems, 

 and between the gelatine acid and its combined base in alkaline ones 

 respectively. The work described in the following paper is an attempt to 

 study the problem in its simplest form ; namely, in a fluid system in which 

 all the components are in solution. Hydrochloric acid and sodium hydroxide 

 were chosen as the acid and base to use in the investigation, since both form 

 highly ionised solutions in water, and since none of the ions resulting carry 

 more than a single charge, thus simplifying any considerations deduced 

 from the law of mass action. It is still an open question how far the law 

 of mass action can be applied to colloidal solutions. It has been shown 

 by Procter (20), Procter and Wilson (21), Wintgen and Kruger(27), that 

 the quantitative relations found by them to exist in the combination of 

 hydrochloric acid with gelatine, under the conditions .of their experiments, 

 fell within the general statement of the law, such combination being 

 regarded as a simple case of salt formation. 



Procter considered that his results were explicable on two hypotheses : — 



(1) That gelatine had a molecular weight of 839, and combined with 

 one molecule of hydrochloric acid to form gelatine hydrochloride ; or 



(2) That if gelatine had a larger molecular weight, some multiple of 839, 

 say 839*-, then the gelatine molecule combined with x molecules of hydro- 

 chloric acid and the ionisation constants of the x hydroxyl groups involved 

 must be the same. The second hypothesis he rejected as improbable. 



We still consider that the molecular weight of gelatine must be greater 

 than Procter has allowed, and we do not regard it as improbable that a 

 number of the hydroxyl ions of the gelatine may have approximately equal 

 ionisation constants. Our experimental results suggest that, up to a given 



VOL. XCIII. — B. G 



