70 



Miss D. J. Lloyd and Mr. C. Mayes. 



concentration of hydrogen ions, a group of hydroxyl ions having approximately 

 equal ionisation constants is involved ; beyond this concentration, and up 

 to a second fixed value, a second group approximating to a second constant 

 is involved ; and beyond this again there is slight evidence of a third group. 

 The factors required in order to bring the second and possible third groups 

 into conformity with the generalised statement of the law of mass action 

 are not yet fully known. 



II. Material and Method. 



The gelatine used in this investigation was Coignet's Gold Label gelatine, 

 purchased in 1914. It was purified by prolonged dialysis in dilute hydro- 

 chloric acid at a reaction of Ch = 10 -4 ' 6 , and subsequent precipitation in 

 strong alcohol. Details of the purification are given elsewhere (Jordan 

 Lloyd, 7). It was dried with absolute alcohol and ether, and kept in a 

 desiccator over pure sulphuric acid. (It is possible to cause the gelatine 

 to lose more water by heating to 100° over phosphorus pentoxide in vacuo.) 

 When dried with absolute alcohol it is a white brittle substance, fibrous 

 in appearance, and containing O'OO to 0"06 per cent, of ash. It forms clear 

 solutions in water, which set to opaque white gels on cooling. The clear 

 solutions set to colourless, glassy, transparent gels in the presence of free 

 acid or base. The solution referred to below as 1 per cent, gelatine contains 

 1 grm. of this purified dry gelatine in 100 c.c. of freshly-boiled distilled 

 water at room temperature. 



The object of our experiments was to determine the amount of hydro- 

 chloric acid or sodium hydroxide which would combine with a constant 

 weight of gelatine, and the method employed throughout was the electrical 

 measurement of the concentration of the free hydrogen ions in solutions 

 containing 1 per cent, of gelatine and known concentrations of hydrochloric 

 acid or sodium hydroxide. The change of hydrogen ion concentration from 

 that of an equally concentrated system containing no gelatine, is a measure 

 of the acid (or base) which has combined with the gelatine. The routine 

 method employed was to take 5 c.c. of a freshly made solution of 2 per cent, 

 gelatine which had cooled but not yet set ; the requisite amount of acid 

 (or base) was added from a pipette calibrated to - 01 c.c, and sufficient freshly- 

 boiled distilled water to make the final volume 10 c.c. The reaction was taken 

 at once by means of a gas chain. A Tinsley potentiometer was used. 



The electrodes used were of a very simple modified Barendrecht type, and 

 were made for us by Mr. A. W. Hall, of the Biochemical Laboratory. Their 

 great advantage is the ease with which they are cleaned, a matter of great 

 importance, as we found it essential to clean and re-coat the electrodes after 



