The Titration Curve of Gelatine. 



71 



every reading. A diagram of the electrodes is given below (fig. 1), and is 

 self-explanatory. 



from n**^' 

 hydrogen «^ 



ground 

 Cf/dST Jomt- 



p/at/num fo/7 

 coated w//-h — 

 p/at/num b/ack 



<Q—p/atwum 

 w/re 



V 



Vic. I 



Y 



The platinum foil was coated very thinly with platinum black according 

 to the directions of Michaelis (16). Only sufficient platinum was deposited 

 to hide the glint of the foil. With such electrodes it was possible to take 

 readings immediately contact had been made with the experimental fluid 

 This is an essential condition for accuracy. With a slow electrode and a 

 delayed reading, values tend to be very irregular. This is particularly 

 marked in alkaline solutions. The experimental fluid was placed in silica 

 cups into which the electrodes dipped. Contact was made by means of a 

 sliding joint. The solutions had a strong tendency to froth near the iso- 

 electric point. All experimental readings were taken at 20° C. 



III. Experimental Results. 

 (a) The Gelatine-Hydrochloric Acid Curve. 



If N represents the normality of a solution of hydrochloric acid and a its 

 degree of ionisation, the concentration of the hydrogen ion present may be 

 represented as N« on the normality scale, or —log N« on the logarithmic scale. 

 If 1 per cent, of gelatine be introduced into such a system, the reaction (Ph) 

 is no longer given by the expression — logN«, but by some lower value. 

 This can be determined by the hydrogen electrode. 



The following experimental values were obtained for the variation of 

 reaction (Ph) with total acid-content (N), gelatine concentration being kept 

 constant at 1 per cent, temperature at 20° C. The constants used in calcula- 

 ting the reaction (Ph) from the observed electromotive force (E) are taken 

 from Michaelis' ' Wasserstoffionen Konzentration ' (16). The values for the 

 concentration of the hydrogen ion are given on both the logarithmic scale 

 Ph, and in terms of normality [H]. From the values of [H] it is possible to 

 calculate the concentration of un-ionised free hydrochloric acid present in the 



G 2 



