78 



Miss D. J. Lloyd and Mr. 0. Mayes. 



experimental points,and the calculations of ri have been made from Pn readings 

 taken from this smooth curve and not from the Pn readings given in 

 Table II. Taking into consideration the poor quality of our experimental 

 determinations of E in alkaline solutions, and the lack of agreement as to the 

 values of « for caustic soda, it has not been considered worth while to correct 

 the values of n' obtained by the Blasel and Matula formula. Fig. 5 shows n 

 plotted against [OH], and will be referred to later in Section IV (I). 



IV. The Calculation of the Ionisation Constants of Gelatine. 

 (a) The Value of K b . 

 The combination of gelatine with hydrochloric acid between OOO and 

 0"25 N concentration of acid may be represented as a curve with three 

 sections. 



Assuming for the moment that gelatine in hydrochloric acid solution 

 behaves as a monacidic base from 0-00 to 0"0-4 N HC1, and combines with 

 the acid to form an ionisable salt (see Introduction), the system may be 

 represented as follows : — 



Let [G(H)OH] represent the un-ionised gelatine base, and [GH'] and 

 [OH'] the two ions of the ionised base ; and assume that the salt GHC1 is 

 completely ionised ; now by the law of mass action for a weak base, 



[GH']x[OH'] 

 [G (H) OH] 



FGH'l TH1 K 

 - ' • r, L, J tri = where Ki = — the hydrolysis constant of the base. 

 [G(H)OHJ Ivi ilj 



Let C = the equivalent concentration of the gelatine. 

 Then [GH'] + [G(H)OH] = C, 



* • ~c~-hTk 1 ' w 



[OH'] _ H 



[GH'] + [G(H)OH] R + Ki' 



Now, in equation (1), [GH'] is equal to N' and is known, H is known, and 

 therefore there are two unknown quantities, C and Kj. If Procter's (20) 

 value of 839 be taken as the reacting weight of gelatine, C = O012. 

 Wilson's (26) later value of 768 makes C equal to - 013. Wintgen and 

 Kruger (27), using the catalysis of methyl acetate as a measure of the 

 concentration of the hydrogen ion, obtain a molecular weight of 839 for 

 gelatine, while, calculating from the experimental results of Pauli and 

 Hirschfeldt, they obtain the value 881/4. They give 2 - 7 x 10 -11 as a value for 

 K ft at 25° C. In the calculations given below, C is taken as 0'0120. 



