738 COMBINATION OF GELATIN WITH HYDROCHLORIC ACID 



normality of 0.0092 with respect to its combination with HCl, or 

 that the combining weight of gelatin is „ Q^g^ , or about 1,090. While 

 the exact height of the maximum is still more or less uncertain, it is 

 probable that this value of the combining weight is more nearly 

 correct than the smaller values given by Procter,^ Wilson,^ Wintgen 

 and Kriiger,^" and Wintgen and Vogel,^"^ because the calculation 

 involves simpler and more probable assumptions. Moreover, the 

 earlier workers did not have ash-free or isoelectric gelatin at their 

 disposal. 



It is possible to calculate from these results an ionization constant 

 for gelatin, assuming it to react as a mono-acid base. The simplest 

 way of doing this is based on the resemblance between the com- 

 bination curve and the dissociation curve of a base. The equation 



kb 

 for the ionization of such a base is a = — - — _ where a represents 



the fraction ionized and kb the ionization constant. Since the ion 

 product of water, kw, is equal to [H+] X [0H~], this becomes 



[H+] -I- K' kb 



Michaelis^2 has pointed out that if a is plotted against log [H+], 

 at the point where a = |, log [H+] = log K. Applying this to Fig. 2, 

 a = ^ where the ordinate = 4.6 or pH = pK = 3 . 625. Accordingly 

 K = 2.4 X 10""^ This is of the same order as the value obtained 

 by Procter and Wilson^^ and is intermediate between the values ob- 

 tained by Wintgen and Kriiger^" and Wintgen and Vogel,^^ but differs 

 by a whole power of 10 from that obtained by Lloyd and Mayes.^ 

 Inasmuch as the use of this constant leads to only rough agreement 

 with the combination curve in Fig. 2, it is not desired to lay any 

 stress on this calculation. It is quite certain that gelatin is not 

 a mono-acid base, though its combination curve may resemble the 

 ionization curve of such a base. 



8 Procter, H. R., /. Chem. Soc, 1914, cv, 313. 

 8 Wilson, J. A., /. Am. Leather Chem. Assn., 1917, xii, 108. 

 i" Wintgen, R., and Kruger, K., Kolloid-Z., 1921, xxviii, 81. 



11 Wintgen, R., and Vogel, H., Kolloid-Z., 1922, xxx, 45. 



12 Michaelis, L., Die Wasserstoffionenkonzcntration, Berlin, 1914, 20. 



13 Procter, PI. R., and Wilson, J. A., /. Chem. Soc., 1916, cix, 307. 



