DAVID I. HITCHCOCK 737 



These results were plotted on a large scale and a smooth curve was 

 drawn through the points. From this curve were read off the con- 

 centrations of free HCl present at the pH actually measured in each 

 gelatin chloride solutibn, and these values were subtracted from the 

 total HCl concentrations of the respective gelatih chloride solutions. 

 The differences accordingly represent the millimoles of HCl comi- 

 bined with the amount of gelatin present in 1 liter, or the cc. of 0.1 m 

 HCl combined with the gelatin present in 100 cc. These values were 

 divided by the number of grams of gelatin in 100 cc. to get the cc. 

 of 0.1 M HCl combined with 1 gm. of gelatin, which is the sam.e as 

 the number of millimoles of HCl combined with 10 gm. of gelatin. 

 Table II indicates the method of calculation, and the final results 

 are plotted in Fig. 2. 



It will be noticed that the points lie fairly close to a smooth curve, 

 except in the most acid region, where a small error in the pH may 

 lead to a large error in the difference between the ordinates of two 

 steep curves. The curve is horizontal between pH 1 and 2, indi- 

 cating that here the gelatin is all combined with the acid. There is 

 no evidence of the discontinuous sections found in the curve of 

 Lloyd and Mayes. ^ This difference is due in part to differences in 

 the experimental curves, but is also due largely to the method of 

 calculation of the combined acid. The method used by Lloyd and 

 Mayes involves the assumption that the uncombined HCl is ionized 

 to the extent indicated by the conductivity ratio for a different 

 concentration of HCl; namely, the concentration of the total HCl 

 present. Moreover, these authors have neglected the difference 

 between the conductivity ratios and the activity coefficients or 

 hydrogen electrode values for HCl, which is clearly brought out by 

 the table given by Lewis" from which they obtained their ionization 

 values. The method of calculation used by Loeb, Tague, and the 

 present writer involves the assumption that the same concentration 

 of uncombined HCl is necessary to furnish the same hydrogen ion 

 concentration, as determined by the hydrogen electrode, whether 

 or not gelatin is present. The latter assumption seems to lead to 

 more reasonable results. 



The maximum height of the curve in Fig. 2, 9.2 millimoles of HCl 

 for 10 gm. of gelatin, imlicates that a 1 per cent gelatin solution l.as a 



