H. ECKWEILER, H. M. NOYES, AND K, G. FALK 293 



TT -0.3377 



pH 



0.0577 + ({t - 18) 0.0002) 



in which tt = reading in volts; 0,3377 = potential of calomel elec- 

 trode; and 0.0577 + ( (/ - 18)0.0002) = correction factor for tem- 

 perature /°, as a thermostat was not used. 



In a number of cases the hydrogen ion concentrations were also 

 determined colorimetrically. Where this was done, the indicators and 

 standard solutions described by Clark and Lubs were used.^ 



The isoelectric points of the various ampholytes were also deter- 

 mined by the method described by Michaelis^ and developed by Soren- 

 sen.8 This consists of adding the pure ampholyte solution to a series 

 of solutions of progressively changing hydrogen ion concentration 

 containing the indicator and noting where no change in color 

 occurred on mixing the unknown ampholyte solution and standard 

 solution. 



Experimental Results. 



The results obtained for the pH values for the solutions of the vari- 

 ous substances in the presence of acid and alkali are given in Table I. 

 Although not stated in the table, it may be recalled that all the solu- 

 tions contained sodium chloride of the concentration 0.05 m. Column 

 1 headed ''Water" contained no added solute but the sodium chloride 

 and acid or alkali. The headings of the remaining columns indicate 

 the solute present, 0.05 m in every case. 



The titration curves plotted from these results are given (Figs. 1, 

 2, and 3). The pH values are given as abscissa?, and as ordinates are 

 shown the number of cc. of 0.1 n hydrochloric acid or sodium hydrox- 

 ide added to 25 cc. of 0.1 m solution of the indicated solute (also 0.1 m 

 with respect to sodium chloride), and the whole diluted to 50 cc. The 

 numbering of the curves corresponds to the numbering of the columns 

 in Table I. Since the results of the amino-acids are so nearly alike, 

 only one curve (Curve 6, Fig. 2) is given. Similarly for the dipeptides, 

 only one curve (Curve 10, Fig. 3) is presented. 



« Clark, W. M., and Lubs, H. A., /. BacL, 1917, ii, 1, 109, 191. 

 ^ Michaelis, L., Biochem. Z., 1912, xlvii, 251. Hasselbalch, K. A., Biochem. Z., 

 1916, Ixxviii, 129. 

 * Sorensen,^ p. 150. 



