300 PROPERTIES OF AMINO-ACIDS AND PEPTIDES 



included among those for which accurate determinations are not 

 possible. 



This raises the question of possible inaccuracies in the determina- 

 tions. Aside from such questions as the presence of small amounts of 

 impurities at or near the isoelectric points of ampholyte solutions 

 where the buffer action is found to be small/° certain points con- 

 nected with the relative accuracies of the chemical and electrical 

 measurements are involved. Comparing, for example, the determi- 

 nation of the hydrogen ion concentration of 0.1 n hydrochloric acid 

 (pH approximately 1.0) and of 0.1 N sodium hydroxide (pH approxi- 

 mately 13.0), the electrical measurement is much more sensitive for 

 the latter. The component determined in both cases is the hydrogen 

 ion. In the preparation of the solutions, the alkaline solution is more 

 readily influenced or changed (perhaps due to the accidental nature 

 of the surroundings) as regards its hydrogen ion concentration. Alka- 

 line solutions may therefore require the presence of buffer mixtures in 

 order to obtain constant and reproducible results under conditions 

 with which analogous acid solutions apparently give accurate values 

 with no buffer present. 



The presence of sodium chloride of the given concentration had no 

 appreciable influence on the determinations as shown by the work of 

 Hamed" and of Fales and Nelson^^ on mixtures of acid and neutral 

 salts, of Sorensen^^ who compared ampholyte solutions with and without 

 salt, and of Tague^^ who measured amino-acid solutions without salt. 



Some determinations by others may be compared with the results 

 given here, although in most cases the results are not comparable for a 

 whole series. The solutions with no added solute besides the acid 

 and alkali agree satisfactorily with such results^^ except that Tague 

 appeared to be more successful in excluding carbon dioxide. The 

 results for glycine agreed well with those of Sorensen^^ and of Tague^* 

 where comparison is possible. 



^° Cf. Michaelis, L., Die Wasserstoffionenkonzentration, Berlin, 1914, for a 

 careful and systematic discussion of these relations. 



11 Harned, H. S., /. Am. Chem. Soc, 1915, xxxvii, 2460. 



1^ Fales, H. A., and Nelson, J. M., /. Am. Chem. Soc, 1915, xxxvii, 2773. 



1^ Sorensen, S. P. L., Compt. rend. lab. Carlsherg, 1909, viii. 



1* Tague, E. L., /. Am. Chem. Soc, 1920, xlii, 173. 



IS Cf. Frary, F. C, and Nietz, A. H., /. Am. Chem. Soc, 1915, xxxvii, 2264. 



