292 PROPERTIES OF AMINO-ACIDS AND PEPTIDES 



acetamide, urea, acetic acid, and aceturic acid. The substances were 

 prepared by the usual methods and showed satisfactory states of 

 purity except for the alanyl-alanine, for which the nitrogen content 

 was found to be 1 per cent less than the calculated value. Attempts 

 at purification did not remedy this, so that evidently an impurity, 

 possibly alanine, was present. 



The solutions studied were prepared so that 1 liter of the final mix- 

 ture contained in every case 0.05 gram molecule of the amino-acid or 

 peptide or other substance, and 0.05 gram molecule of sodium chloride, 

 while the amounts of hydrochloric acid or sodium hydroxide (0.1 n) 

 added were varied. The water used was distilled first from sulfuric 

 acid, then from alkaline permanganate, and condensed in a block tin 

 condenser. 



The concentration of the ampholyte or other substance and of the 

 sodium chloride in the final volume (50 cc. as made up and used) 

 was the same in all cases. The only variable was the amount of acid 

 or alkali which was added, or the ratio between this and the concen- 

 tration of solute. 



The hydrogen ion concentrations of the solutions were determined 

 by electromotive force measurements using calomel electrodes with 

 0.1 M potassium chloride solution, saturated potassium chloride bridge, 

 unknown solution with hydrogen electrode (platinum or gold coated 

 with palladium black) , and a potentiometer of Leeds and Northrup type 

 ''K" in conjunction with a type "H" galvanometer with a loading coil 

 to decrease its period. Cells of the rocking type described by Clark' 

 were used with the modification of having an adapter of about 25 cc. 

 capacity fused on the inlet tube.^ Hydrogen was obtained compressed 

 in cylinders and was purified by passage through (electrically) heated 

 copper gauze and then bubbled through water. 



The customary precautions in purifying the materials, and stand- 

 ardizing the various parts of the apparatus and the solutions were 

 taken. The hydrogen ion concentrations in terms of pH were calcu- 

 lated from the experimental measurements by the formula^ 



3 Clark, W. M., /. Biol. Chem., 1915, xxiii, 475. 



■* The writers desire to thank Professor F. S. Lee of the Department of Physi- 

 ology of Columbia University for presenting to them the excellent rocking appa- 

 ratus used in this work. 



^ Sorensen, S. P. L., Compt. rend. lab. Carlsberg, 1909, viii, 22, 29. 



