1919] 



DODGE TYROSINASE REACTION 77 



sulphonphthalein, the only indicator which I have found 

 which changes at the right point for work with tyrosin, value- 

 less. Sorensen ('07) states that he was unable to use either 

 phenolphthalein or thymolphthalein in the formol titration 

 of tyrosin. 



A method was then devised for the titration of dark-colored 

 solutions, by titrating them until they matched a solution of 

 definite hydrogen ion concentration, thus making possible a 

 graph after the method of Bovie ( '15), which will be described 

 more fully in connection with the discussion of results. The 

 basic principles of the method were worked out in connec- 

 tion with a paper by Dr. B. M. Duggar and the writer (Duggar 

 and Dodge, '19 ). A Kober colorimeter, with artificial light 

 and rheostat for varying the light intensity, was used as in 

 the ammonia determinations cited above. The general method 

 of Walpole ('10), improved by Homer ('17), of comparing 

 colored solutions, was adapted to our purpose. For details 

 and precautions necessary in such color comparisons, see 

 paper cited above (Duggar and Dodge, '19 ). It was found 

 after some experiments that for ordinary work the amount 

 of light absorbed by the water was so slight that it might be 

 dispensed with, making the apparatus much easier to manipu- 

 late. In this case the right cup was set at 20 mm. and low- 

 ered in proportion to the amount of alkali added, as this 

 diluted the color in the cup. This phenomenon was found to 

 make the solutions incomparable as dilution increased, 

 although such a small amount of dilution did not affect 

 the hydrogen ion concentration of the solution appre- 

 ciably. The burette used consisted of a 2-cc. pipette, gradu- 

 ated to 0.02 cc, melted on a glass stopcock, the end of which 

 was drawn out to a tip which gave drops of 0.01 cc. of N/50 

 Ba(OH)2 consistently. This tip was placed directly over the 

 liquid so that a drop of water was rarely necessary to wash 

 down the drop. The solution was stirred after the addition 

 of each drop, by agitating the cup about the prism, care being 

 taken to prevent the spilling of the solution. 



In the work with tyrosin, thymolsulphonphthalein was 

 found to be the most suitable indicator, since it has an alkali 



