238 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



SECTION III. THE QUANTITATIVE ESTIMATION OF 

 OXIDASE. 



COLORIMETRIC METHODS FOR PHENOLASES. 



The estimation of oxidases and of their rate of action 

 presents many difficulties. The following colorimetric 

 methods have been tried. Laborde (1898), and later on 

 Brunn (1909) and Euler and Bolin (1909), used an alcoholic 

 solution of guaiacum resin, which becomes blue on oxidation. 

 Slowtzoff (1900) estimated laccase by the rate of coloration 

 of an aqueous solution of ^-phenylenediamine and m-tolui- 

 dene. The oxidation of phenolphthalin to phenolphthalein 

 was followed by Kastle and Shedd (1901), the latter sub- 

 stance giving an intense red upon the addition of a trace 

 of alkali. Von Czyhlarz and Von Furth (1907) made use 

 of the leucobase of malachite green. By determining the 

 extinction coefficient of the solution from time to time, 

 the rate of the reaction was studied. None of these proved 

 reliable or of general application, both because further 

 oxidation of the coloured oxidation products of the chromo- 

 gens results in the formation of colourless substances, and 

 on account of the natural condition of the sap pressed 

 from plant tissues, which is rarely colourless or free from 

 turbidity after nitration. 



VOLUMETRIC METHODS FOR PHENOLASES. 

 Estimations of an aldehydase were carried out by 

 Medvedew (1897) by titrating the acid formed by the 

 oxidation of salicylaldehyde. Bach (1904) introduced 

 the method of titrating the iodine liberated by the oxida- 

 tion of hydriodic acid, the latter being derived from 

 potassium iodide and acetic acid. 



