292 Prof. A. J. Ewart. Comparative Study of Oxidation by 



acids hinder or decrease the blue reaction, which is however given strongly 

 even in the presence of 1-per-cent. oxalic acid. Hydrogen peroxide is 

 jmrtially antagonistic to this action, and it is possible to obtain acidified 

 solutions of ferric chloride which will give a blue when peroxide of hydrogen 

 has been added, but not when it is absent. The effect probably depends upon 

 the ionic condition of the iron in solution, but the disappearance of an 

 oxidase reaction on the addition of acid does not necessarily mean that the 

 oxidase has been destroyed, any more than ferric chloride is destroyed by the 

 addition of hydrochloric acid. Further, in Moore and Whitley's experiment 

 with expressed potato sap heated to 55° C. for some hours, the sap becomes 

 more acid, and this might in itself be a sufficient explanation of why an 

 addition of peroxide of hydrogen then becomes necessary to obtain an 

 oxidase reaction. 



Further, the case of cobalt chloride and ammonia shows that the addition of 

 alkali to certain plant oxidases might greatly increase their oxidase activity 

 or might convert a non-oxidase combination into an oxidase one. Thus, with 

 a small quantity of ammonia, cobalt chloride forms a green precipitate, slowly 

 oxidising to brown ; but with slight access of ammonia a nearly colourless 

 liquid is formed, oxidising to brown from the surface. If the two liquids are 

 diluted, the latter gives a blue with guaiacum directly, and the former an 

 intense blue on adding H2O2; but without H 2 2 no blue is given, or only a 

 faint trace on long standing. In other words, an oxidisable substance can 

 act as an oxidase, and ammonia, by accelerating the rate of auto-oxidation, . 

 also increases the intensity of oxidase action and converts a peroxidase into 

 an oxidase. 



According to Porodko* per salts (-ie) of iron, copper, manganese, and 

 chromium give a blue with guaiacum in the absence of peroxide of hydrogen, 

 proto salts (-ous) only when it is present. This is, however, by no means a 

 general rule. . Ferrous chloride gives a blue without H2O2, but not cupric 

 sulphate or manganic chloride in moderate dilution. Lead nitrate gives no 

 blue in the presence or absence of hydrogen peroxide, while lead acetate gives 

 a strong blue in its presence. Further, the addition of H2O2 converts the 

 non-oxidase ferrous sulphate or potassium ferrocyanide into ferric compounds, . 

 which give a blue with guaiacum in the absence of hydrogen peroxide. 



Oxidase Sensitisers and Inhibitors. 

 Various neutral salts may exert a powerful action on metallic oxidases 

 either as sensitisers or retardants. Thus an old test for a soluble copper salt 

 given by Purgotti is : Add salt and pour on top an alcoholic solution of 

 * ' Bot. CentralbL,' Beihefte 2, vol. 16, p. 1 (1904). 



