Catalysts of Organic and Inorganic Origin. 295- 



copper sulphate, ferrous sulphate and chromium chloride, which act as 

 oxidases in the presence of hydrogen peroxide but not in its absence. 



Copper sulphate, in the presence of salt and H 2 2 , rapidly causes an oxidase 

 browning in tannic acid, a slight browning is slowly produced with tyrosin, 

 and a full colour sequence with ursol tartrate. The oxidase reactions of 

 copper sulphate and H 2 2 with pyrogallol, hydrocpumone, and gallic acid are 

 approximately the same in the presence as in the absence of salt. 



In the presence of H 2 2 salt slightly accelerates the oxidase action of 

 chromium chloride on hydroquinone and pyrogallol, and a deep blue is given 

 with guaiacum, particularly if bromide is used instead of chloride, but a weaker 

 blue if no chloride or bromide is present. In general sodium and potassium 

 bromides are slightly stronger sensitisers than the chlorides, the iodides are less 

 active* and the fluorides still more so or may even exercise the reverse action. 



Copper acetate, ferrous chloride and potassium ferricyanide, which slowly 

 give a pale blue with guaiacum in the absence of H 2 2 , give a stronger 

 blue rapidly in the presence of salt nearly as well as when H 2 2 is added. 

 Salt is, however, unable to produce a blue in the absence of H 2 2 with 

 manganese sulphate or chloride, with copper oxychloride or with potassium 

 ferrocyanide. 



The relative mass of the oxidase and sensitiser is of importance. Thus if 

 equal masses of ferrous sulphate and of KC1, KI, or KF are present, and the 

 solutions fairly strong, no blue is given with guaiacum, but if the ferrous 

 sulphate is present in relatively dilute solution, a rather pale blue is given 

 with KC1, weaker with KBr and KI, and faint or imperceptible with KF. 

 Hence if the oxidase and sensitiser are not present in the proper proportions 

 some oxidase actions may be prevented or overlooked. Copper sulphate, 

 however, even when present in excess gives direct oxidase reactions in the 

 presence of sensitisers, possibly because unlike ferrous sulphate it has no 

 tendency to auto-oxidation. On the other hand, a sensitiser does not act with 

 all oxidase tests. Thus neither KI, KBr, KC1 nor KF, whether relatively 

 dilute or concentrated, give ferrous sulphate any oxidase action on ursol 

 tartrate or hydroquinone. 



The double fluoride of sodium (XaFHF) inhibits the oxidase action of ferric 

 chloride on guaiacum, ursol tartrate, pyrogallol, hydroquinone and tyrosin, and 

 also its power of decomposing H 2 2 . It strongly retards the oxidase action of 

 potassium ferricyanide and of manganese sulphate and H 2 2 , and although a 

 blue is still given with guaiacum it is much paler. A rather weaker retarding 

 action is also exercised upon the oxidising action of potassium permanganate 



* They cannot be tried with copper, owing to the precipitation of the latter, or used 

 in the presence of H 2 2 , owing to the decomposition of the iodide by the peroxide. 



