Catalysts of Organic and Inorganic Origin. 293 



guaiacum — strong blue colour. Either sodium or potassium chloride will cause 

 copper sulphate to give as deep a blue as in the presence of H 2 02, i.e. converts 

 it from a " peroxidase " to an oxidase. The action is not solely due to cupric 

 chloride being present in the mixed solution, since the blue is deeper than 

 with cupric chloride alone. The intensity of the action decreases slowly with 

 increasing dilution. It is given by a dilution of 1 grm. of copper sulphate in 

 100,000 c.c. of water, faintly in a dilution of 1 in 250,000, very faintly by 

 1 in 100,000, and not at all in a solution of 1 in 10,000,000 ; i.e. guaiacum is 

 about as sensitive in the presence of salt to the oxidase action of copper 

 sulphate as the pulp cells of apples are to its poisonous action. 



Sodium and potassium phosphates are also able to act as sensitisers to such 

 oxidases as potassium ferricyanide, and the influence of a sensitiser may show 

 with some but not necessarily with all test substances (oxidants). The 

 accelerating action of phosphates is particularly marked with tannic acid, if 

 sufficient is added to leave a clear solution. An excess produces a purplish 

 white precipitate and naturally interferes with oxidation. Acid potassium 

 phosphate is a less active oxidase sensitiser than the neutralised solution of 

 the same salt. Neutral potassium phosphate accelerates the oxidation of 

 tannic acid by potassium permanganate, but not by black oxide of manganese, 

 and it acts as a retardant to those soluble metallic oxidases which it pre- 

 cipitates. 



Water itself may act as a sensitiser as well as an oxygen carrier. Thus, 

 if potassium ferricyanide is dissolved in pure boiled glycerine and guaiacum 

 dissolved in absolute alcohol added, even after long standing only a faint blue 

 or none at all appears at the junction of the two liquids, which rapidly inten- 

 sifies on adding a little water. Nasse and Fram* have even gone so far as to 

 ascribe the oxidation entirely to the hydroxidation of water without the 

 presence of free oxygen being necessary, but Porodkof has shown that this is 

 not the case. 



The addition of a neutral solution of potassium phosphate to potassium 

 ferrocyanide does not cause any ferricyanide to appear, but causes it to behave 

 as a weak oxidase to guaiacum, ursol tartrate, pyrogallol and hydroquinone, 

 and as a " peroxidase " to gallic acid, tannic acid and tyrosin. The action in 

 the presence of hydrogen peroxide is, however, in part due to its partial con- 

 version into ferricyanide. Neutral potassium phosphate intensifies the 

 oxidase reaction of potassium ferricyanide and converts it from a non-oxidase 

 to tannic acid and tyrosin into an oxidase to the former and a " peroxidase ' 

 to the latter. 



* ' Pfliiger's Archiv,' vol. 63, p. 203 (1896). 



t 'Bot. Centralbl.,' Beihefte, vol. 10, p. 1 (1904). 



