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



and upon its power of decomposing H 2 2 , the solution in the latter case 

 remaining clear instead of turning brown. 



The single fluoride (ISTaF) is also capable of acting as an antagoniser, 

 particularly shown in the case of ferric chloride with guaiacum and hydro- 

 quinone, while with pyrogallol a violet-blue is given instead of dark brown. 

 On the other hand the addition of sodium fluoride to potassium ferricyanide 

 increases all its oxidase reactions except to pyrogallol, where a retardation 

 is shown, and renders it a weak oxidase to tannic acid and a '■' peroxidase " 

 to hydroquinorie. 



If a dilute solution of lead nitrate is added to a dilute solution of copper 

 acetate and salt in such proportion that two atoms of chlorine are present to 

 each atom of lead, a pale blue is still given with guaiacum, but none if the lead 

 nitrate is in excess, although in neither case is any precipitate formed. Lead 

 nitrate might, therefore, be regarded as an antagoniser to copper acetate and 

 sodium chloride as a sensitiser. An excess of lead nitrate prevents copper 

 acetate giving a distinct blue by itself, but a blue is still given in the presence 

 of H 2 2 . 



In the case of potassium permanganate the addition of sodium fluoride does 

 not affect the guaiacum test or the decomposition of H 2 2> slightly accelerates 

 the oxidation of tannic acid (in the presence of H 2 2 ) and of ursol tartrate, 

 and distinctly retards the oxidation of pyrogallol and hydroquinone. The 

 same substance may, therefore, be a sensitiser to one oxidase and an 

 antagoniser to another, while the action may vary according to the substance 

 oxidised. 



• Sensitisers and Antagonisers to Plant Oxidases. 



It is well known that diastase acts as an oxidase to guaiacum and the same 

 applies to other ferments. Plant oxidases, however, appear to be more 

 specific and less generalised in their action than are metallic oxidases. Hence 

 it is of importance to determine to what extent the specific peculiarities of 

 certain plant oxidases can be ascribed to the presence of accompanying 

 sensitisers and antagonisers or inhibitors. 



In the following Table a general comparison is given between a few of the 

 common plant ferments and oxidases. In the first four cases watery solutions 

 were used, in the last two cases thin slices of potato and apple were rapidly 

 dried in vacuo after squeezing out the sap and were pounded to powder. The 

 powder was added to the test solution. 



The chief peculiarities are that potato oxidase acts strongly on tyrosin and 

 feebly or not at all on tannic acid or on ursol tartrate in the absence of hydrogen 

 peroxide, while apple oxidase, which is generally weaker, acts strongly on 

 tannic acid and ursol tartrate but not at all on tyrosin. 



