OXIDATION AND REDUCTION 587 



The formulae rather suggest adsorption compounds. We may also call to mind the con- 

 troversy between Faraday and de la Rive (see page 306 above). 



To return to the hypophosphite system, hypophosphites do not undergo 

 oxidation in water alone at any measurable rate. But in the presence of finely 

 divided palladium, this takes place. Bach puts it thus : the water is decomposed 

 and its HO is used for oxidation of the hypophosphite, the hydrogen is taken up 

 temporarily by palladium, and then set free. Thus : 

 H x /H OHH H x /OH 



>P\ + = jP< + H 2 + H 2 0. 



0^ X)H OHH 0^ X)H 



Palladium acts as a true catalyst ; minute amounts decompose indefinite amounts of 

 hypophosphite. If an easily reducible substance is present, the nascent hydrogen 

 reduces it. 



If we take an aldehyde in place of hypophosphite, we find that the presence 

 of metals of the platinum group does not accelerate to any great degree the 

 decomposition of water. A further addition is required in the form of an easily 

 reducible substance as " acceptor " for the nascent hydrogen as it is formed ; such 

 substances are methylene blue, indigo, nitrates, and so on. None of these are 

 reduced at any perceptible rate by formaldehyde alone without platinum. In 

 the reaction, the aldehyde is oxidised to the corresponding carboxylic acid. 

 The case of methylene blue is instructive because this dye contains no oxygen, so 

 that the additional atom of oxygen required to convert formaldehyde into formic 

 acid must come from the water by some means. 



The explanation of this fact suggested by Bach (1911, 1) is, shortly, as follows. 

 Water may be looked upon as an unsaturated compound, H 2 O = , since oxygen is 

 quadrivalent, at all events potentially. Since H' and OH' ions are also present 

 in water, as we have seen, it seems probable that " unstable complexes " may be 

 formed thus : 



H x /H- H0' v /H 



>O< and >O< 



H/ X H- HO'/ X H 



The first may be called " hydrogen suboxide " or " oxygen perhydride," analogous 

 to the metallic salts M 4 O, such as Ag 4 O. The second is the hydrate of 

 hydrogen peroxide. We have seen reason in Chapters VII. and VIII. to hold 

 that ions are associated with water molecules. 



The acceleration by platinum of the oxidation of aldehydes to form acids can be explained 

 on the view of Engler and Wohler (1901) that colloidal platinum combines with molecular 

 oxygen to form a peroxide, Pt0 2 , which, in its turn, reacts with water to form the hydrate 



HO-Pt-O = O 



H 



This substance may also be supposed to be formed by reaction of platinum with the H 2 0(OH') 2 

 present in the water. It acts as a powerful oxidising agent on formaldehyde. Since 

 H 2 0(OH'). 2 is used up, the equilibrium is disturbed, more is formed, and so the catalytic 

 process continues. 



If we admit the presence in water of H 4 0, it is not unlikely that platinum metals should 

 form strongly reducing hydrides by combination with this. 



The two complexes of H' and OH' ions with water are only present in very small con- 

 centration, so that unless one of them is used up, say the hydride in reducing methylene blue, 

 the oxidation of the aldehyde can only take place to a very small extent. 



I have already remarked that this view assumes the existence of chemical compounds 

 of rather doubtful nature, and Bach himself states (1913, p. 154) that the question requires 

 further investigation to make the mechanism clear. The view given certainly explains 

 the occurrence of active hydrogen, which is otherwise difficult to account for. 



We naturally turn next to look for the evidence of an enzyme, in milk and 

 tissue cells, which plays a part similar to that of the platinum metals. Since a 

 peroxidase causes the activation of peroxide 1 oxygen, we may call, with Bach 

 (1913, p. 161), an enzyme which causes the activation of perhydride hydrogen a 

 " perhydridase." 



The enzyme shown by Schardinger and Trommsdorff to be responsible for the 

 reduction by fresh milk of methylene blue, if an aldehyde be also present, has been 

 referred to above. Now, reducing action of fresh tissues has been described by 



