Chemical Properties of Hydrogen Peroxide

Chemical Properties of Hydrogen Peroxide are related with the fact that Pure hydrogen peroxide is an extremely unstable substance, and, if suddenly raised in temperature, will explode. The reason is that although indirect determination shows the formation from its elements to be an exothermic process,

(H₂) + (O₂) = H₂O₂ + 46,000 calories,

hydrogen peroxide is endothermic with respect to water and oxygen, so that its decomposition into these substances is accompanied by a very considerable evolution of heat:

H₂O₂ = (H₂O) + ½(O₂) + 23,000 calories.

Any tendency to decompose on account of local overheating will therefore be accentuated by the heat developed in the decomposition, so that the latter will tend to become explosive.

Hydrogen peroxide does not volatilise appreciably from its aqueous solutions at 50° to 60° C., and its rate of decomposition at these temperatures is not accelerated by stirring.

Although its dilute aqueous solutions are neutral, various indications have been observed of a feeble acidic tendency in some of the reactions of hydrogen peroxide. At the ordinary temperature barium hydroxide reacts with a solution of hydrogen peroxide, giving a precipitate of hydrated barium peroxide, BaO₂.8H₂O, but at lower temperatures an unstable compound, BaO₂.H₂O₂, is also obtainable. Calcium peroxide in several stages of hydration has been prepared, notably CaO₂.8H₂O and CaO₂.2H₂O or Ca(OH)₂.H₂O₂. In the presence of ammonia, hydrogen peroxide converts calcium chloride into the peroxide, which is precipitated out from solution. With alkali hydroxides various unstable, highly oxidised products have been obtained, and it is possible that such compounds are involved in the catalytic decomposition of hydrogen peroxide by alkalies.

On addition of 30 per cent, peroxide to sodium hydroxide or ethoxide a precipitate of sodium perhydroxide is obtained to which the formula 2NaHO₂.H₂O₂ is given. This possesses marked basic properties, and on saturation with carbon dioxide yields the acid percarbonate, NaHCO₄. The same compound is formed when hydrogen peroxide is added to sodyl hydroxide, NaO.OH. This latter substance was first prepared by Tafel by the action of sodium peroxide upon well-cooled absolute alcohol.

With potassium hydroxide two compounds have been obtained, namely, 2KHO₂.H₂O₂ and 2KHO₂.3H₂O₂.

On passing dry ammonia into a solution of pure hydrogen peroxide in absolute ether at -10° C., or into pure anhydrous peroxide at 0° C., ammonium perhydroxide, NH₄O.OH, or NH₃.H₂O₂, crystallises out. Further addition of ammonia is stated to convert this into an oily mass which solidifies at about -40° C. and has the composition (NH₄)₂O₂, and is therefore ammonium peroxide, the analogue of sodium peroxide. The crystals of ammonium perhydroxide melt at 24.5° C. to an oily liquid, which is stable in the entire absence of water.

The frequent occurrence of additive compounds of hydrogen peroxide with basic organic compounds also appears indicative of an acidic tendency, although it is quite possible that in such compounds the hydrogen peroxide is playing a similar role to water in the so-called water of crystallisation; indeed, many salts do give rise to additive compounds with hydrogen peroxide, a solution of ammonium sulphate in 30 per cent, hydrogen peroxide on evaporation in a vacuum depositing crystals of the composition, (NH₄)₂SO₄. H₂O₂, and analogous compounds with other salts are producible in a similar manner. In such compounds the hydrogen peroxide is only feebly combined and is removable by the addition of water or by extraction with ether. Quite frequently it happens that the hydrogen peroxide "of crystallisation" is accompanied by water of crystallisation, the sodium salt, 2Na₂SO₄.2H₂O.H₂O₂, corresponding with the above ammonium salt being a case in point.

When a mixture of hydrogen peroxide and hydrochloric acid is evaporated in a platinum dish, a certain amount of chlorplatinic acid is formed. Hydrogen peroxide is capable of displacing acids, notably the halogen acids, in certain circumstances. Addition of saturated copper nitrate solution to the peroxide results in the precipitation of brown hydrated copper oxide with simultaneous evolution of oxygen. A similar precipitate is obtained when copper foil is introduced into a solution of hydrogen peroxide containing alkali nitrates or sulphates. The reaction is presumed to take place in several stages; thus, in the case of sodium sulphate solution

Na₂SO₄ + H₂O₂ = Na₂O₂ + H₂SO₄;
Na₂O₂ + H₂O = 2NaOH + H₂O₂;
Cu + H₂SO₄ + H₂O₂ = CuSO₄ + 2H₂O;
CuSO₄ + 2NaOH = Na₂SO₄ + CuO.H₂O.

Hydrogen peroxide vapour affects a photographic plate in the same way as exposure to light, the sensitised film detecting the presence of as little as 3×10^-9 grams of vapour per cubic centimetre; in this power is probably to be found the explanation of the photographic action of some oxidisable metals. The effect is a chemical one, probably analogous to the action of hydrogen peroxide on silver oxide (see later).

Towards protoplasm hydrogen peroxide exerts a decidedly poisonous action, and so causes the death of organic ferments such as yeast, although it does not necessarily affect the activity of the enzymes. Cases have been observed of seeds, moulds, and phagocytes the growth of which was favoured by very dilute solutions of hydrogen peroxide, but it is possible that such results may be due to the presence in the organism of some organic catalyst (catalase) which causes the decomposition of the peroxide and so prevents its usual action. In dilute aqueous solution hydrogen peroxide forms an excellent antiseptic wash for open sores and wounds.

Some of the most striking Chemical Properties of Hydrogen Peroxide are connected with its ready decomposition and the closely related oxidising power which have proved very attractive fields for physico-chemical investigations.