Physical Properties of Hydrogen Peroxide

Anhydrous hydrogen peroxide is an almost colourless syrupy liquid, which in bulk is seen to have a faint greenish-blue colour rather more marked than that of water. The odour of the pure substance is stated to recall that of nitric acid, but the dilute solutions are without odour.

Pure hydrogen peroxide readily assumes an undercooled condition and may refuse to crystallise even at temperatures as low as -30° C., although its melting-point is -1.70° C. If, however, a small quantity be solidified by strongly cooling, e.g. with a mixture of solid carbon dioxide and ether, the product can be used as a nucleus to induce crystallisation at temperatures just below the melting-point. The solid then separates in prismatic crystals, and fractional crystallisation may be applied as an effective method of final purification. Slow evaporation of the liquid can occur at the ordinary temperature, and, under reduced pressure, distillation may be effected with only slight decomposition, the boiling-point under 26 mm. of mercury being 69.2° C. Hydrogen peroxide is denser than water, the following data being given for the anhydrous liquid (99.93 per cent.):

Temperature °C	-12.13	-6.23	0.10	3.00	5.55	8.30	12.60	15.30	19.90
Density of H2O2	1.4774	1.4705	1.4631	1.4579	1.4570	1.4541	1.4490	1.4465	1.4419

The variation in density with dilution is shown in the following table.

Water is miscible with hydrogen peroxide in all proportions, a slight evolution of heat accompanying the process; although dilute solutions are neutral in reaction, pure hydrogen peroxide appears to be feebly acidic.12 The aqueous solutions have a characteristic unpleasant taste and, if fairly concentrated, attack the skin, causing a prickling sensation and the formation of opaque white patches which slowly fade. Ether, alcohol, and acetic acid are also good solvents for hydrogen peroxide.

Density of Aqueous hydrogen peroxide

H2O2 per cent.	Density at 0° C.	Density at 18° C.
0.00	0.99987	0.99862
10.57	1.0419	1.0372
22.33	1.0894	1.0815
40.14	1.1655	1.1552
56.70	1.2404	1.2270
61.20	1.2610	1.2465
73.44	1.3235	1.3071
84.86	1.3839	1.3662
90.42	1.4144	1.3955
98.89	1.4596	1.4404
99.93	1.4632	. . .

In the following table are given the more important physical constants of hydrogen peroxide as compared with water, compiled from the data supplied by Maass and Hatcher. These may be taken as entirely replacing the earlier data of Bruhl and others.

Physical properties of hydrogen peroxide

Physical Constant.	Hydrogen Peroxide (100 per Cent.).	Water
Melting-point	-1.70° C.	0° C
Density of liquid at 0° C.	1.4633	0.99987
Mean coefficient of expansion (-10° to + 20° C.)	0.00107	0.0000
Density of solid	1.644	0.9168
Specific heat of liquid	0.579	1.000
Latent heat of fusion of solid	74 calories	79.7 calories
Specific heat of solid	0.470	0.5057
Surface tension at 0° C	78.76 dynes	75.5
Molecular association at 0° C	3.48	3.58
Viscosity at 0°	0.01828 dynes	0.01793 dynes
Refractive index D 22°	1.4139	1.3330
D 20°	1.40624	. . .

A study of the freezing-point curve of solutions of hydrogen peroxide in water indicates the existence of a definite hydrate H₂O₂.2H₂O, melting at -51° C.

In aqueous solution, hydrogen peroxide possesses only a very feeble electrical conductivity comparable with that of water, and cannot be regarded as an electrolyte. As a solvent, however, it exhibits certain characteristics similar to water; thus it favours the dissociation of the solute, especially of acids, whilst with salts it tends to form compounds in a manner corresponding with the hydration of substances in aqueous solution. The absorption spectrum of hydrogen peroxide for ultra-violet light has been examined.

Hydrogen peroxide is diamagnetic, and that to a greater extent than water. Representing magnetic susceptibility by K, the values for air, water and peroxide at 10° C. are as follow:

K_air, 0.25×10^-6; K_water, 7.2×10^-7; K_H2O2, 8.8×10^-6×10^-7.

This high value for hydrogen peroxide is in harmony with the observation of Henrichson, that as a general rule unsaturation gives a diamagnetic tendency to a molecule.