Solubility of Gases in Water

Solubility of Gases in Water is usually expressed in one of two ways, namely, as Bunsen's Absorption Coefficient, β, or as Ostwald's Solubility expression, Z. These terms have already been defined.

Gases may be divided broadly into two classes, namely, those that combine with water, such as ammonia, hydrogen chloride, sulphur dioxide, and the like; and those that are chemically inert. Amongst the latter class may be placed oxygen, nitrogen, etc. The former group usually possess high absorption coefficients, whilst the latter are frequently so slightly soluble, that for many chemical purposes they may be regarded as insoluble and may be collected and even stored over water. Xenon lies between the two groups, its solubility being greater than that of any other chemically neutral gas.

Gas	0°	5°	10°	15°	20°	25°	30°
Hydrogen	0.0214	0.0204	0.0195	0.0188	0.0182	0.0175	0.0170
Nitrogen	0.0239	0.0215	0.0196	0.0179	0.0164	0.0150	0.0138
Oxygen	0.0496	0.0439	0.0390	0.0350	0.0317	0.0290	0.0268
CO	0.0354	0.0315	0.0282	0.0254	0.0232	0.0214	0.0200
CH4	0.0556	0.0481	0.0418	0.0369	0.0331	0.0301	0.0276
NO	0.0738	0.0646	0.0571	0.0515	0.0471	0.0430	0.0400
N2O	. . .	1.1403	0.9479	0.7896	0.6654	0.5752	. . .
CO2	1.713	1.424	1.194	1.019	0.878	0.759	0.665
Chlorine (β1)	4.610	(3.232)	3.095	2.635	2.260	1.985	1.769
H2S	4.621	3.935	3.362	2.913	2.554	2.257	2.014
SO2	79.79	67.48	56.65	47.28	39.37	32.79	27.16
HCl	525.2	496	474	456	443	432	. . .
NH3	1299	1019	910	802	710	635	. . .

Solubility of Gases in Water falls with rise of temperature, but in the case of the inert gases minimum values exist, the solubility falling in the cases of xenon and argon as the temperature rises from 0° to 40° C. Above this point the solubility rises again. For krypton the minimum lies between 30° and 40° C.; for helium at 10° C., and for neon probably in the neighbourhood of 0° C.

In the preceding table are given the solubilities of a few typical gases in water at various temperatures. Unless otherwise stated the solubility co-efficient is employed. β₁ signifies measurement of the gaseous volume at N.T.P., but the pressure includes the vapour pressure of the liquid also, whereas β signifies the volume under the pressure of the gas itself of 760 mm.

Winkler directed attention to the fact that, in the case of many chemically inert and "permanent" gases, the percentage decrease in the solubility coefficient between 0° and 20° C. is approximately proportional to the cube root of the molecular weight of the gas. Thus:

	Percentage decrease in β A.	Cube Root of Molecular Weight. B.	A/B
Hydrogen	15.32	1.259	12.2
Nitrogen	34.33	3.037	11.3
Oxygen	36.55	3.175	11.5
Carbon monoxide	34.44	3.037	11.3
Nitric oxide	36.24	3.107	11.7

The decrease in is attributed by Winkler to a fall in the viscosity of the solvent consequent upon rise of temperature. The two factors may be equated as follows:



where η₀ and η_t represent the viscosities of water at 0° and t° C. respectively, and k is a constant, M being the molecular weight. If β_t is calculated from this expression, the value agrees well with that found experimentally between 0° and 60° C. The following values for k have been deduced:

	k
Argon	4.4
Hydrogen, oxygen, and diatomic gases	3.8
CO2 and triatomic gases	3.2

Helium is exceptional, however. As a rule, for those gases that deviate markedly from Boyle's law, the value for k tends to rise with temperature.

The Inert Gases. - The study of the solubility of the gases of group 0 of the Periodic Table has proved very interesting. The data published by Antropoff are given in the following table:

Solubilities of the Inert Gases in Water

Temperature, ° C	Helium	Neon	Argon	Krypton		Xenon
				Sample I	Sample II
0	0.0134	0.0114	0.0561	0.1249	0.1166	0.2189
10	0.0100	0.0118	0.0438	0.0965	0.0877	0.1500
20	0.0138	0.0147	0.0379	0.0788	0.0670	0.1109
30	0.0161	0.0158	0.0348	0.0762	0.0597	0.0900
40	0.0191	0.0203	0.0338	0.740	0.0561	0.0812
50	0.0226	0.0317	0.0343	0.0823	0.0610	0.0878

Solubilities of the inert gases in water (Antropoff, 1910).

These results are shown graphically in Fig. A remarkable feature of these determinations is the minimum of solubility shown by every gas. Estreicher had already in 1899 suggested that such a minimum ought to exist, and indeed found such to be the case with helium. The minimum for xenon and argon lies at 40° C.; for krypton between 30° and 40° C.; for helium, at 10° C., and for neon probably at 0° C. As a general rule, the solubility rises with the atomic weight, helium and neon being exceptional, possibly on account of erroneous determination. It is also worthy of note that the solubility of xenon is greater than that of any other gas which does not form a compound with the solvent.

The solubility of a gas increases with the pressure; for gases chemically neutral towards water,

β ∞ p

since under moderate pressures Boyle's Law is obeyed with fair accuracy, it follows that a volume of water at a given temperature will absorb the same volume of gas whatever the pressure. This is Henry's Law, enunciated in 1803. The same year Dalton observed that the amount of a gas absorbed from a gaseous mixture is directly proportional to its partial pressure. In both of these cases it is assumed that the gas does not combine with the water. If combination takes place these laws are not obeyed.

The methods employed for the determination of Solubility of Gases in Water and gases in water may be either chemical or physical. The former are useful in cases where chemically reactive gases such as oxygen, etc., are concerned. For neutral gases, like nitrogen, physical methods are essential.

The presence of dissolved salts tends to reduce the solubility of neutral gases in water. This is capable of explanation on the assumption that the electrolyte is hydrated, and that the water thus " fixed " is no longer able to absorb the gas. If this theory is accepted, it becomes possible to calculate the degree of hydration of the salt. A solution of alcohol in water is peculiar in its behaviour towards oxygen. Although oxygen is several times more soluble in alcohol than in water, addition of alcohol to water reduces the absorption of oxygen, a minimum being reached with about 30 per cent, of alcohol. Further addition of this liquid raises the absorption coefficient.