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Atomistry » Oxygen » Water » Solubility of Gases in Water » |
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.
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:
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 ηt represent the viscosities of water at 0° and t° C. respectively, and k is a constant, M being the molecular weight. If βt 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:
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
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. |
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