Box 34 Gas solubility

The solubility of gases in liquids is often treated as an equilibrium process. Take the dissolution of carbonyl sulphide (OCS) as an example:

OCS(0

where OCS(g) and OCS(aq) represent the concentration of the substance carbonyl sulphide in the gas and liquid phase. This equilibrium relationship is often called Henry's law, after the English physical chemist who worked c. 1800. The Henry's law constant (KH) describes the equilibrium. Using pressure (p) to describe the concentration (c) of OCS(g) in the gas phase, we have:

cOCS(aI POCS(g

If we take the atmosphere as the unit of pressure and mol l-1 as the unit of concentration, the Henry's law constant will have the units mol l-1 atm-1. The larger the values of this constant, the more soluble the gas. Table 1 shows that a gas like hydrogen peroxide is very soluble, oxygen very much less so.

Many quite important gases have only limited solubility, but often they can react in water, which enhances their solubility. Take the simple dissolution of formaldehyde (HCHO)2 which readily hydrolyses to methylene glycol (H2C(OH)2):

HCHO(g

The second equilibrium lies so far to the right that solubility is enhanced by a factor of about 2000 (Box 3.2).

Table 1 Some Henry's law constants at 15°C

The second equilibrium lies so far to the right that solubility is enhanced by a factor of about 2000 (Box 3.2).

Table 1 Some Henry's law constants at 15°C

Gas

Kh (mol l-1 atm-1)

Hydrogen peroxide

2 x 105

Ammonia

90

Formaldehyde

1.7

Dimethyl sulphide

0.14

Carbonyl sulphide

0.035

Ozone

0.02

Oxygen

0.0015

Carbon monoxide

0.001

phide (OCS). This can be produced by reaction between carbon disulphide (CS2) and water:

and, although the flux to the atmosphere is smaller than that of DMS, its stability means that it will accumulate to higher concentrations. These sulphur gases have low solubility in water (Box 3.4), making them able easily to escape from the oceans into the atmosphere.

Halogenated organic compounds are well known in the atmosphere. Although these have an obvious human source, being present in cleaning fluids, fire extinguishers and aerosol propellants, they also have a wide range of biological sources. Methyl chloride (CH3Cl) is the most abundant halocarbon in the atmosphere and arises primarily from poorly understood marine sources, although terrestrial microbiological processes and biomass burning also contribute. Bromine- and iodine-containing organic compounds are also released from the oceans and the distribution of this marine iodine over land-masses represents an important source of this essential trace element for mammals. As one might predict, the iodine-deficiency disease, goitre, has been common in regions remote from the oceans.

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