Methane and aliphatic hydrocarbons

Unpolluted air contains about 0.03% by volume of CO2 which corresponds to its partial pressure of 3 X 10 MPa. Under these conditions approximately only 0.7 mg 1 CO2 is dissolved at 10°C in distilled water. Although the content of carbon dioxide in the air of large towns and in the neighbourhood of industrial sites is higher, it is not the reason of high concentrations of CO2 in waters. 

Another source of CO2 can be thermal decomposition of carbonate minerals, or their destruction by acid waters during the oxidation of sulphide ores. 

Hydrogen carbonates get into water via chemical weathering of aluminosilicates by CO2 and HjO and the reaction between carbonate minerals and CO2. 

Carbonates occur in natural waters at higher concentrations only very rarely. Concentrations which can be analytically determined are formed during intensive photosynthetic assimilation of green organisms in waters when dissolved CO2 is exhausted and thus, the carbonate equilibrium is shifted in favour of the formation of CO3 ions. By this process the pH value is increased (pH 8.3). 

Carbon dioxide is dissolved in the molecular form as a free hydrated CO2 and is usually denoted by the symbol C02(aq). Slightly less than 1% reacts with water to form non-dissociated molecules of H2CO3. Carbon dioxide dissolved in water is called free carbon dioxide and this term is used for the sum of the concentrations of free hydrated CO2 and H2CO3. 

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