SODIUM CARBONATE CAS N°: 497-19-8 - UNEP Chemicals

OECD SIDS SODIUM CARBONATE
The carbonate ions will react with water, resulting in the formation of bicarbonate and hydroxide,
until an equilibrium is established (McKee et al., 1963). It is obvious that both the sodium and
bicarbonate ion have a wide natural occurrence (UNEP, 1995).
Background concentration of carbonate
If carbonate is dissolved in water a re-equilibration takes place according to the following
equations:
HCO3 - ↔ CO3 2- + H + pKa = 10.33
CO2 + H2O ↔ HCO3 - + H + pKa = 6.35
Only a small fraction of the dissolved CO2 is present as H2CO3, the major part is present as CO2.
The amount of CO2 in water is in equilibrium with the partial pressure of CO2 in the atmosphere.
The CO2 / HCO3 - / CO3 2- equilibria are the major buffer of the pH of freshwater and seawater
throughout the world.
Based on the above equations, CO2 is the predominant species at a pH smaller than 6.35, while
HCO3 - is the predominant species at a pH in the range of 6.35-10.33 and CO3 2- is the predominant
species at a pH higher than 10.33.
The natural concentration of CO2 / HCO3 - / CO3 2- in freshwater is influenced by geochemical and
biological processes. Many minerals are deposited as salts of the carbonate ion and for this reason
the dissolution of these minerals is a continuous source of carbonate in freshwater. Carbon dioxide
is produced in aquatic ecosystems from microbial decay of organic matter. On the other hand plants
utilise dissolved carbon dioxide for the synthesis of biomass (photosynthesis). Because many
factors influence the natural concentration of CO2 / HCO3 - / CO3 2- in freshwater, significant
variations of the concentrations do occur.
If the pH is between 7 and 9 then the bicarbonate ion is the most important species responsible for
the buffer capacity of aquatic ecosystems. UNEP (1995) reported the bicarbonate concentration for
a total number of 77 rivers in North-America, South-America, Asia, Africa, Europe and Oceania.
The 10 th –percentile, mean and 90 th -percentile were 20, 106 and 195 mg/l, respectively.
Background concentration of sodium
The sodium ion is ubiquitously present in the environment and it has been measured extensively in
aquatic ecosystems. Sodium and chloride concentrations in water are tightly linked. They both
originate from natural weathering of rock, from atmospheric transport of oceanic inputs and from a
wide variety of anthropogenic sources. The sodium concentration was reported for a total number
of 75 rivers in North and South America, Africa, Asia, Europe and Oceania, with a 10 th percentile
of 1.5 mg/l, mean of 28 mg/l and 90 th percentile of 68 mg/l (UNEP, 1995).
Anthropogenic addition of sodium carbonate
The use of sodium carbonate could potentially result in an aquatic emission of sodium carbonate
and it could locally increase the sodium and carbonate concentration in the aquatic environment.
Specific analytical data or other reliable data about the use of sodium carbonate and the related
emissions of sodium and carbonate have not been found.
As indicated before, the emission of sodium carbonate to the aquatic environment will increase the
pH of the water. To underline the importance of the buffer capacity, a table is included with the
concentration of sodium carbonate needed to increase the pH to a value of 9.0, 10.0 and 11.0 at
different bicarbonate concentrations. The data of Table 1 were based on calculations (De Groot et
al., 2002).
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