Solubility of Calcium Species in Natural Waters

Calcium carbonate has intrinsically low solubility in water, Ksp = 6 x 10 (mol/L) at 25°C. However, because the carbonate anion is basic, CaCOs becomes increasingly soluble as the pH drops. It is known that even unpolluted rain water has pH 5.6 and the rainwater is slightly acidic. The reaction may be summarized as 

This reaction is responsible for formation, over thousands of years, of caves and gorges in limestone areas, as CO2-laden rainwater very slowly dissolves the rock. The process is slow because the equilibrium constant is small. This constant can be 

Due to the dynamic equilibrium between the atmospheric carbon dioxide and the oceanic bicarbonate and carbonate anions, the greatest amount of soluble calcium cations is contained in the ocean. This mass is four orders of magnitude higher than the total mass of bound calcium in living and dead matter of both terrestrial and aquatic organisms. The average calcium content in the seawater is 408 mg/L, and the overall pool is 559 x lO tons. 

The actual calcium concentration in the marine waters is about 30 times higher than in the riverine waters. This is owed to the limited solubility of calcium carbonate and, especially, related to the extensive calcium uptake by planktonic organisms followed by calcium deposition as pellets. These processes have facilitated the vast accumulation of calcium as a component of massive layers of limestone, dolomite, marl, calcareous clay, and other Ca-containing rocks (see Box 8). 

Why calcium carbonate does not spontaneously precipitate (After Bunce, 1994) 

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