Note on calcium carbonate equilibrium and the Ryznar index

1 Note on calcium carbonate equilibrium and the Ryznar index 

Natural water is not pure and contains a number of dissolved chemical elements. The most common is calcium bicarbonate. It was seen earlier that one of the first salts that is liable to precipitate out in water is calcium carbonate. Corrosion problems are often hidden or thrown off by the formation of a relatively thick layer of calcium carbonate. 

In fact, the practical balance of calcium bicarbonate with carbon dioxide (CO2) is governed by fairly complex relationships. A shift in the balance can cause chemical reactions that either dissolve calcium carbonate (i.e. aggressivity) or precipitate it (i.e. scale build up). These reactions may be compounded by electrochemical corrosion processes that are specific to metals. 

Calcium bicarlponate is unstable in an aqueous solution and can break down as shown in the reversible reaction below  

To keep a given calcium bicarbonate concentration in solution, the water must contain a corresponding amount of dissolved free CO2. This is the balancing CO2 - 

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