Frequently, the minerals causing hardness in water are hydrogen carbonate and carbonate ions in equilibrium with calcium ions. If this equilibrium is disturbed and shifts in one direction, precipitation of calcium carbonate is the result, forming a chalky layer of limescale on the inside of water pipes. This can have negative consequences for water systems (the accumulation of mineral deposits in piping). If the equilibrium shifts in the opposite direction, solid calcium carbonate deposits are dissolved upon contact with water. This dissolves untreated or black steel pipes causing corrosion by dissolution of the protective layer present on the inner surface of the pipes consisting of a mixture of lime scale and rust. In this case, the water is considered “aggressive” and causes the metal in piping and fixtures to leach into the water.
Water containing either excessive or insufficient amounts of the solubility product of calcium carbonate (i.e., calcite) possesses either lime-precipitating or lime-dissolving properties. When the calcite saturation is in a state of equilibrium, the calcium in particular is able to remain in solution in the form of calcium hydrogen carbonate. The equilibrium is depicted in the chemical formula below:
CO2 + H2O ⇔ H2CO3 ⇔ H+ + HCO3- ⇔ 2 H+ + CO32-
A certain quantity of carbonic acid (H2CO3) is necessary to maintain the equilibrium. Excessive amounts of carbonic acid are “aggressive” meaning that the carbonic acid will dissolve the lime deposits. If the carbonic acid content is too low, the lime will precipitate out and form lime deposits.
The carbonic acid in water present in a state of equilibrium is related to the sum of bound carbonic acid (W-000.14.031 Bound Carbon Dioxide (Carbonate, Hydrogen Carbonate) in Water) and free carbonic acid (W-000.15.031 Free Carbon Dioxide in Water) and can therefore be calculated. The carbonic acid aggressive towards lime deposits can also be determined analytically through treatment with calcium carbonate.
The method describes how to determine the content of carbon dioxide in water capable of causing the dissolution of lime and thus corrosion in certain types of piping.
Water intended for use as an ingredient in the production of beer (brewing liquor) or other foods
Through the addition of solid calcium carbonate to a water sample followed by constant stirring for a period of time, either part of the salt will dissolve or the water will remain unchanged. By examining the sample prior to and after the addition of calcium carbonate, one can quantitatively determine whether it is lime-aggressive or not.