As mentioned in Chapter 10 on water softening, as long as the concentrations of CaCO3 and Mg(OH)2 exceed their solubilities, the solids may continue to precipitate. This condition can cause scale to form, a solid that deposits due to precipitation of ions in solution. To prevent scale formation, the water must be stabilized. A water is said to be stable when it neither dissolves nor deposits precipitates. If the pH is high, stabilization may be accomplished using one of several acids or using CO2, a process called recarbonation. If the pH is low, stabilization may be accomplished using lime or some other bases.
Because of the universal presence of carbon dioxide, any water body is affected by the reaction products of carbon dioxide and water. The species produced from this reaction form the carbonate system equilibria. As discussed later, the stability or instability of water can be gaged using these equilibria. Thus, this chapter discusses this concept. It also discusses criteria for stability and the recarbonation process after water softening.
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