I understand that limestone (calcium carbonate and/or magnesium carbonate can be used to neutralize a strong acid. Why can I simply use limestone to raise the pH above 7.0 (neutral)?
A strong acid, such as hydrochloric acid (HCl), can react directly with various forms of limestone, (CaCO3, MgCO3 or CaCO3 MgCO3). For example, the reaction of CaCO3 with HCl is: CaCO3 + 2HCl –> CaCl2 + H2CO3. The carbonic acid produced is unstable and breaks down into carbon dioxide (gas) and water: H2CO3 -> CO2 + H2O. However, CO2 is soluble in water and produces a weak carbonic acid, so you ultimately find that the neutralization of a strong acid with limestone results in the generation of a weak acid. Consequently, you can never reach a point above a pH of 7.0. Note: Carbon dioxide (CO2) which is a component of the atmosphere will combine with rainwater to produce carbonic acid, H2CO3, which can dissolve limestone over time. Also, sulfur dioxide (SO2) in the air can also dissolve in the water, which produces a strong acidic solution of H2SO4 (sulfuric acid). This result of this process is referred to as acid rain.
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