Question

Explain the effect carbonate has on the solubility of calcium ions in natural waters. Show the general reaction. (ii) How is this effect different at low and high pH?

Answer 1

At pH = 7.0 calcium carbonate is water insoluble. Calcium carbonate has a solubility of 14 mg/L

CaCO₃(s) + H₂O(l) <==> Ca²⁺(aq) + CO₃^2-(aq) Equilibrium lies towards left-side at pH = 7.0 (general equation). The solubility increases by a factor of five in presence of carbon dioxide as shown by a reaction as below.

H₂O (l) + CO₂ (g) => H₂CO₃(aq)

CaCO₃ (s) + H₂CO₃ (aq) => Ca(HCO₃)₂ (carbon weathering) CaCO₃ solubility greatly increased here.

And the total reaction mechanism:

CaCO₃ (s) + CO₂ (g) + 2H₂O (l) => Ca²⁺ (aq) + 2 HCO₃^- (aq) => Ca(HCO₃)₂ The product is calcium hydrogen carbonate. These are very complex reactions.

At low pH:

H⁺(aq) + Ca(HCO₃)₂ <==> Ca²⁺(aq) + H₂O (l) + CO₂(aq) and H⁺(aq) + CO₃^2-(aq) <==> HCO₃^-(aq)

acidity regulated now, calcium bicarbonate adjusts pH. At low pH protons (H⁺) are dominant and are neutralized by bicarbonate.

At high pH:

OH-(aq) + Ca(HCO₃)₂ <==> Ca²⁺(aq) + H₂O (l) + CO₂(aq) [Apply HCO₃^- <==> H⁺(aq) + CO₃^2-(aq)] and

[ H⁺ (aq) + OH^-(aq) => H₂O(aq)]; [CO₃^2- (aq) + H₂O(l) <==> HCO₃^-(aq) + OH^-(aq)]

at high pH hydroxyl ions are dominant and are neutralized by protons from bicarbonate.

Finally the bicarbonate-carbonate system acts as a buffer to mainatain specific pH region values. Our blood is also pH regulated by this bicarbonate-carbonate system.

Hope i have been successful in explaining the basic concept.

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