Question

The surface water in your backyard pong has a pH=7.7 and its chemistry is dominated by the carbonate system. For the carbonate system, a pKa₁=6.35 and a pKa₂=10.33 give important information about the equilibrium condition of the carbonate system at various pH values.

a. Which carbonate species ([H₂CO₃], [HCO₃^-], and [CO₃^-2] would you expect to be the dominate species? Why?

b. The pond is not equilibrated with the atmosphere at pH=7.7. What changes will take place if the water equilibrates with the atmosphere (e.g., you turned on a bubbler in the pond)?

c. What would you expect in pH readings if your pond water began to experienc an algal bloom? Explain how the pH would change over 24-hour cycle and why? Write any chemical equations that explain the observed pH readings in the water.

Answer 1

a). More is the pKa, weaker is the acid. It signifies that the second species with pKa 10.33 is a weaker acid as compared to the first species with pKa 6.35, but both are acidic.

So, the carbonate species expected is H₂CO₃ (carbonic acid).

b). There will be negligible change in the pH of the pond water due to Oxygen, nitrogen etc. as there is very little exchange of gases between water and atmosphere.

Carbon dioxide has very low value of Henry's constant. This implies that at low pressures also carbon dioxide dissolves in water.

P is proportional to K_H.

Hence, this CO₂ after dissolution forms carbonic acid and this will lower the pH of the pond.

c). The carbon dioxide when reacts with water forms carbonic acid which lowers the pH of water.

CO₂ + H₂O ---------> H₂CO₃

Since, algae photosynthesise like plants, they take up carbon dioxide (an acidic oxide) and expels out the oxygen gas. As a result, carbonic acid does not form and the pH of the water will increase significantly.

Within 24 hours pH of the water will be as such that will be dangerous for aquatic life.