Carbon Dioxide - Carbonic Acid Equilibrium

Carbonic acid is formed when atmospheric CO2 is dissolved in water. The chemical equilibria are

1) Gas dissolution

2) Carbonic acid formation

3) Carbonic acid equilibrium

In an open system, the partial pressure of CO2 (g) is relatively constant at P(CO2) = 0.000355 Atmosphere.

The equilibrium expression is the Henry's Law equation (@ 25 °C)

Thus

Using this one may obtain

Dependent Carbonate Equilibrium

Using carbonate mass balance and acid equilibrium expressions, the fractional amounts of all carbonate species can be found as a function of [H+]

The pH dependent fractions are

Of interest is the total carbonate as a function of pH.

Remember that [H2CO3] is "fixed" by the atmosphere in an open equilibrium system and

Total carbonate increases with pH.

Solubility of Calcium Carbonate

(A Systematic Treatment)

Chemistry

Charge Balance

Mass Balance

Equilibrium Equations

WOW!

This is obviously a difficult system of equations to solve. Though it could be done, let's look for the approximations (AKA, the easy way out)

Molar Solubility of Calcium Carbonate Approximations

Open system; The trick to solving for carbonate equilibria is to first identify the major species. That is, we should be able to look at all equilibria and determine which carbonate species are in greatest abundance.

The major equilibrium process here is

By examining the alpha plot, we see that HCO3- is a maximum at pH=8.3. This will help. Solving…

Closed System: The real trick here, if there is one, is to find the pH. With the pH, we can easily solve any carbonate problem with the fractional amounts. For example, the calcium carbonate solubility is a function of the fractional amount of carbonate as carbonate and the total amount of carbonate. In a closed system at fixed pH