# Dissolution of Calcite (CaCO3)

What is the equilibrium composition of water after dissolution of calcite in a closed and in an open CO2 system at 25? The two systems are defined by:

 • closed system: H2O + Calcite • open system: H2O + CO2(g) + Calcite

Dissolution in the Closed System

We start with pure water (button H2O) and switch to molar units (checkbox mol). To open the mineral table click on Minerals, then enter the initial amount of 5 mmol/L calcite.1

Run the calculation with Start. The results are displayed in the schema on the right-hand side:

 pH = 9.91 Ca = 0.123 mM DIC = 0.123 mM

Thus, in the closed system, 0.123 mM calcite dissolve. The complete water composition (speciation) is shown in the output tables Ions (button next).

Dissolution in the Open System

Repeat the calculation, but before Start switch to the “Open CO2 System” as shown in the right screenshot.

[The default value, pCO2 = 3.408, is equivalent to the CO2 partial pressure of the atmosphere.2]

The results are shown in the schema below:

 pH = 8.22 DIC = 1.06 mM Ca = 0.53 mM

In the open system, 0.53 mM calcite dissolve. That is about 4 times more than in the closed system. The cause: The water in contact with CO2 gas is slightly acidic (pH 5.61) which enhances the mineral dissolution.

Note the big difference between total Ca and DIC (which does not exist in the closed system). The high DIC value results from three contributions:

1. The CO2 exchange – prior to the calcite dissolution – contributes 0.0157 mM (see Output 1).

2. The dissolution of 0.53 mmol calcite.

3. The increased pH (due to calcite dissolution) sucks more CO2 from the atmosphere into the water.

Taken together, we get:

 DIC  =  0.0157 mM + 0.53 mM + 0.514 mM  =  1.06 mM

Results – Equilibrium Speciation

Closed System Open CO2 System
pH   9.91 8.22
Ca total mM 0.123 0.530
DIC mM 0.123 1.057
CO2 3 mM 2.2×10-5 0.0133
HCO3- mM 0.0831 1.023
CO3-2 mM 0.0338 0.009
Ca+2 mM 0.117 0.519
CaCO3(aq) 4 mM 0.0055 0.005
CaHCO3+ mM 0.00011 0.006
CaOH+ mM 0.00015 1.2×10-5

The value of Ca total is identical with the amount of dissolved CaCO3 (calcite).

Remarks & Footnotes

1. The amount of calcite that dissolves is independent of the initial inventory – provided it exceeds the solubility limit. So you can enter any value greater than 2 mM.

2. Textbooks often prefer “rounded” values such as pCO2 = 3.5 or partial pressure of 0.00035 atm.

3. In this table, the dissolved species “CO2” symbolizes the composite carbonic acid.

4. CaCO3(aq) abbreviates the dissolved aqueous complex. It should not be confused with the mineral phase calcite, usually abbreviated by CaCO3(s)