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°C? 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 New) and switch to molar units (activate 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, a first schematic overview appears. After click on next, the results are shown again in the output table. Here the right column is relevant:

pH = 9.91     and     Ca = DIC = 0.123 mM

Thus, in the closed system, 0.123 mM calcite dissolve. The complete speciation is shown in the table Ions.

Dissolution in the Open System

aqion_input_calcite

Repeat the calculation as above. Before Start, however, 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:

pH = 8.22
DIC = 1.06 mM
Ca = 0.53 mM

In the open system, 0.53 mmol/L calcite dissolve. That is about 5 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 difference between total Ca and DIC. Explanation: The higher DIC value (1.06 mM) has three contributions: First, the CO2 exchange – prior to the calcite dissolution – contributes 0.0157 mM (see middle column in output table). Second, 0.53 mmol calcite is dissolved. Third, the increased pH (due to calcite dissolution) sucks more CO2 from the atmosphere into the water. In sum:

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 0.53
DIC mM 0.123 1.06
CO2 3 mM 2.2×10-5 0.0133
HCO3- mM 0.0831 1.02
CO3-2 mM 0.0338 0.009
Ca+2 mM 0.117 0.518
CaCO3(aq) 4 mM 0.0055 0.0055
CaHCO3+ mM 0.00011 0.0057
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. Note: The amount of calcite that dissolves is independent of the initial inventory – provided it exceeds the solubility limit. So you can enter any initial value above 2 mM or so.

  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 species CaCO3(aq). It should not be confused with the mineral phase calcite, usually abbreviated by CaCO3(s).

[last modified: 2015-01-25]