Calcite Carbonate System

The program aqion is shipped with five examples for the calcite saturation problem. These are the input waters calcite-1 to calcite-5: 1


calcite-1 calcite-2 calcite-3 calcite-4 calcite-5
pH_0 6.90 8.20 7.90 8.40 8.90
T °C 20.0 15.0 20.0 17.0 20.0
Teval °C 10.0 11.0 5.5 12.0 10.0
DIC 2 mM 6.680 1.661 2.456 0.941 0.329
Ca mM 3.50 1.24 1.22 0.57 0.30
Mg mM 0.75 0.41 0.35 0.10 0.15
Na mM 2.05 0.90 0.15 0.21 0.30
K mM 0.15 0.07 0.05 0.02 0.05
Cl mM 2.50 0.90 0.16 0.34 0.30
NO3 mM 0.50 0.55 0.07 0.08 0.20
SO4 mM 1.20 0.58 0.35 0.10 0.20


The abbreviations are:

T temperature of water sample in °C
Teval evaluation temperature for the calcite saturation state 3
pH_0 pH of the sample water at temperature T
DIC dissolved inorganic carbon

The task is to calculate the calcite saturation state of each sample water at the evalution temperature Teval.

Calculation Procedure

The calculation of the calcite saturation state with aqion is simple:

  • click on Open to select calcite-1.sol
  • click on Start to run the equilibrium calculations (charge balance is okay)
  • click on next to get the composition of the equilibrium solutions
  • click on next to get the calcite saturation state

Repeat this procedure with calcite-2 to calcite-5.

Results

The calculated parameters of the calcite-carbonate system can be compared with the values of the German norm “DIN 38404-C10R3” (April 1995) for the five sample waters:


calcite-1 calcite-2 calcite-3 calcite-4 calcite-5
DIN aqion DIN aqion DIN aqion DIN aqion DIN aqion
pH 6.98 6.98 8.25 8.25 8.05 8.05 8.47 8.46 9.08 9.08
pH_S 7.07 7.07 8.00 7.99 7.90 7.90 8.46 8.46 9.22 9.22
SI -0.15 -0.13 0.27 0.28 0.17 0.17 0.01 0.01 -0.15 -0.16
CCPP 0.20 0.18 -0.03 -0.03 -0.03 -0.03 0.00 0.00 0.01 0.01
state corrosive scale forming scale forming calcite equilibrium corrosive


The abbreviations are:

pH calculated pH of water at evaluation temperature Teval
pH_S calculated saturation pH (that enters LSI)
SI saturation index of calcite
CCPP Calcium Carbonate Precipitation Potential in mmol/L

The last line contains the saturation state of the water: corrosive, scale-forming, or in equilibrium with calcite. The classification is based on the Langelier Saturation Index (LSI).

The above table shows that the calculated parameters are in agreement with the DIN values. Note, however, that the DIN values (from 1995) are based on iterative models developed before the advent of modern hydrochemistry programs such like PhreeqC (which can handle an “unlimited” number of aqueous species and phases). aqion rests upon PhreeqC and the thermodynamic database wateq4f. In this way, it is free of several limitations imposed on the older iterative models of the DIN norm.

Footnotes

1 These five sample waters are taken from the German industry standard norm “DIN 38404-C10R3” (Table 6, April 1995). These waters are commonly used to test numerical models for the calcite carbonate system.

2 In addition to the DIC value the “DIN 38404-C10R3” provides also the alkalinity (more precisely the ANC to pH 4.3) for each water. However, the system is completely determined by either DIC or alkalinity. In the present example we prefer the DIC (but, alternatively, aqion also allows you to start from alkalinity as well).

3 The evaluation temperature Teval, which usually differs from the sampling temperature T, is entered in the input window separately.

[last modified: 2013-12-14]