Calcite Carbonate System
Five Example Waters (Input)
The program aqion comes with five examples for the calcite saturation problem. These are the input waters calcite-1 to calcite-5:1
The abbreviations are:
|pH_0||pH of the sample water at temperature T|
|T||temperature of water sample in °C|
|Teval||evaluation temperature for the calcite saturation state3|
|DIC||dissolved inorganic carbon)|
The task is to calculate the calcite saturation state of each sample water at the evaluation temperature Teval.
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.
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:
|state||corrosive||scale forming||scale forming||calcite equilibrium||corrosive|
The abbreviations are:
|pH||calculated pH at evaluation temperature Teval|
|pH_S||calculated saturation pH (that enters the 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 perfect 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 as PhreeqC (which can handle an “unlimited” number of aqueous species and phases). aqion is based on PhreeqC and the thermodynamic database wateq4f. In this way, it is free from restrictions imposed on the older models of the DIN standard.
Remarks and Footnotes
These five sample waters are taken from the German industry standard DIN 38404-C10R3 (Table 6, April 1995). These waters are used to test numerical models regarding the calcite-carbonate system. ↩
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 with aqion you can also start from the alkalinity). ↩