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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

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		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
DIC2	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

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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 T_eval.

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:

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	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	8.23	8.05	8.04	8.47	8.46	9.08	9.06
pH_S	7.07	7.07	8.00	7.98	7.90	7.89	8.46	8.45	9.22	9.21
SI	-0.15	-0.13	0.27	0.26	0.17	0.17	0.01	0.00	-0.15	-0.16
CCPP	0.20	0.17	-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

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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