Overview Water Sample Carbonate System pH Calculations Acids and Bases Dilution and Mixing Titration Dosage / Addition Mineral Phases Environmental Chem

Validation of Water Sample Data

This example demonstrates how a water analysis can be (routinely) checked and interpreted by hydrochemistry. We start with the water C9.sol, which is part of the collection of example files supplied with aqion.

We restrict our demonstration to the following four steps:

1. input of sample water (raw data of water analysis)
2. charge-balance error (and parameter adjustment)
3. aqueous composition
4. carbonate system & calcite saturation

An example of an incomplete water sample is given here.

Step 1:  Input of Water Sample Data

Just after starting the program, the main input panel opens where you can enter the parameters of the aqueous solution: pH, temperature and, of course, the element concentrations.

Alternatively, you can load an existing aqueous solution (input water) by clicking the Open button. In this example we select the file C9.sol.¹

The content of this file feeds the window as shown in the right screenshot. In the top line you find the name of the solution: C9 (where the file extension sol is skipped).

There are two possibilities (two buttons) to start a sequence of hydro­chemical calculations:

• Start: charge balance, chemical equilibrium without/with minerals, carbonate-calcite system, etc.
• Reac: addition of chemicals, pH calculations, titration curves

Click on Start.

Step 2:  Charge Balance Error (CBE)

The very first verification checks the cation-anion balance. The result is shown as CBE in the right image.

In this example, the charge-balance error is -0.77%, caused by a small excess of anions. It is now possible to establish full charge balance, i.e., an equilibrium solution with CBE ≈ 0.

Charge balance can only be established if the corres­ponding left checkbox is activated. Then you can select a parameter from the drop-down list. The default parameter is pH; however, in this example, the program suggests the parameter DIC (because the carbonate content was given as alkalinity, which is an approximation for the DIC).

Click on the next ≫ button.

Charge-Balance Adjustment

In this example, full charge balance is established by adjusting the chosen parameter DIC to 4.63 mM (before calcite precipitation) and 4.56 mM (after calcite precipitation). Also note the respective pH changes.

Technically, the program performs two subsequent calculations before displaying the results in the right panel:

• Output 1: parameter adjustment to achieve full charge balance (i.e. chemical equilibrium without mineral phases)

• Output 2: chemical equilibrium with mineral phases (incl. redox reactions)

In this case, the water is supersaturated with calcite (which precipitates as indicated in Output 2). Precipitation is often accompanied with changes of pH, and these changes are also displayed.

By clicking on next ≫ you will get the complete aqueous composition of the input and output waters.

Step 3:  Output Table

The structure of the output table in the right screenshot is explained here. It contains three water compositions labeled as:

Input. This column represents the input water (raw data).

Output 1. This column represents the aqueous solution after charge balance adjustment. It is the result of an equilibrium calculation.

Output 2. This column displays the aqueous solution after charge balance adjustment and equilibrium with mineral phases. In this example, Ca and DIC decrease due to the precipitation of calcite.

More Data. While this table shows the total concen­trations of the elements, additional information is provided in separate tables (in the top menu bar):

• Ions – ions and aqueous complexes (complete speciation)²
• Minerals – precipitated minerals and saturation indices (SI)

By clicking on next ≫ you will get the results shown in step 4.

Remarks & Footnotes