Charge-Balance Adjustment
Step 1: Calculate the Charge Balance Error
The first thing aqion does when you click on the button Start is to calculate the charge-balance error (CBE) of the input water. The screenshot below shows the CBE of the example water ib.sol.1
The output panel displays:
- CBE in percent
- sum of cations in meq/L
- sum of anions in meq/L
The user can now select a chemical parameter, which will be re-adjusted to establish charge balance. The following parameters are available in the drop-down list:2
| • pH value | (default) |
| • Ca | |
| • Mg | |
| • Na | |
| • K | |
| • SO4 | [S(6)] |
| • DIC | [C(4)] |
| • Cl | |
| • NO3 | [N(5)] |
| • NH4 | [N(-3)] |
| • … etc. |
Note: The button Details opens a guidance for charge-balance adjustment. It provides a hint which parameter, cation or anion, should be selected to establish charge balance.
Step 2: Adjust Charge Balance
The example above has an error of -1.89. If we select the parameter “DIC” and click on button next ≫, the right screenshot pops up. It tells us that DIC is decreased:3
5.894 mM ⇒ 5.455 mM
The line below displays the amount added (as the difference of both values):
ΔDIC = -0.439 mM
A positive (negative) Δ-value signals addition (removal) of the element. The obtained solution is fully charge-balanced (and is referred to as Output 1).
In addition, aqion performs a second calculation (Output 2) to check whether minerals precipitate or not. In this example, the equilibrium solution is supersaturated with two minerals: calcite and siderite. The precipitation causes the final pH change from 7.00 to 6.95.
Remarks
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The complete speciation of the example water ib.sol is presented as PowerPoint. ↩
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The symbols in square braces represent the corresponding valence states. ↩
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The concentration units can be switched between mg/L and mmol/L (by the drop-down field to the right of the Δ-value). ↩