Charge-Balance Adjustment

First Step:  Calculate Charge Balance Error

The first thing aqion does when you click on the Start button 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

aqion_CBE panel

The output panel displays:

  • CBE in percent
  • sum of cations in meq/L
  • sum of anions in meq/L

Now, the user has the possibility to re-adjust one chemical parameter in order to obtain complete charge balance. For this purpose you should select one of the following parameters from the corresponding pull-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 improve the discrepancy.

Second Step:  Adjust Charge Balance

aqion_CBE adjustment

The example above has an error of -1.89%. If we select the parameter DIC and click on 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: Fe(OH)3 and Calcite. The precipitation causes the final pH change from 7.00 to 7.94.

Remarks

  1. The complete speciation of the example water “ib.sol” is presented as PowerPoint

  2. The symbols in square braces represent the corresponding valence states

  3. The concentration units can be switched between mg/L and mmol/L (by the dropdown field to the right of the Δ-value). 

[last modified: 2018-01-14]