Addition of Chemicals (pH Calculator)

The reaction module (button Reac) allows the addition of chemicals to any aqueous solution (input water). In the simplest case the input water is “pure water” (H2O). In this way you get the pH of an acid, base, salt or another chemical compound.

However, you can also start with an initial water of any composition. Here is the example calculation where 0.3 mM KOH (potassium hydroxide) is added to a specific solution, namely calcite-3.sol:1

addition of KOH: input and output panels

The addition of KOH increases the pH from 7.90 to 9.03 (output 1). In this way, calcite becomes super­saturated and precipitates, changing the pH back to 7.83 (output 2).

More Reactants. By activating the checkbox More reactions, up to three additional reactants can be added. An example is given here.

Plot Titration Curve. By activating the checkbox Plot Titration Curve, small portions of the reactant will be successively added to obtain pH plots. An example is given here.

List of Chemical Compounds (Reagents)

In total, aqion provides more than 500 chemicals2 that the user can add to the aqueous solution. The full list is arranged in alphabetical order, for example:


The addition/dosage can be done in units of mmol/L or mg/L. Typical pH values of common acids and bases are given in the pH Table.

“Forced Mineral Dissolution”

The Reac module includes also mineral phases denoted by the prefix “Mineral_”, such as

Mineral_Al(OH)3 Al(OH)3
Mineral_Brucite Mg(OH)2
Mineral_Calcite CaCO3
Mineral_Fe(OH)3 Fe(OH)3
Mineral_Gypsum CaSO4
Mineral_Hydroxyapatite Ca5(PO4)3OH
Mineral_Pyrochroite Mn(OH)2
Mineral_Rhodochrosite MnCO3
Mineral_Siderite FeCO3

Suppose one sets 0.5 mmol/L of “Mineral_Gypsum” then exactly 0.5 mmol/L CaSO4 will be added to the input water. This approach is distinctly different from the thermodynamics of mineral dissolution and is called “forced mineral dissolution”.

In thermodynamics (which is used here), the mineral dissolves only as long as the solution remains undersaturated with the mineral (SI < 0). In other words, the (maximum) amount that dissolves is dictated by equilibrium thermodynamics and cannot be chosen arbitrarily.

The “forced mineral dissolution” bypasses the equilibrium thermodynamics. The selected amount of addition will always dissolve completely, regardless of whether SI > 0 or not. In this “crude way” one can simulate e.g. strongly supersaturated solutions.


  1. This water belongs to the examples that are delivered with the program. 

  2. In addition, aqion PRO provides about 60 organic compounds (acids and their salts). 

[last modified: 2021-05-23]