Overview Data Input Data Output DIC Charge Balance Physico-Chem Params Reactions Redox Mineral Phases

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 where 0.3 mM KOH (potassium hydroxide) is added to a specific solution, namely calcite-3.sol:¹

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).

Checkboxes

• show all reactants — extends the available list of reactants (up to about 600)

• temperature only — the temperature T of the solution will be changed to a target value (without adding reactants)

• plot titration curve — the reactant is added sequentially to generate pH and other plots (example)

• more reactions — up to three additional reactants can be added (example)

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:

…

H2SO4

H3AsO3

H3AsO4

H3BO3

H3PO4

H4SiO4

HBr

HCl

HF

HNO3

K2CO3

K2SO4

…

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 enters 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” you can simulate e.g. strongly supersaturated solutions.

Footnotes