Aqion

Aqion is a hydrochemistry software tool. It bridges the gap between scientific software (such like PhreeqCParkhurst, D.L. and C.A.J. Appelo: User's Guide to PHREEQC (version 2), a computer program for speciation, batch-reaction, one-dimensional transport and inverse geochemical calculations. USGS Water-Resources Investigations Report 99-4259, 1999)
and the calculation/handling of "simple" water-related tasks in daily routine practice. The software aqion is free for private users, education and companies.

Motivation & history

First. Most of the hydrochemical software is designed for experts and scientists. In order to flatten the steep learning curve aqion provides an introduction to fundamental water-related topics in form of a "chemical pocket calculator".

Second. The program mediates between two terminological concepts: The calculations are performed in the "scientific realm" of thermodynamics (activities, speciation, log K values, ionic strength, etc.). Then, the output is translated into the "language" of common use: molar and mass concentrations, alkalinity, buffer capacities, water hardness, conductivity and others.

History. Version 1.0 was released in January 2012 (after a half-year test run in 2011). The project is active with 1-2 updates per month.

Features

* Validates aqueous solutions (charge balance error, parameter adjustment)
* Calculates physico-chemical parameters: alkalinity, buffer capacities ( ANC, BNC), water hardness, ionic strength
* Calculates aqueous speciation and complexation
* Calculates pH of solutions after addition of chemicals (acids, bases, salts)
* Calculates the calcite-carbonate system (closed/open system, Langelier Saturation Index)
* Calculates mineral dissolution, precipitation, and saturation indices
* Calculates mixing of two waters
* Calculates reduction-oxidation ( redox) reactions
* Plots titration curves

Fields of application

* Water analysis and water quality
* Geochemical modeling (in simplest form)
* Education

Limits of application

* only inorganic species (no organic chemistry)
* only equilibrium thermodynamics (no chemical kinetics)
* only aqueous solutions with ionic strength ≤ 0.7 mol/L (no brines)

Basic algorithm & numerical solver

There are two fundamental approaches in hydrochemistry: Law of mass action (LMA) and Gibbs energy minimization (GEM).
The program aqion belongs to the category LMA approach. In a nutshell: A system of ''N_B'' independent basis components ''j'' (i.e. primary species), that combines to form ''N_S'' secondary species ''i'', is represented by a set of mass-action and mass-balance equations:

(1)       mass action law:        $\ \,=\, K_i \, \prod_^ \,\^$        with i = 1 ... N_S

(2)       mass balance law:        \,=\, + \sum_^\, \nu_ \,        with j = 1 ... N_B

where ''K_i'' is the equilibrium constant of formation of the secondary species ''i'', and ν_i,j represents the stoichiometric coefficient of basis species ''j'' in secondary species ''i'' (the values of ν_j,i can be positive or negative). Here, activities ''a_i'' are symbolized by curly brackets while concentrations ''c_i'' by rectangular brackets Both quantities are related by the

(3)       activity correction:        \ \,=\, \gamma_ \,

with γ_i as the activity coefficient calculated by the Debye–Hückel equation and/or Davies equation. Inserting Eq.(1) into Eq.(2) yields a nonlinear polynomial function f_j for the ''j-''th basis species:

(4)       f_j (c_1,c_2,...,c_) \,=\, - - \sum^_ \frac\, K_i\, \prod^_ \^ \,=\,0

which is the ''objective function'' of the Newton–Raphson method.

To solve Eq.(4) aqion adopts the numerical solver from the open-source software ''PhreeqC''.
The equilibrium constants ''K_i'' are taken from the thermodynamic database ''wateq4f''.

Examples, test & verification

The software aqion is shipped with a set of example solutions (input waters) and a tutorial how to attack typical water-related problems (online-manual with about 40 examples). More examples and exercises for testing and re-run can be found in classical textbooks of hydrochemistry.Appelo, C. A. J. and D. Postma: ''Geochemistry, Groundwater, and Pollution''. Taylor & Francis, 2005,

The program was verified by benchmark tests of specific industry standards.DIN 38404-10: German standard methods for the examination of water, waste water and sludge - Physical and physicochemical parameters (group C) - Determination of calcite saturation of water (C 10)

Screenshots

File:Aqion_input_panel_3.5.png, Input panel for chemical elements
File:aqion pH calculator.jpg, Output of pH calculator (example)
File:Aqion_calcite_carbonate_system_3.5.png, Calculated parameters of the calcite carbonate system
File:Titration NaOH.gif, titration curves (example: addition of NaOH to a given input water)

References

External links

PHREEQC

Online calculator
for pH, aqueous speciation, saturation indices, alkalinity, EC

{{Chemistry software

Computational chemistry software
Science education software