# Aqion

Aqion is a hydrochemistry software tool. It bridges the gap between scientific software (such like PhreeqC) 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 CO$_2$ 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: $\{i\}\,=\,K_{i}\,\prod _{j=1}^{N_{B}}\,\{j\}^{\nu _{i,j}}$ with i = 1 ... N$_S$
(2) mass balance law: $[j]_{TOT}\,=\,[j]+\sum _{i=1}^{N_{S}}\,\nu _{i,j}\,[i]$ 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 {i} while concentrations c$_i$ by rectangular brackets [i]. Both quantities are related by the
(3) activity correction: $\{i\}\,=\,\gamma _{i}\,[i]$
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) $f_{j}(c_{1},c_{2},...,c_{N_{B}})\,=\,[j]_{TOT}-[j]-\sum _{i=1}^{N_{S}}{\frac {\nu _{i,j}}{\gamma _{i}}}\,K_{i}\,\prod _{k=1}^{N_{B}}\{k\}^{\nu _{i,k}}\,=\,0$
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.
The program was verified by benchmark tests of specific industry standards.

## Screenshots

* Input panel for chemical elements
* Output of pH calculator (example)
* Calculated parameters of the calcite carbonate system
* titration curves (example: addition of NaOH to a given input water)