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 CO₂ 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.