Aqion is a hydrochemistry software tool. It bridges the gap between scientific software (such like PhreeqC^[1]) 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^[2] (no brines)

Basic algorithm & numerical solver

There are two fundamental approaches in hydrochemistry: Law of mass action (LMA) and Gibbs energy minimization (GEM).^[3] 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:        ${\displaystyle \{i\}\,=\,K_{i}\,\prod _{j=1}^{N_{B))\,\{j\}^{\nu _{i,j))}$        with i = 1 ... N_S

(2)       mass balance law:        ${\displaystyle [j]_{TOT}\,=\,[j]+\sum _{i=1}^{N_{S))\,\nu _{i,j}\,[i]}$        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:        ${\displaystyle \{i\}\,=\,\gamma _{i}\,[i]}$

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)       ${\displaystyle f_{j}(c_{1},c_{2},...,c_{N_{B)))\,=\,[j]_{TOT}-[j]-\sum _{i=1}^{N_{S)){\frac {\nu _{i,j)){\gamma _{i))}\,K_{i}\,\prod _{k=1}^{N_{B))\{k\}^{\nu _{i,k))\,=\,0}$

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.^[1][4] The equilibrium constants K_i are taken from the thermodynamic database wateq4f. ^[5]

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.^[6][7][8]

The program was verified by benchmark tests of specific industry standards.^[9]

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)