Aqion

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.

• 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

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

• only inorganic species (no organic chemistry)
• only equilibrium thermodynamics (no chemical kinetics)
• only aqueous solutions with ionic strength ≤ 0.7 mol/LNote: The upper limit is sea water. (no brines)

There are two fundamental approaches in hydrochemistry: Law of mass action (LMA) and Gibbs energy minimization (GEM).http://www.kristall.uni-frankfurt.de/media/handouts/GEM-lecture.PDF{{dead link|date=October 2016 |bot=InternetArchiveBot |fix-attempted=yes }}

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:        $\backslash \{i\backslash \}\; \backslash ,=\backslash ,\; K\_i\; \backslash ,\; \backslash prod\_\{j=1\}^\{N\_B\}\; \backslash ,\backslash \{j\backslash \}^\{\backslash nu\_\{i,j\}\}$        with i = 1 ... N_S

(2)       mass balance law:        $[j]\_\{TOT\}\; \backslash ,=\backslash ,\; [j]\; +\; \backslash sum\_\{i=1\}^\{N\_S\}\backslash ,\; \backslash nu\_\{i,j\}\; \backslash ,[i]$        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:        $\backslash \{i\backslash \}\; \backslash ,=\backslash ,\; \backslash gamma\_\{i\}\; \backslash ,\; [i]$

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)       $f\_j\; (c\_1,c\_2,...,c\_\{N\_B\})\; \backslash ,=\backslash ,\; [j]\_\{TOT\}\; -\; [j]\; -\; \backslash sum^\{N\_S\}\_\{i=1\}\; \backslash frac\{\backslash nu\_\{i,j\}\}\{\backslash gamma\_i\}\backslash ,\; K\_i\backslash ,\; \backslash prod^\{N\_B\}\_\{k=1\}\; \backslash \{k\backslash \}^\{\backslash nu\_\{i,k\}\}\; \backslash ,=\backslash ,0$

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.Remark: To keep things apart, the numerical solver of PhreeqC is outsourced from aqion.exe into a separate DLL.

The equilibrium constants K_i are taken from the thermodynamic database wateq4f.

Ball J. W. and D. K. Nordstrom: WATEQ4F – "User’s manual with revised thermodynamic data base and test cases for calculating speciation of major, trace and redox elements in natural waters", USGS Open-File Report 90-129, 185 p, 1991.

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.Stumm, W. and J. J. Morgan: Aquatic Chemistry, Chemical Equilibria and Rates in Natural Waters (3rd ed.), John Wiley & Sons, Inc., New York, 1996, {{ISBN|978-0471511854}}Morel, F. M. M. and J. G. Hering: Principles and Applications of Aquatic Chemistry (2nd ed.), John Wiley, New York, 1993, {{ISBN|978-0471548966}}Appelo, C. A. J. and D. Postma: Geochemistry, Groundwater, and Pollution. Taylor & Francis, 2005, {{ISBN|978-0415364287}}

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)

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)

{{Reflist}}

• [http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/ PHREEQC]
• [http://www.aqion.onl/ Online calculator] for pH, aqueous speciation, saturation indices, alkalinity, EC

{{Chemistry software}}

Category:Computational chemistry software

Category:Science education software