Aqion

aqion

Initial release	January 1, 2012
Stable release	version 5.18 / May 2016
Written in	C++
Operating system	Windows
Size	1.5 MB
Available in	English, German
Website	aqion.de

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 2 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 programm 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)       $f_j (c_1,c_2,...,c_{N_B}) \,=\, [j]_{TOT} - [j] - \sum^{N_S}_{i=1} \frac{\nu_{i,j}}{\gamma_i}\, K_i\, \prod^{N_B}_{k=1} \{k\}^{\nu_{i,k}} \,=\,0$

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)

References

1. Parkhurst, 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
2. Note: The upper limit is sea water.
3. http://www.kristall.uni-frankfurt.de/media/handouts/GEM-lecture.PDF
4. Remark: To keep things apart, the numerical solver of PhreeqC is outsourced from aqion.exe into a separate DLL.
5. 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.
6. 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
7. Morel, F. M. M. and J. G. Hering: Principles and Applications of Aquatic Chemistry (2nd ed.), John Wiley, New York, 1993, ISBN 978-0471548966
8. Appelo, C. A. J. and D. Postma: Geochemistry, Groundwater, and Pollution. Taylor & Francis, 2005, ISBN 978-0415364287
9. 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)

External links

• PHREEQC