Need help recreating this speciation diagram

[emma_k]

Hello everyone. I'm new to GWB and am attempting to recreate a diagram that was made in my lab years ago. Essentially, I am looking to develop a plot (see attached) of the speciation of Cu(II) as a function of sulfate concentration. I know that this plot was made in Excel but the data derived from GWB. Once I figure this out I'm also hoping to do the same thing with Zn(II). It seems like it shouldn't be too difficult, but I'm struggling to find guiding resources as most plots are speciation varied by pH. If someone could explain what program in GWB they would use and how to input that data and avoid charge errors, it would be extremely helpful. Thanks for your time!

[Jia Wang]

Hello Emma,

Most speciation diagrams like this are created in the React application, available in the GWB Standard and Professional packages. I am guessing that the user set a reaction path to titrate sulfate into your system and see how the concentration of Cu-complexes and free Cu(II) change over the course of the addition. By the way, if you set up a React simulation described above, you can directly generate the diagrams in Gtplot and don't need to render the diagram separately in Excel if you just want to plot in common concentration units (i.e. mmol/kg, molal, etc). If you would like more information on React titration paths, please look at section 3.1 Titration paths in the GWB Reaction Modeling User Guide. You can find this under the "Help" menu on any of the GWB applications or in the Docs pane of your GWB dashboard. If this is your first time using React, I would suggest reading through section 3 as a starting point. More information regarding Gtplot can be found in section 6 of the same user guide. 

If you aim to reproduce the diagram exactly, it would be very beneficial if you can find the input file that outputted the results used to make this diagram. 

Just an fyi, the pH speciation diagrams are a different type of reaction paths available in React. You can find more information regarding it in section 3.6 of the same user guide mentioned above. 

Hope this helps,
Jia Wang
Aqueous Solutions LLC

[emma_k]

Hi Jia,

Thank you so much for your former response. I've revisited the project and had a few additional questions if you or anyone else could assist. I unfortunately do not have a file for the above image, however we are not even sure if it is accurate for the situation we're attempting to model, so I'm basically starting from scratch.

The scenario is wanting to titrate in 0.18 M of NaSO4 into a solution of 5 mM Cu(2+) and 1 mM NaNO3 at pH5 to determine the concentration of free copper and copper complexes. I'm going to include screenshots of what my React setup is. I've looked through the manuals but am still struggling with error messages. I was first receiving an error regarding sulfate needing to be in the basis, however it is not in my initial solution so I set this as 0. I am also confused on what my charge balance ion would be.

Thanks so much for any help on this issue!

Emma

[Jia Wang]

Hello Emma,

You're welcome. I am glad you find the comments helpful. Here are a few suggestions to get you started. 

In your Basis pane, you would want to set a small, insignificant concentration of sulfate in your initial system. The program solves a set of equations by numerical iteration and cannot start with a zero value for component concentration mass.

If you're titrating Na2SO4 into the solution, you should check to make sure that it is added correctly with the right unit in the Reactants pane to ensure that your reactants are properly charge balanced. If the titrant is Na2SO4, then for every mole of of SO4-- added, you should also include 2 times the amount for Na+. Is 0.18 molar the concentration of your titrant or is that the endpoint concentration of sulfate you want to titrate to for your simulation? The plot in your original post stops at 10 mM of sulfate, which seems to me you want to only titrate a total of 10 millimoles sulfate per liter.

Typically, the "balance on" species chosen has high abundance and low uncertainty. For example, React is set up to use Cl- as the default charge balancing ion since most Cl- is usually abundant and can be calculated from charge balance based on other species concentrations measured in chemical analyses. In your case, I think your system is in excess of cation and choosing NO3- rather than Na+ as your balancing ion seems to help. You can also turn off charge balancing as well. 

You might consider whether or not the complexation reaction occurring is a kinetic reaction. If it is, you may want to consider implementing a kinetic approach with your reactions. You can refer to section 4 Kinetic Reaction Paths in the GWB Reaction modeling User Guide for more information.

You may also want to check the dataset you're using to see if the equilibrium constants are appropriate for your experiment. The default dataset (thermo.tdat) used by the GWB is published by Lawrence Livermore National Laboratory. In particular, you might check if the equilibrium constant for the reaction of CuSO4 is what you expect. There are other datasets installed with the dataset from published from other sources that may vary from the dataset you are using. You can find all the default thermo datasets installed with the GWB in the Gtdata folder. To view the datasets, you can use the GWB thermo dataset editor TEdit. For more information on this, please see section 9 in the GWB Essentials User Guide. 

Hope this helps,
Jia