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Mixing samples in GSS


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I am trying to mix two samples at different temperatures and different pHs. I followed an earlier post on calculating pH for mixing calculations in GSS, however I am not able to calculate the pH after creating the new mixed sample. I made sure that H+ was incuded as a basis species, but the pH does not show up after I select Data>Calcuate (selecting both HCO3 and H+)for the newly calculated mixed sample. Any suggestions? We are trying to find the most efficient way to mix samples at various proportions and be able to quickly calculate the pH, temperature, and speciation of important fluid components.





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Hi Elisabeth,


Sorry for the delayed response. To calculate the pH of mixtures in GSS, there are a few things you need to do. First, define the chemistry of each of your samples (including pH and temperature) then set the mass of each sample (+ analyte - System parameters - Mass Solution) so that you can specify your mixing ratios. Then you need to calculate the concentration of total H+ component in the fluid (+ analyte - Calculate... - Components in fluid - H+). Next mix your samples (Analysis - Mix Samples), being sure to check "Save as sample". This treats your mixed fluid as a new sample which can be manipulated like any of the original fluids. Then just calculate the pH (+ analyte - System parameters - pH). GSS will give a warning that pH is already known for your original samples, just go ahead and say Yes to all, and the pH of the mixture will be calculated.


There is an error in the current release, unfortunately, in the step where GSS calculates the amount of H+ component. The calculated value is total moles of H+, but the unit that shows up right now is molal. You'll want to change the unit without performing a unit conversion, since you'll want to preserve the calculated value. To do so, right-click the unit field for the H+ component and select units - mol, then when GSS asks to convert from molal to moles say no. GSS will then correctly solve for pH of the mixed sample given solution mass and total moles H+.


The temperature and component concentrations of your mixed fluid will be calculated automatically, as will the mass of the mixed fluid. To find the concentration of an individual species in the mixture, just go to + analyte - Calculate - Species concentrations.


In this way you can calculate the pH, temperature, and concentration of various species in a mixture of samples. Just change the solutions mass, then repeat the above steps, to consider various proportions.



You can also use React to mix two fluids together. First define a fluid in the Basis pane. Once you run the equilibrium model, you can "pick up" the results to be used as a reactant, which will be titrated into a new, second fluid which you define in the now empty Basis pane. Now when you run the model, a small amount of the reactant fluid will be added incrementally to the fluid in the Basis pane. In the Reactants pane, you can set a value for "reactants times" to specify the final proportions of the two fluids. In this way you set up a reaction path in which one fluid is gradually added to another, allowing you to see how the pH, temperature, and speciation change.


The flash option is a variant of this calculation type. With the flash option, you can mix two fluids in any mixing ratio - from one end-member to the other.


For more information and examples, you should look at section 3.4.4 (Mixing samples) of the GWB Essentials Guide and sections 3.7 ("flash diagrams") and 3.9 (Picking up the results of a run) in the GWB Reaction Modeling Guide.


Hope this helps,


Brian Farrell

Aqueous Solutions LLC

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