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Vanadium Speciation using thermo.dat

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I am trying to calculate the percent of each vanadium species as a function of the total vanadium concentration for a particular groundwater sample (script is attached). However, when I sum all the V species predicted, the sum of their concentrations is lower than total input V concentration. For the sample attached, the total V concentration is 0.19 nmol/kg, but the sum of the predicted species is 0.17 nmol/kg, or 22 pmol/kg lower. There is probably something obvious I am missing, but I have not encountered this problem before.


Thank you!



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Hi Kat,


When comparing the mass of a component to the masses of species making up that component, you need to account for the stoichiometry of each species. Taking the V+++ component as an example, the V+++, VO2(OH)2-, and VO3OH-- species are each composed of one mole of V+++. Other species, like (VO)2(OH)5-, (VO)2(OH)2++, and V2(OH)2++++, are composed of two moles of the V+++ component. Before you add up species concentrations in Excel, you need to multiply the concentration of each by the number of moles of the component which go into it. In your case, (VO)2(OH)5- is the third most abundant species, with a concentration of .022 nmol/kg. By not accounting for the fact that this species (along with the others I mentioned) is composed of two moles of V+++, your sum works out to 0.168 instead of 0.19 nmol/kg, which is the total you entered on the Basis pane.


For a reference on the subject, try checking out section 3.3.3, Mass balance equations, in the Geochemical and Biogeochemical Reaction Modeling text.


Hope this helps,


Brian Farrell

Aqueous Solutions

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