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Is the entry for HPO4-- the laboratory measured 'orthophosphate'or 'SRP' ?

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I am examining solubility constraints for dissolved phosphate in some groundwater samples.  The field pH of these waters is circa 7.0-7.4.

When setting up the input data the only phosphorus input I can see is  'HPO4--'.  I am currently assuming this is my laboratory measured result for orthophosphate or soluble reactive phosphorus (SRP), but I want to check that I should not be first calculating the proportion of HPO4-- versus H2PO4- using pH and the dissociation constant.  

So my question is: is the HPO4-- entry for lab orthophosphate (or SRP ) or is it the concentration of the dissolved HPO4-- ion (as calculated using dissociation constant and pH)?

(As an aside I am aware of the paper by Sahai & Schoonen 2020 (Accuracy of Thermodynamic Databases for Hydroxyapatite Dissolution Constant, ASTROBIOLOGY
Volume 20, Number 1, 2020 which flags up an eight order magnitude difference in the equilibrium solubility constant for hydroxyapatite in the different databases)

Thank you


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Hello Nick,

Are you talking about entering data into GSS or another GWB application?

The software by default sets the total concentration (the mass of the thermodynamic component). What you enter for HPO4-- is a bulk constraint equal to the sum of H2PO4-, HPO4--, PO4---, etc. species.

The software does allow you to specify the HPO4-- free ion concentration and calculate additional mass for other phosphorus species. To specify a free species concentration in GSS, you can right-click on the unit (e.g. mmol/kg) and select "Free". In another application like SpecE8 or React, you would left-click on the unit and select "Free".

For more information regarding free vs. bulk concentrations, please see section 7.2 Equilibrium Model in the GWB Essentials Guide.

Best regards,
Jia Wang
Aqueous Solutions LLC


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Hello and thank you Jia

Yes I am entering HPO4-- into a GSS spreadsheet.

You have answered my question perfectly, i.e. what I enter for HPO4-- is in my case my laboratory measurement of soluble reactive phosphorus (which in the groundwaters I am studying is also almost identical to laboratory orthophosphate result).

Thank you for your help and swift response.



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