Nick Rukin

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. Nick

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 Nick