SolveSAPHE-r2 (Solver Suite for Alkalinity-PH Equations Release 2) provides a collection of Fortran 90 routines to solve the total alkalinity–pH equation, which relates total alkalinity and pH for a given set of total concentrations of the acid–base systems that contribute to total alkalinity in a given water sample. Carbonate alkalinity can now be expressed by using the total concentration of dissolved inorganic carbon (CT, as in SolveSAPHE v.1) or by specifying either concentration of CO2, HCO3− or CO32−. The package provides two different iterative solvers for the alkalinity-pH equation, designed to assure convergence with any physically meaningful chemical composition, and in a completely autonomous way. The AlkT & CO32− pair is completely diagnosed and if there are two roots, both are calculated. Although the AlkT & CT pair can also be processed with SolveSAPHE-r2, this is considerably slower than with SolveSAPHE v.1, due to the extra safeguarding that had to be introduced for the other three pairs This package was first published as a supplement to the discussion paper: Guy Munhoven (2021) SolveSAPHE-r2: revisiting and extending the Solver Suite for Alkalinity-PH Equations for usage with CO2, HCO3− and CO32− input data, Geosci. Model Dev. Discuss., 2021 (doi:10.5194/gmd-2020-447).

Financial support for the development of SolveSAPHE was provided by the Belgian Fund for Scientific Research – FNRS (project "SERENATA", CDR grant J.0123.19). Guy Munhoven is a Research Associate with the Belgian Fund for Scientific Research – FNRS.