Weak barium and radium hydrolysis using an ion exchange method and its uncertainty assessment

•The hydrolysis of Ba2+ and Ra2+ was studied at 25 °C using ion exchange method in NaOH and NaClO4 mixtures.•The hypothesis that BaOH+ and RaOH+ are not formed in the studied system was tested and rejected.•A model for ion pairing was developed and apparent BaOH+ and RaOH+ stability constants were derived.•Systematic uncertainties have a much greater contribution to the BaOH+ and RaOH+ stability constant uncertainty budget.•A model for uncertainty assessment in ion paring was suggested.