experiments in 0.1, 0.01, and 0.001 M NaNO3, resp.; ion exchange: Na+ + X- = NaX logK=20 by assumption, H+ + X- = HX logK=22.9 [SLW87], Ni2+ + 2X- = NiX2 logK=40.8 (calculated from Na-K-exchange data of Ewin et al. (1981) and K-Ni-exchange data of Bansai (1982))