Ionization Constants of T2O and D2O at 25° from Cell emf's. Interpretation of the Hydrogen Isotope Effects in emf's

The ionization constants of T2O and D2O were determined to be 6.1 × 10−16 and 1.43 × 10−15 at 25°, corresponding to K(H2O) / K(T2O) = 16.4 and K(H2O) / K(D2O) = 7.1 (concentrations in aquamolalities m′, moles of solute per 55.51 moles of isotopic water). Emf's were measured at 25° with X = H, D, and T for cells of the types: Pt–X2(p) | NaOX(m2′)+NaCl(m3′) | AgCl–Ag and Pt–X2(p) | XCl(m1′) | AgCl–Ag; m1′ = m2′ = m3′ = 0.1. The isotope differences [E°(isotope) − E° (H2O)] in the alkaline cell are 45.8 ± 0.5 mV for D2O and 65.4 ± 0.3 mV for T2O. In the acid cell the differences are − 4.4 ± 0.1 mV for D2O and − 6.4 ± 0.2 mV for T2O. Published data on the H2O–D2O emf isotope effects from 5° to 50° in alkaline and acid cells are correlated with spectral and structural information by methods similar to those used in the treatment of vapor-pressure isotope effects. These methods and the H2O–D2O data are used to predict H2O–T2O isotope effects in emf's and a value of K(H2O) / K(T2O) which agree satisfactorily with experiment. Free energies of transfer of NaCl from D2O to H2O and from T2O to H2O are discussed.