As suggested in a previous study under the title "Simple Relationship Between the Properties of Isotopic Water", viscosity results verify the fact that the structural properties of liquid H 2 O and D 2 O are nearly identical once a zero-point-energy-induced thermal offset effect is taken into account. This means that the viscosities of these two isotopic forms must be compared at different temperatures, rather than at the same temperature. Only in this way can the expected (M D 2 O /M H 2 O ) 1/2 viscosity ratio be retrieved. Application of this most simple idea, with no additional parameter adjustment, to H 2 O viscosity data, or equivalently to any of the existing empirical viscosity equations for H 2 O, leads to D 2 O viscosities having better than 1% accuracy over a wide temperature range. This isotopic correlation concept has also been used here to predict viscosities of liquid T 2 O, no viscosity data apparently being available for this substance.

In an earlier paper on this subject, it was conjectured that if temperatures of H 2 O and D 2 O are scaled according to their temperatures of maximum density, then the structural properties of these two liquids should be very closely the same. In this paper it is found that the idea is valid for the densities within the four decimal point experimental precision of the D 2 O data. If this structural effect is not taken into account, a comparison of properties, such as viscosities or self-diffusion coefficients, of isotopic liquid water would have a reduced meaning. Comparing data at the same temperature, as has traditionally been done, simply gives rise to misleading isotope effects.