Ionic Diffusion Coefficients of Cs⁺, Pb²⁺, Sm³⁺, Ni²⁺, SeO²-₄ and TcO⁻₄ in Free Water Determined from Conductivity Measurements

The ionic diffusion coefficients for Cs⁺, Pb²⁺, Sm³⁺, Ni²⁺, SeO²-₄ and TcO⁻₄, in free water (Do) were determined at room temperature by measuring conductivity. The Do values were derived based on the Nernst-Einstein equation. The limiting ionic equivalent conductivities (λ₀) for Cs⁺, Pb²⁺, Sm³⁺, Ni²⁺ and SeO²-₄ were derived based on Kolrausch's square-root law and that for TcO⁻₄ was derived from the relation between the concentration of TcO⁻₄ in the solution and the λ because of its weak electrolyte. The Do values were in a range of 1.4 to 2.2x10⁻⁹ m²/s. The Do values for Cs⁺, Pb²⁺, Sm³⁺, Ni²⁺ and SeO²-₄ and those found in the literature showed good agreement. The Do for TcO⁻₄, which was previously unknown, was determined to be 1.95x10⁻⁹ m²/s, which is similar to that for analogous chemical species. The variation in Do has been expected to be approximately one order of magnitude at constant temperature and all Do values determined in this work were in this range. The variation in activation energy for diffusion for some important ions was less than a factor of two and it is concluded that the temperature dependence of Do is approximately the same.