Volume Change of Ion Pair Formation: Rubidium Nitrate and Thallium(I) Nitrate in Water

Density data for dilute aqueous solutions of RbNO(3) and TlNO(3) at 25 degrees C lead to the following equations for the apparent molal volumes: RbNO(3) : phi(V) = 43.0(eta) + 2.46c(1/2) + 0.15c (0-0.41 M) TlNO(3) : phi(V) = 39.5(0) + 10.83c(1/2) - 25.46c + 26.0c(3/2) (0.03- 0.25 M) From these expressions and a semiempirical equation for phi(V) of a completely dissociated 1:1 electrolyte, values of Delta phi(V) for ion pair formation at c = 0.10 are calculated to be + 5 ml-mole(-1) for RbNO(3) and+12 ml-mole(-1) for TlNO(3). Comparison of Delta phi(V) with the electrostriction volume effect suggests that virtually all of the electrostriction is removed in the formation of the TlNO(3) ion pair but only about half with RbNO(3). Values of Delta phi(V) are in poor agreement with those estimated from electrostatic theories of ion pair formation.