Dealing with Ionic Strength Changes in Chemical Kinetics, Including Extrapolation to Zero Ionic Strength

Few kinetic studies are conducted in aqueous solution without the addition of an inert salt to maintain constant ionic strength. The immediate reason is to maintain constant activity coefficients, which are strongly ionic strength dependent. Rate constants are subsequently reported at a given ionic strength, or in some cases, a study of ionic strength dependence is used for extrapolation to determine the rate constants, <i>k</i>₀, at zero ionic strength, at infinite dilution. This is in line with the more commonly determined thermodynamic equilibrium constant, <i>K</i>₀. In both instances, the goal is to determine the intrinsic rate or equilibrium constant of a molecular chemical interaction without interference from other compounds in the solution. The addition of inert salts has several disadvantages and limitations: inert salts are usually not completely inert and can interfere with the process under investigation; they can be expensive; and, most importantly, they never maintain constant ionic strength. In fact, they do not reduce the changes in absolute ionic strength at all; they only reduce relative ionic strength changes. Here, we present a new method that avoids these constraints by incorporating ionic strength and activity coefficient changes directly into the numerical analysis of the measured data. Using the Ni²⁺-oxalate reaction as a case study, we apply activity coefficient corrections (Debye-Hückel, Davies, SIT) within numerical integration of the rate equations, enabling <i>k</i>₀ to be determined from a single experiment at minimal ionic strength. Replicate measurements yield consistent results, log<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>(</mml:mo><mml:msubsup><mml:mrow><mml:mi>k</mml:mi></mml:mrow><mml:mrow><mml:mn>0</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup><mml:mo>)</mml:mo></mml:math> = 5.20 ± 0.02. This approach eliminates inert salt addition, simplifies experimental procedures, and provides a generalizable framework for obtaining physically meaningful rate constants.