Master of Science (MS)
Semester of Degree Completion
David W. Ebdon
Stoichiometric ion-association constants were determined at several ionic strengths for (NaF) over the temperature range 15-40°C and for (MgF)+ and (CaF)+ over the temperature range 25-45°C using fluoride ion selective electrode measurements.
Thermodynamic association constants, K°, were obtained through extrapolation of the stoichiometric values to infinite dilution. Values of K° for (NaF) are 1.70, 2.16 and 2.54 at 15, 25 and 40°C, respectively. The K° values for (MgF)+ are 73.4, 88.0 and 98.2, and for (CaF)+ are 13.7, 17.5 and 21.9 at 25, 35 and 45°C., respectively.
The stoichiometric association constants for (NaF) can be reproduced by the equation log K' = log K° - 2AI + BI, where A is the Debye-Huckel limiting slope, B is an empirical constant and K° is the thermodynamic association constant.
The stoichiometric association constants for CaF+ and MgF+ can be reproduced by the equation: log K'= log K° - 4AI1/2 + BI.
A linear correlation between ΔH' (the enthalpy of formation of the ion pair from the free ions) and the ionic strength of the solutions has been obtained for each of the salts studied. The results show that as the ionic strength increases, ΔH' for sodium fluoride ion association changes from positive to negative, for magnesium fluoride also from positive to negative and for calcium fluoride from higher to lower positive values. These correlations can be used to calculate the stoichiometric association constants at various temperatures and ionic strengths.
Zamorano-Santander, Waldo, "Thermodynamics of Ion Association in Aqueous Fluoride Solutions. Determination of Stability Constants for (NaF), (MgF)+ and (CaF)+ Ion Pairs" (1985). Masters Theses. 2737.