Sodium fluoride ion-pairs in seawater

Abstract

Laboratory investigations were conducted on the formation of NaF° ion-pairs at the ionic strength of seawater using specific ion electrodes. Sodium and fluoride ion electrodes produced results which are consistent with the ion-pairing model for these ionic interactions. The stoichiometric association constant for NaF°, K ∗ NaF , was determined at 15, 25, and 35°C. It was assumed that K ∗ NaF was a function of temperature, pressure, and ionic strength but not of solution composition. The value for K ∗ NaF at 25°C and I = 0.7 m is 0.045 ± 0.006. K ∗ NaF increased with decreasing temperature. This result was used to recompute values of K ∗ MgF and K ∗ CaF accounting for the presence of NaF° ion-pairs. The value for K ∗ NaF indicates that 1.1% of the fluoride in seawater is ion-paired with sodium at 25°C and 35‰ salinity. This fraction increases to approximately 2% at the lower temperatures found in the deep ocean. The percentage of free fluoride in natural seawater was measured at 15, 25, and 35°C to verify the speciation calculated from equilibrium constants.