Fluoride in Sea Water: Distribution in the North Atlantic Ocean and Formation of Ion Pairs with Sodium

Abstract

Two problems concerning fluoride in sea water were considered in order to advance the understanding of the chemistry of fluoride in the marine environment. Anomalously high fluoride concentrations have been observed at some locations in the North Atlantic Ocean. If these anomalous concentrations continued to exist they could provide an opportunity to examine some of the geochemical processes of fluoride in sea water. Considerations of the geochemistry and physical chemistry of fluoride in sea water are hindered by an inadequate account of the interactions of fluoride and sodium. Water samples were collected on several cruises in the North Atlantic and the fluoride to chlorinity ratio measured. Particular emphasis was made to sample at locations where anomalously high ratios had been previously reported. The results of this survey showed that the fluoride to chlorinity ratio was constant, within analytical uncertainties, and indicated that there were no anomalous fluoride concentrations present near the Mid-Atlantic Ridge or at several other locations where they had been reported. There appears to be no obvious reason to consider the previously reported concentrations invalid. It was concluded that fluoride was a conservative element for the sea water samples analyzed although there may be episodic inputs which may change the fluoride to chlorinity ratio in restricted locations by a significant amount. Laboratory investigations were conducted on the formation of NaF° ion-pairs at the ionic strength of sea water and 15, 25, and 35°C. It was assumed that the stoichiometric association constant was a function of temperature, pressure, and ionic strength but not of solution composition. It was also assumed that NaCl and KF were completely dissociated. The value for K*NaF at 25°C and μ= 0.7 m is 0.045 ± 0.006. K*NaF increased with decreasing temperature. This value for K*NaF indicates that 1.1% of the fluoride in sea water is ion-paired with sodium at 25 C and 35 0/00 • This fraction increases to approximately 2% at the lower temperatures found in the deep ocean.