Fluoride, in seawater

Fletsch and Richards [51] determined fluoride in seawater spectrophotometri-cally as the cerium alizarin complex. The cerium alizarin complex and chelate was formed in 20% aqueous acetone at pH 4.35 (sodium acetate buffer) and, after 20-60 min, the extinction measured at 625 nm (2.5 cm cell) against water. The calibration graph was rectilinear for 8-200 ig/l fluoride the mean standard deviation was 10 xg/l at a concentration of 1100 ig/l fluoride. 

Ion selective electrodes are emerging as a method of preference for the determination of fluoride in seawater [53-58]. 

The concentration of fluoride in seawater is approximately 1.4 ppm or 7 x icr5 mol/1 and the fluoride electrode has been shown to give a Nerns-tian response at this level - and indeed three orders of magnitude lower than this level [64]. 

Table2.2. Determination of fluoride in seawater under a variety of conditions to demonstrate the reliability of the method [58] ...

The level of fluoride in seawater is at least two orders of magnitude higher than this residual level, hence contamination does not significantly influence the results. 

Ke and Regier [71] have described a direct potentiometric determination of fluoride in seawater after extraction with 8-hydroxyquinoline. This procedure was applied to samples of seawater, fluoridated tap-water, well-water, and effluent from a phosphate reduction plant. Interfering metals, e.g., calcium, magnesium, iron, and aluminium were removed by extraction into a solution of 8-hydroxyquinoline in 2-butoxyethanol-chloroform after addition of glycine-sodium hydroxide buffer solution (pH 10.5 to 10.8). A buffer solution (sodium nitrate-l,2-diamino-cyclohexane-N,N,N. AT-tetra-acetic acid-acetic acid pH 5.5) was then added to adjust the total ionic strength and the fluoride ions were determined by means of a solid membrane fluoride-selective electrode (Orion, model 94-09). Results were in close agreement with and more reproducible than those obtained after distillation [72]. Omission of the extraction led to lower results. Four determinations can be made in one hour. 

Photoactivation analysis has also been used to determine fluoride in seawater [73]. In this method a sample and simulated seawater standards containing known amounts of fluoride are freeze-dried, and then irradiated simultaneously and identically, for 20 min, with high-energy photons. The half-life of 18F (110 min) allows sufficient time for radiochemical separation from the seawater matrix before counting. The specific activities of sample and standards being the same, the amount of fluoride in the unknown may be calculated. The limit of detection is 7 ng fluoride, and the precision is sufficient to permit detection of variations in the fluoride content of oceans. The method can be adapted for the simultaneous determination of fluorine, bromine, and iodine. 

Complexation of fluoride by metal ions in seawater has previously been overcome by the addition of TISAB solution. The reagent is presumed to release the bound fluoride by preferential complexation of the metal ions with EDTA type ligands present in the TISAB. Examination of the metal ions present in seawater [66,67] suggests that magnesium is the major species forming fluoride complexes. Theoretical calculations demonstrate that even this species is unlikely to interfere. 

Fluoride has been determined in seawater in amounts down to 8 ng/ml by a method based on the formation of A1F in an electrothermal graphite furnace, followed by molecular absorption at 227.45 nm [74]. 

Holm et al. [74] used a spectrometry for the determination of 237neptunium in seawater. The actinides are preconcentrated from a large seawater sample by hydroxide precipitation. The neptunium was isolated by ion exchange, fluoride precipitation, and extraction with TTA. 238Neptunium or 235neptunium was used to determine the radiochemical yield. 

A full imderstanding of the speciation of dissolved iron requires consideration of ligands other than water and hydroxide. The most important ones are listed in Table 5.6 along with their concentration ranges in seawater and freshwater. For Fe(III) in seawater at pH > 4, the formation of complexes with hydroxide is most important, but at pH <4, sulfete, chloride, and fluoride pairing predominates (Figure 5.15b). To predict the equilibrimn speciation at low pH, these anions need to be added to the mass balance equation fiar Fe(III) (Eq. 5.20). Seawater with low pH tends to have low O2 concentrations. Under these conditions, most of the dissolved iron is present as Fe( II), which undergoes complexation with sulfide and carbonate. 

Table 5), and several are now being used, or are potentially useful, for measuring key ocean elements. The most common use of direct potentiometry (as compared with potentiometric titrations) is for measurement of pH (Culberson, 1981). Most other cation electrodes are subject to some degree of interference from other major ions. Electrodes for sodium, potassium, calcium, and magnesium have been used successfully. Copper, cadmium, and lead electrodes in seawater have been tested, with variable success. Anion-selective electrodes for chloride, bromide, fluoride, sulfate, sulfide, and silver ions have also been tested but have not yet found wide application. 

In dilute solutions and at neutral pH, dissolved fluorides are usually present as the fluoride ion (F ). As pH decreases, the proportion of F decreases, while hydrogen fluoride (HF ) and nondissociated hydrogen fluoride increase. Levels of nondissociated hydrogen fluoride also increase in concentrated solutions. In seawater, fluorides exist in equilibrium. Calcium carbonate precipitation dominates the removal of dissolved fluoride from seawater. The next most important removal mechanism is incorporation into calcium phosphates. Undissolved fluoride... 

Fluoride content in natural waters in the northeastern part of the U.S. ranges from 0.02 to 0.1 ppm, while in the west and midwest river waters it ranges from 0 to 6.5 ppm, with an average of 0.2 ppm. Groundwaters contain from 0.1 to 8.7 ppm, depending on the rocks from which the waters flow. The level of F in seawater is about 1.2 ppm. 

Seawater contains many dissolved substances, mostly dissolved sodium chloride. In Chapter 4, you learned that sodium chloride is an ionic compound. Another ionic compound found dissolved in seawater is magnesium chloride. Some common ionic compounds used in everyday life are potassium chloride, a salt substitute used by people avoiding sodium for health reasons potassium iodide, added to table salt to prevent iodine deficiency and sodium fluoride, added to many toothpastes to strengthen tooth enamel. You will learn how to use the language of chemistry to name and write the formulas of ionic compounds. 

Froelich, P. N., K. H. Kim, R. A. Jahnke, W. C. Burnett, A. Soutar, and M. Deakin (1983). Pore water fluoride in Peru continental margin sediments Uptake from seawater. Geochim. Cosmochim. Acta 47,1605-1612. Froelich, P. N., M. A. Arthur, W. C. Burnett, M. Deakin,... 

Magnesium is the third most abundant element in seawater, behind sodium and chorine, and has an average concentration of approximately 1300 ppm. Table 3.2 displays the major and some minor elemental constituents of seawater. Eleven major constituent ions account for 99.5% of the total solutes present in seawater. These 11 are chloride, sulfate, bicarbonate, bromide, fluoride, sodium, magnesium, calcium, potassium, strontium, and boron, and they largely determine the chemistry of seawater. 

Froelich PN, Kim KH, Jahnke R, Burnett WC, Soutar A, Deakin M (1983) Pore water fluoride in Pern continental margin sediments uptake from seawater. Geochim Cosmochim Acta 47 1605-1612... 

Our conclusion from this discussion is that synthetic seawater buffers containing sulphate but not fluoride, as originally suggested by Hansson (1973), should be used for pH measurement in seawater and estuarine water, ue., that the pH(T) scale should be used. With this choice the problems caused by the uncertainty in the stability constant for HF are avoided. Preparation of the buffers and assignment of pH is treated below. 

Of continuing interest is monitoring of fluoride, F, equally in seawater and sweet water, where, despite the principal progress of modem insfrumental analysis, still the most popular methods for its determination are based on the classical F -ISE, applicable not only to reliable quantification (see Table 6.1, reference 10), but also for speciation of some fluoride complexes (of the Me F typc> where Me is Mg, Ca, Sr, Ba), detectable at the decimolar level. As certain rarity, an amperometric biosensor for F can also be mentioned, being modified with a flower tissue and having utilised the above-mentioned bio-inhibition mechanism. 

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