Fluoride in natural waters

Bayon, M. M., Rodriguez Garcia, A., Garcia Alonso, J. I., and Sanz-Medel, A., Indirect determination of trace amounts of fluoride in natural waters by ion chromatography a comparison of on-line post-column fluorimetry and ICP-MS detectors, Analyst, 124, 27, 1999. 

The concentration of fluorides in natural waters ranges from hundredths to tenths mg 1. Concentrations exceeding 1 mg 1 are exceptional. 

Most of fluorides in natural water are found in the form of Al(III) and Fe(III) fluorides. Fluoride boimd to Al(lll) or Fe(lll) is not determined by these colorimetric methods. To mask of Al(lll) or Fe(lll), complexing agents such as DCTA (frans-l,2-amino-cyclohexane-N,N,N, N -tetraacetic acid) cannot be used in these methods due to the decomposition of the colored ternary complexes by DCTA. Therefore the sensitive methods using optical detection usually employ separation step. 

Dissolved mineral salts The principal ions found in water are calcium, magnesium, sodium, bicarbonate, sulphate, chloride and nitrate. A few parts per million of iron or manganese may sometimes be present and there may be traces of potassium salts, whose behaviour is very similar to that of sodium salts. From the corrosion point of view the small quantities of other acid radicals present, e.g. nitrite, phosphate, iodide, bromide and fluoride, have little significance. Larger concentrations of some of these ions, notably nitrite and phosphate, may act as corrosion inhibitors, but the small quantities present in natural waters will have little effect. Some of the minor constituents have other beneficial or harmful effects, e.g. there is an optimum concentration of fluoride for control of dental caries and very low iodide or high nitrate concentrations are objectionable on medical grounds. 

Two factors characterized most of the waters sampled in the monitoring program. The factor loadings for Factor one indicate that the following chemical species participate in correlated behavior that permits the separations and distinctions described above alkalinity, bicarbonate, B, Cl, conductance, F, Li, Mo, and Na. To simplify discussions in the plots shown earlier this group of species was called the salinity factor. Specific conductance in natural waters usually correlates well with hardness and not as well with bicarbonate, but the current study finds specific conductance closely related to bicarbonate and unrelated to hardness (Ca, Mg, sulfate). This indicates that the ions responsible for increased conductance are probably not calcium or magnesium, rather they are more likely sodium, fluoride, and chloride. 

Because of the low natural levels of fluoride in some water supplies and correspondingly high levels of dental caries, many authorities worldwide have permitted, or instigated, fluoridation of water supplies, although this has met some opposition, partly because of the potential health or dental effects including fluorosis. In order to prevent dental caries, fluoride is deliberately added to salt or milk in some countries. 

Fluorine is widely distributed in Nature, representing about 0.065% of the earth s crust, making it the 13th most abundant element. It is more abundant than chlorine and much more abundant than common metals such as zinc and copper. Fluorine occurs in many minerals in which fluoride ion replaces hydroxide. The conversion of hydroxyapatite to flu-oroapatite strengthens tooth enamel. However, this would result in an increased brittleness in bones. An untested theory is that the widespread use of fluoride in drinking water, which had remarkable benefits in preventing dental caries in the 1960s, may be the cause of the rise in osteoporosis in the elderly population. 

Complex Ion Formation. Formation of complexes of lead with the various anions such as chloride, fluoride, carbonate, bicarbonate, and hydroxide increases the concentration of lead in natural waters by preventing lead from taking part in other chemical reactions, primarily adsorption, that would lower its... 

Silicate minerals are more soluble in natural waters having high fluoride concentrations and low pH values than in other waters. High concentrations of dissolved silica may be maintained by the formation of hexafluorosilicic acid ... 

Roberson, C. E., and Barnes, R. B. Stability of fluoride complex with silica and its distribution in natural water systems. Chem. Geol. 21, 239-256 (1978). 

Types of Solvent.—In order that a particular solvent may permit a substance dissolved in it to behave as an acid, the solvent itself ifiust be a base, or proton acceptor. A solvent of this kind is said to be proto-philic in character instances of protophilic solvents are water and alcohols, acetone, ether, liquid ammonia, amines and, to some extent, formic and acetic acids. On the other hand, solvents which permit the manifestation of basic properties by a dissolved substance must be proton donors, or acidic such solvents arc protogenic in nature. Water and alcohols arc examples of such solvents, but the most marked protogenic solvents are those of a strongly acidic character, e.g., pure acetic, formic and sulfuric acids, and liquid hydrogen chloride and fluoride. Certain solvents, water and alcohols, in particular, are amphiprotic, for they can act both as proton donors and acceptors these solvents permit substances to show both acidic and basic properties, whereas a purely protophilic solvent, e.g., ether, or a completely protogenic one, e.g., hydrogen fluoride, would permit the manifestation of either acidic or basic functions only. In addition to the types of solvent already considered, there is another class which can neither supply nor take up protons these are called aprotic solvents, and their neutral character makes them especially useful when it is desired to study the interaction of an acidic and a basic substance without interference by the solvent. 

The administration of fluoride in drinking water at concentrations of approximately 1 ppm significantly reduces dental caries. The anticaries benefits are similar to those due to natural fluoride in drinking water. Fluoridated drinking water produces the following a 60% lower dental caries rate, a 75% decrease in the loss of 6-year molars, and a 90%i reduction in the incidence of proximal caries of the four upper anterior teeth. 

The natural level of fluoride in drinking water where the child lives should be known before dietary fluoride is prescribed. At present, it is suggested that fluoride supplements be limited to where drinking water contains 60% or less of the optimal level of fluorides recommended for community water in the geographic area. 

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. 

Fig. 16.5 Relationship of DMFT and index of fluorosis to water fluoride content. The left y-axis indicates the number of decayed missing and filled teeth from caries (DMFT) and the right y-axis indicates the index of fluorosis, a measure of the deleterious effect of fluoride on the enamel surface (see text). The x-axis indicates the ppm fluoride found naturally in the drinking water supply. Triangles indicate DMFT and circles indicate the fluorosis index in the same populations. The curves showing the decrease in caries and decrease in fluorosis intersect at 1 ppm fluoride in the water supply on the x-axis (Copy of Fig. 3 from Hodge HC, Smith FA. (1954). Some public health aspects of water fluoridation. American Association of the Advancement of Science Publication No 19 Washington DC 1954 AAAS 1954, pp. 79-109)...

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