Fluoride In Sea Water Research Articles

The solubility of CaF2 in seawater of normal and increased salinity (associated with genesis of fluorite in sedimentary rocks)

The solubility of crystalline and precipitated calcium fluoride in seawater and brines of the first stages of its evaporative concentration (before the beginning of gypsum and halite setting) was experimentally studied. It was established that in the entire studied range of salinity, seawater and its derivatives are strongly undersaturated by the calcium fluoride, which excludes its spontaneous precipitation in drying isolated sea basins. A necessary condition for the formation of sedimentary fluorite is the entry into the drying sea basins of significant amounts of dissolved fluorine from external sources, which can be river runoff, volcanic emanations, and hydrothermal solutions.

The hydrothermal chemistry of fluoride in seawater

A series of hydrothermal experiments were performed at temperatures from 150–350°C (500 bars) to assess the effect of temperature on the solubility of F in seawater. Changes in dissolved F were measured along with pH and the major element chemistry of seawater. The major element chemistry of seawater changes with temperature owing to homogeneous nucleation and precipitation of carbonate, sulfate, and hydroxyl-sulfate minerals. Thus, to identify better the role such minerals play on F variability, predicted and measured changes in major elements were assessed simultaneously with temperature dependent changes in dissolved F.Dissolved F was observed to decrease at temperatures from 150–250°C. At temperatures greater than 250°C, however, dissolved F concentration increased and at 300°C, F achieved a concentration equivalent to that of unaltered seawater. This suggests that a F-rich mineral precipitates at temperatures in the vicinity of 250°C, and then entirely dissolves at temperatures greater than 250°C. Temperature dependent changes in dissolved F are best accounted for by 2F− ⇔ CO3−2 and F− ⇔ OHt¯ exchange between the fluid and magnesium carbonate and magnesium-hydroxide-sulfate-hydrate (MHSH) phases, respectively. At 250°C, F(-aq)− and MgF(aq)+ dominate the distribution of F-bearing aqueous species, while at temperatures greater than this, HF(aq)° dominates. Thus, the sharp decrease in F(aq)− ion activity at temperatures ≥ 250°C, may account for the dissolution of F-bearing minerals.Results from the seawater heating experiments show that F is a particularly reactive element and precipitates as a component in a number of different minerals over a range of temperatures. Reactions of this sort likely occur during seawater recharge of subseafloor hydrothermal systems and limit the amount of seawater derived F which can participate in alteration processes at relatively deep crustal levels. Results of thermodynamic calculations, however, indicate that rock-fluid interaction processes, especially at low to moderate temperatures, can also provide sinks for F in the form of fluor-components in basalt alteration phases, such as F-talc. Considering the tendency of F to partition into the fluid with increasing temperature, the relative absence of F in hot spring fluids suggests the existence of phases with unusually strong affinity for F in subseafloor reaction zones at mid-ocean ridges. Additional experiments are needed to confirm results of theoretical calculations involving F partitioning during fluid-mineral equilibria at elevated temperatures and pressures.

Determination of fluoride in sea-water by molecular absorption spectrometry of aluminium monofluoride after removal cation and anion interferences

Three procedures are proposed for the determination of trace levels of fluoride in sea-water, based on the formation of aluminium monofluoride in an electrothermal graphite furnace, followed by measurement of its molecular absorption at 227.45 nm. They involve the use of dilution, a matrix modifier, or a matrix modifier and an ion-exchange resin, and are all acceptably sensitive and specific for fluoride. Interferences from cations and anions are removed by a simple 20-fold dilution of the sample. At 10-fold sample dilution, chloride interference can be removed by adding 0.3 M ammonium nitrate together with 0.01 M aluminium + 0.01 M strontium as a matrix modifier. The same matrix modifier is valid for use with 5-fold sample dilution and a cation-exchange step to avoid matrix affects from cations and chloride. The detection limit is about 8–10 ng/ml fluoride and the determination limit is 20 ng/ml. The precision of peak-height measurement at 0.2 μg/ml is 5–7%.

Determination of titanium in sea water using adsorptive cathodic stripping voltammetry

A procedure is presented for determining titanium dissolved in sea water and fresh water using cathodic stripping voltammetry preceded by adsorptive collection of its mandelic acid complexes. The optimized conditions include a pH of 3.3, a mandelic acid concentration of 4 mM and an adsorption potential of −0.15 V. The peak potential is −0.9 V. Differential pulse modulation is used, with a pulse rate of 10 s −1 and a scan rate of 20 mV s −1. The limit of detection is 0.03 nM Ti when a 10-min adsorption time is used. The sensitivity is higher for fresh water than sea water owing to interfering Ti complexation by fluoride in sea water. Interference caused by surface-active organic compounds, which lower the sensitivity owing to adsorption on the mercury electrode, is removed by UV photolysis of the sample prior to the analysis. The method was tested by determining Ti in samples from the North Sea and the Indian Ocean.

Direct determination of fluoride in sea water with a fluoride selective ion electrode by a method of standard additions

Direct determination of fluoride in sea water with a fluoride selective ion electrode by a method of standard additions

Sodium fluoride ion-pairs in seawater

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.

Fluoride in sea water: intercalibration study based on electrometric and spectrophotometric methods

Fluoride was shown to be a conservative constituent of sea water. Samples from the eastern tropical North Pacific, the Caribbean, the Labrador Basin, and the Reykjanes Ridge region indicated that the most probable F Cl ratio in sea water is 6.71·10 -5 with an overall uncertainty of ±0.07·10 -5 based on 99% confidence limits of ±0.02·10 -5 and an allowance of ±0.05· 10 -5 for systematic error. Double-blind measurements from laboratories using electrometric and spectrophotometric methods agreed closely. Mean F Cl ratios found were all within 0.7% of the most probable value.

A standard addition titration method for the potentiometric determination of fluoride in sea water

A standard addition titration method for the potentiometric determination of the total fluoride concentration in sea water with a lanthanum fluoride electrode is described. The fluoride concentration of standard sea water was found to be 0.0734 ±0.0004 mmol kg −1. Equations for the computer treatment of the titration data are given. The calibration of the electrode for single point potentiometric measurements at different sea water salinities is discussed.

Spectrophotometric determination of fluoride in seawater

Spectrophotometric determination of fluoride in seawater

Determination of the stability constants of MgF + and CaF + using a fluoride ion selective electrode

The stability constants of MgF + and CaF + have been determined at 25°C in aqueous solutions of ionic strength 0·1, 0·4, 0·7 and 1·0M held constant by means of NaCl. No buffering agents were used but the pH was in the range 5·5–6·0. An Orion fluoride ion selective electrode, model 94-09, was used to measure the fluoride ion activity with a saturated calomel electrode as reference. The complexation of fluoride in sea water is calculated on the basis of the measurements.

Fluoride in Seawater: Measurement with Lanthanum Fluoride Electrode

Fluoride ion concentrations are quickly and easily determined in seawater by means of a fluoride-selective electrode. Samples require little pretreatment; a determination takes 15 minutes; error limits are 5 percent; equipment required is rugged, inexpensive, and motion-insensitive; and minimum demands are made on the operator.

The automatic determination of fluoride in sea water and other natural waters

A Technicon AutoAnalyzer has been used for the determination of 0–1.5 μg fluoride/ml in sea water and other natural waters. Photometric measurement is made on the blue complex formed by reaction with the chelate formed between lanthanum and alizarin fluorine blue, The method has a coefficient of variation of ca. 0.9% at a fluoride level of 1.5 μm/ml.

Fluoride inhibition of skeleton formation in the sea urchin

1. 1. The formation of the skeleton of plutei of the sea urchin Lytechinus variegatus and Arbacia punctulata was inhibited by a solution of from 1.5 × 10 −2 M to 2 × 10 −3 M sodium fluoride in sea water. 2. 2. This inhibition was reversed by the addition of fresh sea water but not by the addition of an isotonic NaCl solution or by the addition of magnesium, calcium or phosphate ions. 3. 3. The inhibition of calcification was pH sensitive, the range for inhibition being about pH 7.2 to 8.7. 4. 4. Chelating agents (alanine, glycine, acetylacetone, oxylate and citrate) with known affinities for calcium and magnesium, were tested with negative results, except for near toxic levels of citrate. 5. 5. Inhibitors of the glycolytic cycle (monoiodoacetate, dinitrocresol, chloropicrin and iodoacetamide) were tested and none inhibited skeleton formation. 6. 6. Beryllium was found to have no effect on this system, although it is known to inhibit calcification. 7. 7. A possible mode of action of fluoride is through the formation of a magnesium-fluorophosphate complex, cutting off the source of inorganic phosphate necessary for calcification.

The determination of microgram quantities of fluoride : The determination 0f fluoride in waters and sewage effluent using the aluminium-haematoxylin complex

The application of the aluminium-haematoxylin reagent to the determination of fluoride in sea-water, river and potable waters, and sewage effluent is described. It has been shown that down to a level of about 0.1 p.p.m. the fluoride can be determined directly (i.e. without prior separation by distillation); there is some loss in accuracy for amounts of the order of 0.2–0.5 p.p.m. With sea-water, calibra tions should be carried out in a blank of artificial sea-water. The fluoride contents of a number of British rivers etc., are given.