Fluoride potentiometric method

The fluorine titration of chromium oxide was carried out at the Elf-Atochem Research Center, Pieire-Benite.The catalyst mineralization was carried out in a Parr bomb by reaction with sodium peroxyde. Fluorine ions were then titrated by a potentiometric method with a specific fluoride electrode. 

The potentiometric method also surpasses the others for speed, simplicity, precision and accuracy as indicated in Table 12.1. Furthermore, it is particularly suited to the continuous monitoring of fluoride levels in drinking water. The spectrophotometric methods are lengthy because of the time required to develop a stable colour (up to 1 hour), the alizarin red-S complex being especially poor in this respect. It was noted, however, that for the three bleaching methods (1-3) the rate of change of absorbance by the blank closely followed that of solutions containing fluoride, i.e. the difference between the blank and a sample absorbance is nearly constant. 

Formation constants for silver(I)-olefin complexes have been obtained in aqueous solution by potentiometric methods by Hartley and Venanzi 45>. The ligands allylammonium perchlorate, but-2-enyl ammonium perchlorate, allyl alcohol and but-2-en-l-ol, were shown to have affinities for silver(I) comparable with acetate and fluoride ions the complexes are much weaker than the corresponding platinum(II) complexes, possibly due to the non-directional characteristic of the silver component of the a-bond (5s-orbital). The small difference between the formation constants for the allylammonium and but-2-enylammonium complexes and the larger difference between the alcohol complexes was assumed to be an enthalpy effect, arising from an electrostatic repulsion in the case of the unsaturated ammonium cations. 

Using the proper choice of separation column, ion chromatography appears to be applicable for the sequential multicomponent analysis of other anions such as sulfate, chloride, fluoride and nitrate. The detection limits will be substantially lower than the classical gravimetric and potentiometric methods currently used. Ion selective electrodes are available for chloride and fluoride. 

The formation of fluoride complexes in aqueous NaCl solutions at various ionic strengths has been studied by potentiometric methods (Bilal et al., 1979 Bilal and Becker, 1979). Monofluoro and difluoro complexes are observed, but evidence is not found for higher fluoride complexes. The stability constants, 1, for the monofluoro complexes of La and Nd are 4.8 0.4 x 10 M, while those for Tb, Er and Lu are in the range 1.2-1.8 x 10 M . The /3i values vary with ionic strength. Stability constants, 2, for the difluoro complexes all increase monotonically across the rare earth series from 1.3 x 10 to 3.lxl0 M . The... 

NiO solubility in molten KF-LiF at 550 C determined by potentiometric method is 1.3x10 This value considerably exceeds values obtained for the above chloride melts. The increase of NiO solubility occurred due to formation of fluoride complexes which are more stable than chloride ones. 

In potentiometric methods, the potential between a reference and an indicator electrode is measured, which corresponds to the analyte activity. Because of their usefulness in food analysis, ion-selective electrodes (ISEs) that measure anions like bromide, chloride, and fluoride or cations like potassium, sodium, and calcium stand out among indicator electrodes. The characteristics and advantages of ISE include the ability to measure different anions and cations directly, the fact that they do not consume the analyte, the fact that analyses are independent of sample volume when taking direct measurements, and that moreover turbidity, color, and viscosity do not affect the measurement. Potentiometric methods are also... 

The accepted by EPA potentiometric methods involve selective electrodes for fluorides, cyanides, nitrates, ammonia, and sulfides detection [20]. The potentiometric characteristics of the anion-selective sensors are strongly dependent on the anion receptor design and properties [25]. At this time, molecular recognition of anions by synthetic receptors is an expanding research area. 

Direct potentiometric method The sample and standard solutions are introduced into the potentiometric measuring cell mixed with background electrolyte. The fluoride ion-selective electrode and appropriate reference electrode (saturated calomel electrode or silver/silver chloride electrode with double jimction is recommended) are dipped into the solution and the electromotive force is measured. An EMF versus log (fluoride ion concentration) calibration curve is used for evaluation. 

Sarma and Rao [63] recently studied the potentiometric fluoride measuring method by analyzing well waters with a commercial analyzer. In their experiments the detection limit of fluoride ions was 1 pM in case of the samples studied. 

The limited number of well functioning, classical or spectrophotometric methods is available for measuring fluoride ion concentration in different samples. Therefore, after the invention of lanthanum fluoride crystal-based ISE [15], its use as a detector in standardized methods becomes almost general. For example, the Environmental Protection Agency (ERA) METHOD 9214 [44] is for measuring the concentration of fluoride ions in water samples as well as in soil extracts. It is a direct potentiometric method using the ion-selective fluoride electrode and the conventional or double junction reference electrode. The lower limit of detection is 0.025 mg dm. Fluoride concentrations from 0.025 to 500 mg dm can be measured. 

In some cases, tea plants accumulate fluoride ions from the soil. To investigate the risk of drinking too much tea, the Slovenian research team [45] measured the fluoride ion concentration of teas cooked from different tea blends using the simple potentiometric method. 

Potentiometric titration is actually a form of the multiple known subtraction method. The main advantage of titration procedures, similar to multiple addition techniques in general, is the improved precision, especially at high determinand concentrations. ISEs are suitable for end-point indication in all combination titrations (acid-base, precipitation, complexometric), provided that either the titrand or the titrant is sensed by an ISE. If both the titrant and the titrand are electro-inactive, an electrometric indicator must be added (for example Fe ion can be titrated with EDTA using the fluoride ISE when a small amount of fluoride is added to the sample solution [126]). 

Ion solvation has been studied extensively by potentiometry [28, 31]. Among the potentiometric indicator electrodes used as sensors for ion solvation are metal and metal amalgam electrodes for the relevant metal ions, pH glass electrodes and pH-ISFETs for H+ (see Fig. 6.8), univalent cation-sensitive glass electrodes for alkali metal ions, a CuS solid-membrane electrode for Cu2+, an LaF3-based fluoride electrode for l , and some other ISEs. So far, method (2) has been employed most often. The advantage of potentiometry is that the number and the variety of target ions increase by the use of ISEs. 

The fluoride ion selective electrode is the most popular means of fluoride ion determination after sample destruction by any method but it does have limitations. It can be used either directly to measure the fluoride potential6 or as an end-point detector in a potentiometric titration with a lanthanum(l II) reagent as titrant.4,7 Problems can be experienced with potential drift in direct potentiometry, especially at low fluoride ion concentrations. Titration methods often yield sluggish end points unless water miscible solvents are used to decrease solubilities and increase potentia 1 breaks and sulfate and phosphate can interfere. End-point determination can be facilitated by using a computerized Gran plotting procedure.4... 

Galvis-Sanchez, A.C., Santos, J.R., Rangel, A.O.S.S. Standard addition flow method for potentiometric measurements at low concentration levels application to the determination of fluoride in food samples. Talanta 133, 1-6 (2015)... 

When electrolyte concentrations are not too great, it is often useful to swamp both samples and standards with a measured excess of an inert electrolyte. The added effect of the electrolyte from the sample matrix becomes negligible under these circumstances, and the empirical calibration curve yields results in terms of concentration. This approach has been used, for example, in the potentiometric determination of fluoride ion in drinking water. Both samples and standards are diluted with a solution that contains sodium chloride, an acetate buffer, and a citrate buffer the diluent is sufficiently concentrated so that the samples and standaids have essentially identical ionic strengths. This method provides a rapid means of measuring fluoride concentrations in the part-per-million range with an accuracy of about 5% relative. 

The solubilities of nickel and thorium(IV) oxides in molten KF-LiF mixture at 550 °C were investigated by the potentiometric titration method using a membrane oxygen electrode [379, 380]. The solubility product of NiO in the fluoride melt was equal to 1.3 X 10-6, which considerably exceeded the corresponding values in the chloride melts discussed above. The solubility of NiO seemingly increased owing to the fixation of Ni2+ cations in strong fluoride complexes. 

The potentiometric experiments used the method of competing reactions with fluoride and changes in activity coefficients between the media used being determined using Th which does not hydrolyse in the proton concentration range examined. The stabil-... 

Almost all flowing-stream methods reported to date for free and total fluoride determination and applied to quality control and environmental monitoring are related to potentiometric detection using the fluoride ISE owing to its high selectivity. Spec-trophotometric alternatives based either on the Alizarin complexone methodology or on the inhibitory effect of fluoride on the formation of different metal chelates have also accomplished a great approval. 

A method, free from Fe or Cr interference, (MacDonald and Savage 1979) involves a prior oxidation of plutonium to Pu(VI) with an excess of Ce(IV). Sulfamic acid is added to avoid side reactions with nitrites. Fluoride residues from the dissolution of solid samples are complexed hy additon of Al(in). The excess Ce(IV) is then reduced by a slight excess of arsenite in the presence of Os(Vni) as catalyst. Excess As(III) is oxidized by permanganate, also in a very quantity. Finally, oxalic acid is added to reduce the excess Mn(VII). Pu(VI) is then reduced quantitatively to Pu(IV) with Fe(II) and the excess Fe(II) backtitrated with Cr(VI). All steps are followed potentiometrically and carefully timed. The Pu mass fraction, Cpu, in g/g, in the sample aliquant submitted to titration is calculated according to the following equation ... 

While all analysts have the ability to learn and certify in all of the methods, generally, the analysts specialize in one or more of the following areas 1) analysis of nitrates and nitrites (7) 2) trace metal analysis by ICP-MS (8) 3) anions analysis by Ion chromatography (P) 3) Total Suspended Solids (TSS) 10) and Total Dissolved Solids (TDS) (ii) 4) Biological Oxygen Demand (BOD) 12) and, 5) determination of mineral cations by flame atomic absorption) 12). Other tests such as potentiometric measurement of fluoride 14) and ammonia 15), alkalinity 16) and cyanide 17) are non-systematically assigned to analysts. 

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