Fluoride colorimetric methods

Colorimetric Methods are used only for the estimation of very small percentages of vanadium, e.g. in vanadium steels and alloys. The most important depend on the intensity of the reddish-brown colour produced by the action of hydrogen peroxide on an acid vanadate solution.3 If chromium is present, an equal amount must be introduced into the standard vanadium solution under the same conditions of temperature, acid concentration, etc. Phosphoric acid is added to destroy any yellow colour due to ferric iron, and either hydrofluoric acid or ammonium fluoride to destroy any colour produced by titanium.4 A colorimetric method for the simultaneous estimation of small quantities of titanium and vanadium has also been worked out.5 Other colorimetric processes are based on (a) the formation of a yellow to black coloration, due to aniline black, in the presence of aniline hydrochloride and potassium chlorate or other oxidising agent,6 and (b) the orange coloration finally produced when an acid solution of a vanadate is brought into contact with strychnine sulphate.7... 

The amount of fluoride in the local water supply was determined by the four colorimetric methods in a comparative study A, B, C and D. Five replications were made for each test. To preclude bias from variations in the sample over the time required for the analysis, all samples were taken from a single 10-gal carboxy of water. The results in ppm are ... 

Fluoride ion electrode method, internal standard, dilutions at 0.1, 0.02, 0.01. Colorimetric method with internal standard, dilution at 0.004. 

This is a colorimetric method and colour development is virtually instantaneous and no waiting is required before measuring fluoride concentration. Colour determinations are made photometrically, using a spectrophotometer. A curve developed from standards can be used for determining the fluoride concentration of a sample or the concentration can be calculated on the basis of a pair of standards. The latter technique makes use of the fact that the relationship between fluoride concentration and absorbance (within the range of the method) is linear and thus that two points can define accurately the position of the line. 

The good agreement found between fluoride in rat bones using an electrode and diffusion—colorimetric methods established the prospective utility of the fluoride ion-selective electrode. After dissolution in hydrochloric acid and pH-ionic strength adjustment, the potentials of ashed bone samples are related to fluoride content from calibration standards [289]. 

Neutron activated gamma-ray spectrometric analysis (Na, Cl, Al, Mn, Ca, and P), atomic absorption spectrophotometry (K, Mg, Zn, Cu, and Fe), or a fluoride-specific electrode (F). a Atomic absorption spectrophotometry (Ca), and colorimetric method (P). 

Dedicated benchtop NMR analyzers for a variety of applications are available. These include an analyzer to determine fluoride in toothpaste quantitatively, such as the MQC from Oxford Instruments, a 23 MHz benchtop NMR, and another to determine water droplet size distribution in oil/water emulsions. Fluoride is often added to toothpaste as sodium fluoride or sodium mono-fluorophosphate to prevent tooth decay. The fluorine analyzer can determine fluorine at the level of a few hundred ppm. Toothpaste is squeezed into a glass sample tube and the quantitative determination of fluorine takes less than 1 min. The NMR method uses no solvents or reagents and is independent of the sample color and clarity, unlike the colorimetric methods and other instrumental methods such as ion chromatography (IC) that are used for this purpose. In the water droplet size distribution analyzer, droplets as small as 0.25 pm can be measured. The shelf life and palatability of products such as margarine, mayonnaise, salad dressings, and soft cheese depend on the size of... 

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. 

Several colorimetric procedures for fluoride are available, but it is usually desirable to distill the sample from concentrated sulfuric acid prior to analysis to eliminate interferences. One method is based upon bleaching a dye formed by the reaction of zirconium and sodium 2-(p-sulfophenylazo)-l,8-dihydroxy-3,6-naphthalenedisulfonate (SPADNS reagent) (28). 

The estimation of small quantities of tantalum in niobium compounds is more difficult, and cannot be carried out colorimetrically. The usual method is to convert the material into the potassium double fluoride, and then to take advantage of the fact that a white precipitate of potassium tantalum oxyfluoride, K4Ta405F14 (see p. 132), is thrown down when a solution of potassium tantalum fluoride, KaTaF7, is boiled.7 Powell and Schoeller 8 find this test imperfect, and have modified the procedure (based on the differential hydrolytic dissociation of oxalo-niobic acid and oxalo-tantalic acid in the presence of tannin in slightly add solution) for the detection and estimation of traces of tantalum in niobium compounds. 

Analysis of major elements (except Si) and total phosphorus on bomb-digested samples was accomplished by inductively coupled plasma emission spectrometry (ICP, ARL model 34,000). Silicon was analyzed colorimetrically (14). Phosphorus in total digests was also determined colorimetrically by the method of Murphy and Riley (15), as modified by Erickson (16). To avoid interference from fluoride ion used in the digestion technique, sample volumes were restricted to <1.5 mL in the standard P analytical protocol. 

Methods 1 and 2 are colorimetric techniques based on the reaction between fluoride and a dye. Methods 3 and 4 are discussed in Chapters 1.9 and 1.11, respectively. 

The accuracy of the method was indicated by the value of (F )g, which was 1.05 ppm, and which would correspond to a value of 1.3 ppm for the sample of salinity, S = 35%o. For comparison, the calculated value for salinity of 35%o would be 1.28, based upon the reported value for standard seawater (13). The precision was estimated for samples 6S and 6B for which the mean and standard deviations were 34.2 0.8 and 35.0 0.3, respectively the corresponding relative deviations were 2.3 and 0.9%. There was no significant variation in fluoride values during the 24 hr after being stored in a plastic container and refrigerated at < 4°C. In addition, fluoride was also determined for one unique sample colorimetrically, using an lanthanum-alizarin complexone reagent (14). Data are compared in Table I. 

This view is supported by another unique feature, the excessively high fluoride concentrations in the north discharge canal. It appears the concentration of fluoride was about 340 ppm. This is a value that was consistently obtained by two methods (specific ion electrode and colorimetric analysis), but a 100-fold dilution was required for the first method and a 250-fold one for the second. We recognize that the problems of errors are magnified by dilutions of this magnitude, but it is evident that the fluoride concentrations were unusually excessive. 

Once in solution, the preferred method for measurement of boron is inductively coupled plasma atomic emission spectroscopy (ICP-AES) or inductively coupled plasma mass spectrometry (ICP-MS). The most widely used nonspectrophotometric method for analysis of boron is probably ICP-MS because it uses a small volume of sample, is fast, and can detect boron concentrations down to 0.15 pgL . When expensive ICP equipment is not available, colorimetric or spectrophotometric methods can be used. However, these methods are often subject to interference (e.g., nitrate, chloride, fluoride), and thus must be used with caution. Azomethine-H has been used to determine boron in environmental samples (Lopez et al. 1993), especially water samples. Another simple, sensitive spectrophotometric method uses Alizarin Red S (Garcia-Campana et al. 1992). 

Application of the oxygen-flask combustion method to the determination of fluorine in organic combination has now been made by a number of workers. The fluoride in solution after combustion may be determined by thorium nitrate titration or colorimetrically as the alizarin complexan chelate or with a chloranilate (see Halogen Acids and Salts, p. 302). A method based upon the alizarin complexan chelate is given in Appendix IV. 

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