Fluorides, colorimetric determination

Pretreatment of the collected particulate matter may be required for chemical analysis. Pretreatment generally involves extraction of the particulate matter into a liquid. The solution may be further treated to transform the material into a form suitable for analysis. Trace metals may be determined by atomic absorption spectroscopy (AA), emission spectroscopy, polarogra-phy, and anodic stripping voltammetry. Analysis of anions is possible by colorimetric techniques and ion chromatography. Sulfate (S04 ), sulfite (SO-, ), nitrate (NO3 ), chloride Cl ), and fluoride (F ) may be determined by ion chromatography (15). 

Chlorine and fluorine in beryllium metal are isolated by pyrohydrolysis or by distillation (21). Fluoride and chloride in the condensate are determined by ion-selective electrode or colorimetrically. 

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. 

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 ... 

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 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 products of hydrolysis of sulphonyl halides, i.e. sulphonic and hydrohalic acids, can be easily titrated with bases, which accelerate the hydrolysis as well. Thus Cundiff and Markunas titrated potentiometrically benzenesulphonyl chloride (and sulphuric acid) in pyridine with tetrabutylammonium hydroxide in benzene/methanol251. Jansseune and Janssen252 titrated sulphonyl fluorides in butylamine with potassium methoxide. Krivoruchko253 estimated 2-chloroethanesulphonyl chloride in air by hydrolysing it with alcoholic potassium hydroxide and determining the chloride ion colorimetrically or nephelometrically. Jansseune and Janssen252 also hydrolysed sulphonyl fluorides with alkali, acidified the mixture, distilled the HF and titrated it with alkali. 

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). 

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... 

Shinkai futher elaborated his ferrocenyl system and created a colorimetric system capable of visually detecting fluoride binding, thus devising an optical sensor for fluoride. Taking advantage of a redox reaction between the dye molecule methylene blue and ferrocenyl boronic acid, it was possible to visually determine fluoride concentrations. Decolorization of the dye occurs over a 4 x 10- -3 X 10 mM fluoride range (monitored by UV-vis spectroscopy as a decreasing absorbance at 665 nm). 

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. 

Fluoride may be determined colorimetrically by its bleaching effect on peroxo-titanic acid. 

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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