Spectrophotometry color observation

Methods are described for determining the extent to which original natural color is preserved in processing and subsequent storage of foods. Color differences may be evaluated indirectly in terms of some physical characteristic of the sample or extracted fraction thereof that is largely responsible for the color characteristics. For evaluation more directly in terms of what the observer actually sees, color differences are measured by reflectance spectrophotometry and photoelectric colorimetry and expressed as differences in psychophysical indexes such as luminous reflectance and chromaticity. The reflectance spectro-photometric method provides time-constant records in research investigation on foods, while photoelectric colorimeters and reflectometers may prove useful in industrial color applications. Psychophysical notation may be converted by standard methods to the colorimetrically more descriptive terms of Munsell hue, value, and chroma. Here color charts are useful for a direct evaluation of results. 

The color of the quinonoid compounds that may be obtained by disproportionation can be sufficiently like that of the radicals to cause confusion if visual observation or broad-band spectrophotometry is used.11 For example, Preckel and Selwood, using paramagnetism as a measure of the amount of radical, reported that solutions of triphenyl-methyl derivatives more or less rapidly lost their paramagnetism. The decomposed solutions were still highly colored, but the color was no longer dependent on the temperature as it is in the case of a radical-dimer equilibrium mixture. What is more striking, and an even more subtle and dirtier trick on the part of nature, is the fact that Preckel and Selwood s non-paramagnetic solutions were still rapidly bleached by exposure to the air. It is clear that radical-like reactivity is not a safe criterion for the presence of radicals. It is also clear that the ebullioscopic method is particularly unsatisfactory in view of the excellent chance for decomposition. 

Compared with spectrophotometry, the NMR method has a number of advantages (i) The procedure is very rapid, and it can be used by observing the variation of chemical shifts of diverse nuclei such as 3H, 13C, 19F, and nO. (ii) It is insensitive to colored impurities and slight decomposition of the indicator, (iii) In principle, it can be used over the whole range of known acidity. The medium effect, which may be important in 1H NMR, becomes negligible in the case of 13C NMR spectroscopy. The method can be used with a wide variety of weak bases having a lone-pair containing heteroatoms as well as simple aromatic hydrocarbons. 

The selectivity of the trap towards hydroxyl radicals was demonstrated by several control experiments using different radicals, showing that the formation of the respective hydroxylation product, 5-hydroxy-6-0-zso-propyl-y-tocopherol (57), was caused exclusively by hydroxyl radicals, but not by hydroperoxyl, alkylperoxyl, alkoxyl, nitroxyl, or superoxide anion radicals. These radicals caused the formation of spin adducts from standard nitrone-and pyrroline-based spin traps, whereas a chemical change of spin trap 56 was only observed in the case of hydroxyl radicals. This result was independent of the use of monophasic, biphasic, or micellar reaction systems in all OH radical generating test systems, the trapping product 57 was found. For quantitation, compound 57 was extracted with petrol ether, separated by adsorption onto basic alumina and subsequently oxidized in a quantitative reaction to a-tocored, the deeply red-colored 5,6-tocopheryldione, which was subsequently determined by UV spectrophotometry (Scheme 23). 

After LLE into ethanolic KOH, the antioxidant BHT (32a) used in aircraft fuel was determined in the presence of Cu(n) ions, by UVV spectrophotometry at 368 nm. Linearity was observed in the 0 to 30 ppm range, RSD <2% . A UVV spectrophotometric method for determination of -cyclodextrin (126) is based on the formation of a complex with phenolphthalein (19, Section Vin.A.3). Both the intensity and hnear range are affected by the pH and the concentration of 19 in solution. The method was considered to be inadequate for precise determinations of 126 of purity higher than 98% see also application of a-cyclodextrin (85) for analysis of phenolics in Section IV.B.4 . Phenol in the presence of sodium nitroprusside, Na2[Fe(CN)5NO], and hydroxylamine, at pH 10.26-11.46, developed a blue coloration that could be applied for quantitative analysis (kmax 700 nm, e 1.68 x 10" LmoU cm , Sandell sensitivity 0.0052 p,g phenol cm ). Beer s law was found to be valid from 0.1 to 6.5 ppm". ... 

The results of catalytic epoxidation of various olefins, using f-BuOOH as the terminal oxidant and Mn(Me2EBC)Cl2 as the catalyst, are summarized in Table 3.4. The color of the reaction mixture turns to purple upon addition of f-BuOOH and the ultraviolet-visible spectrophotometry shows that the manganese is present predominantly in the tetravalent state, and no dioxygen evolution was observed. 

As a chromophore (from the Greek chromo = color, phero = to carry along, to bear), porphyrin (Figure 13.2) can be described by its characteristic signature observed by UV-visible spectrophotometry. The intense color of porphyrin derivatives arises from a highly delocalized n system involving 18 electrons (bold lines) which, because of its subsequent aromatic character, partially explains the survival of this structure through evolution. 

Rapid scan spectrophotometry has been used to study the base hydrolysis of ajS5-(salicylato)(tetraethylenepentamine) cobalt(III) (17). The instantaneous color change observed on addition of base to the complex has been attributed to formation of the phenoxide species and this point has been confirmed. Subsequent aquation and base hydrolysis of the phenoxide species then occurs with = 0.116 s and /cqh = 3.32 M s at 25"C. 

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