Spectro-photometric color

Aminothiazole present in urine or blood plasma forms a colored Schiff base when 5-nitrofurfural is added the colorimetric analysis of the Schiff base allows the quantitative determination of this thiazole (1571). The Schiff base may also be dosed by polarographic of spectro-photometric methods (1572). 

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 indirect methods discussed thus far have dealt with measurement of color only as it can be correlated with physical characteristics of materials and the effect of these materials on radiant energy. As has been pointed out, the reflectance spectro-photometric curve describes a property of the material. A change in the reflectance spectrophotometric properties may not always result in a change in visual color. The reason is that color of the object is not an unchangeable characteristic of the object itself, dependent only upon these reflectance properties, but is also dependent upon the quality of the illuminating light and the sensitivity of the observer s eye. Thus the measurement and description of visual color are psychophysical problems... 

Objective Evaluation of Color. In recent years a method has been devised and internationally adopted (International Commission on Illumination, I.C.I.) that makes possible objective specification of color in terms of equivalent stimuli. It provides a common language for description of the color of an object illuminated by a standard illuminant and viewed by a standard observer (H). Reflectance spectro-photometric curves, such as those described above, provide the necessary data. The results are expressed in one of two systems the tristimulus system in which the equivalent stimulus is a mixture of three standard primaries, or the heterogeneous-homogeneous system in which the equivalent stimulus is a mixture of light from a standard heterogeneous illuminant and a pure spectrum color (dominant wave-length-purity system). These systems provide a means of expressing the objective time-constant spectrophotometric results in numerical form, more suitable for tabulation and correlation studies. In the application to food work, the necessary experimental data have been obtained with spectrophotometers or certain photoelectric colorimeters. 

Fe(phenanthroline)3 standards for spectro-photometric analysis. Volumetric flasks contain Fe(phenanthroline)f+ with Fe concentrations ranging from 1 mg/L (left) to 10 rng/L (right). The absorbance, as evidenced by the intensity of the color, is proportional to the iron concentration. 

Color Characteristics of the Reaction Mixtures. Initial experiments in air involving the reactions of syringyl alcohol, vanillyl alcohol, and a-methylvanillyl alcohol with alkali showed both visually and spectro-photometrically that the reaction mixture of syringyl alcohol, the hardwood lignin model, and alkali was more intensely colored than either of the reaction mixtures of the guaiacyl compounds and alkali. 

Color. Det color in accordance with ASTM D1209. The sample is compared spectro-photometrically against platinum-cobalt standard solns... 

For purification, the crystalline hexaatomic sulfur is dissolved in benzene (approx. 100 mg./l.) toluene is not suitable (see Properties). The benzene solution is extracted with a series of reagents, three or four extractions being made with each reagent except where otherwise indicated. (For 200 ml. of benzene solution, 50-ml. portions of reagent are used). The reagents are employed in the following order (1) water, (2) 10% aqueous potassium triiodide, until the benzene phase remains colored from iodine, (3) 10% aqueous potassium iodide, (4) water, (5) 5% aqueous lead acetate, twice, (6) water, (7) 5 % aqueous potassium hydroxide, once, (8) water. The purified solution is dried over magnesium sulfate and should be used immediately. The approximate concentration of Sg can be determined spectro-photometrically at 300 mju (e = 181 l./g.-atom) after suit-... 

Even when none of the reactants or products of the enzyme reaction of interest lends itself to an appropriate absorption change, it is often possible to determine one of these components by adding another enzyme or color reagent that acts on the product of the reaction in such a way as to bring about the change one of the products of the enzyme reaction is made a substrate or reactant of a second reaction (indicator reaction) that can be followed spectro-photometrically. 

The colorimetric methods recommended for the determination of total fusel oil contents in beverages are based on a color reaction. In this reaction, 2-methyl-l-propanol, 2-methyl-l-butanol, and 3-methyl-l-butanol lose water during heating in a strongly acidic solution and the unsaturated hydrocarbons formed yield colored complexes with vanillin, salicylaldehyde, and 4-dimethyl-amino-benzaldehyde. The absorbance is measured spectro-photometrically at 445 and 560 nm. 

Nitrate and nitrite The nitrate and nitrite content of cheeses that have been treated with these salts can be determined by reduction with cadmium followed by reaction with sulfonamide and N-l,nap-hthylethylenediamine hydrochloride. This produces a colored compound that can be quantified spectro-photometrically. 

A different approach was taken by Ciba-Geigy (Hermann 1996). Left-over printing pastes were collected and combined, and their color coordinates spectro-photometrically measured. Computer software was developed that defines a specifically segmented color space in which all color can be defined and desired shades matched from one, two or three dyes in the fresh and recycled paste. The reclaimed paste could be incorporated without problems. 

The elution order of metal cations should follow the order of decreasing com-plexation. This is indeed the case for the separation in Figure 7.17. Here, spectro-photometric detection at 530 nm was used after post-column addition of pyridyl-azoresorcinol (PAR) as a complexing reagent. At highly alkaline pH values the eluted metal ions form colored complexes with PAR that are more stable than the complexes with the mobile phase. However, the use of post-column detection requires somewhat more complicated equipment. 

Four different methods are applicable to an investigation of the kinetics of reaction (21). One approach is to determine the amount of chloride ion in solution as a function of time by any of the various analytical methods for chloride ion determination. Another is to follow the change in color of the reaction mixture over a period of time. In practice this is done spectro-photometrically at one or more wavelengths known to give the maximum difference in optical density between reactant and product. It is also apparent from Eq. (21) that the reaction results in an increase in the number of ions in solution. The rate of reaction can therefore be followed by intermittent conductivity measurements. The fourth method that can be applied is one taking advantage of the acidity of [Co(NH3)6H20] +(pKa 5.7). The amount of this acid product in solution can be determined at various... 

Photometric methods are widely employed in the determination of microamounts of beryllium. These methods are used primarily when simplicity and speed are preferred. Many colored reagents are employed, as well as reagents such as acetylacetone, salicyclic and sulfosalicyclic acids, whose compounds with beryllium display an intense absorption in the UV range. Colorimetric determination of beryllium in coal has been described by Abernethy and Hattman [1]. This spectro-photometric method is sufficiently sensitive for the determination of microgram quantities and may be desirable for small laboratories without access to spectro-graphic equipment. 

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