Colour formation

The colour sequence already described, for the reduction of van-adium(V) to vanadium(II) by zinc and acid, gives a very characteristic test for vanadium. Addition of a few drops of hydrogen peroxide to a vanadate V) gives a red colour (formation of a peroxo-complex) (cf. titanium, which gives an orange-yellow colour). 

Development of chromophojic groups leading to colour formation. 

Ultraviolet absorbers are a form of light screen which absorb primarily in the ultraviolet range. It is a requirement for most ultraviolet absorbers that absorption in the visible range should be negligible if this were not so the resultant instant colour formation could be worse than that experienced after prolonged exposure of the polymer. 

Both novolaks and resols are prepared in similar equipment, shown dia-grammatically in Figure 23.16. The resin kettle may be constructed from copper, nickel or stainless steel where novolaks are being manufactured. Stainless steel may also be used for resols but where colour formation is unimportant the cheaper mild steel may be used. 

Fast sulphon black F ( C.I.26990). This dyestuff is the sodium salt of 1-hydroxy-8-( 2-hydroxynaphthylazo) -2- (sulphonaphthylazo) -3,6-disulph onic acid. The colour reaction seems virtually specific for copper ions. In ammoniacal solution it forms complexes with only copper and nickel the presence of ammonia or pyridine is required for colour formation. In the direct titration of copper in ammoniacal solution the colour change at the end point is from magenta or [depending upon the concentration of copper(II) ions] pale blue to bright green. The indicator action with nickel is poor. Metal ions, such as those of Cd, Pb, Ni, Zn, Ca, and Ba, may be titrated using this indicator by the prior addition of a reasonable excess of standard copper(II) solution. 

We studied the polyamidation of nylon 4,6, and varied the reaction time, reaction temperature, partical size, starting molecular weight, and type of reactor gas. At the same time we looked at the molecular weight broadening and the degradation with colour formation. In order to have good heat and mass transfer the reactions were mainly conducted on fine powder in a fluidized bed reactor and with dry nitrogen as carrier gas. 

Use is made of colour changes resulting from reaction of pollutant and chemical reagents colour intensity indicates concentration of pollutant in the sample. Reaction can take place in solution or on solid supports in tubes or on paper strips, e.g. litmus or indicator paper. Quantitative assessment of colour formation can also be determined using visible spectroscopy. Instruments are calibrated... 

Scheme 10.2 Colour formation in the ring-opening of crystal violet lactone and related compounds...

Colour formation reactions of this type are utilised in carbonless copy paper, which is based on the principle of colour formation on the copy as a result of pressure of writing or typing in the master sheet. In such systems, the underside of the master sheet contains the colour former, for example compound 243, encased in microcapsules, which are tiny spheres with a hard polymer outer shell. Pressure on the master sheet breaks the microcapsules and allows the colour former to come into contact with an acidic reagent coated on the copy sheet, thus causing an irreversible colour formation reaction. 

TGA, iodometric, mid-IR, luminescence (fluorescence and phosphorescence) and colour formation (yellowness index according to standard method ASTM 1925) were all employed in a study of aspects of the thermal degradation of EVA copolymers [67], Figure 23 compares a set of spectra from the luminescence analysis reported in this work. In the initial spectra (Figure 23(a)) of the EVA copolymer, two excitation maxima at 237 and 283 nm are observed, which both give rise to one emission spectrum with a maximum at 366 nm weak shoulders... 

During virtually the whole of the polymerisation the solutions remained colourless. A yellow colour always developed towards the end of the polymerisation its origin and phenomenology will be discussed in the next paper of this series. No colour formation was observed if water was present at a concentration higher than that of the perchloric acid. 

Since we could not prepare a stable solution of the ester, we attempted its preparation in the styrene solution to be polymerised. Silver perchlorate was dissolved in this and the reaction was started by the crushing of a phial containing 1-phenylethyl bromide (under our conditions styrene was not polymerised by the silver perchlorate alone). The solutions became cloudy because of the formation of colloidal silver bromide, but no colour formation could be observed until the end of the polymerisation then the solutions became yellow, very like the reaction mixtures in which perchloric acid had been used as catalyst. The ester was found to be as effective a catalyst as anhydrous perchloric acid. Equal concentrations of the ester and the acid produced very similar polymerisations as shown in the Figure. The accelerating parts of the curves obtained with the ester as catalyst are readily explained by the fact that the reaction between silver perchlorate and 1-phenylethyl bromide is not instantaneous and therefore a steady increase in catalyst concentration characterises the first part of the polymerisation. 

Although AS and AES can be detected at a low UV wavelength, sensitivity is lacking and a more suitable detection was achieved using indirect photometric detection, post-column colour formation reactions, or a pre-column derivatisation, suppressed conductivity detection and refractive index detection [1,42,43]. A comparison of detection limits for the determination of these anionic surfactants shows that photometric and conductivity detectors are better (picomole or nanogram range) than refractive index or fluorometry detectors by about a factor of 1000 [40],... 

The various chemical processes influencing the colour formation in tea have been vigorously investigated. The self-association of black tea polyphenol theaflavin and its com-plexation with caffeine [175], and the role of epicatechin quinone in the synthesis and degradation of theaflavin [176] have been studied in detail. 

The spectra were recorded in the positive-ion mode in the range of m/z 120-1 500. Some chromatograms illustrating the effect of aldehydes on the interaction of mv3gl and B2-3 -gallate are shown in Fig. 2.120. The chromatograms demonstrate that different aldehydes influence differently the formation of anthocyanin-flavanol pigments. The results of HPLC-MS measurements are compiled in Table 2.94. Because free aldehydes display an unpleasant aroma in Port wine these reactions may improve the quality of wines and contribute to the colour formation [266],... 

Fig. 3.5 Schematic representation of mechanism for colour formation in Konig s reaction...

Figure 1.22 Colour formation in Crystal Violet Lactone/developer system.

Thus, much research regarding environmental stress had been done with red grapes, limiting the impact of stress to changes in colour formation, which is easily measured in grapes and wines by straightforward photometric analysis. 

A variety of processes exist in which photographic images are produced as a result of a photoinitiated chain reaction. Such reactions, which are characterized by three fundamental steps (initiation, propagation and termination), provide the basis for certain processes in which photopolymerization, photocrosslinking or colour formation or destruction represents the main image forming stage. 

Tannins Peroxidase, laccase Teas, wines Astringents, bitter taste, colour formation... 

The brown stain on western hemlock sapwood is not controlled by sodium azide (18). A number of chemicals (reducing agents and acids) prevent the colour formation in laboratory experiments but... 

The authors found a 12-fold enhancement of the brown colour (420 nm) of the reaction medium. The reactions, however, were not performed at the same pH. Therefore the quantification by the authors is not very useful, because the formation of brown colour is known to be dependent on the pH the higher the pH, the higher the rate of the colour formation. The same authors reported the use of sugar phosphates, in which the phosphate is covalently bound to the sugar moiety, also leading to a more rapid browning. 

In an autoanalyzer the solutions were mixed with the TNBS-solution (0.3% w/w) and a phosphate "reaction" buffer (0.1M pH—8). The colour of the yellow compound formed was measured using a spectrophotometer at 427 nm. To enhance the colour formation and to stabilize the bubbles, which were formed in the apparatus to separate the samples, sodium sulphite and Brij Wetting Agent (ex. Merck) were added to the phosphate "reaction" buffer. 

Colour formation, which lies well beyond the early-stage Maillard reaction, has been described by Wedzicha and Leong118 by the following scheme (DH = deoxy-hexosone) ... 

The derived rate constants were applied to the glucose-glycine system, where a good fit was obtained. In accord with the above scheme, ARP and 3-DG were formed simultaneously, while there was a lag phase for colour (melanoidins, M) and 1-DG. 3-DG proved to be the main pathway for colour formation. 

Colour formation is the primary characteristic of the Maillard reaction, yet even now our knowledge of the coloured moieties responsible for the coloration is only rudimentary. 

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