Apparent colourfulness

Figure 3.2 Effect of dyebath pH on apparent colour yield in a five-dip indigo dyeing process [29]...

Petrick and Wilson [137] recently developed an HPLC method using both UV-Vis and ESI-MS detection able to separate and detect 15 basic dyes and 13 disperse dyes. Separation was carried out in RP-mode using an acidified (formic acid) water-acetonitrile mobile phase in gradient mode. The method enabled the discrimination of fibers with the same apparent colour based on their different chromatographic and mass spectrometric profile. 

Sometimes the absorbance is also assessed at 580 nm and a ratio calculated for the values at 520 and 580 nm, which is called the blue index. With the red/black juices, where the product may have to be diluted, the intensity of the colour is assessed by dilution in an acidic buffer to ensure that the true colour intensity is recorded. If this is not done the colour can be reduced by the shift in pH affecting the apparent colour of the product. The analysis of the colour in juices is detailed in IFU Handbook method No. 78. 

Thompson and Gratton discuss various technical aspects of the phase shift fluorometric method for determining lifetimes. Rayleigh scattering shows apparent colour effects due to the wavelength dependence of electron transit times in the PM tube. 

The fibrillation on the fabric surface changes the light reflectance behaviour. This gives rise to the bloom or frosted look on a peach-touch lyocell fabric. The degree of fibrihahon will influence the apparent colour. 

Dezincification is readily apparent, since the yellow colour of the brass is replaced by the characteristic red of copper, which may take the form of small plugs or of layers that in some cases can extend over the whole of the surface (Fig. 1.60). In plug-type dezincification a mechanically weak, porous residue of copper is produced, which may remain in situ or become removed by the pressure of water, leading to a perforation. In the layer type the transformation of the alloy into a mechanically weak layer of copper results in loss of strength, and failure may occur by splitting when the metal is subjected to water pressure or to external stress. 

In the pH range 7-11, in which the dye itself exhibits a blue colour, many metal ions form red complexes these colours are extremely sensitive, as is shown, for example, by the fact that 10 6 — 10 7 molar solutions of magnesium ion give a distinct red colour with the indicator. From the practical viewpoint, it is more convenient to define the apparent indicator constant K ln, which varies with pH, as ... 

The reaction was followed by observing the appearance of the yellow colour of Ce(IV) at 400 m, with Pb(IV) present in excess concentration. Pb(IV) was varied in the region 8.6 x 10 M to 4.4 x 10 M while Ce(III) was kept at 4 x 10 M. A practical difficulty encountered was the photochemical instability of Ce(IV) acetate. Under the above conditions and in the temperature range 30-47 °C, the reaction is strictly first-order in each reactant. The observed rate coefficient at 30.0 °C is 1.48x10" l.mole sec and the apparent activation energy and... 

McMillan has reviewed the chemistry of Ag(II) and Ag(IlI). Paramagnetism and electron spin resonance studies confirm the presence of Ag(ll) (as opposed to equimolar Ag(I)+Ag(III)). The colours of Ag(II) solutions in various mineral acids indicate the existence of complexes, the oxidising power of which is apparent from their decomposition even at 0 °C, although high acidity promotes stability. Rapid isotope exchange between Ag(I) and Ag(n) is considered to result from the equilibrium... 

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