Colour transition

If a given indicator is half dissociated, the value of H0 is equal to the decadic logarithm of its acidity constant times —1. For practical measurements an indicator may be used only in the range of its colour transition, i.e. at most in the range of the ratio cB/cBH+ from 10 to 10 1. For a larger interval of H0 values several indicators have to be used. 

When the Zincon ion-pair is exposed to an aqueous sample containing the analyte, the latter diffuses into the sensor membrane to react with the indicator, and gives a colour transition from pink to blue at near neutral pH. The pKa value of Zincon for the color transition from pink to blue is above 13, therefore, the sensor membrane is virtually insensitive to pH changes. However, due to the high complexation constant of Zincon for copper and zinc, the response of sensor membrane is irreversible and must be evaluated kinetically12. 

Indicator Oxidized colour Reduced colour Transition potential (volts) Solution conditions... 

Since the band structure which develops upon doping induces changes not only in the conductivity but also in the optical absorption (see Fig. 9.8), conducting polymers may be exploited for electrochromic displays, which are optical devices with marked colour transitions. An example is illustrated diagramatically in Fig. 9.18. 

There is the issue of using an unfamiliar, highly coloured transition metal complex, which can be toxic, and needs to be removed. Interesting attempts have been made to address this issue via polymer supported catalysts. This removes the colour issue substantially (not completely), but polymerisation... 

Colour transitions of indicators Acid-alkali and iodometric/iodimetric titration Pipetting technique... 

The last system presented will be the polyalkylthio-phene, for which a comparative study between the doped and undoped state is available. To end this section, the question of the colour transition in solution will be specifically discussed in 1.3.4. 

The colour transition was primarily explained with different single chain processes. An isomerisation process was first considered and the colour transition was supposed to be accompanied by a cis-trans isomerisation. The coil conformation being rich in cis structure while the red or blue solution was made of macromolecules of rich trans structure. If this first... 

Figure 13 Colour changes in the titration of with EDTA using murexide as indicator. The effect of pH on pCa at the colour transition point...

Al oxides, more strongly coloured Transition to C, more like B than C... 

There is some discussion in the previous chapter as to whether the carbonation front is truly as well defined as the indicator shows it to be (see Figure 3.1(b)) but for most practical considerations it is a reliable technique. Some aggregates can cause problems usually making the colour transition difficult to see. Also very poorly consolidated concrete and concrete underground exposed to dissolved carbonates in the water may not show clearly defined carbonation fronts due to the non-uniform progress of the carbonation front. 

A comparison of fingerprints illustrates the colour change with an increase of absorbance in correspondence of the red channel and a decrease of absorbance in green and blue channels. This behaviour reveals the colour transition of biladiene due to the change of protonation state as previously discussed. 

Table 5 Colour transition of L (Darkness to Lightness), a (greenness to redness) and b (blueness to yellowness) of wrapped tomato at room condition...

From the calculation, h° = 1. 19 is located in the red region but lies more toward +b axis. In theory, if the b value increases, the degree of h also increased. The colour region on a hue colour wheel is actually lying more toward +fc axis where the red colour is less intense. Hence, it could be affirmed that the higher /> value denotes lower red colour intensity. As stated in Table 5, the colour drastically changed in tomato that was wrapped with control starch film. The colour transition of tomato from light red (yellowish red) to dark red (blue red) was a little bit severe... 

Transitions between iigand and metai orbitals in transition metai com-piexes. These are called metal to ligand, and ligand to metal charge transfer (MLCT and LMCT) transitions, respectively. These can be fully-allowed transitions and are usually the source of colour in intensely coloured transition metal complexes, e.g. the LMCT transitions in chromate, Cr04 , and permanganate, MnOJ ions, and the intense absorptions in ferroin (tris(phenan-throline) iron(II)) and tris(2,2 -bipyridyl)mthenium(II) (see Chap. 4). 

So far no generally accepted conclusion has been disclosed to explain the relation between the exact structure of the polymer and the colour transition. We considered to choose a suitable model compound to examine the optical and thermal properties of the polymer. E-l,4-diphenylbutenyne (1) seems to be appropiate for this purpose. It has similar structure as the repetetive unit of the polyacetylehe backbone, 2,... 

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