Origin of the Color Change

The appearance of the color or its change is due to structural changes of the indicator after the capture or the loss of a proton. The new structure is endowed with an increase or a decrease in its resonance possibilities with respect to the initial one. This is the origin of the color change. 

From the simple qualitative point of view, we have known for more than a century that colored molecules bring characteristic atom groups called chromophores (part responsible for bringing the color). They are unsaturated groups, such as 

Sometimes the sole presence of one of these groups is not sufficient to confer a color to the molecule that brings it. In order to develop a color, it must possess a supplementary group called an auxochrome (increasing the color). In most cases, but not necessarily, the coloration change due to the medium s acidity is related to the appearance of a p-quinonic structure a or, more rarely, to that of an o-quinonic structure b  

A first example is that of methyl orange (helianthin), whose bicolor indicator is the sodium salt of the 4-dimethylaminoazobenzen-4 -sulfonic acid whose color-change interval is 4.4 pH 6.2. The structures that participate in the equilibrium 

The acidic form (red) exhibits two principal limit forms, one with an azo rest and the other one with a p-quinonediimine rest. The basic form (yellow) presents a principal limit form with an azo structure. Another example is provided by phe-nolphthalein, which is a unicolor indicator. It is a derivative of triphenylmethane. Its acidic form, symbolized by H2ln, 

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The systematic investigation of natural colors led to the discovery of several dyes and dye classes which are able to change their color. Thus, it was found that the origin of the color of hydrangea blossoms is an anthocyan dye. 

Procedure If the sample concentration is higher than 2 mg/dm , then 1-2 drops of phenolphthalein indicator is added to 100 cm sample solution, and the pH of the solution is adjusted to the range of the color-change of this indicator with 0.05 M sulfuric acid or 0.1 M sodium hydroxide. Then 1 cm potassium chromate (10%) is added and the sample titrated with silver nitrate solution. At the endpoint, the original lemon yellow color changes to reddish brown. In order to get a blank value, 100 cm distilled water is titrated in the same way. 

Explain the origin of the rule of thumb that indicator color changes occur at pACHln 1. 

No attempt was made to apply the Arrhenius equation to the color change data because of the relatively large ( 1 AE unit) variation of the color uniformity ("evenness") of the original uncoated silk fabric. At the lower temperatures, 70°C and 80°C, the measured color changes were close to the uncertainty in the color of the unexposed silk fabric itself and therefore too few temperature data points were available for meaningful calculations. 

Figure 19.18 Thennosensitive hybrid microgel containing gold nanoparticles. The color changes originate from nanoparticle aggregation induced by the swelling/deswelling of the thermosensitive microgel.

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