Color indicator bicolor indicators

These relations show that the acid and base concentrations ratio depends on the pH through the indicator pKa value. The solution color depends on the pH value. Some indicators exhibit only one colored form they are called unicolor indicators. For example, this is the case of phenolphthalein. The theory concerning them is the same as that concerning bicolor indicators except for the influence of their concentration on the color-change interval (see Sect. 8.4). 

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... 

Figure 4.16 Hebeloma danicum (arrows) and Laccaria bicolor (arrowheads) fruiting beside remains of paper, which indicates not only antecedent human excreta but also a squatting pose (see text). Photograph was taken in a Pinus forest, Shiga, Japan, on October 20, 1981. (See color insert following p. 178.)...

The color-change interval of a bicolor pH indicator is independent of its total concentration. The assertion is correct only when the Henderson relation is legitimate. 

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