This range is termed the color change interval. The substance identifiers displayed in the InfoCard are the best available substance name, EC number, CAS number and/or the … A substance that changes color of the solution in response to a chemical change. Phenolphthalein, (C20H14O4), an organic compound of the phthalein family that is widely employed as an acid-base indicator. Phenolphthalein indicator used in acid-base titration. The phenolphthalein indicator allows chemists to visually identify whether a substance is an acid or a base. However, under alkaline conditions, the equilibrium is to the right, and the concentration of the anion becomes sufficient for the pink colour to be observed. As an indicator of a solution’s pH, phenolphthalein is colourless below pH 8.5 and attains a pink to deep red hue above pH 9.0. This color change is not at a fixed pH, but rather, it occurs gradually over a range of pH values. Since, it doesn't react with acid, it is colourless in acid. Phenolphthalein is an weak acid which reacts only with strong bases and changes colour. Phenolphthalein is a potent laxative, which Substance identity Substance identity. In an acid-base titration neutralization occurs at pH 7. Phenolphthalein is a colourless, weak acid which dissociates in water forming pink anions. Since the pH versus concentration curve is so steep around the equivalence point any indicator that changes color in this general region can be used as an acid-base indicator. Under acidic conditions, the equilibrium is to the left,and the concentration of the anions is too low for the pink colour to be observed. The color change in phenolphthalein is a result of ionization, and this alters the shape of the phenolphthalein molecules. Each pH indicator is defined by a useful pH range. And Bromthymol Blue has a useful range from 6.0 (yellow) to 7.6 (blue). For example Phenolphthalein changes from colorless at 8.2 to pink at 10. 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