indicators, acid-base, organic compounds that, in aqueous solution, exhibit color changes indicative of the acidity or basicity of the solution. Common indicators include p -nitrophenol, which is colorless from p H 1 to 5 and yellow from p H 5 to 9; methyl orange, yellow in basic and neutral solutions and reddish below p H 3.7; phenolphthalein, colorless in acid and neutral solutions, pink at about p H 8.5, and purplish at p H 10; and litmus. Most indicators are also used in large amounts for dyeing; small quantities are nonetheless invaluable for use as indicators in chemical laboratories.
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