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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