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
pH 1 to 5 and yellow from
pH 5 to 9; methyl orange, yellow in basic and neutral solutions and reddish below
pH 3.7; phenolphthalein, colorless in acid and neutral solutions, pink at about
pH 8.5, and purplish at
pH 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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