Synthetic indicators are artificially prepared chemical substances that change colour at specific pH values to indicate whether a solution is acidic or basic. The two most important synthetic indicators are phenolphthalein and methyl orange. Unlike natural indicators (litmus, turmeric), synthetic indicators are prepared in laboratories and give sharper, more precise colour changes.
Two synthetic indicators: phenolphthalein (colourless in acid, pink in base) and methyl orange (red in acid, yellow in base).
Phenolphthalein pH range: 8.2–10.0.
Methyl orange pH range: 3.1–4.4.
Phenolphthalein is used in weak acid–strong base titrations.
Methyl orange is used in strong acid–weak base titrations.
Synthetic indicators are prepared artificially — unlike natural indicators from plants.
They give sharp, precise colour changes at defined pH values.
Other synthetic indicators: methyl red, bromothymol blue, litmus (also available synthetically).
Phenolphthalein is used in titrations where the endpoint is a base (alkaline solution). It is prepared by reacting phenol with phthalic anhydride in the presence of H₂SO₄.
Methyl orange is used in titrations involving strong acids and weak bases. It gives a clear red → orange → yellow colour transition.
Phenolphthalein: • Colour in acid: Colourless • Colour in base: Pink/magenta • pH range: 8.2 – 10.0 • Best used for: Strong acid–strong base and weak acid–strong base titrations
Methyl Orange: • Colour in acid: Red • Colour in base: Yellow • pH range: 3.1 – 4.4 • Best used for: Strong acid–weak base titrations
Methyl Red: • Colour in acid: Red • Colour in base: Yellow • pH range: 4.4 – 6.2
Bromothymol Blue: • Colour in acid: Yellow • Colour in base: Blue • pH range: 6.0 – 7.6 (useful near neutral pH)
Natural indicators: • Obtained from plants (litmus from lichens, turmeric from turmeric plant) • Less precise colour changes • Limited pH range • Examples: litmus, turmeric, red cabbage juice, China rose
Synthetic indicators: • Prepared artificially in laboratories • Sharp, precise colour changes at defined pH values • Wide variety for different pH ranges • Examples: phenolphthalein, methyl orange, methyl red, bromothymol blue
In laboratory titrations, synthetic indicators (especially phenolphthalein and methyl orange) are preferred because they give a sharp, unambiguous endpoint.
Two synthetic indicators are: (1) Phenolphthalein — colourless in acid, pink/magenta in base (pH range 8.2–10). (2) Methyl orange — red in acid, yellow in base (pH range 3.1–4.4).
Phenolphthalein is colourless in acidic solutions (and neutral solutions) and turns pink/magenta in basic (alkaline) solutions. It is used as an endpoint indicator in titrations.
Methyl orange turns red in acidic solutions and yellow in basic solutions. It shows an orange colour in the transition zone around pH 3.1–4.4.
A synthetic indicator is an artificially prepared chemical substance that changes colour at specific pH values to show whether a solution is acidic or basic. Examples: phenolphthalein and methyl orange.
Methyl orange is used in strong acid–weak base titrations because the equivalence point in such titrations is on the acidic side (pH < 7), which falls within the methyl orange transition range (3.1–4.4).
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