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What it is. This phenolphthalein indicator is a 1% solution in isopropyl alcohol (IPA), available in 1 pint and 20 L sizes. It is the classic colorless-to-pink indicator for titrating acids with strong bases.
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How the color change works. Phenolphthalein is colorless below about pH 8.2 and turns pink to fuchsia above about pH 10. In a titration of acid with sodium or potassium hydroxide, the first persistent faint pink marks the endpoint.
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Chemical identity. Phenolphthalein (CAS 77-09-8, C20H14O4, 318.32 g/mol) is a triphenylmethane dye. In base, it loses protons and opens its lactone ring to form the colored form; in strongly basic solution, it slowly fades back to colorless.
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Why it is dissolved in alcohol. Phenolphthalein is only slightly soluble in water but dissolves readily in alcohols. A 1% solution in isopropyl alcohol stays clear and stable and disperses quickly into the titration sample.
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Where its endpoint is useful. Because it changes color near pH 8.2 to 10, phenolphthalein suits titrations of strong and weak acids with strong bases, such as acetic acid in vinegar, acid value in oils, and standardizing sodium hydroxide with potassium hydrogen phthalate.
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Standardizing sodium hydroxide. Phenolphthalein is the standard indicator for standardizing sodium or potassium hydroxide against potassium hydrogen phthalate, a stable primary standard acid. Lab Pro's Potassium Hydroxide 0.1 N Solution, 1 Gallon is one base commonly checked this way.
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Phenolphthalein alkalinity. In water analysis, titrating with acid to the phenolphthalein endpoint near pH 8.3 measures phenolphthalein alkalinity, mainly hydroxide plus half the carbonate. Continuing to the methyl orange endpoint gives total alkalinity; see Lab Pro's Methyl Orange Indicator, 0.1% (w/v) Aqueous.
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Two sizes. The 1 pint size (ZP0523-16) suits teaching labs and benchtop titration, while the 20 L size (ZP0523-640) supports QC labs and production testing with high titration volumes.
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Flammability and handling. Because the solvent is isopropyl alcohol, the solution is flammable. Keep it away from heat and flames. Phenolphthalein is treated as a suspected carcinogen, so avoid ingestion and skin contact; wear gloves.
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Where it is used. Phenolphthalein is used in acid-base titrations in teaching, QC, and food labs; for acid value and free fatty acid testing of oils; for alkalinity in water; for concrete carbonation testing; and in demonstrations of pH indicators.
Technical Specifications
| Chemical Name |
Phenolphthalein, 1% solution in isopropyl alcohol |
| CAS Numbers |
77-09-8 (phenolphthalein); 67-63-0 (isopropyl alcohol) |
| Synonyms (dye) |
3,3-Bis(4-hydroxyphenyl)-1(3H)-isobenzofuranone |
| Formula (dye) |
C20H14O4 |
| Molecular Weight (dye) |
318.32 g/mol |
| Concentration |
1% phenolphthalein |
| Solvent |
Isopropyl alcohol |
| Transition Range |
pH 8.2 (colorless) to about 10 (pink to fuchsia) |
| Typical Endpoint |
First persistent faint pink |
| Appearance |
Clear, colorless liquid |
| Flash Point (IPA solvent) |
About 12 °C |
| Typical Dose |
2 to 3 drops per titration |
| Hazard Profile |
Flammable liquid; phenolphthalein is a suspected carcinogen (confirm on SDS) |
| DOT |
Confirm on SDS (flammable IPA-based solution) |
| Package Sizes |
1 pint; 20 L |
Available Variants
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ZP0523-16: 1 pint for teaching labs and benchtop titration.
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ZP0523-640: 20 L for QC labs and high-volume testing.
Applications / Common Uses
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Acid-base titration: Indicates strong-base titration endpoints.
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NaOH standardization: Signals the KHP standardization endpoint.
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Acid value of oils: Indicates free fatty acid titrations.
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Water alkalinity: Marks the phenolphthalein alkalinity endpoint.
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Concrete carbonation: Shows carbonated (colorless) versus alkaline (pink) concrete.
Storage, Handling & Shipping
Caution: This solution is flammable because it contains isopropyl alcohol. Phenolphthalein is a suspected carcinogen. Keep away from heat and flames; avoid ingestion and skin contact; wear safety glasses and gloves.
Store tightly closed in a cool, well-ventilated flammables area away from heat, sparks, open flames, and oxidizers. Dispense small amounts into a dropper vessel for use, and never return used indicator to stock.
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Incompatible materials: Strong oxidizers and strong bases.
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Personal protection: Safety glasses and gloves.
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Spill response: Remove ignition sources and absorb with non-combustible material.
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First aid: Eyes: rinse for several minutes. Skin: wash with soap and water. Ingestion: seek medical advice.
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