Definition

1. Noun:
   • A chemical compound used as an indicator dye: Resorcinolphthalein is a synthetic organic compound that appears as a yellow dye. It is notable for its use as an absorption indicator in analytical chemistry, specifically in precipitation titrations involving silver ions and chloride ions. Its key property is a visible color change (to pink) that signals the endpoint of the titration when chloride ions are depleted.

Examples of Usage

• Noun:
  • The chemist used resorcinolphthalein as an absorption indicator for the argentometric titration.
  • A dilute solution of resorcinolphthalein appears yellow but turns pink at the titration endpoint.

Advanced Usage

• As an absorption indicator: This term specifies its primary function. An absorption indicator changes color when it is adsorbed onto the surface of a newly formed precipitate.
  • In the Volhard method, resorcinolphthalein can serve as the absorption indicator.

Variants and Related Words

• Fluorescein: This is the more common name for resorcinolphthalein. Fluorescein is widely used not only in chemistry as an indicator but also in medicine (e.g., in eye exams to detect corneal injuries) and in tracing water flow due to its intense fluorescence.
• D&C Yellow No. 7: A designated name for fluorescein when used as a colorant in drugs and cosmetics.

Synonyms

• Fluorescein
• Acid Yellow 73 (a common name in dye chemistry)

Noun

1. a yellow dye that is visible even when highly diluted; used as an absorption indicator when silver nitrate solution is added to sodium chloride in order to precipitate silver chloride (turns pink when no chloride ions are left in solution and negative fluorescein ions are then absorbed)