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4. Acid-Base Reactions

Equivalence Point Indicators

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Acid-base titration setup: The pink color is caused by the phenolphthalein indicator.

Before you begin the titration, you must choose a suitable pH indicator, preferably one that will experience a color change (known as the “end point”) close to the reaction’s equivalence point; this is the point at which equivalent amounts of the reactants and products have reacted. Below are some common equivalence point indicators:

  • strong acid – strong base titration: phenolphthalein indicator
  • weak acid – weak base titration: bromthymol blue indicator
  • strong acid-weak base titration: methyl orange indicator the base is off the scale (e.g., pH > 13.5) and the acid has pH > 5.5: alizarine yellow indicator
  • the base is off the scale (e.g., pH > 13.5) and the acid has pH > 5.5: alizarine yellow indicator
  • the base is off the scale (e.g., pH > 13.5) and the acid has pH > 5.5: alizarine yellow indicator
  • the acid is off the scale (e.g., pH < 0.5) and the base has pH < 8.5: thymol blue indicator

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