Indicators in Chemistry

Indicators in Chemistry

Introduction

  • Overview of using indicators to test substances in our homes.
  • Four indicators will be used: Phenolphthalein, universal indicator, hydrian strip (hydrian paper), and red cabbage indicator.

Phenolphthalein

  • Colorless in an acid.
  • Turns hot pink in a base.
  • Color change occurs between pH 8 and 9.
    • Hint of pink appears at pH 8.
    • Fully fuchsia at pH 9.

Universal Indicator

  • Changes color depending on the "power of the hydrogen" (pH).
  • Example setup:
    • HCl (strongest acid)
    • Vinegar (weak acid)
    • Distilled water (neutral)
    • Ammonia
    • NaOH

Hydrian Strip (Hydrian Paper)

  • Universal indicator on paper.
  • Dip the paper into the substance to test the pH.
  • Compare the color of the strip to the scale on the side to determine pH.
  • Most effective and precise measurement method.

Red Cabbage Indicator

  • Made from red cabbage, which contains anthocyanins.
  • Color scheme differs from universal indicator.
    • Strong acid: Red
    • Acid: Pink
    • Neutral: Purple (original color of red cabbage)
    • Ammonia: Green
    • Strong base: Yellow
  • Different color scheme due to different compounds.

Testing with Indicators

Carbonated Water

  • Universal indicator: Orange (indicates acidity).
  • Phenolphthalein: Turns white (indicates acidity).
  • Cabbage juice: Purple (fairly neutral to slightly acidic).
  • Hydrian paper: pH of about 6.

Kitchen Cleaner

  • Universal indicator: Teal/dark green.
  • Phenolphthalein: Pink (indicates base).
  • Cabbage juice: Teal color.
  • Hydrian paper: pH between 9 and 11 (base).

Orange Juice

  • Universal indicator: Indicates acidity.
  • Phenolphthalein: No significant change (indicates acidity).
  • Cabbage juice: Turns pink (reacts with the acid in the orange juice).
  • Hydrian paper: pH in the 3 to 4 range (weak acid).

Drain Cleaner

  • Observation: Phenolphthalein already turns pink.
  • Universal indicator: Dark blue/purplish color.
  • Phenolphthalein: Strong pink color (strong base).
  • Cabbage juice: Yellow (strong base).
  • Hydrian strip: pH between 12 and 13, possibly even 14 (strong base).

Coffee

  • Universal indicator: Orange color.
  • Phenolphthalein: No change.
  • Cabbage juice: Almost brown; Secondary reaction may be occurring.
  • Hydrian strip: pH of about 5 (very weak acid).

Key Takeaways

  • Different indicators have different behaviors.
  • Phenolphthalein: Clear in acid, hot pink in base.
  • Universal indicator: Red in acidic solution, purplish-blue in basic solution.
  • The further from center of the pH scale, the stronger the acid or base.