Comprehensive Notes on Indicators, Acids, and Bases

Indicators

Learning Objectives

  • Indicators are used to identify acids and bases in solution. Observations of indicator color classify solutions as acidic, basic, or neutral.
  • Strong acids/bases differ from weak acids/bases in the degree of ionization or dissociation.
  • Common strong acids: HCl, HNO3, and H2SO4.
  • Common weak acids: CH3COOH, H3PO4, and other organic acids.
  • Common strong bases: Group 1 and 2 oxides and hydroxides.
  • Common weak bases: NH3 and Na2CO3.
  • "Strong" and "concentrated" are different terms when referring to acids and bases.

What are Indicators?

  • Indicators change color in the presence of an acid or an alkali.
  • They are often weak acids or bases themselves.
  • Many different indicators exist.
  • Many indicators come from plants (fruits, flowers, and vegetables).
  • Universal indicator is a mixture of indicators.
    • It displays a range of colors from pH 1–14.
  • Geraniums:
    • Acidic soil: red flowers
    • Alkali soil: blue flowers

Universal Indicator

  • Universal indicator composition:
    • Phenolphthalein
    • Methyl red
    • Bromothymol blue
    • Thymol blue
    • Sodium hydroxide
    • Propan-1-ol
    • Water

Components of Universal Indicator

  • Methyl red:
    • Azo dye that gives a red color under acidic conditions.
  • Propan-1-ol:
    • A solvent.
  • Sodium hydroxide:
    • Adjusts the pH of the indicator solution.
    • Ensures green color at exactly pH 7.
  • Phenolphthalein:
    • Gives a deep violet color at very high pH.
    • Made from naphthalene, xylene, and phenol from crude oil/coal tar.
  • Bromothymol blue monosodium salt:
    • Gives a deep blue color under alkaline conditions.
  • Thymol blue monosodium salt:
    • Differentiates low and very low pH by becoming red at pH 2.
    • Yellow at pH 7, contributing to the green color at neutral pH.

Litmus

  • Litmus is a mixture of dyes from lichen.
  • Red in acidic substances, blue in basic substances.
  • Usually used as paper strips:
    • Red litmus: slightly acidic
    • Blue litmus: slightly basic
  • Use one of each strip to test solutions.
  • Wet litmus paper tests for acidic or basic gases.
  • 7-hydroxyphenoxazone is the active compound in litmus.

Litmus Details

  • Litmus dye is extracted from lichens.
  • Orcinol, from lichens, converts into orcein dyes with ammonia and oxygen (NH3, O2).
  • The same conversion with Ca(OH)2, K2CO3, and CaSO4 produces litmus dye.
  • Litmus dye is a complex mixture of polymeric compounds. The main constituent consists of several hydroxyorcein units linked together.
  • The chromophore of litmus dye is 7-hydroxyphenoxazone.
    • In acidic solutions, it gains a proton (H^+) and turns red (pH < 5).
    • In alkaline solutions, it loses a proton and turns blue (pH > 8).

Phenolphthalein

  • Phenolphthalein is a weak acid.
    • Can lose H^+ ions in solution.
    • The molecule is colorless.
    • The phenolphthalein ion is pink.
  • Adding a base shifts the equilibrium to the right.
    • More ionization as H^+ ions are removed.
    • Turns pink.

Methyl Orange

  • Methyl orange is an azo compound.
    • Characterized by the –N=N- group between two aromatic rings.
    • It is a weak base.
  • Nitrogen of the azo group accepts a proton in acidic solution.
    • Changes the color from yellow to red.

Indicators in Food

  • Many naturally occurring compounds act as indicators.
  • Fruits, vegetables, and flowers contain anthocyanins and other pigments that indicate a wide range of pH.

Red Cabbage Indicator

  • Compounds giving red cabbage its color can be extracted and used as a pH indicator.
  • Method:
    1. Chop the cabbage.
    2. Boil for a few minutes.
    3. Strain and let cool.
    4. Use as an indicator!
  • Colors:
    • Acidic (pH < 3): Red
    • Violet (pH 4-7)
    • Blue (pH 7-8)
    • Yellow-Green (pH > 8)
  • The red cabbage extract determines if substances are acidic or alkaline.
  • Anthocyanin pigments' structures change subtly at varying pH, leading to a range of colors.

Hydrangea Color Changes

  • Hydrangeas change color depending on soil acidity.
  • The main pigments are anthocyanins.
    • Delphinidin 3-glucoside produces a pink-red color.
  • Varying shades depend on aluminum ions in the soil.
    • Aluminum is usually in the form of silicates or oxides.
    • Lower soil pH releases aluminum ions.
  • Hydrangea sepals are sensitive to soil acidity.
    • Above pH 5.5: Pink
    • At or below pH 5.5: Blue
  • Aluminum ions form a complex with delphinidin 3-glucoside and phenolic copigments, making the sepals blue.
  • Without the complex, the anthocyanin colors the sepals pink.
  • A mix of complexed and unbound anthocyanins results in purple.
  • To make blue:
    • Reduce soil pH by adding sulfur or aluminum sulfate.
  • To make pink:
    • Increase soil pH by adding lime (calcium oxides and hydroxides).
  • Soil pH changes take weeks to alter hydrangeas' colors.