Acid-Base Titration Notes

Acid-Base Titration

• Neutralization reaction: reaction when an acid and base react.
• Indicator: Indicates when the acid and alkali are mixed in the right proportions to neutralize each other.
• Endpoint: When the indicator changes color.
• Equivalence point: Point at which chemically equal amounts of acid and base have reacted; the endpoint occurs just after the equivalence point.

Acid-Base Titration Curve

• Titration curve: plot of pH of the titre vs. volume of the titrant.
• Shows how the pH of a solution changes as another solution is added.
• Plot depends on the nature of the acid and base.

Strong Acid-Strong Base

• Equivalence point at pH 7. Example: NaOH and HCI.

Strong Acid-Weak Base

• Rapid fall of pH to start. Example: HCI and NH3.

Weak Acid-Strong Base

• Equivalence point above pH 7. Example: CH_3COOH and NaOH.

Weak Acid-Weak Base

• The change will never be sharp enough

Indicator

• Equilibrium reaction: HIn \leftrightarrow H^+ + In^-
  • Colour 1 observed in acidic media.
  • Colour 2 observed in basic media.
• Indicator: weak organic acid used to indicate the equivalence point.

Selecting an Indicator

• Color changes over a range of 1.5 to 2 pH units; want a small range for accuracy.
• Select one whose pH range of color change includes the equivalence point.

Strong acid vs strong base

• Either indicator can detect the endpoint

Strong acid vs weak base

• Methyl orange will change from yellow to orange near the equivalence point
• Choose an indicator which changes colour on the steep part of the curve

Weak acid vs strong base

• Phenolphthalein will go from pink to colourless at the equivalence point

Weak acid vs weak base

• Indicators are not used due to the gradual color change.