Selection of an Indicator for Titrations
Understanding Key Terms: Equivalence Point vs. End Point
Before selecting an indicator, it's essential to distinguish two key terms:
- Equivalence Point: The point in a titration where the acid and base have reacted in exact stoichiometric proportions.
- End Point: The moment when the indicator changes color, signaling the titration's completion. Ideally, the end point should closely align with the equivalence point.
$$
\text{HCl(aq) + NaOH(aq) → NaCl(aq) + H}_2\text{O(l)}
$$
The equivalence point occurs when 1 mole of HCl reacts with 1 mole of NaOH.
The Role of pH at the Equivalence Point
The pH at the equivalence point depends on the type of acid and base involved:
- Strong Acid + Strong Base: $pH = 7$ (neutral solution of salt and water).
- Weak Acid + Strong Base: $pH > 7$ (basic solution due to conjugate base hydrolysis).
- Strong Acid + Weak Base: $pH < 7$ (acidic solution due to conjugate acid hydrolysis).
- Weak Acid + Weak Base: pH depends on the relative strengths of the acid and base, often near neutral.
Determine the pH at the equivalence point to guide your choice of indicator.
How Indicators Work
Think of an indicator as a chemical "mood ring." It changes color depending on the pH of the solution.
- Indicators are weak acids or bases that exist in two forms: protonated and deprotonated.
- Each form has a distinct color. $$
\text{HInd(aq)} ⇌ \text{H}^+\text{(aq)} + \text{Ind}^-\text{(aq)}
$$ - The color change occurs over a pH range of approximately pKa ± 1.
Phenolphthalein is:
- Colorless in its acidic form (HInd).
- Pink in its basic form (Ind⁻).
- Suppose you're titrating acetic acid (CH₃COOH) with NaOH.
- The equivalence point is at pH ≈ 8.5.
- Phenolphthalein is a suitable indicator because its transition range (8.3–10.0) includes this pH.
Choosing the Right Indicator
Step 1: Identify the pH at the Equivalence Point
Determine the type of acid and base in the titration to estimate the equivalence point's pH.
Step 2: Match the Indicator’s Transition Range
Select an indicator whose transition range overlaps with the equivalence point's pH.
Choosing an indicator with a transition range far from the equivalence point can lead to inaccurate results.
Which indicator would you select for a titration between a strong acid and a weak base? Why?
Limitations of Indicators
Indicators are not always precise. For example:
- Weak Acid + Weak Base Titrations:
- The pH change near the equivalence point is too gradual for a clear color change.
- Use a pH meter instead.
- Highly Dilute Solutions: The color change may be faint and hard to observe.
