The indicator's pKa range should lie within the steep section of the titration curve.
Practise Acids, Bases & Electrochemistry All Acids, Bases & Electrochemistry topics
A suitable indicator must have a transition range that lies within the vertical portion of the titration curve, so that the color change occurs very close to the equivalence point. For a weak acid titrated with a strong base, phenolphthalein is usually chosen because its range lies in the basic region near the equivalence pH. For a strong acid titrated with a weak base, the equivalence point is acidic, so methyl orange or methyl red is more appropriate. Choosing the wrong indicator gives an end point far from the equivalence point and a significant error. This decision is an essential skill in analytical chemistry.
Key ideas: Pick an indicator whose transition range lies within the steep jump of the titration curve, so pKa(In) is close to the equivalence pH. Strong-strong: almost any. Weak acid with strong base: phenolphthalein. Weak base with strong acid: methyl red or methyl orange. Weak with weak: no sharp jump, so no visual indicator works well.
Common mistakes and connections: Mistake: choosing an indicator because it changes near pH 7 regardless of the system. Also, an indicator that changes before the steep region leads to underestimation. Check the pH at 0.1 mL before and after equivalence to verify the range falls inside. Ties together curve shape and the hydrolysis pH at equivalence.
In a dilute weak acid titration the steep section is short and sits at high pH, so phenolphthalein suits it but methyl orange, changing near pH 4, does not. The interactive lesson shows this as a live animation or chart.
Methyl orange would be a poor choice for titrating acetic acid, because its color change occurs far below the basic equivalence point.
The indicator's pKa range should lie within the steep section of the titration curve.
Choosing Indicators is a advanced topic in Acids, Bases & Electrochemistry. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in Acids, Bases & Electrochemistry first: Weak Acid Strong Base Titration, Acid-Base Indicators, Universal Indicator.
The next topics in Acids, Bases & Electrochemistry are Buffer Solutions and Henderson-Hasselbalch Equation.
Yes. The interactive lesson includes a self-check question on Choosing Indicators, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Choosing Indicators is topic 30 of 105 in Acids, Bases & Electrochemistry.