Which indicator would be most suitable for a strong acid-strong base titration: Methyl orange (pH transition range: 3.1 - 4.4) or Cresol red (pH transition range: 7.2 - 8.8)?

Question

BlackTom AI · Accepted Answer

Equivalence point for a strong acid + strong base titration lies at pH ≈ 7, and the pH changes very steeply through the equivalence region. This steep jump means many indicators with transition ranges that lie anywhere across the steep portion will show a sudden color change at the endpoint.

1) Relevant knowledge points
- For strong acid vs. strong base (equal concentrations), the equivalence pH = 7.  
- The titration curve near the equivalence is very steep: pH can change several units over a very small added volume.  
- An indicator is suitable if its pH transition range falls within (or overlaps) the steep rise of the titration curve.

2) Step-by-step reasoning
- At equivalence pH ≈ 7.  
- Typical strong acid/strong base titration curves (at typical concentrations like 0.01–0.1 M) show a fast pH rise that spans roughly from low acidic values up through neutral and into basic values in a narrow volume window.  
- Methyl orange transitions at pH 3.1–4.4: this is on the acidic side, but because the pH jump includes that region for many practical concentrations, the color change can coincide with the endpoint for some sets of concentrations.  
- Cresol red transitions at pH 7.2–8.8: this straddles neutral-to-basic and includes pH near 7, so it also commonly indicates the endpoint.

3) Calculations / intermediate steps
- No specific numeric equilibrium calculation is needed here because the key is the shape of the titration curve (large, rapid pH change around 7). The practical consequence: both indicators’ transition ranges can be crossed during that rapid change.

4) Why the correct answer is correct
- Because the steep equivalence jump typically covers both the methyl orange range (acidic end of the jump) and the cresol red range (near-neutral to slightly basic end), either indicator can produce a clear endpoint color change in a strong acid–strong base titration.

5) Why other choices are incorrect
- Picking only methyl orange (a) is too restrictive: while it can work, cresol red can also work.  
- Picking only cresol red (b) is too restrictive for the same reason.  
- Saying neither (d) is incorrect because at least one indicator (in fact both here) will change color at the equivalence.

Conclusion: Either indicator can be suitable for a strong acid–strong base titration; this matches the provided answer “Either.”

Which indicator would be most suitable for a strong acid-strong base titration: Methyl orange (pH transition range: 3.1 - 4.4) or Cresol red (pH transition range: 7.2 - 8.8)?单项选择题

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