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Difficulty: MediumAcid-Base Titrations, Indicators, and Volumetric Calculations

Complete the statement below regarding indicator selection and color transition during an acid-base titration.

Answer:In the volumetric titration of a strong acid against a weak base, 【methyl orange】 is the most suitable indicator, and it changes color from 【yellow】 to red at the acidic equivalence point when acid is added to the base.

Answer

In the volumetric titration of a strong acid against a weak base, methyl orange is the most suitable indicator, and it changes color from yellow to red at the acidic equivalence point when acid is added to the base.
Titrations involving a strong acid and a weak base yield an acidic equivalence point (pH 3−5\text{pH } 3 - 5). Methyl orange is ideal because its transition range is pH 3.1−4.4\text{pH } 3.1 - 4.4. In the initial basic environment of the conical flask, methyl orange is yellow, and it shifts to red/pink as the acid endpoint is reached.

Step-by-Step Solution

1
Determine the pH of the equivalence point for the titration system.
Titrating a strong acid against a weak base produces a salt that undergoes hydrolysis, yielding an acidic equivalence point with pH<7\text{pH} < 7 (typically between pH 3\text{pH } 3 and 55).
The conjugate acid of the weak base hydrolyzes in water to release hydrogen ions H+\text{H}^+.
2
Match the equivalence point pH range with the appropriate indicator.
Methyl orange has an effective indicator range of pH 3.1 to 4.4\text{pH } 3.1\text{ to } 4.4, which overlaps with the sharp pH drop of this titration curve.
An indicator is suitable only if its working range coincides with the steep vertical region of the titration curve.
3
Identify the initial and endpoint colors of methyl orange.
Before titration begins (in basic solution), methyl orange is yellow. At the endpoint after excess acid is introduced, it turns red.
Methyl orange is yellow in alkaline media (pH>4.4\text{pH} > 4.4) and turns pink/red in acidic media (pH<3.1\text{pH} < 3.1).

Key Concept

Indicator Selection and Endpoint Color Changes in Volumetric Analysis
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Topics

Acid-Base Titrations, Indicators, and Volumetric Calculations

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