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Difficulty: Very hardAcid-Base Titrations, Indicators, and Volumetric Calculations

Complete the statement below regarding the volumetric analysis of a mixture containing sodium carbonate and sodium hydrogen carbonate using double indicators.

Answer:During the titration of a mixture of sodium carbonate and sodium hydrogen carbonate against standard hydrochloric acid, the first equivalence point marking the half-neutralization of sodium carbonate to sodium hydrogen carbonate is indicated by 【phenolphthalein】, which turns from pink to 【colorless】 at a pH range of approximately 8.3 to 10.0.

Answer

The first equivalence point is detected using phenolphthalein indicator, which transitions from pink to colorless (or colourless).
Phenolphthalein is the correct indicator for the first equivalence point because its transition interval (pH 8.3–10.0) matches the pH of the sodium hydrogen carbonate solution formed when all carbonate ions are neutralized. Its color changes from pink in alkaline medium to colorless at the endpoint.

Step-by-Step Solution

1
Identify the chemical reaction occurring at the first equivalence point of the carbonate-bicarbonate titration.
At the first endpoint, all CO32−\text{CO}_3^{2-} ions are protonated to form HCO3−\text{HCO}_3^- according to CO32−+H+→HCO3−\text{CO}_3^{2-} + \text{H}^+ \rightarrow \text{HCO}_3^-. The pH at this stage is weakly alkaline (around pH 8.3).
Sodium hydrogen carbonate is an amphiprotic salt that forms a solution with a pH near 8.3, requiring an indicator with a pH transition range in the alkaline region.
2
Determine the appropriate indicator and its observed color change at this pH transition.
Phenolphthalein has a pH transition range of 8.3–10.0. In basic solution containing carbonate ions, it displays a pink color, and turns colorless once all carbonate ions are converted to hydrogen carbonate ions.
Methyl orange changes color at pH 3.1–4.4, which corresponds to the second equivalence point where hydrogen carbonate is completely converted to carbon dioxide and water.

Key Concept

Double Indicator Titration of Carbonate and Hydrogen Carbonate Mixtures
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Topics

Acid-Base Titrations, Indicators, and Volumetric Calculations

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