The best indicator for titrating HCl with NH₄OH is?

Q: The best indicator for titrating HCl with NH₄OH is?

Methyl orange - In acid–base titration, the choice of indicator depends on the strength of the acid and the strength of the base involved. Hydrochloric acid (HCl) is a strong acid, while ammonium hydroxide (NH₄OH) is a weak base. When these two are titrated, the equivalence point does not occur at pH 7. Instead, due to the weak base and strong acid combination, the equivalence point falls on the acidic side, generally around pH 4.5 to 6.0. An appropriate indicator must change color near the equivalence point to give a clear endpoint. Among the given options: Methyl orange has a pH transition range of 3.1 to 4.4, which fits very well with the acidic equivalence point of HCl vs NH₄OH titration. This makes methyl orange the most suitable indicator. Litmus changes color around pH 5 to 8, which is too broad and less precise for analytical titrations. Phenol red changes between pH 6.8 to 8.4, which is closer to neutral and basic ranges, not suitable for a strong acid–weak base titration. Phenolphthalein has a range of 8.3 to 10.0, which works best for strong acid–strong base titrations but is useless in this case, as the equivalence point lies in acidic pH. Therefore, methyl orange is the best choice of indicator when titrating HCl (strong acid) with NH₄OH (weak base). This ensures a sharp and accurate color change exactly at the equivalence point. Such careful selection of indicators is a fundamental principle in analytical chemistry because it ensures accuracy in volumetric analysis and chemical measurements.

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Accepted Answer

Methyl orange - In acid&ndash;base titration, the choice of indicator depends on the strength of the acid and the strength of the base involved. Hydrochloric acid (HCl) is a strong acid, while ammonium hydroxide (NH₄OH) is a weak base. When these two are titrated, the equivalence point does not occur at pH 7. Instead, due to the weak base and strong acid combination, the equivalence point falls on the acidic side, generally around pH 4.5 to 6.0.
An appropriate indicator must change color near the equivalence point to give a clear endpoint. Among the given options:


Methyl orange has a pH transition range of 3.1 to 4.4, which fits very well with the acidic equivalence point of HCl vs NH₄OH titration. This makes methyl orange the most suitable indicator.


Litmus changes color around pH 5 to 8, which is too broad and less precise for analytical titrations.


Phenol red changes between pH 6.8 to 8.4, which is closer to neutral and basic ranges, not suitable for a strong acid&ndash;weak base titration.


Phenolphthalein has a range of 8.3 to 10.0, which works best for strong acid&ndash;strong base titrations but is useless in this case, as the equivalence point lies in acidic pH.


Therefore, methyl orange is the best choice of indicator when titrating HCl (strong acid) with NH₄OH (weak base). This ensures a sharp and accurate color change exactly at the equivalence point.
Such careful selection of indicators is a fundamental principle in analytical chemistry because it ensures accuracy in volumetric analysis and chemical measurements.

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