Question

For the titration between oxalic acid and sodium hydroxide, the indicator used is
A ) Potassium permanganate
B ) Phenolphthalein
C ) Litmus
D ) Methyl orange

Answer 1

Hint: Indicators are substances having different colours in different media (acidic and basic media). Thus, indicators can tell if the solution is acidic or basic. Indicators can be natural indicators such as turmeric or beetroot. Indicators can also be synthetic indicators such as phenolphthalein indicator and methyl orange.

Complete step by step answer:
Compare the pH range of the indicators with the pH range for the equivalence point of the titration between oxalic acid and sodium hydroxide.
Oxalic acid is a weak organic acid Sodium hydroxide is a strong base. The titration of oxalic acid and sodium hydroxide is the titration of a weak acid with strong base.

\[{\rm{HOOC}} - {\rm{COOH + 2 NaOH }} \to {\rm{ NaOOC}} - {\rm{COONa + 2}}{{\rm{H}}_2}{\rm{O }}\]

Phenolphthalein indicator is used in the pH range \[8.3 - 10\]. The equivalence point for the titration between oxalic acid and phenolphthalein is in the same pH range.
The pH range for methyl orange is \[{\rm{3}}.{\rm{2}} - {\rm{4}}.{\rm{4}}\] . For litmus, the pH range is \[{\rm{4}}.{\rm{5}} - {\rm{8}}.{\rm{3}}\] . Potassium permanganate indicator is not used for acid base titrations. Potassium permanganate indicator is used for redox titrations.
Hence, the indicator used for the titration of oxalic acid with sodium hydroxide is phenolphthalein.
During the titration, oxalic acid is taken in the burette. Aqueous solution of sodium hydroxide is placed in the conical flask and one or two drops of phenolphthalein indicator are added. Phenolphthalein indicator is pink in basic solutions and becomes colorless in acid solution. The end point is obtained when colour changes from dark pink to nearly colourless solution.
Methyl orange indicator is used in the pH range \[3.2 - 4.4\]. The equivalence point for the titration between oxalic acid and Methyl orange is in the pH range \[8.3 - 10.0\]. Hence, methyl orange is not a suitable indicator for this titration.

So, the correct answer is (B).

Note: Acid base indicators are also known as pH indicators. These indicators change colour with pH. Acid base indicators are usually weak acids or weak bases, which, upon dissolution in water, dissociate slightly to form ions. In the tirations, indicators are used to signal the end point in the titration, at which complete neutralization of acid with base takes place.