Question

The indicator which is used to find the strength of caustic soda solution with the help of oxalic acid is:
(A)- Methyl orange
(B)- Phenolphthalein
(C)- Potassium permanganate
(D)- None of the above

Answer 1

Hint: Caustic soda is alkaline in nature. Its main uses are in the manufacture of pulp and paper, soap and detergents, petroleum products and chemical production. Other applications include water treatment, food, textiles, metal processing, glass making and others.

Complete step by step solution:
Our question is about the use of indicators, so first, we have to find out what are indicators.
Indicators are substances whose solutions change colour with the response to different pH. These are called acid-base indicators. Usually, these acids or bases are weak, but their conjugate base or acid forms have different colours due to differences in their absorption spectra. Many indicators are extracted from plants. For example, red cabbage juice and tea pigments show different colours for different pH values. The colour of tea darkens when the solution is basic, but the colour becomes lighter when lemon juice is added which is acidic in nature. Red cabbage juice turns blue in a basic solution, but it shows a distinct red colour when the solution is acidic.
In our question, we have 3 different types of indicators. Let's discuss them one by one:
-Methyl orange: Methyl orange is a pH indicator commonly used in titration because of its clear and distinct colour variance at different values of pH. Methyl orange shows red colour in acidic medium and yellow colour when the medium is basic. In a solution that decreases in acidity, methyl orange moves from red to orange in colour and finally to yellow with the opposite occurring for a solution increasing in acidity.
-Phenolphthalein: Phenolphthalein is used as an indicator in acid-base titration. It turns colourless in acidic solutions and pink in basic solutions. It belongs to the class of dyes commonly known as phthalein dyes. Phenolphthalein is slightly soluble in water and usually is dissolved in alcohols. In a highly basic solution, phenolphthalein's slow change from pink to colourless. For that reason, it is used in chemistry classes for the study of reaction kinetics.
-Potassium permanganate: Potassium permanganate is an inorganic compound. It is a purplish-black crystalline solid that dissolves in water to give bright pink or purple solutions. It is widely used in the chemical industry and also in laboratories as a strong oxidizing agent.
From the above discussion, we can say that Phenolphthalein is used to find the strength of caustic soda solution with the help of oxalic acid.

Therefore, the correct answer is (B)- Phenolphthalein.

Note: Indicators are not always best to find the strength of a solution because of many different reasons like if the substance if contaminated, the colour may be wrong, acid-base indicators can show only one or two colour changes, they can indicate only acidity or alkalinity of the solution but not its pH value.