Let's examine each statement one by one to choose the most appropriate answer :

Statement (I) : Potassium hydrogen phthalate is a primary standard for standardisation of sodium hydroxide solution.

This statement is correct. Potassium hydrogen phthalate (often abbreviated as KHP, with the chemical formula $$ C_8H_5KO_4 $$) is commonly used as a primary standard in titrations because it is pure, stable, non-hygroscopic, and has a high molar mass. It reacts with sodium hydroxide (NaOH) in a stoichiometric reaction, where one mole of KHP reacts with one mole of NaOH. The equation for the reaction is as follows :

$$ C_8H_5KO_4 + NaOH \rightarrow C_8H_4KO_4Na + H_2O $$

Statement (II) : In this titration phenolphthalein can be used as indicator.

This statement is also correct. Phenolphthalein is a suitable indicator for the titration between KHP and NaOH. It changes color in the pH range of about 8.2 to 10, which is appropriate for the endpoint of a titration involving a weak acid (KHP) and a strong base (NaOH). In an acidic solution, phenolphthalein is colorless, but it turns pink in a basic solution, signaling the end of the titration when slight excess NaOH is present.

Given that both statements are correct, the most appropriate answer is :

• Option A : Both Statement I and Statement II are correct.