Acetyl salicylic acid (CH3COO.C6H4.COOH) undergoes hydrolysis when boiled gently with aqueous NaOH, — Physical Chemistry — Kinetics Chemistry Question

Acetyl salicylic acid (CH₃COO.C₆H₄.COOH) undergoes hydrolysis when boiled gently with aqueous NaOH, which forms a basis for its estimation.

Chemicals and solutions
* Plain aspirin tablets
* Hydrochloric acid, c = 0.1 mol dm–3
* Sodium hydroxide, aqueous solution, c = 1 mol dm–3
* Borax (AR Grade)
* Phenol red, indicator

Preparation of HCl aqueous solution (c = 0.1 mol dm–3)
A volume of 9 cm₃ of concentrated HCl is diluted to 1000 cm₃ using freshly prepared distilled water in a standard volumetric flask.

Preparation of NaOH aqueous solution (c = 0.1 mol dm–3)
Weigh rapidly approximately 10.5 g of NaOH in a small beaker. Dissolve it in minimum amount of distilled water. Transfer the solution in a 250 cm₃ flask and dilute the solution using boiled out distilled water.

Procedure

Standardisation of HCl solution
Weigh 0.15 g of borax accurately and transfer it quantitatively in a clean 250 cm₃ conical flask. Add 50 cm₃ of distilled water to the flask. Titrate the resulting solution with HCl solution using methyl red indicator until the colour changes from yellow to red.

Blank titration
Dilute the 25 cm₃ of NaOH solution (c = 1 mol dm–3) in a 250 cm₃ standard flask using freshly boiled distilled water. Pipette out 25 cm₃ of the diluted NaOH solution and titrate it against the HCl solution using phenol red as indicator until the colour changes from red to yellow.

Titration of sample aliquot
Weigh accurately about 1.5 g of the crushed tablet sample and transfer it quantitatively in a 250 cm₃ beaker. Add 25 cm₃ of NaOH solution (c = 1 mol dm–3) using pipette and swirl the content. Boil the mixture gently on water bath for 15 min and then cool the solution. Transfer the solution to a 250 cm₃ standard flask. Dilute the solution up to the mark with distilled water and mix well. Titrate 25 cm₃ of the diluted solution against the standardised HCl solution using phenol red indicator until the colour changes from red to yellow.