For many reasons, materials packaged for domestic use are often “diluted” by inert substances, often — Physical Chemistry — Thermodynamics Chemistry Question

For many reasons, materials packaged for domestic use are often “diluted” by inert substances, often referred to as fillers. In the case of drugs, one reason for this procedure is to provide the correct dosage in a tablet of acceptable size. For example, aspirin, acetylsalicylic acid, is often mixed with a filler in commercial preparations. The aim of this experiment is to determine the percentage of aspirin in a readily available tablet.

Aspirin or acetylsalicylic acid can be considered to be the product of reaction of acetic acid (CH₃COOH) and salicylic acid (HOC₆H₄COOH). When treated with a solution of sodium hydroxide, aspirin is hydrolyzed and the two acids are simultaneously neutralized.

CH₃COOC₆H₄COOH + 2 NaOH → CH₃COONa + HOC₆H₄COONa + H₂O

If an excess of NaOH solution is used in this reaction, the amount of excess can be determined by a back titration with H₂SO₄. It is essential, however, that the H₂SO₄ used in this titration does not also react with sodium acetate and sodium salicylate, both of which contain basic anions. This can be avoided by the selection of either phenol red (pH range 6.8 – 8.4) or phenolphthalein (pH range 8.3 – 10.0) as the indicator.

Procedure
Weigh out accurately sufficient aspirin tablets to give a mass of about 1.5 g. Record the number of tablets and the mass.

Transfer the tablets to a 150 cm₃ conical flask. Add a 25 cm₃ aliquot of a carefully prepared NaOH solution together with a similar volume of water. Heat gently for about 10 minutes to hydrolyze the acetylsalicylic acid, according to the equation above. Cool the reaction mixture by holding the flask under running water and carefully transfer the contents, without loss, to a 250 cm₃ volumetric flask. Rinse the reaction vessel several times with water, adding the washings to the volumetric flask. Dilute the solution to the calibration mark and mix well by shaking.

Take a 25 cm₃ aliquot of the diluted reaction mixture and transfer it to a clean conical flask. Titrate the aliquot with H₂SO₄ solution with the concentration of 0.05 mol dm–3 using phenol red or phenolphthalein as the indicator. Record the actual concentration of the acid and the titre obtained. Repeat the determination until consistent titres are determined. Calculate the average titre.

Using a pipette and a volumetric flask, dilute a sample of the NaOH solution (c = 1 mol dm–3) to final concentration of 0.1 mol dm–3. Titrate 25 cm₃ aliquots of the dilute solution with the H₂SO₄ solution (c = 0.05 mol dm–3) using the same indicator as before.