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Prepared by

Hoda Abd El-Shafy Shilkamy Diab

Lecturer assistant of physical chemistry

Chemistry depatment- Faculty of Science – Sohag university

Sec. 8 Determination of the percentage of saycalic acid in aspirin pH- metrically

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Aspirin is the trade name of drugs containing organic chemical compound named acetylsalicylic acid. It is widely used as an analgesic to relieve minor aches and pains, as an antipyretic to reduce fever, and as an anti-inflammatory medication.

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The maximum allowable amount of free salicylic acid in an aspirin sample is 0.15% salicylic acid.

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Acetylsalicylic acid

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Some data on SA and ASA relevant to this exercise are the following:

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Compound Salicylic Acid (SA) Acetylsalicylic Acid (ASA)

Formula C7H6O3 C9H8O4

Molar Mass 138.12 180.15

Ka 1.08 x 10-3 2.72 x 10-5

pKa 2.99 4.57

Solubility (g/100 mL) 0.18 0.25

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Chemically, aspirin is monoprotic acid and its strength (pKa=3.5) is comparable to that of formic acid. It solubility in water is not high (ca. 3.3 g/L at 20°C).

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(N × ml ) saycalic acid = (N × ml) NaOH

(N ×10 ) salcalic acid = ( 0.05 N ×V )NaOH

g/l = ??

% = ??

ppm = ??