Pharmaceutical Chemistry Laboratory
Weigh a sample on an analytical balance, make it up in a volumetric flask, pipette an aliquot and titrate it with standardised sodium hydroxide to a phenolphthalein end-point — then calculate the assay from your own readings.
Citric acid is a weak triprotic acid (pKa 3.13, 4.76, 6.40). Sodium hydroxide neutralises all three carboxyl groups: C₆H₈O₇ + 3 NaOH → Na₃C₆H₅O₇ + 3 H₂O, so 3 mol of NaOH react with 1 mol of citric acid.
At the equivalence point the solution contains sodium citrate, a weak base, so the pH is alkaline (about 9). Phenolphthalein (colourless below pH 8.2, pink above about 9) marks it; indicators that change in acid solution do not.
A known mass of sample is dissolved and made up to 100.00 mL; a 20.00 mL aliquot is titrated. The titre gives the moles of NaOH, hence the moles and mass of citric acid in the aliquot, scaled up by 100/20 to the whole sample.
This is an illustrative scaled-down method. The official monographs (Ph. Eur. 0456 / USP Citric Acid Monohydrate) dissolve 0.550 g in 50 mL water and titrate directly with 1 M NaOH to phenolphthalein (1 mL ≡ 64.03 mg C₆H₈O₇), with limits of 99.5–100.5 % calculated on the anhydrous substance. Here the result is expressed as the monohydrate and the limit is a teaching value.
Model assumptions: concentrations stand in for activities, 25 °C, additive volumes. The sample, its purity and the NaOH molarity are simulated teaching values.
Citric acid monohydrate (simulated sample CA-01) · Sodium hydroxide, 0.1012 M · Phenolphthalein solution · Purified water
Curriculum
Listed in the UoK Pharm-D practical syllabus (PHC-607 Pharmaceutical Analysis – I, 4th year, 1st semester: “Acid base titration”). The syllabus names no sample — citric acid is our illustrative choice. Not faculty-reviewed.
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