Measuring Ascorbic Acid in Vitamin C Tablets by Titration
Iodine titration provides a straightforward way to estimate the ascorbic acid content of a vitamin C tablet. The method is suitable for teaching laboratories, undergraduate practicals, and community science projects because the reaction has a simple 1:1 stoichiometry and produces a clear starch endpoint.
For Australian students, the experiment also offers a useful way to compare a tablet’s measured content with the amount printed on its label. Products purchased from a local chemist, supermarket, or pharmacy may contain fillers, sweeteners, flavourings, and colourants, so careful sample preparation is essential.
Choosing The Chemistry And Equipment
Ascorbic acid reduces iodine to iodide while it is oxidised to dehydroascorbic acid:
C₆H₈O₆ + I₂ → C₆H₆O₆ + 2HI
One mole of iodine reacts with one mole of ascorbic acid. A typical procedure uses a dilute iodine solution and soluble starch as the indicator. While vitamin C remains present, iodine is consumed and the solution stays colourless or pale yellow. Once all the ascorbic acid has reacted, a small excess of iodine forms a persistent blue-black complex with starch.
You will need a burette, stand and clamp, pipette, volumetric flask, conical flask, funnel, beakers, balance, distilled or deionised water, potassium iodide, iodine solution, starch indicator, and the tablet sample. Wear safety glasses and a lab coat, and handle iodine carefully because it stains skin and benches and can irritate the eyes.
Preparing The Tablet Sample
Record the tablet’s labelled vitamin C mass before beginning. Crush one tablet to a fine, uniform powder using a clean mortar and pestle. Transfer the powder quantitatively into a volumetric flask, rinsing the mortar and funnel several times so that little material remains behind.
Add distilled water, swirl until the soluble material has dispersed, then dilute to the calibration mark. Some binders may remain undissolved. Filter the mixture if necessary, but record the final volume and ensure the filtrate is mixed thoroughly before taking an aliquot. If the tablet contains strong colouring, a reagent blank or a visual comparison with a known endpoint can help distinguish the iodine colour from the product’s own colour.
Protect the solution from bright light and analyse it promptly. Oxygen, light, and prolonged exposure to air can gradually reduce the reliability of the ascorbic acid measurement.
Making And Standardising Iodine
An approximately 0.005 mol/L iodine solution is convenient for many tablet samples. Iodine is commonly dissolved with potassium iodide, which forms triiodide in water and improves its solubility. Because iodine solutions can change concentration during storage, standardisation is preferable to relying only on the preparation calculation.
A practical standardisation uses a primary standard such as accurately weighed ascorbic acid, or an established sodium thiosulfate procedure. Titrate the standard using the same indicator and technique intended for the tablet sample. Calculate the actual iodine concentration from the known amount of standard and the titre volume.
Starch should be freshly prepared or supplied as a laboratory indicator. Add it near the endpoint rather than at the beginning when possible. High iodine concentrations can produce a strongly coloured complex that makes the endpoint less distinct.
Running The Titration
Rinse the burette with a small portion of the iodine solution, fill it, remove air bubbles from the tip, and record the initial reading. Use a pipette to transfer a measured aliquot of the tablet solution into a conical flask. Add water if needed to create a convenient volume and add a few drops of starch indicator near the expected endpoint.
Titrate while swirling continuously. The endpoint is the first blue-black colour that remains for about 20–30 seconds. Near the endpoint, add iodine dropwise because a single extra drop can noticeably affect a small titre. Record the final burette reading and calculate the titre by subtraction.
Repeat the titration until at least two concordant titres are obtained, commonly within about 0.10 mL of each other for a school or teaching laboratory. A rough trial run can identify the approximate endpoint, but it should not be included in the average.
Calculating And Checking The Result
Convert the average titre from millilitres to litres, then calculate the moles of iodine:
n(I₂) = c(I₂) × V(I₂)
The reaction ratio is 1:1, so the same number of moles represents ascorbic acid in the aliquot. Multiply by the molar mass of ascorbic acid, 176.12 g/mol, to obtain the mass in that aliquot. Finally, apply the dilution factor to estimate the amount in the whole tablet.
| Measurement | Example value |
|---|---|
| Standardised iodine concentration | 0.00500 mol/L |
| Average titre | 17.60 mL |
| Tablet solution volume | 100.0 mL |
| Aliquot volume | 10.00 mL |
| Moles of iodine in aliquot | 0.0000880 mol |
| Ascorbic acid in aliquot | 15.49 mg |
| Estimated amount per tablet | 154.9 mg |
For example, 0.00500 × 0.01760 = 0.0000880 mol of iodine. Multiplying by 176.12 g/mol gives 0.01549 g, or 15.49 mg, in the 10.00 mL aliquot. Since that aliquot represents one-tenth of a 100.0 mL sample, the estimated tablet content is 154.9 mg.
Interpreting Results In Australian Context
Compare the experimental value with the tablet’s declared amount, allowing for measurement uncertainty and the product’s stated tolerances. Australian products are regulated under the Therapeutic Goods Administration, but a practical result can still differ because of tablet age, storage conditions, incomplete dissolution, endpoint judgement, iodine instability, or oxidation of vitamin C. A tablet kept in a hot car in summer may be a poorer sample than one stored as directed in a cool, dry cupboard.
When selecting a product, note whether it is a supermarket supplement, a pharmacy brand, or a chewable tablet marketed for children. Australian labels may list vitamin C as ascorbic acid, sodium ascorbate, or another form; these forms do not all behave identically in a simple direct iodine titration. Effervescent tablets can also contain acids and carbonate salts that cause vigorous fizzing, so allow the solution to finish reacting before transferring an aliquot.
In a school laboratory in Sydney, Perth, or regional Queensland, use the same units and recording standards expected in Australian science courses: millilitres, litres, grams, and significant figures. The most dependable result comes from a standardised iodine solution, a clear sample solution, concordant titres, and a calculation that explicitly includes the aliquot and dilution factors. A practical takeaway is to record every volume, protect the sample from light, and repeat the titration until the endpoint is consistent.