Titrating an Antacid Tablet

The packet says each tablet contains 750 mg of calcium carbonate. Is that true? And how much stomach acid can one tablet actually neutralise? A back titration answers both questions with standard school equipment, and it’s a great example of chemistry checking a real product claim.

Why a back titration?

You can’t titrate an antacid tablet directly. It’s a solid that dissolves slowly, it contains insoluble binders and fillers, and the reaction with acid releases carbon dioxide, which would interfere with the end point.

Instead:

1. Dissolve the tablet in a known excess of hydrochloric acid. All the base in the tablet reacts, and some acid is left over.
2. Titrate the leftover acid with sodium hydroxide.
3. The acid that reacted with the tablet = acid added − acid left over.

This technique is called a back titration. It’s used whenever the substance being analysed is insoluble, reacts slowly, or is volatile.

The chemistry

For a calcium carbonate antacid:

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

For the back titration:

HCl + NaOH → NaCl + H₂O

Other antacids contain magnesium hydroxide, aluminium hydroxide or magnesium carbonate. The method works for all of them, as long as you know the reaction ratio. See stomach acid chemistry for how antacids work in the body.

Equipment and chemicals

• Antacid tablets (calcium carbonate type, with the stated content on the packet)
• Mortar and pestle
• Balance (±0.01 g or better)
• 250 cm³ conical flasks
• 50.00 cm³ pipette (or burette) for the acid, 50.00 cm³ burette for the alkali
• 0.500 mol/dm³ hydrochloric acid (standardised)
• 0.500 mol/dm³ sodium hydroxide (standardised)
• Phenolphthalein indicator
• Hot plate or Bunsen burner, tripod and gauze
• Eye protection

Method

1. Weigh one tablet and record its mass.
2. Crush it to a fine powder in a mortar and pestle. Transfer all of it into a conical flask, rinsing the mortar with a little distilled water into the flask.
3. Add exactly 50.00 cm³ of 0.500 mol/dm³ HCl using a pipette or burette. Swirl. The powder will fizz as CO₂ is released.
4. Warm the flask gently for a few minutes (don’t boil dry) to help the tablet react completely and to drive off dissolved carbon dioxide, which would otherwise react with the sodium hydroxide and give a false result. Let it cool.
5. Add 2–3 drops of phenolphthalein.
6. Titrate with 0.500 mol/dm³ NaOH until the first permanent pale pink.
7. Repeat with further tablets (at least three) to check consistency.

Note: binders in the tablet may leave the solution slightly cloudy. That’s fine; watch for the colour change through the cloudiness, ideally against a white tile.

Sample results

	Tablet 1	Tablet 2	Tablet 3
Mass of tablet (g)	1.312	1.305	1.318
Volume of HCl added (cm³)	50.00	50.00	50.00
Final burette reading (cm³)	21.45	43.10	21.30
Initial burette reading (cm³)	0.05	21.45	0.00
Titre of NaOH (cm³)	21.40	21.65	21.30

Mean titre (all three agree within about 0.35 cm³, acceptable for a tablet-to-tablet comparison) = 21.45 cm³.

Calculation

Step 1. Moles of HCl added n(HCl) = 0.500 × 0.05000 = 0.02500 mol

Step 2. Moles of HCl left over n(NaOH) = 0.500 × 0.02145 = 0.010725 mol = n(HCl left over)

Step 3. Moles of HCl that reacted with the tablet 0.02500 − 0.010725 = 0.014275 mol

Step 4. Moles of CaCO₃ Ratio CaCO₃ : HCl = 1 : 2, so n(CaCO₃) = 0.014275 ÷ 2 = 0.0071375 mol

Step 5. Mass of CaCO₃ M(CaCO₃) = 100.09 g/mol Mass = 0.0071375 × 100.09 = 0.714 g = 714 mg

Step 6. Compare with the label Label claim: 750 mg. Our result: 714 mg, which is 95% of the claim.

Step 7. Acid-neutralising capacity One tablet neutralised 0.0143 mol of HCl. Stomach acid is around 0.01–0.1 mol/dm³, so one tablet can neutralise the acid in roughly 0.14–1.4 dm³ of gastric juice, far more than the stomach contains at any one time. That’s why one or two tablets are usually enough.

For more worked examples of this method, see titration calculations.

Evaluation

Why might the result be lower than the label?

• The tablet didn’t react completely (lumps or binders coating particles).
• Some powder was lost while crushing or transferring.
• Carbon dioxide wasn’t fully driven off, so extra NaOH was used to react with it (this would actually make the calculated tablet content lower, since more NaOH means less acid apparently used by the tablet).
• The label may state a nominal amount; manufacturers typically aim within a tolerance.

Why might the result be higher?

• The tablet contains other bases (some brands add magnesium carbonate or hydroxide), which also react with the acid and would be counted as “calcium carbonate” in this calculation.
• Overshooting the end point doesn’t make the result higher; it makes the titre larger, which makes the calculated tablet content lower. Check your reasoning carefully in evaluation questions.

Improving the method:

• Grind the tablet finely and wash all particles into the flask.
• Warm to complete the reaction and remove CO₂.
• Standardise both the HCl and NaOH beforehand.
• Repeat with several tablets and average.

Extension ideas

1. Compare brands. Which gives the most neutralising capacity per penny or per tablet?
2. Compare antacid types. Calcium carbonate vs magnesium hydroxide vs sodium hydrogencarbonate, taking the mole ratios into account.
3. Rate of neutralisation. Measure the pH of acid over time after adding a whole tablet vs a crushed tablet, using a pH probe. This connects to rates of reaction and surface area.
4. Chewable vs swallowed. Why do many antacids say “chew before swallowing”?

Linking to real quality control

Pharmaceutical companies test their antacids in a very similar way. National pharmacopoeias (official standards for medicines) describe “acid-neutralising capacity” tests in which a dose of antacid is reacted with a measured excess of hydrochloric acid and the leftover acid is titrated back, often with a pH meter to find the end point. Products must meet a minimum capacity and contain their stated amount of active ingredient within a set tolerance. Your school version is a simplified form of a genuine regulatory test.

Safety

• Wear eye protection throughout.
• 0.500 mol/dm³ hydrochloric acid and sodium hydroxide are irritants; avoid skin contact and wash off splashes with water.
• Take care when warming the flask; use tongs or a heat-resistant glove.
• Don’t eat the tablets used in the experiment.
• Dispose of the final solutions by diluting with plenty of water, following your school’s rules.

Key takeaways

• A back titration adds a known excess of acid, then titrates what’s left, to analyse solids like antacid tablets.
• Acid used by the tablet = acid added − acid remaining.
• Apply the correct mole ratio (2 HCl per CaCO₃) to find the amount of active ingredient.
• Warming removes CO₂ and helps the tablet react fully.
• The same approach can compare brands and check label claims, a real-world use of acid–base titration.