Chemistry Labs
Undergraduate · 25 min

Gravimetric determination of sulfate as BaSO₄

Precipitate sulfate quantitatively with barium chloride, then filter, ignite and weigh the BaSO₄: the mass of a pure, stoichiometric solid reveals how much sulfate the sample contained.

Goal

Link the mass of dried BaSO₄ to the amount of sulfate via the gravimetric factor M(SO₄²⁻)/M(BaSO₄), and justify each quantitative-transfer step.

Apparatus and reagents

A sample containing sulfate, 5% barium chloride solution, dilute hydrochloric acid, beakers and a glass rod, ashless filter paper or a crucible, drying oven or muffle furnace and an analytical balance.

Procedure

  1. Acidify the sulfate sample slightly with dilute HCl, then add hot BaCl₂ solution slowly while stirring — the slider stands for the volume added.
  2. Let the precipitate digest: keeping it hot allows small BaSO₄ crystals to dissolve and redeposit on larger ones, giving particles easy to filter.
  3. Add a small excess of precipitant, then test the supernatant with a drop of BaCl₂ — no new clouding means precipitation is complete.
  4. Filter, wash until free of chloride (AgNO₃ test), ignite to constant mass and weigh; m(SO₄²⁻) = m(BaSO₄) × 96.06/233.39.

What to observe

  • Fine white particles form where the BaCl₂ stream enters and slowly settle; the dense precipitate collects as a compact layer at the bottom.
  • Precipitation in hot, slightly acidic solution gives larger, purer crystals than adding all the barium at once to a cold solution.

Explanation

BaSO₄ has Ksp ≈ 1.1·10⁻¹⁰, so a modest excess of Ba²⁺ drives sulfate almost completely into the solid. Because the precipitate is chemically defined and stable to ignition, its mass converts directly to sulfate through the gravimetric factor 96.06/233.39 ≈ 0.4116 — no calibration curve is needed, which makes gravimetry an absolute method.

History of the experiment

Gravimetry is the oldest quantitative method in analytical chemistry: in the 1820s Jöns Jacob Berzelius used careful precipitation and weighing to determine the atomic masses of much of the periodic table. The BaSO₄ method for sulfate became a reference procedure — so insoluble (Ksp ≈ 1.1·10⁻¹⁰) and so stable to ignition that it is still taught as the model gravimetric analysis.

Chemists behind it

Jöns Jacob Berzelius

Related topics

Gravimetric analysisErrors and statistical treatment of data

Virtual experiment: a simplified model to build intuition. It does not replace real lab work or safety training; never repeat chemistry at home without supervision.