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Sulfate by gravimetry: ignite to constant mass

Precipitate the sample’s sulfate as BaSO₄ — slowly, from a hot dilute solution — digest, filter and wash, then ignite and weigh repeatedly to constant mass and compute the sulfate content with the gravimetric factor. Nickel by dimethylglyoxime is available too.

Difficulty ●●○ ≈ 20 min

Aim Determine the mass fraction of SO₄²⁻ in a soluble sulfate sample as barium sulfate, and compare how the precipitation conditions affect the result.
Analytical balance 0.1 mg · constant mass within ±0.3 mg

Gravimetry: precipitate, digest, heat to constant mass

Sample # 1

BaX2++SOX4X2−→BaSOX4(s)\ce{Ba^2+ + SO4^2- -> BaSO4(s)} · hot dilute BaCl₂, slightly acidic

19.4034gd = 0.1 mg
Sample weighed (by difference)
0.3916 g
Empty crucible (at constant mass)
19.4034 g
BaSOX4\ce{BaSO4} · digest, filter, ignite at 800 °C to constant mass
GF = 0.4116
② Precipitation conditions
Computed: the ideal precipitate mass = moles of analyte in the sample × the precipitate’s molar mass (from its formula), and the gravimetric factor = molar mass of analyte ÷ molar mass of precipitate. The loss to the wash uses BaSO₄’s solubility in pure water, √Ksp (Ksp = 1.1 × 10⁻¹⁰), with the wash liquid half saturated; occlusion (cold, concentrated, fast +1.5 %; hot, dilute, slow +0.2 %) and fines passing the filter without digestion (−1.0 %) are illustrative magnitudes that show why technique matters. Each weighing after heating approaches constant mass (adsorbed water and filter-paper ash decrease cycle by cycle) with a deterministic balance repeatability of 0.05 mg. The sample’s content is set by its number.

Procedure

  1. Note the sample mass (about 0.4 g, weighed by difference) and the mass of the empty crucible, already at constant mass.
  2. Choose the precipitation conditions: add BaCl₂ slowly to a hot, dilute solution and digest for an hour; use a moderate volume of wash liquid.
  3. Precipitate, filter and wash, then “heat and weigh” repeatedly until two successive weighings agree within 0.3 mg.
  4. Compute the sulfate mass fraction from the precipitate mass and the factor 0.4116, then reveal the true value.
  5. Repeat with “cold, concentrated, all at once”, without digestion, or with 200 mL of wash, and compare the error sources.

What to notice

  • The first weighing after ignition is clearly too heavy (adsorbed water and paper ash); it falls with every heating and reaches constant mass after three or four.
  • Fast precipitation increases occlusion and reads high; without digestion fine crystals pass the filter and it reads low.
  • More wash liquid dissolves more precipitate, though for BaSO₄ the loss is only a few tenths of a milligram.

In a real lab

  • Barium salts are toxic: wear gloves, and precipitate the Ba²⁺ in the waste with sulfate before disposal.
  • Use ashless quantitative filter paper; char the paper at low heat first, then raise to about 800 °C, so that carbon does not reduce BaSO₄ to BaS.
  • Cool the crucible from the furnace to room temperature in a desiccator before weighing, for the same time on every cycle.
  • NO₃⁻, ClO₃⁻ and Fe³⁺ in the sample coprecipitate with BaSO₄: remove them first or precipitate from dilute hydrochloric acid.

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