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VKC Notes · Analytical chemistry · Geo-Scientist P-I

17.4 Gravimetric Analysis: Solubility Product, Precipitation, Co-precipitation, Organic Precipitants

★★★ 2021–2025: 19 parts, 160 marks, all in Geo-Scientist Paper I. Every year has a 5-mark Ksp or gravimetric-factor numerical and a 10–15-mark question on precipitate quality or a named estimation (BaSO₄, Ni–DMG, oxine).

Syllabus line. Geo-Scientist Paper I: “General principles: solubility, solubility product and common ion effect, effect of temperature on the solubility … Stoichiometry, calculation of results from gravimetric data. Properties of precipitates. Nucleation and crystal growth, factors influencing completion of precipitation. Co-precipitation and post-precipitation, purification and washing of precipitates. Precipitation from homogeneous solution. A few common gravimetric estimations: chloride as silver chloride, sulphate as barium sulphate, aluminium as oxinate and nickel as dimethyl glyoximate.” Salt hydrolysis is in 17.1.

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CSE 2024 Q7(a) = asked there; tap to open the solved answerSN = line kept in the short notes

17.4.1 Solubility product, common ion and temperature ★★★

(17.4.1)
  • Precipitation starts when the ionic product Q > Ksp. A common ion lowers the solubility (s = Ksp/[Cl⁻] for AgCl in excess Cl⁻): this is why a slight excess of precipitant is added.SN
  • A very large excess is avoided: complexation redissolves the solid (AgCl + Cl⁻ → [AgCl₂]⁻; Al(OH)₃ + OH⁻ → [Al(OH)₄]⁻), adsorption and occlusion of the excess reagent increase, and the higher ionic strength raises solubility (diverse-ion effect) Geo 2021 Q11(c) 5m.SN
  • Temperature: d ln Ksp/dT = ΔH°diss/RT². Most precipitates (BaSO₄, AgCl) dissolve endothermically, so they are more soluble hot: precipitate hot (lower relative supersaturation, bigger crystals), but filter cold when the loss matters (MgNH₄PO₄); a few (CaSO₄, Ce₂(SO₄)₃) are less soluble hot Geo 2021 Q11(d) 5m.SN

Solved PYQs Geo 2025 Q1(e) 5m Geo 2023 Q1(e) 5m

  1. Ag₂CrO₄, 2.5 × 10⁻² g L⁻¹, M = 331.73: s = 7.54 × 10⁻⁵ mol L⁻¹; Ksp = (2s)²s = 4s³ = 1.71 × 10⁻¹². (Its red colour after AgCl is complete is Mohr’s end point.)SN
  2. Pb₃(PO₄)₂, Ksp = 1.5 × 10⁻³²: (3s)³(2s)² = 108s⁵; s = (1.5 × 10⁻³²/108)1/5 = 1.69 × 10⁻⁷ mol L⁻¹ × 811.5 g mol⁻¹ = 1.37 × 10⁻⁴ g L⁻¹ (neglecting hydrolysis of PO₄³⁻).

17.4.2 Requisites, gravimetric factor and calculations ★★★

  • A good weighing form: so insoluble that loss is below the balance limit; filterable crystals; pure; of known, constant composition after drying or ignition; not hygroscopic; high formula mass relative to the analyte (small GF, smaller weighing error).SN
(17.4.2)

a, b make the analyte atoms balance: Fe₃O₄ from Fe₂O₃ needs 2/3 (two Fe₃O₄ ≡ three Fe₂O₃ ≡ six Fe).

Solved PYQs Geo 2025 Q6(a) 15m Geo 2024 Q5(c) 10m Geo 2022 Q6(a) 10m Geo 2022 Q1(e) 5m

  1. Sulphate as BaSO₄ (filtering crucible): Ba²⁺ + SO₄²⁻ → BaSO₄↓ from hot, dilute (≈ 0.05 M) HCl solution with hot dilute BaCl₂ added slowly with stirring; digest; wash with hot water until Cl⁻-free; ignite at about 800 °C to constant mass. GF(SO₄²⁻) = 96.06/233.39 = 0.4116. Low results: solubility in too much acid, fine particles through the filter, reduction to BaS (by filter-paper carbon), co-precipitated Na₂SO₄/Fe₂(SO₄)₃ (lighter or lose SO₃). High: co-precipitated BaCl₂ or Ba(NO₃)₂, occluded water. Remedy: dilute hot solutions, slow addition, digestion, a sintered crucible instead of paper Geo 2025 Q6(a).SN
  2. Magnetite: 1.500 g ore → 0.8525 g Fe₂O₃; GF = 2 × 231.54/(3 × 159.69) = 0.9666; Fe₃O₄ = 0.8240 g = 54.94 %.SN
  3. Rock, 2.0000 g: PbSO₄ 0.3188 g × 0.6831 = 0.2178 g Pb (10.89 %); Fe₂O₃ 0.7282 × 0.6993 = 0.5092 g Fe (25.46 %); CuSCN 0.4787 × 0.5224 = 0.2501 g Cu (12.50 %).
  4. 0.830 g AgCl from 25 mL: 0.830/143.32 = 5.79 × 10⁻³ mol Cl⁻; [Cl⁻] = 0.232 M.

17.4.3 Nucleation, particle growth and how to get big crystals ★★★

(17.4.3)
(Q − S)/Srategrowthnucleationlow: growth wins,few large crystalshigh: colloid
Fig. 17.4.1 Nucleation and growth against relative supersaturation.
Draw it in 60 s: two curves from the origin: a straight line (growth) and a steep exponential (nucleation) that overtakes it; left side “big crystals”, right side “colloid”.
  • Nucleation: a few ions come together to form the smallest stable particles. Particle growth: ions deposit on existing nuclei. Nucleation rises steeply with (Q − S)/S, growth only linearly; low relative supersaturation gives few nuclei that grow into large, pure, filterable crystals; high gives a colloid Geo 2022 Q6(b) 10m.SN
  • To keep (Q − S)/S low (lower Q, raise S): dilute sample and dilute reagent; add the reagent slowly with good stirring (no local excess); precipitate hot; adjust pH so S is a little higher at first (BaSO₄ from dilute HCl; CaC₂O₄ in acid, then raise pH slowly); precipitation from homogeneous solution Geo 2021 Q11(a) 10m.SN
  • Homogeneous precipitation: the precipitant is made slowly and uniformly inside the solution, so Q never rises locally: urea hydrolysis CO(NH₂)₂ + 3H₂O → CO₂ + 2NH₄⁺ + 2OH⁻ at 90 °C raises pH evenly (Al, Fe hydroxides, CaC₂O₄); sulfamic acid gives SO₄²⁻; dimethyl sulfate gives sulfate; trimethyl phosphate gives PO₄³⁻.SN
  • Digestion (standing hot in the mother liquor): small crystals dissolve and big ones grow (Ostwald ripening); occluded impurities escape; colloids coagulate.

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