17.7 Analysis of Ores, Alloys, Petroleum, Coal and Coke

17.7.1 The gravimetric factor: one tool for every ore ★★★

• a and b balance the key element: Fe₃O₄ weighed as Fe₂O₃: 2 Fe₃O₄ ≡ 6 Fe ≡ 3 Fe₂O₃, so G = 2 × 231.54/(3 × 159.70) = 0.9666. Ni weighed as Ni(DMG)₂: G = 58.69/288.91 = 0.2031. Zn as ZnNH₄PO₄: 0.3665; as Zn₂P₂O₇: 2 × 65.38/304.70 = 0.4291.SN
• Magnetite ore: 1.3155 g, dissolved in HCl, oxidised by HNO₃, NH₃ precipitates Fe(OH)₃, ignited to 0.7273 g Fe₂O₃: Fe₃O₄ = 0.7273 × 0.9666 = 0.7030 g → 53.44 % (w/w) Geo 2024 Q5(c)(ii) 5m.SN

17.7.2 Dolomite: CaCO₃ and MgCO₃ by EDTA ★★★ 15 marks

1. Dissolve: 0.5 g in dilute HCl (CO₂ escapes); evaporate to dryness to make silica insoluble; take up in HCl, filter off SiO₂. Precipitate R₂O₃ (Fe, Al hydroxides) with NH₄Cl + NH₃ and filter (or mask with triethanolamine); dilute the filtrate to 250 mL.SN
2. Ca + Mg: 25 mL aliquot, NH₃–NH₄Cl buffer pH 10, Eriochrome Black T; titrate with standard EDTA from wine red to pure blue: V₁.SN
3. Ca alone: second aliquot made pH 12–13 with NaOH (Mg precipitates as Mg(OH)₂), murexide (or Patton–Reeder) indicator; titrate pink → violet: V₂. Mg ≡ V₁ − V₂.SN
4. Basis: EDTA binds every metal 1 : 1, so 1 mol EDTA ≡ 1 mol CaCO₃ (100.09 g) ≡ 1 mol MgCO₃ (84.31 g): % CaCO₃ = M V₂ × 100.09 × 10 × 100/(1000 w); % MgCO₃ = M(V₁ − V₂) × 84.31 × 10 × 100/(1000 w) (factor 10 for the 25 of 250 mL aliquot) Geo 2021 Q3(b) 15m.SN

17.7.3 Bauxite: Al₂O₃, Fe₂O₃ and TiO₂ ★★★

• Decomposition: fuse 0.25–0.5 g with Na₂CO₃ + borax (or K₂S₂O₇ for Ti); dissolve the melt in dilute HCl; dehydrate and filter silica. Pour into hot excess NaOH: Fe(OH)₃ and TiO₂·xH₂O stay behind, Al passes as aluminate [Al(OH)₄]⁻.SN
• Al₂O₃ by EDTA back-titration: Al³⁺ reacts with EDTA slowly and blocks metal indicators, so add a measured excess of EDTA at pH ≈ 3, boil, buffer to pH 5–5.5 (acetate), titrate the unused EDTA with standard Zn²⁺, xylenol orange yellow → red-violet. mmol Al = (MV)_EDTA − (MV)_Zn; 1 mmol Al ≡ 50.98 mg Al₂O₃ Geo 2023 Q4(b) 10m.SN
• Fe₂O₃: reduce Fe³⁺ with SnCl₂ (excess removed by HgCl₂) and titrate with K₂Cr₂O₇ (diphenylamine sulfonate); or colorimetrically as the thiocyanate or 1,10-phenanthroline complex.
• TiO₂ (colorimetric peroxide method): Ti(IV) in dilute H₂SO₄ + H₂O₂ gives a yellow-orange peroxo complex [TiO(H₂O₂)]²⁺; read at 410 nm against standards made from TiO₂ or K₂TiF₆; H₃PO₄ masks the yellow of Fe³⁺; F⁻ must be absent (bleaches it) Geo 2025 Q5(c) 5m.SN

17.7.4 Pyrolusite: MnO₂ and MnO ★★

• MnO₂ (available oxygen): ≈ 0.2 g powdered ore warmed with a measured excess of standard Mohr's salt in dilute H₂SO₄ until no black particles remain; back-titrate the unused Fe²⁺ with KMnO₄ (self-indicator, first pink). mmol MnO₂ = ½[(MV)_Fe − 5(MV)_KMnO₄]; % Mn = mmol MnO₂ × 54.94 × 100/mg sample. Or use standard oxalate at 60–70 °C Geo 2022 Q1(h) 5m.SN
• MnO: total Mn (as Mn²⁺ → MnO₄⁻ by bismuthate or periodate, then titration or 525 nm) minus Mn present as MnO₂, converted to MnO.

17.7.5 Brass, bronze, steel, type metal ★★★

• Cu in brass, iodometric: dissolve in HNO₃, boil off NOₓ, neutralise with NH₃, acidify with acetic acid, add KI: 2 Cu²⁺ + 4 I⁻ → 2 CuI + I₂; titrate I₂ with thiosulfate, starch near the end (KSCN sharpens it). 2.5 g in 250 mL, 25 mL used 16 mL of 0.1010 N: 1.616 meq Cu in 25 mL; 1.616 × 63.54 × 10 = 1.0268 g; 41.07 % Cu Geo 2021 Q2(b) 5m.SN
• Zn in brass, gravimetric: dissolve in 1 : 1 HNO₃ (Sn left as metastannic acid, filter); fume with H₂SO₄ (PbSO₄, filter); remove Cu electrolytically on a Pt cathode and Fe as Fe(OH)₃; at pH 6–7 add (NH₄)₂HPO₄ hot: ZnNH₄PO₄, dry at 105 °C (×0.3665) or ignite to Zn₂P₂O₇ (×0.4291) Geo 2021 Q1(d) 5m.SN
• Bronze (Cu, Sn with Zn, Fe, Mn, Al, Ni): Sn as SnO₂; Cu electrolytically or iodometrically; Fe, Al as hydroxides; Ni by DMG; Zn by EDTA; Mn by periodate (525 nm).
• Ni in steel, gravimetric Geo 2025 Q1(g) 5m: 0.5 g in HCl + HNO₃; add tartaric acid (holds Fe³⁺, Cr³⁺); warm to 60–80 °C, add 1 % alcoholic dimethylglyoxime, then NH₃ until just ammoniacal: red Ni(DMG)₂; digest, filter on a sintered crucible, dry at 110–120 °C. Avoid large excess of reagent (it crystallises out); Pd interferes.SN

• Steel, others: Cr and Mn together by persulfate–Ag⁺ oxidation to Cr₂O₇²⁻ and MnO₄⁻ and absorbance at 440 and 545 nm (13.8); P as the yellow ammonium phosphomolybdate or molybdenum blue.
• Type metal: printer's alloy of Pb (major, 50–86 %), Sb (11–30 %, hardens and stops shrinkage) and Sn (3–20 %, fluidity), e.g. linotype Pb 84, Sb 12, Sn 4. Non-destructive: XRF on the solid (Pb by L lines, Sb, Sn by K lines) against alloy standards; neutron activation also Geo 2023 Q1(h) 5m.SN