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Manganese in steel by spectrophotometry

Oxidise the manganese in a steel sample to purple permanganate, compare the spectra of a standard and the steel solution to choose the analytical wavelength, then calibrate with KMnO₄ standards and report the steel’s manganese content. The steel’s chromium quietly spoils the wrong wavelength.

Difficulty ●●○ ≈ 25 min

Aim Determine the mass fraction of manganese in a steel by external calibration, and show how the choice of analytical wavelength avoids interference from chromium (as dichromate).
Oxidised to MnO₄⁻ with periodate · external calibration · 1.00 cm cell

Manganese in steel by spectrophotometry

Steel sample # 1 0.4952 g of steel dissolved in acid, Mn oxidised to MnO₄⁻ with KIO₄, made up to 250.0 mL.

① Choose the analytical wavelength

526 nm400450500550600Wavelength / nm00.20.40.60.8A
  • KMnO₄ standard
  • Steel solution

Spectra of a 3.00 × 10⁻⁴ M KMnO₄ standard and the steel solution (model)

② Standards from 1.000 × 10⁻³ M KMnO₄ stock (50 mL flasks)

50 mL20 °C · A1.050 mL20 °C · A2.050 mL20 °C · A3.050 mL20 °C · A4.050 mL20 °C · A5.0250 mL20 °C · ASteel solution
Standards and the steel solution (colours computed)
Computed: the steel solution holds MnO₄⁻ plus some Cr₂O₇²⁻ (the steel’s chromium is oxidised too); absorbance = Σ εbc + a 0.004 reagent blank, plus stray light and deterministic noise of σ_T = 0.0005. Pipettes and flasks carry deterministic errors inside their class A tolerances. The Mn (0.35–1.4 %) and Cr (0.3–1.2 %) contents are set by the sample number. The KMnO₄ and K₂Cr₂O₇ bands are teaching fits anchored to literature ε (MnO₄⁻: 526/546 nm, ε ≈ 95 at 440 nm; Cr₂O₇²⁻: ε ≈ 369 at 440 nm and ≈ 11 at 545 nm).

Procedure

  1. Note the sample number and mass: about 0.5 g of steel has been dissolved in nitric–phosphoric acid, its manganese oxidised to MnO₄⁻ with potassium periodate, and the solution made up to 250.0 mL.
  2. Compare the spectra of a KMnO₄ standard and the steel solution and choose the analytical wavelength from 440, 480, 526, 546 and 580 nm.
  3. Make five standards of 1–5 × 10⁻⁴ M from 1.000 × 10⁻³ M KMnO₄ stock and measure them and two or three reagent blanks at the chosen wavelength.
  4. Measure the steel solution 3–5 times, read its MnO₄⁻ concentration with a confidence interval from the calibration line, and convert to the manganese mass fraction of the steel.
  5. Reveal the true value, then repeat at another wavelength (440 nm, say) and compare.

What to notice

  • KMnO₄ has two almost equal peaks at 526 and 546 nm; near 440 nm the steel solution absorbs noticeably more than the standard — that extra is Cr₂O₇²⁻.
  • At 440 nm the calibration slope is small and the steel result comes out clearly high; at 546 nm chromium hardly absorbs and the result is accurate.
  • After a change of wavelength every old reading is void: the calibration and the sample must be measured at the same wavelength under the same conditions.

In a real lab

  • The nitric acid, phosphoric acid and potassium periodate are corrosive or oxidising: wear goggles and gloves and heat in a fume hood; phosphoric acid also complexes Fe³⁺ and removes its yellow colour.
  • Rinse the cell with the solution to be measured and wipe the optical faces; zero on the reagent blank before each series.
  • Permanganate slowly fades in the presence of organic matter or reducing agents: make standards and samples fresh and measure them promptly.
  • Cr(VI) is toxic and carcinogenic: collect the waste separately for reduction and disposal.

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