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Iron in a tablet by redox titration

Dissolve an iron(II) sulfate tablet in dilute sulfuric acid and titrate it with potassium dichromate (diphenylamine sulfonate indicator) or permanganate (its own indicator) to find its iron in milligrams. A platinum electrode shows the solution potential throughout: compare how early and how sharply the violet endpoint appears with and without phosphoric acid.

Difficulty ●●○ ≈ 20 min

Aim Determine the iron in one tablet (mg), and use the potential recorded by a platinum electrode to explain what phosphoric acid does.
1 M H₂SO₄ · Pt electrode versus SHE · label: 65 mg of iron

Iron in a tablet by redox titration

Flask: one tablet dissolved in 50 mL of 1 M H₂SO₄, plus 5 mL of 85 % H₃PO₄ and 5 drops of 0.2 % diphenylamine sulfonate.

CrX2OX7X2−+6 FeX2++14 HX+⟶2 CrX3++6 FeX3++7 HX2O\ce{Cr2O7^2-} + \ce{6Fe^2+} + \ce{14H+} \longrightarrow \ce{2Cr^3+} + \ce{6Fe^3+} + \ce{7H2O}
05101520253035404550STIR50100150200
Burette reading 0.00mL
Pt electrode E (vs SHE) —V
Colouralmost colourless
Indicator oxidised (violet) 0 %

The solution turns green as Cr³⁺ forms; the endpoint is the first violet that survives swirling. Then try it without phosphoric acid.

Potential recorded by the Pt electrode as you titrate, with the indicator range

Indicator range 051015202530Titrant volume / mL0.40.60.811.21.41.6E / V (vs SHE)
Computed: after every drop the single solution potential follows from the electron balance and the Nernst equations (the same kernel as the redox titration curve widget), with E°′(Fe³⁺/Fe²⁺) = 0.68 V in 1 M H₂SO₄ and about 0.61 V with phosphoric acid. Diphenylamine sulfonate has E° = 0.85 V and n = 2; the Nernst equation gives the fraction oxidised. The flask colour comes from the bands of pale-yellow iron(III) sulfato complexes (colourless phosphate complexes with H₃PO₄), Cr₂O₇²⁻, green Cr³⁺ (a teaching fit), MnO₄⁻ and the violet indicator via Beer–Lambert and CIE colorimetry. The tablet holds 60–70 mg of iron (label: 65 mg), set by its number, all as Fe²⁺. Not modelled: titrant used by the indicator itself, tablet binders oxidised by KMnO₄, and air oxidation; the brief colour where a drop lands is illustrative.

Procedure

  1. Choose a tablet number. The flask holds one tablet dissolved in 50 mL of 1 M H₂SO₄, with 5 mL of 85 % H₃PO₄ and 5 drops of 0.2 % diphenylamine sulfonate; the burette holds 0.008000 M K₂Cr₂O₇.
  2. Make a rough run in 1.00 mL steps: watch the solution turn green as Cr³⁺ forms, and note roughly where violet appears. Start again and add single drops near the end.
  3. The endpoint is the first violet that survives swirling. Record the volume, compute m(Fe) = 6 × c × V × 55.845 g/mol, then reveal the true value.
  4. Titrate again without phosphoric acid: note the colour before the violet appears, where the violet appears, and where the electrode trace meets the indicator band.
  5. Titrate once with 0.01000 M KMnO₄ and no indicator; m(Fe) = 5 × c × V × 55.845 g/mol.

What to notice

  • Before equivalence the iron couple sets the potential: E = E°′ + 0.05916 lg([Fe³⁺]/[Fe²⁺]). Half-way, E = E°′ — 0.68 V in 1 M H₂SO₄, about 0.61 V with phosphoric acid.
  • With phosphoric acid the violet appears within a drop of equivalence; without it the indicator starts to change about 0.10 mL (two drops, −0.42 %) early, the colour turns greyish first, and the endpoint drags and comes early.
  • Permanganate needs no indicator: one drop past equivalence (about +0.2 % here) leaves enough MnO₄⁻ to turn the flask faint pink.

In a real lab

  • K₂Cr₂O₇ is a primary standard: dry it at 150 °C and weigh it directly (2.3534 g per litre for 0.008000 M); KMnO₄ is not, and is standardised against sodium oxalate at 55–60 °C.
  • Chromium(VI) is toxic and carcinogenic: wear gloves, and collect chromium waste for reduction to Cr(III) rather than pouring it down the drain.
  • KMnO₄ also oxidises chloride and the organic excipients of a tablet (binders, sugar coating), reading high; dichromate does not oxidise chloride in dilute HCl and reacts far more slowly with organic matter at room temperature.
  • Fe²⁺ is slowly oxidised by air: dissolve the tablet straight into the acid and titrate promptly. Any Fe³⁺ already present is not titrated — for total iron, reduce it first with SnCl₂.
  • The indicator is itself oxidised (two molecules couple before the violet forms), so it uses a little titrant; careful work subtracts an indicator blank.

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