Redox titration: ethanedioic acid vs potassium manganate(VII)
A self-indicating redox titration. Heat ethanedioic acid (oxalic acid) with dilute sulfuric acid to about 70°C, then titrate against acidified potassium manganate(VII) — no indicator needed, because KMnO4 is its own: the flask stays colourless until the very last drop leaves it permanently pale pink.
WHAT YOU'LL LEARN
- ✓Why a titration can be 'self-indicating' — KMnO4's own intense purple colour disappearing on reduction to near-colourless Mn2+ does the job an indicator would otherwise do
- ✓Why the flask is heated to about 70°C first (the MnO4-/oxalate reaction starts slowly, then autocatalyses) and why boiling would ruin the result
- ✓Why dilute sulfuric acid (FA 3), not water, is added before titrating — the half-equation needs H+ directly, and without enough acid present MnO4- is reduced to brown MnO2 instead
- ✓Using a titre and a known mole ratio (MnO4- : oxalic acid = 2 : 5) to work back to an unknown concentration, then to Mr, then to water of crystallisation
ON YOUR BENCH
- □50 cm³ burette + stand and clamp
- □25 cm³ pipette + filler
- □Two 250 cm³ conical flasks
- □25 cm³ measuring cylinder
- □100 cm³ beaker
- □White tile
- □Thermometer, –10°C to 110°C
- □Tripod, gauze and Bunsen burner + heatproof mat
- □0.0200 mol/dm³ acidified potassium manganate(VII), FA 2 (in the burette)
- □Aqueous ethanedioic acid, FA 1 (the unknown you're standardising)
- □1.00 mol/dm³ sulfuric acid, FA 3
- □Distilled water wash bottle
The protocol, step by step
The same guide is printed and waiting at your bench.
- 01
Rinse and fill the burette
Rinse the burette with a little FA 2 (not water — water left inside dilutes the standard). Fill through a funnel, run a little through the tap to clear the jet of air, then read the initial level to the nearest 0.05 cm³.
- 02
Pipette FA 1 and add FA 3
Pipette exactly 25.0 cm³ of FA 1 into a conical flask. Measure out FA 3 (dilute sulfuric acid) with the measuring cylinder and add it to the flask — the reaction needs H+ directly (see the half-equation), and without enough acid present MnO4- reduces to brown MnO2 instead of colourless Mn2+.
- 03
Heat to about 70°C
Stand the flask on a tripod and gauze over a Bunsen and warm to roughly 70°C, checking with the thermometer. Warm, not boiling: the reaction between MnO4- and oxalate is slow at room temperature but speeds up once some Mn2+ has formed (it's autocatalytic) — heating gives it a head start without decomposing the acid or boiling off your measured volume.
- 04
Rough titration
Move the hot flask to the white tile under the burette. Run in FA 2 about 1 cm³ at a time, swirling constantly. The purple colour decolourises instantly at first — keep going until one drop leaves the whole flask a permanent pale pink. Record this rough titre.
- 05
Accurate titrations
Repeat with a fresh 25.0 cm³ portion of FA 1, fresh FA 3, and a fresh heating step each time. Run in FA 2 quickly to about 2 cm³ before your rough titre, then add it drop by drop, swirling after each. Near the endpoint each drop takes a moment to decolourise — stop the instant one drop leaves a permanent pale pink.
- 06
Get concordant results
Repeat until two accurate titres agree within 0.10 cm³ of each other. Average only the concordant accurate titres — the rough titre never counts towards the mean.
- 07
Calculate
2MnO4- + 5(COOH)2 + 6H+ → 2Mn2+ + 10CO2 + 8H2O. Moles MnO4- = 0.0200 × (mean titre/1000). Moles oxalic acid = that × 5/2. Concentration of FA 1 = moles ÷ 0.025 dm³. From the known mass concentration of the hydrate, Mr follows, and then x in (COOH)2·xH2O = (Mr − 90) ÷ 18.
What you should see
- →A flask that stays colourless (or very faintly, fleetingly tinted) through the whole titration, then turns and holds pale pink on a single drop. No indicator was added — none is needed.
- →Titres around 24–25 cm³ if FA 1 is close to 0.0492 mol/dm³ and FA 2 to 0.0200 mol/dm³, which is what this paper's own supervisor's results give.
- →If the flask goes straight to a deep, obviously purple colour rather than a pale pink, you've overshot — the excess FA 2 is no longer trace-level. Record it, then creep up more slowly on the next run.
- →Working back from a titre of 24.6 cm³: concentration of FA 1 = 0.0492 mol/dm³, Mr of the hydrate ≈ 126, and x = (126 − 90)/18 = 2 — the dihydrate.
| Rough | Accurate 1 | Accurate 2 | |
|---|---|---|---|
| Final reading /cm³ | 25.10 | 24.65 | 24.60 |
| Initial reading /cm³ | 0.00 | 0.05 | 0.05 |
| Titre /cm³ | 25.10 | 24.60 | 24.55 |
| Concordant? | — | ✓ | ✓ |
⚠ BEFORE YOU START
- •Wear goggles throughout — acidified KMnO4 stains skin and clothing, and dilute H2SO4 is an irritant.
- •Let the flask cool slightly before handling it directly; it's been on a Bunsen at ~70°C.
- •Never point a hot flask's neck towards yourself or a neighbour when swirling.
- •Report and mop up any KMnO4 spills immediately — it will stain benches as well as skin.
Try it, then run it for real.
Practise the whole thing on the virtual bench, then book real lab time and do it with your own hands.