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.
Reading it isn't running it.
Collect the apparatus, build the setup and take your own readings — as many times as it takes.