Double-indicator titration: phosphoric acid against sodium hydroxide
Phosphoric acid holds three protons and gives up the first two at clearly separate points. Put two indicators in one flask and catch both end-points in a single run — red to orange, then yellow to green.
WHAT YOU'LL LEARN
- ✓Why a polyprotic acid gives more than one end-point, and roughly where each one falls
- ✓Running methyl orange and thymolphthalein together without either masking the other
- ✓Reading the second end-point as a total from the initial reading, not from the first end-point
- ✓Comparing two unknown concentrations purely from the volumes they consume
ON YOUR BENCH
- □50 cm³ burette + stand and clamp
- □Small funnel to fill the burette
- □25 cm³ measuring cylinder
- □250 cm³ conical flask
- □100 cm³ beaker
- □White tile
- □Solution B — dilute phosphoric acid, 0.10 mol/dm³
- □Solution C — aqueous sodium hydroxide, 0.15 mol/dm³
- □Solution D — aqueous sodium hydroxide, 0.30 mol/dm³
- □Methyl orange and thymolphthalein indicators
- □Dropping pipettes, distilled water
The protocol, step by step
The same guide is printed and waiting at your bench.
- 01
Rinse and fill the burette
Rinse with distilled water and then with solution C. Fill, then run some out so the level sits between the 0.0 and 1.0 cm³ marks — you want the whole scale available, because the second end-point is roughly double the first.
- 02
Measure out the acid
Use the 25 cm³ measuring cylinder to pour 25 cm³ of solution B into the conical flask. A measuring cylinder is what the paper supplies — noting that a pipette would be more accurate is itself a mark.
- 03
Add both indicators
Five drops of methyl orange and five drops of thymolphthalein into the same flask. Methyl orange is red in acid; thymolphthalein is colourless until well into alkali, so it sits out of the way until you need it.
- 04
Titrate to the first end-point
Stand the flask on the white tile and run in solution C, swirling, until the colour changes from red to orange. That is the first end-point — the first proton has gone. Record the reading.
- 05
Carry on to the second end-point
Keep adding. Orange fades to yellow as methyl orange finishes its change, and nothing much seems to happen for a while. Then thymolphthalein starts to turn and yellow shifts to green — the second end-point. Record the reading.
- 06
Repeat with solution D
Empty the flask, rinse it with distilled water, and run the whole thing again with solution D in the burette. D is more concentrated, so expect to use about half as much.
- 07
Work out the volumes
Both recorded readings are burette readings, so subtract the initial reading from each. The second end-point volume is a total from the start, not a step from the first end-point.
What you should see
- →With solution C: first end-point near 16.7 cm³, second near 33.3 cm³ — almost exactly double, because the second proton takes the same amount of base as the first.
- →With solution D: both volumes roughly halve, to about 8.3 and 16.7 cm³, because D is twice the concentration of C.
- →The stretch between orange and yellow is long and dull. That is the buffer region between the two equivalence points, and it is supposed to feel like nothing is happening.
- →Doubling the acid to 50 cm³ would put the second end-point near 67 cm³ — more than a 50 cm³ burette holds, which is exactly why the question asks about it.
| Experiment 1 (C) | Experiment 2 (D) | |
|---|---|---|
| Initial reading /cm³ | 0.4 | 0.2 |
| First end-point reading /cm³ | 17.1 | 8.5 |
| Second end-point reading /cm³ | 33.7 | 16.9 |
| Volume to first end-point /cm³ | 16.7 | 8.3 |
| Total volume to second end-point /cm³ | 33.3 | 16.7 |
⚠ BEFORE YOU START
- •Both sodium hydroxide solutions are irritants — goggles on, and rinse splashes immediately.
- •Methyl orange is toxic and flammable; keep it away from flames and use the dropper provided.
- •Fill the burette below eye level, never above your head.
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.