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