Question 1 — Double-indicator titration
Titrate acidic solution B against two different sodium hydroxide solutions, catching both end-points with two indicators in the same flask.
RUN IT ON THE BENCH
Your readings feed straight into the answer table below as you record them.
ON YOUR BENCH — FROM THE CONFIDENTIAL INSTRUCTIONS
Question 1 — solutions
- solution B(80 cm³)
- solution C(100 cm³)
- solution D(100 cm³)
- methyl orange indicator
- thymolphthalein indicator
Question 1 — apparatus
- 25 cm³ measuring cylinder(1)
- 100 cm³ beaker(1)
- 250 cm³ conical flask(1)
- burette(1)
- stand and clamp for burette(1)
- small funnel to fill burette(1)
- white tile(1)
- dropping pipettes
- access to water and distilled water
THE BENCH — 25 cm³ SOLUTION B + BOTH INDICATORS · SOLUTION C IN THE BURETTE
Why are there two end-points at all?
Phosphoric acid is triprotic — H₃PO₄ has three protons, and they come off one at a time at very different pH. The first is easy, the second harder, the third so hard it never happens in water. So the acid is neutralised in two visible instalments, and one indicator can only ever catch one of them.
Why put two indicators in the same flask?
Methyl orange changes around pH 3.1–4.4, right where the first proton finishes. Thymolphthalein is colourless until about pH 9.3, which is where the second finishes. Because thymolphthalein is invisible below that, it sits in the flask doing nothing while methyl orange does its work — then takes over. Red → orange is the first end-point; yellow → green is the second.
Why does nothing seem to happen between the two?
Between the end-points you are in the H₂PO₄⁻ / HPO₄²⁻ buffer, and buffers resist pH change — that is the definition. You add cubic centimetre after cubic centimetre and the colour barely shifts. Then the buffer runs out and the second change arrives quickly. That long dull stretch is the chemistry working, not you doing it wrong.
YOUR TASKS · 0/2
- ✓Experiment 1 — record both end-points with solution C
- ✓Experiment 2 — record both end-points with solution D
TABLE 1.1 — YOUR READINGS
| EXP 1 (C) | EXP 2 (D) | |
|---|---|---|
| first end-point reading | — | — |
| second end-point reading | — | — |
| initial reading | 0.0 | 0.0 |
| volume to first | — | — |
| total volume to second | — | — |
Copy these into the question paper. Both end-point figures are burette readings — the volumes are what you get after subtracting the initial reading.
YOUR ANSWERS
Complete Table 1.1 with your burette readings for both experiments, and calculate the volume added to reach each end-point.
| Experiment 1 using solution C | Experiment 2 using solution D | |
|---|---|---|
| burette reading at first end-point /cm³ | ||
| burette reading at second end-point /cm³ | ||
| initial burette reading /cm³ | ||
| volume added from burette to reach first end-point /cm³ | ||
| total volume added from burette to reach second end-point /cm³ |
Explain why the conical flask is rinsed with distilled water at the start of Experiment 2.
Explain how the volume added from the burette to reach the first end-point would be different if the burette was not rinsed with solution C.
Explain why the conical flask is placed on a white tile during the titration.
Compare the concentration of solution C used in Experiment 1 with the concentration of solution D used in Experiment 2. Explain your answer.
Deduce the volume of solution C required to reach the first end-point if Experiment 1 is repeated using 50 cm³ of solution B instead of 25 cm³.
State why using 50 cm³ of solution B would cause a problem when finding the volume of solution C needed to reach the second end-point.
A student warms solution B in the conical flask before titrating. State the effect, if any, on the volume of solution D required to reach the second end-point, and explain.
State one change to the apparatus that will improve the accuracy of the results.