Indicators and Titration Curves
Senior 6 · Chemistry
Indicators and Titration Curves
Lesson 3 of 3
25 min
Two students titrate the same 25.0 cm³ of 0.100 mol dm⁻³ ethanoic acid with 0.100 mol dm⁻³ sodium hydroxide. One adds phenolphthalein and gets a sharp pink end point at 25.00 cm³. The other adds methyl orange: the red starts turning orange almost at once, and the solution is fully yellow by about 7.6 cm³. Same acid, same alkali, same burette; so why do the answers disagree?
An indicator only works if its colour change happens on the vertical section of the titration curve. In this lesson you will use the curves from the last lesson to choose an indicator for each type of titration, and see how two indicators in one flask can analyse a mixture.
Definition
Suitable indicator: an indicator whose whole colour-change range lies inside the vertical section of the titration curve, so that one drop of titrant changes its colour completely. Example: phenolphthalein (8.3–10.0) for ethanoic acid with NaOH (vertical section about 7.2–10.3).
Indicator (end-point) error: the difference between the volume at the end point and the true equivalence volume. Example: about 17 cm³ if methyl orange is used for ethanoic acid with NaOH.
Double-indicator titration: a titration that uses two indicators, one after the other, to find two end points in the same flask. Example: phenolphthalein then methyl orange for sodium carbonate with hydrochloric acid.
Figure 3.1 lays the ranges of methyl orange and phenolphthalein over the curve for ethanoic acid with NaOH.
Figure 3.1: Only an indicator whose band is crossed by the vertical section changes colour at the equivalence point.
| Titration (0.100 mol dm⁻³) | Vertical section (pH) | pH at equivalence | Suitable | Unsuitable |
|---|---|---|---|---|
| strong acid + strong base | about 3.7–10.3 | 7.00 | methyl orange, methyl red, bromothymol blue or phenolphthalein | litmus, universal indicator |
| weak acid + strong base | about 7.2–10.3 | 8.73 | phenolphthalein, thymolphthalein | methyl orange, methyl red |
| strong acid + weak base | about 3.7–6.9 | 5.28 | methyl orange, methyl red | phenolphthalein |
| weak acid + weak base | none | 7.00 | no indicator; use a pH meter or a conductivity meter | every indicator |
A quick check: the indicator's p should be close to the pH at the equivalence point. For a strong acid with a weak base, phenolphthalein fails badly: with HCl and ammonia it starts to turn pink only after 27.8 cm³ of ammonia, and would need far more to turn fully pink, because the solution beyond equivalence is an buffer that never reaches pH 10.
The carbonate ion accepts two protons, one at a time:
So the curve has two steps (Figure 3.2). Phenolphthalein turns colourless at the first; methyl orange, added afterwards, turns from yellow to orange-red at the second. The second volume is exactly twice the first for pure sodium carbonate. In Senior 3 you titrated sodium carbonate with methyl orange alone, which detects only the second, total end point.
Figure 3.2: Sodium carbonate titrated with hydrochloric acid (calculated by Ganzaa with p values of 6.35 and 10.33 for carbonic acid).
The first step is gentle: the pink fades gradually over several cm³ before it disappears at 25.0 cm³, so take the end point when the last trace of pink goes, and compare against a flask of known colour. The second step is sharper; boiling the solution briefly near the end drives off and sharpens it further.
Worked example
Problem. Choose an indicator from the table in Lesson 1 for each titration, and justify your choice: (a) 0.100 mol dm⁻³ with 0.100 mol dm⁻³ KOH; (b) 0.100 mol dm⁻³ propanoic acid () with 0.100 mol dm⁻³ NaOH; (c) 0.100 mol dm⁻³ HCl with 0.100 mol dm⁻³ methylamine solution, whose equivalence point is at about pH 6.0.
Solution.
(a) Strong acid with strong base: the vertical section runs from about pH 3.7 to 10.3, so any of methyl orange, methyl red, bromothymol blue or phenolphthalein works. Bromothymol blue (6.0–7.6) is centred closest to the equivalence pH of 7.00.
(b) Weak acid with strong base: the salt sodium propanoate is alkaline, so the equivalence point is above 7 (about 8.8) and the vertical section starts above 7. Phenolphthalein. Methyl orange would change in the buffer region.
(c) Strong acid with weak base: the equivalence point is acidic, at about pH 6.0. Methyl red (4.4–6.2) brackets it; phenolphthalein would change far too late.
Answer: (a) any of the four, best bromothymol blue; (b) phenolphthalein; (c) methyl red.
Worked example
Problem. 25.0 cm³ of a solution containing both NaOH and is titrated with 0.100 mol dm⁻³ HCl. With phenolphthalein the pink disappears after 22.0 cm³. Methyl orange is then added and the titration continued: it turns orange after a further 8.0 cm³. Calculate the concentration of each solute in mol dm⁻³.
Solution. Up to the phenolphthalein end point, HCl has neutralised all the NaOH and converted all the carbonate to hydrogencarbonate:
The extra 8.0 cm³ converts that hydrogencarbonate to , one per original carbonate ion:
So 8.0 cm³ of the first 22.0 cm³ was used by carbonate, and 14.0 cm³ by hydroxide:
Answer: : 0.032 mol dm⁻³; NaOH: 0.0560 mol dm⁻³. This is how a laboratory checks how much sodium hydroxide in a stored solution has absorbed from the air.
Warning
Safety in the titration laboratory. Sodium hydroxide, even at 0.1 mol dm⁻³, damages the eye faster than acids of the same strength. Wear safety glasses, fill the burette below eye level using a funnel, then remove the funnel, and never pipette by mouth; use a pipette filler. If any acid or alkali splashes into an eye, rinse it immediately with gently flowing clean water for at least 15 minutes, holding the eyelids open, and get medical help; for an alkali such as sodium hydroxide, CDC advises keeping on rinsing the eye for 30 minutes. On skin, remove contaminated clothing and rinse with plenty of water.
Common mistakes. Choosing an indicator because its range includes pH 7, instead of checking the vertical section; adding the second indicator in a double-indicator titration before the first end point is reached; and forgetting that, for carbonate, the volume to the methyl orange end point counts only the second proton when it is measured from the phenolphthalein end point.
Tip
Match the indicator to the salt at equivalence. Strong + strong: neutral salt, almost any indicator. Weak acid + strong base: alkaline salt, so choose an indicator that changes in alkali (phenolphthalein). Strong acid + weak base: acidic salt, so choose one that changes in acid (methyl orange or methyl red). Weak + weak: no indicator at all.
Allery Chemistry: Titration curves and indicators. Watch how the presenter lays each indicator's range over the vertical section of the curve to decide whether it is suitable.
Practice
Further reading
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