Chemistry · Rung 20 of 24

Chromatography and titration

Two techniques that answer a laboratory's two questions: what is there and how much. Chromatography answers the first. The solvent climbs the paper dragging the pigments and each is torn between sticking to the paper and going with the solvent; the one that clings least climbs highest. It is not about weight or colour, it is that tug of war, and it is the same principle as the chromatographs in a modern lab. Titration answers the second: base is added drop by drop until the acid is neutralised, the volume used is measured, and from the balanced equation you know how much acid there was. The key is the equivalence point, in the middle of the curve's jump: before it there is acid left over, after it base left over, and right there they have consumed each other exactly. The pH changes several units with a single drop, which is why a burette measures to laboratory precision. The indicator is chosen so that its colour change falls inside that jump.

Where this sits

Comes after
Separating mixtures, Acids and bases

Exercises from this rung

Generated by the app, from easiest to hardest

  1. At what pH does bromothymol blue change colour?

    • 6–7.6
    • 8.2–10
    • 3.1–4.4
    • 4.4–6.2

    Why Between 6 and 7.6, going from yellow to blue. Each indicator is a weak acid whose protonated and deprotonated forms have different colours, so the range where it turns comes from its own pKa. That is why not just any will do: you have to pick the one that changes inside that titration's jump.

    easier

  2. You titrate vinegar, a weak acid, with caustic soda. Which indicator do you choose?

    • methyl orange
    • bromothymol blue
    • methyl red
    • phenolphthalein

    Why Phenolphthalein, which turns between 8.2 and 10. A weak acid titrated with a strong base leaves a basic salt, so its equivalence point sits above 7, and an indicator turning at 4 would call the end far too early. It is the commonest lab mistake in this whole topic.

    in between

  3. You titrate hydrochloric acid with caustic soda. Which indicator do you choose?

    • bromothymol blue
    • phenolphthalein
    • methyl orange
    • methyl red

    Why Bromothymol blue, which turns between 6 and 7.6, right around the 7 where a strong acid and a strong base meet. Here the jump is so large and so vertical that almost any indicator would do, but the one that lands in the middle leaves the least error.

    harder