Using Bromcresol green as an indicator, should one stop when the blue color changes to green or to yellow for more accurate results.

Shouldn't one aim to get a green because its far closer to the end point than a yellow could be. For example, if the solution turned yellow and an additional drop was added, it would be indiscernible.


Neither of your options is correct.

Ideally an acid base indicator would be colorless at one pH and colored after the transition. Such an indicator is phenolphthalein. So when using phenolphthalein it is indeed impossible to detect when all the phenolphthalein is colored. So you titrate to a point where there is the faintest pink color.

So for bromcresol green you should either stop when the solution starts to turn from blue to blue green (ph about 4.6), or as the solution turns from green-yellow to yellow (ph about 5). It would very difficult to judge the deepest green (at about 4.8) accurately.

So which transition you would choose would depend on balancing between which side of pH 4.8 that you wanted to end on, and which transition was easier to detect visually.

You can look at a list of some acid/base indicators at Wikipedia. There are few that go between colorless to colored. Most are color-1 to color-2.

I also point out that you question contains a fallacy.

Should titration stop at the intermediate color or a drop past the intermediate

The burette column is marked "to contain" an amount of fluid along its length. So you can let a partial drop form, then touch the tip of the burette to the flask and get a partial drop of titrant. You don't have to titrate in "quantized" drops.

  • 1
    $\begingroup$ ‘Ideally an […] indicator would be colorless at one pH and colored after the transition.’ [citation needed] $\endgroup$ – Jan Nov 11 '15 at 20:59
  • $\begingroup$ @Jan - Sorry I don't have any analytical chemistry books handy. I'll point out though that a titration is highly technique oriented. You need to watch how the titrant is dispersing in the solution at first as you're swirling the flask. The closer to the end point you watch how a single drop diffuses into the solution without swirling, then swirl to equilibrate. So end-point detection isn't just comparing two colors. $\endgroup$ – MaxW Nov 11 '15 at 21:23
  • 1
    $\begingroup$ I'm just questioning why colourless → colour be better than e.g. yellow to dark blue ;) $\endgroup$ – Jan Nov 11 '15 at 21:27
  • 1
    $\begingroup$ @MaxW Burettes are calibrated ‘to deliver’ (TD, Ex), not ‘to contain’ (TC, In). $\endgroup$ – Loong Nov 11 '15 at 22:12
  • 1
    $\begingroup$ @Long - I disagree. The "amount" of solution that a burette delvers is the difference between two readings of the column which has nothing to do with the tip. So there shouldn't be any part of a drop of titrant hanging on the burette tip when you start the titration nor when you finish. // With a pipette you clear hanging fluid from the tip when full, and at the end, but whatever "hangs" inside the pipette is supposed to stay there. // I reworded to make the point more clearly. $\endgroup$ – MaxW Nov 11 '15 at 22:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.