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# A2 AQA Acid, Bases Buffers Coloured Indicator help watch

1. I've been practicing mostly the calculations and the theory needed for it, and I know how to approach those sort of questions but when it comes to "Which indicator to use" or "Give the colour change at the end point of a titration when So and So is used as an indicator"

Can anyone point in to the right way of approaching this sort of question. I usually take a shot in the dark which that part of the question comes up.
2. You mean when they give you a list of indicators to choosr for a curve?
I've been practicing mostly the calculations and the theory needed for it, and I know how to approach those sort of questions but when it comes to "Which indicator to use" or "Give the colour change at the end point of a titration when So and So is used as an indicator"

Can anyone point in to the right way of approaching this sort of question. I usually take a shot in the dark which that part of the question comes up.
Indicator theory tells you that ka = [H+][In-]/[HIn]

So the midpoint of this indicator in a titration is when [In-]=[HIn]

... and therefore ka = [H+]

... and so pH = pKa
4. (Original post by charco)
Indicator theory tells you that ka = [H+][In-]/[HIn]

So the midpoint of this indicator in a titration is when [In-]=[HIn]

... and therefore ka = [H+]

... and so pH = pKa
I'm not sure what [ln-] is. Is it just the concentration of base
5. (Original post by Manexopi)
You mean when they give you a list of indicators to choosr for a curve?
Yeah, those sort of questions I take a guess at since there usually at the last part.
Yeah, those sort of questions I take a guess at since there usually at the last part.
Oh right. What you do is look at the end point for each indicator and compare it to the equivalence point/end point on the titration curve you are being asked for (the part of the curve that is vertical) If the two ranges are similar/same, then that is the idicator you use.

I'm not sure if I explained it well enough.
I'm not sure what [ln-] is. Is it just the concentration of base
Indicators are weak acids whose molecular form (HIn) and conjugate base form (In-) have different colours

They establish an equilibrium like all weak acids:

HIn <==> H+ + In-

The position of equilibrium depends on the pH of the solution.

When the concentration of hydrogen ions equals the concentration of the conjugate base both colours are seen- it is the mid-point.

At this point pH = pKa (of the indicator)

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Updated: February 26, 2016
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