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# Is pKin the same as the equivalence point? watch

1. Trying to write up my investigation, and can't really remember the theory behind indicators.
2. (Original post by Pheonixfeather3)
Trying to write up my investigation, and can't really remember the theory behind indicators.
Indicators (represented here as HIn) are weak acids (or bases) whose acid and conjugate base have different colours.

The mid-point of the indicator is when pH = pKa (of the indicator)

This is because:

HIn <==> H+ + In-

and the mid point has equal amounts of HIn and In-

ka = [H+][In-]/[HIn]

and if

[In-] = [HIn]

Then

ka = [H+]

and

pKa = pH
3. (Original post by charco)
Indicators (represented here as HIn) are weak acids (or bases) whose acid and conjugate base have different colours.

The mid-point of the indicator is when pH = pKa (of the indicator)

This is because:

HIn <==> H+ + In-

and the mid point has equal amounts of HIn and In-

ka = [H+][In-]/[HIn]

and if

[In-] = [HIn]

Then

ka = [H+]

and

pKa = pH
But is pKin the same as the equivalence point. Like for phenolphthalein pKin=9.4, so is the equivalence point at pH=9.4?
4. (Original post by Pheonixfeather3)
But is pKin the same as the equivalence point. Like for phenolphthalein pKin=9.4, so is the equivalence point at pH=9.4?
The equivalence point of what?
5. I suspect s/he means stoichiometric equivalence.

In which case, no. Most certainly not.

pKind is the pH at which half of the indicator has changed from one form to the other. Equivalence point, if my suspicion is right, is the point at which all of the indicator has changed.

Imagine having 1 mol of a weak acid HA. Add 0.5 mol of NaOH and you'll have 0.5 mol HA and 0.5 mol A-. At that point, [HA] = [A-] and Ka = [H+] x [A-]/[HA], i.e. Ka = [H+] and pKa = pH. This is the pH at which half of the HA has converted to A-.

Now re-read the entire last paragraph substituting every reference for A for ind.

Equivalence point is when 1 mol of NaOH was added to the 1 mol of HA and 0 mol of HA remains and 1 mol of A- has formed. This is (loosely) related to the end point of a titration.
6. (Original post by Pheonixfeather3)
But is pKin the same as the equivalence point. Like for phenolphthalein pKin=9.4, so is the equivalence point at pH=9.4?
The true equivalence point of any titration has the pH equal to that of the salt solution. This may be hydrolysed and be basic, neutral or acidic, depending on the reactants.

The indicator is chosen so that the large inflexion that occurs near the equivalence point causes the pH to pass through the pKa value of the indicator.

Most inflexions occur over a 3-4 pH range, so provided that this coincides with the pKa of the indicator then it is effective.

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