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Titration curves and acid-base indicators

Choosing the best indicator for different titrations depending on the pH at the equivalence point.  Created by Jay.

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• What about for a WA/WB titration? What would the curve look like and what pH indicators could you use?
• The pH curve for a WA/WB reaction would be such that an equivalence point is hard to determine, i.e. the line isn't really straight in the middle like the other acid-base combinations
• Could you please explain what pKa is? Just a little background would be awesome. Overall great set of videos and super helpful and clear!
• Ka is the acid dissociation constant, which is primarily relevant to weak acids, because they only partially dissociate to release H+.
If HA (aq) represents a weak acid, then in aqueous solution it exists in an equilibrium of,
HA (aq) ⇌ A- (aq) + H+ (aq)
This equilibrium can be expressed in terms of Ka, because HA (aq) is a weak acid.
Ka = [A- (aq)] * [H+ (aq)]
[HA (aq)]
At a given temperature, Ka is a constant.
When you've calculated Ka from the formula and equilibrial concentrations of the reaction species, you can calculate pKa, because Ka and pKa are related in the same way as [H+ (aq)] and pH,
pKa = - log( Ka )
As Ka increases, pKa decreases, because they're inversely proportional.
• If i don' know where the equivalence point is, how will indicator help me then?
• You can make a good guess.
For a strong acid-strong base titration, the equivalence point is at pH 7. The pH range of phenolphthalein is about 8.3 to 10.0, but the titration curve is so steep at the equivalence point that phenolphthalein makes a good indicator.
For a strong base-weak acid titration, the equivalence point is probably near pH 9. Phenolphthalein is great for this titration.
For a strong acid-weak base titration, the equivalence point is probably near pH 5. Here you could use an indicator like methyl red (pH 4.4 to 6.2).
• For a SA/SB titration, I see why all three indicators can be used, but would there be any advantage in using Bromothymol Blue over either phenophtalein or methyl red? Would it be much more accurate than the other two indicators at determining when the equivalence point is reached?
• In theory, yes. In practice, no.
Usually, one drop of tiitrant past the equivalence point changes the pH so much that all three indicators will change colour.
• Does the colour goes away if past the range of the indicator? For example, if I used Bromthymol blue as an indicator, will the color disappear once the pH goes past 7.6?
• Most indicators I've worked with don't change color significantly after you go out of their useful range.

This appears to be also true for Bromothymol Blue, which google image searching suggests just becomes a darker shade of blue as the solution gets more basic at least as far as pH 14 -- e.g. http://kenpitts.net/bio/energy/bromthymol_small.jpg ...
• Why doesn't a titration of HCL against Na2CO3 go to completion when phenolphthalein is used as indicator
• When you titrate HCl against Na2CO3, the reaction proceeds in two steps. The first step involves the production of NaHCO3 and this step has an equivalence point of about pH 8.3, in the range covered by phenolphthalein. Hence, the phenolphthalein changes colour at this first equivalence point.

As more acid is dripped in, the NaHCO3 reacts to liberate CO2. The equivalence point for this second step is pH 3.7. However, the phenolphthalein has already changed colour so you will not detect this second equivalence point when using phenolphthalein.

The way around this is to use a different indicator, such as methyl orange, that doesn't change colour until the second equivalence point is reached, at which point the reaction has gone to completion.
• At he says : " Because you have this really steep titration curve you could have used any of the three acid-base indicators to find the equivalence point for your titration. "

At the equivalence point pH = 7. If we used Methyl red the indicator wouldn't have changed color since its range is 4.4-6.2, no?