Acid-base equilibria
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I was doing an experiment to determine the ionization constant, KIn, for an indicator, bromophenol blue solution.
A set of test-tubes is prepared with pairs of tubes containing different concentrations of HIn or In, so they (each pair) differ in colour when viewed from the side.
Then, a buffer solution with known pH is added to some bromophenol blue solution, which is then compared to the pairs of test-tubes to find out the set with the colour that is the closest. The ratio of (In) to (HIn) can then be used to calculate the pKIn value.
My teacher says that the buffer solution used for the experiment should have a pKa value very close to the pKIn value of the indicator, otherwise the colour of the indicator in the buffer solution could never match that of the indicator solution prepared.
I don't understand why these two values have to be close to each other, and why can't the colours be matched?
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Ulex replies
At first sight, it might seem that to do as your teacher suggests rather defeats the object of the experiment since if you already know the answer, why do the measurement!
Actually the suggestion makes very good sense and will save you a great deal of time.
Bromophenol blue, like most acid-alkali indicators, changes colour over a range of almost 2 pH units, going from blue (above 4.6), through various shades of green, to yellow (below 2.8). To cover this pH range with a buffer solution you need one with a pKIn value somewhere between these values. If your buffer's pKIn is outside these values, then it is going to be difficult to make up buffer solutions that cover the range you want.
Remember that in a buffer solution when there are equal concentrations of the acid and base forms, the pH = pKIn. Changing the ratio of acid to base in the buffer by a factor of 10 changes the pH by 1. So if your buffer pKIn was outside your indicator's pH range, you would have difficulty making up buffer solutions that differed by more than 2 pH units from this value.
If your buffer solution corresponded to a pH which is outside the pH range of the indicator, you would get blue or yellow but no green. You could fiddle about for ages trying to find a buffer which gave you a green colour. If, however, you have some idea what the answer is, you can get a more accurate answer by using the method you describe.
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updated: 24 April 2009
