Acid/Base Indicators

1. Acid/Base Indicators

2. What is an Indicator? • They are themselves acids and bases. • During a titration, the acid/base indicator is also being titrated. • It changes from the protonated to the deprotonated form during titration with a base and vice versa during titration with an acid. • As the indicator is also being titrated, you must only use a few drops, or too much titrant will get consumed by the indicator.

3. What is an Indicator? • An acid/base indicator turns different colors at different pH. • Since they have different colors depending on pH, they are very useful as visual endpoints in acid/base titrations.

4. Thymol Blue: pKIn1 = 1.7; pKIn2 = 8.9

5. What is an Indicator? • The protonated and deprotonated form of the indicators absorb light of different wavelengths. • As the pH changes, the ratio of protonated to deprotonated forms changes, so the color of the solution changes. • We call the protonated indicator HIn, and the deprotonated indicator In or In-.

6. What is an Indicator? • Since the indicator is itself an acid or base, it has a pKa value. • This is typically called the pKHIn • The color change will occur within ±1 pH unit of the pKHIn • If the pH = pKHIn , then there is a 50:50 ratio of HIn to In-. • The H-H equation becomes:

7. What is an Indicator? • HIn has one color, In has a different color, and there is a transition region between the two colors. • So when you get within ±1 pH unit of the pKHIn, you will start to see the color change gradually between the acidic and basic forms.

8. Choosing and Using Indicators (Ch 10.4) • An acid base indicator changes colors within ±1 pH unit of the pKHIn. • If the pH < pKHIn -1, then the indicator is protonated with the acidic color. • If the pH > pKHIn -1, then the indicator is deprotonated with the basic color. • If the pH is in between these, there is a mixture of both and there is a mixture of colors as well.

9. Choosing and Using Indicators (Ch 10.4) • To pick an appropriate indicator, you need to have a good idea of what pH the titration equivalence point will be. • For example, a strong acid-strong base titration will have a pH of 7.00 at the equivalence point. • So knowing this, how do you choose an indicator?

10. Choosing and Using Indicators (Ch 10.4) • You choose an indicator whose pKHIn is close to the equivalence point pH. • This way the visual endpoint the indicator gives will be very close to the actual equivalence point. • The error is the indicator error, and it can be positive or negative. • If you choose well, the error will be much less than 1%.

11. Choosing and Using Indicators (Ch 10.4) • If you choose well, the error will be much less than 1%. • This is because the pH of a titration changes dramatically near the equivalence point. • A very small addition of the titrant causes a large change in the pH. • So if the indicator changes colors near this drastic pH change, the indicator error will be small.

12. Choosing and Using Indicators (Ch 10.4) • So looking at a chart of indicators, choose one whose pKHIn or whose transition range overlaps with the suspected or known equivalence point pH. • Looking at the chart, what would be good indicators to use for the titration of HCl with NaOH, and what color change would indicate the endpoint?