Hello. I have a question that came up when I was reviewing indicators.
If you have a small amount of [HIn] indicator (weakly acidic) in an acidic buffered solution whose range covers the pH at the half equivalence point, when would it change color?
According to HendersonHasslbach equation, it seems like you could keep adding base without any change but logically the pH would increase after X amount of base added.
Can anyone clarify this concept? I appologize for any confusion in the question... I am basically asking for any insight into using indicators to find the half equivalence point.
Thanks in advance!
ok, well, you need to remember what an indicator is. it's a molecule that, in its protonated form, is a different color from its deprotonated form. when you use it in a titration to find the equivalence point (or, in reality, the end point) of a weak acid or base, here's what happens:
(let's use a weak acid as an example)
1. starts off at low pH. this pH is below the pKa of the indicator so the indicator is protonated and is a certain color (whatever color the protonated form is). so now you start titrating with a base.
2. since it's a weak acid the solution is buffered so as you add base, pH doesn't change much until all of the weak acid has been deprotonated.
3. once it's all deprotonated, the solution is no longer buffered, so adding another drop of base makes the pH go up a LOT (like several units in just one drop, depending on the concentration of the base you're using). this is the end point
4. thus, at the end point, the pH is now very high. assuming you chose the correct indicator (basically the pKa has to be between the starting pH and the ending pH, and not too close to the starting pH), the pH is now above the pKa of the indicator, so now the indicator exists primarily in its deprotonated form, so your solution will be whatever color the deprotonated form of the indicator is.
so now, can you use this to find the half equivalence point?
well, no, you can't. why? because remember, the pH doesn't change much between the start of titration and the equivalence point. in order for the indicator to show a significant color change at the half equivalence point, the pKa of the indicator pretty much has to be exactly the same as the pKa of the weak acid or base you're titrating. if it's not exactly the same, then the color change will occur either before or after the half equivalence point.
remember, you're changing the pH slowly, and the pH at which the indicator changes color depends on its OWN pKa, not the pKa of the analyte, so it's only useful when the pH changes so much that you're sure to have crossed the pKa of the indicator. big pH changes only happen at equivalence points. if you need to know small changes in pH, you use a pH electrode which is much more sensitive and has a much broader range.
does that make sense?
