Calculating Ka for Acid-Base Indicators

[magma_saber]

Homework Statement

Estimate the Ka the using the assumption that the color change can be seen by our eyes when [ln-]/[Hln] = 1/10
to see the acid form or [ln-]/[Hln] = 10/1 to see the color associated with the base.
Use these numbers with your measured [H+] to calculate a Ka for each indicator you measured.

Homework Equations

acid-base base-acid H+
Phenolphthalein 8.7 7.4 8.91E-09

I got the hydronium concentration by taking taking the average of the ph values and doing 10^-ph. How do i get the ka estimates?

Do i set up the equation like this: ka = 1/[H+] or something like this?

 

[Borek]

Describe the experiment that you did. I am almost sure you have not posted all information that you have.

 

[magma_saber]

 

[Borek]

Something is wrong with these results.

$$pH = pK_a + \log \frac {[In^-]} {[HIn]}$$

This is http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch]Henderson-Hasselblach[/PLAIN] equation[/ulr]. Basically it is used for buffer calculations, but it will be usefull here. You are to assume that [In^-]/[HIn] ratio is 0.1 in one case and 10 in the other. That in turn means that pH values at which you observe both color changes should be 2 pH units apart (log(0.1) = -1 and log(10) = 1). They are not in neither case. Sure, there is always some experimental error, but the differences in your case are 1.3 and 5.1, completely off I am afraid.

Basic idea in this kind of experiment is that pKa value lies somewhere between measured color change pH values, most likely at the middle. But with these numbers it won't work.

 

[magma_saber]

ya i know that the phs are wrong. The bromophenol blue was really off. When calculating the H+ concentration, i only used 4.2, not 9.3.