15.5 Acid-base indicator

Question 1

What is an acid-base indicator and what is his symbol?

Answer 1

It’s a weak acid (HIn)

Question 2

How does it work?

Answer 2

When the proton (H) is attached to the molecule (weak acid - HIn) they show one color. When the proton is absent (conjugate base - In) they show another color. This is how we can tell wether is it acid or basic.

Question 3

Write the equation of an indicator. And its Ka.

Answer 3

HIn (aq) -> H+ (aq) + In-(aq)
Ka= ([H+] [In-]) / [HIn]
Ka/[H+] = [In-]) / [HIn]

Question 4

Suppose we add a few drops of phenolphthalein Ka= 1.010-8 to an acidic solution, whose pH is 1.0 = 1.010-1 . The indicator turns from transparent to pink when the proton is absent. What color will it be? O

Answer 4

The solution will stay transparent

Question 5

At what ratio of [In-] / [HIn] can we see color change?

Answer 5

1/10

Question 6

Bromothymol blue an indicator with ka= 1.0*10-7 is yellow in its [HIn] form and blue in its [In-] form. Suppose we put a few drops of this indicator in a strongly acidic soln. If the soln is then titrated with NaOH , at what pH will the indicator color change first be visible?

Answer 6

At pH of 6

Question 7

How do we calculate the [H+] from pH?

What is the [H+] of pH=4.3

Answer 7

[H+]= 10^pH
[H+] = 10^4.3 = 2*10^4

Question 8

What is the pKa of the indicator used for a titration where the equivalence pt is at pH=7.00 (we want the solution to change color at the equivalence point obviously)

Answer 8

Initially, [HIn] is present and [In-] /[HIn]= 1/10 → pH=pKa+ log (1/10)
pKa=7+1=8, thus the indicator should have pKa=8.

Question 9

Write the equation that allow us to chose an indicator that starts to change color at equivalence point.

Answer 9

pH = pka + log (1/10)

pH at eq point