17.3.6 Acid-Base Indicators

Question 1

Acid-Base Indicators

Answer 1

• A pH indicator is a solution that changes color at a specific pH.
• A pH indicator can be used to determine the equivalence point of a titration.
• Universal indicator is a mix of several dyes that show color changes throughout the pH scale.

Question 2

note

Answer 2

• A pH indicator is a solution that changes color at a specific pH. An indicator behaves chemically as any other weak acid or weak base.
• A pH indicator has the unique property that its acid is a different color than its conjugate base. A color change indicates that a protonation or deprotonation has taken place.
• On a molecular level, a color change indicates a change in molecular structure causing a difference in the way the molecule absorbs or reflects light in the visible range.
• When the indicator molecule has gone from an acidic form to a basic form or vice versa, a color change is seen.
• A pH indicator can be used to determine the equivalence point of a titration. A wide variety of pH indicators are available for different pH ranges. An indicator must be chosen that undergoes its color change in the pH range of interest.
• Universal indicator is a mixture of several dyes that show color changes throughout the pH scale.

Question 3

If we rearrange the Ka expression for the reaction below we get [H3O+ ] / (Ka ) = [HIn] / [ In− ].

Which statement about this rearranged Ka expression is not correct?

Answer 3

If [H3O+ ] > Ka, you have an indicator in the deprotonated form. If [H3O+ ] < Ka, you have an indicator in the protonated form.

Question 4

Consider the following picture which shows the color transition for phenolphthalein. Based on what you see, would the following statement be true or false?

“Phenolphthalein would be an ideal acid-base indicator for use by a person suffering from ‘blue-green’ color blindness.”

Answer 4

true

Question 5

Which statement about the natural indicators is not correct?

Answer 5

Phenolphthalein is a colorless indicator in its basic form and has a reddish, fuchsia color in its acidic form.

Question 6

Which statement about an indicator is not correct?

Answer 6

An indicator can be a weak acid that undergoes a color change through oxidation.

Question 7

Which statement about an indicator is not correct?

Answer 7

The color change is due primarily to how the altered molecule emits light.

Question 8

Which statement about an indicator is not correct?

Answer 8

Any indicator is significantly different from a weak acid or base.

Question 9

Look at the strong acid-strong base titration reaction.

Which statement about this event is not correct?

Answer 9

Phenolphthalein indicator would change colors during the buffer region of this titration.

Question 10

Look at the Ka expression for the reaction of an indicator in aqueous solution.

Ka = [ In− ] [H3O+ ] / [HIn]

Which statement about this expression and the reaction is not correct?

Answer 10

If [H3O+ ] = Ka, there is a color change for the indicator and the pH > pKa.

Question 11

Which of the following best defines a pH indicator?

Answer 11

a substance whose acid is a different color than its conjugate base

Question 12

Look at the indicator dissociation reaction.

Which statement about this reaction is not correct?

Answer 12

In− is a product of the protonation of HIn.