Acid-Base Indicators

What are indicators? Contains a weak acid and its conjugate base

Weak acid—one color Conjugate base—different color

Changes color based on what solution (acidic or basic) it is placed in.

Color within a solution is dependent on the amount of acid and conjugate base----SO pH dependent.

Used when specific pH measurements not needed

How do indicators work?

HIn + H2O H3O+ + In-

(yellow) (red)

Acidic Solution — Yellow color is seen, reaction shifts to the LEFT

Basic Solution — Red color is seen, reaction shifts to the RIGHT

Neutral Solution Same concentration of indicator acid/anion Color mixture between the two colors

Common Indicators

Phenolphthalein

Litmus indicator Broad pH range Litmus—

Dye originating from lichens pH < 4.5 (red), pH > 8.3 (blue)

Indicators and Titrations

pH rapidly shifts past the equivalence point

Shift affects indicator equilibrium—creates a color change

Different indicators have different pH ranges where equilibrium changes (color change) Select indicator changing color close to the pH

at the equivalence point.

Indicator pH range

Methyl Orange 3.2—4.4

Methyl Red 4.8—6.0

Bromthylmol Blue 6.0—8.0

Phenolphthalein 8.2—10.00