Acids and Bases (2 of 2)

Part 1 of Acids and Bases is located here.

Dissociation Constant

At equilibrium, the rate of dissociation of the product AB is equal to the rate of associate of the reactant A and B

∴ at equilibrium:

Kd = [A] [B] / [AB]

Which is derived from an example such as: AB ↔ A + B @ Equilibrium

pH Indicators

pH indicators work because of the dissociation of the H⁺ ions in the indicator itself.

Common Indicators	Range
Phenolphthalein	8 – 10
Methyl Red	4.5 – 6
Bromothymol	6 – 7.6

Buffer Solutions

Buffer solutions resist changes in pH.

The buffer solution absorbs the proton

Example:

HA + H₂O ↔ H₃O⁺ + A^–

Ka = [H₃O⁺] [A^–] / [HA]

-log(Ka) = -log([H₃O⁺] ([A^–] / [HA]))

-log(Ka) = -log([H₃O⁺]) – log([A^–] / [HA])

pKa = pH – log([A^–] / [HA])

pH = pKa + log([A^–] / [HA])

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