The pH of a base in aqueous solution can be estimated based on knowledge of its dissociation constant and molarity.
NOTE: The following applies to monoprotic bases. A weak diprotic base can be treated as a monoprotic base with regards to hydroxyl ion concentration if the Kb1 value > 200 * Kb2 (page 51).
Strength of a Base
Dissociation Constant (Kb)
weak
< 1
strong
>= 1
Once the concentration of hydroxyl [OH-] is known for a base, the pH can be calculated:
pH =
= pKw - pOH
where:
• pOH = -LOG10(concentration of OH-)
• Kw = dissociation constant for water = 10^(-14)
• pKw = 14.00
[OH-] for a strong base =
= (molarity of the base in solution)
[OH-] for a weak base =
= 0.5 * (X – Kb)
X =
= SQRT((Kb^2) + (4 * Kb * (molarity of the base in solution)))
where:
• The equation for a weak base is referred to as the quadratic formula.
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