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The buffer equation, or Henderson-Hasselbalch equation, calculates the pH of a buffer solution based on the concentrations of its components.
For Acidic Buffers
Buffer Equation
where:
[pKa] is the negative logarithm of the acid dissociation constant.
[A-] is the concentration of the conjugate base.
[HA] is the concentration of the weak acid.
Derivation
Starting from the acid dissociation expression:
Rearranged to solve for [H+][\text{H}^+][H+]:
Taking negative logarithms:
Simplifies to the Henderson-Hasselbalch equation.
For Basic Buffers
Buffer Equation
where:
[pkb] is the negative logarithm of the base dissociation constant.
[BH+] is the concentration of the conjugate acid.
[B] is the concentration of the weak base.
Applications
Designing Buffers: Calculate the required proportions of acid and conjugate base to achieve a desired pH.
Predicting pH Changes: Assess how changes in component concentrations affect pH.
Limitations
Assumptions: The equation assumes that activities equal concentrations, which is valid at low ionic strengths.
Temperature Dependence: pKa\text{p}K_apKa values change with temperature; calculations should account for this.
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