Intermediate Topic 542 of 1102 Acids, Bases & Electrochemistry
Henderson-Hasselbalch Equation
pH equals pKa plus log of base over acid concentration ratio.
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In-depth explanation of Henderson-Hasselbalch Equation
The Henderson-Hasselbalch equation is pH = pKa + log([A-]/[HA]), derived by rearranging the expression for the acid dissociation constant. It gives the pH of a buffer directly from the pKa and the ratio of conjugate base to acid concentrations. When the two concentrations are equal the logarithm is zero and the pH equals the pKa. The equation is an approximation that assumes the buffer components are not significantly changed by ionization, which holds for moderate concentrations. It is widely used to design buffers and to calculate the pH after acid or base has been added.
Going deeper: Henderson-Hasselbalch Equation
Key ideas: pH = pKa + log([A-]/[HA]). Because both species share one volume, mole ratios work as well as concentrations. To make a buffer at target pH, choose an acid with pKa within 1 unit and set the ratio to 10^(pH - pKa). For base buffers use pOH = pKb + log([BH+]/[B]).
Common mistakes and connections: Common errors: inverting the ratio, using it for very dilute or very strong acid solutions, and using it when either component is under about 5 percent of the other. After adding strong acid or base, first do the stoichiometry in moles, then apply the equation. It is just the Ka expression rearranged.
Visualization of Henderson-Hasselbalch Equation
Henderson-Hasselbalch: pH = pKa + log(base/acid). With the ammonium/ammonia pair, equal amounts give pH 9.25 and each tenfold ratio shifts pH by one unit. The interactive lesson shows this as a live animation or chart.
Worked example and use case
A solution containing 0.20 M acetate and 0.10 M acetic acid has pH = 4.76 + log 2, which is about 5.06.
Frequently asked questions about Henderson-Hasselbalch Equation
What is Henderson-Hasselbalch Equation?
pH equals pKa plus log of base over acid concentration ratio.
Is Henderson-Hasselbalch Equation a beginner, intermediate or advanced topic?
Henderson-Hasselbalch Equation is a intermediate topic in Acids, Bases & Electrochemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Henderson-Hasselbalch Equation?
Study these earlier topics in Acids, Bases & Electrochemistry first: Universal Indicator, Choosing Indicators, Buffer Solutions.
What comes after Henderson-Hasselbalch Equation?
The next topics in Acids, Bases & Electrochemistry are Buffer Capacity and Blood Buffer System.
Can I practise Henderson-Hasselbalch Equation with quizzes?
Yes. The interactive lesson includes a self-check question on Henderson-Hasselbalch Equation, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Where Henderson-Hasselbalch Equation fits in Acids, Bases & Electrochemistry
Henderson-Hasselbalch Equation is topic 32 of 105 in Acids, Bases & Electrochemistry.