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Buffer & Acid-Base Equilibrium Lab

Select a buffer system, adjust acid and conjugate base concentrations, and watch the Henderson-Hasselbalch equation calculate pH in real time. Add strong acid or base to explore buffer capacity, compare to unbuffered water, and find the point where the buffer breaks.

Guided Experiment: Henderson-Hasselbalch Verification

How will the pH of an acetic acid buffer change as you vary the ratio of [A⁻] to [HA]? What pH do you expect when the concentrations are equal?

Write your hypothesis in the Lab Report panel, then click Next.

Buffer Visualization

Effective range01234567891011121314pKapH 4.764.76Species DistributionHA 50%A⁻ 50%Weak Acid (HA)Conjugate Base (A⁻)[HA] = 0.1000 M[A⁻] = 0.1000 MRatio [A⁻]/[HA] = 1.000Buffer Capacityβ = 0.1152

Controls

[CH₃COOH] (Acid)0.100 M
[CH₃COO⁻] (Base)0.100 M
Volume1.00 L
Amount0.0 mmol

Results

pH=pKa+log⁡[A−][HA]=4.76+log⁡0.10000.1000=4.76\text{pH} = \text{p}K_a + \log\frac{[\text{A}^-]}{[\text{HA}]} = 4.76 + \log\frac{0.1000}{0.1000} = 4.76
pH
4.76
pKa
4.76
[A⁻]/[HA] Ratio
1.000
Buffer Capacity (β)
0.115
[H⁺]
1.74e-5 M
[OH⁻]
5.75e-10 M
Buffer is effective (pH 4.76 is within pKa ± 1 range: 3.76 to 5.76)
Acetic Acid / Acetate (pKa = 4.76)

Buffer Capacity Graph

pKa012345678020406080100120mmol HCl AddedpHBuffered solutionUnbuffered (pure water)pKa

Data Table

(0 rows)
#TrialBuffer System[HA] (M)[A⁻] (M)pHAdded (mmol)ΔpH
0 / 500
0 / 500
0 / 500

Reference Guide

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation relates the pH of a buffer solution to the pKa of the weak acid and the ratio of conjugate base to weak acid concentrations.

pH=pKa+log⁡10[A−][HA]\text{pH} = \text{p}K_a + \log_{10}\frac{[\text{A}^-]}{[\text{HA}]}

When [A-] equals [HA], the log term is zero and pH equals pKa exactly. Doubling the base-to-acid ratio increases pH by about 0.3 units.

Buffer Capacity

Buffer capacity measures how much strong acid or base a buffer can absorb before the pH changes significantly.

β=2.303⋅Ctotal⋅Ka[H+](Ka+[H+])2\beta = 2.303 \cdot C_{\text{total}} \cdot \frac{K_a [\text{H}^+]}{(K_a + [\text{H}^+])^2}

Capacity is highest when [HA] = [A-] (pH = pKa) and increases with total buffer concentration. The effective buffering range is approximately pKa plus or minus 1 pH unit.

Adding Strong Acid or Base

When strong acid (HCl) is added to a buffer, H+ ions react with the conjugate base.

A−+H+→HA\text{A}^- + \text{H}^+ \rightarrow \text{HA}

When strong base (NaOH) is added, OH- reacts with the weak acid.

HA+OH−→A−+H2O\text{HA} + \text{OH}^- \rightarrow \text{A}^- + \text{H}_2\text{O}

The buffer is "broken" when one component is fully consumed and pH changes sharply.

Common Buffer Systems

Acetic acid / Acetate (pKa = 4.76) buffers near pH 4-5. Used in food preservation and biochemistry.

Phosphate (pKa = 7.20) buffers near physiological pH. Critical in biological systems and PBS buffers.

Carbonate (pKa = 6.35) is the primary blood buffer system, maintaining blood pH near 7.4.

Ammonium / Ammonia (pKa = 9.25) buffers in the basic range. Used in analytical chemistry and some industrial processes.