Buffer Capacity Estimator

An offline tool for students, researchers, and lab technicians to estimate a buffer's resistance to pH change upon addition of acid or base. Supports concentration units (M, mM, µM) and runs entirely in your browser. Educational only. Follow your lab SOPs and SDS.

Calculator

Calculation Method

Simplified is suitable for most common buffers (pKa 3-11).

Weak Acid Concentration [HA]

Enter a positive number. Accepts dot, comma, or scientific notation.

Conjugate Base Concentration [A⁻]

Acid pKa at Working Temperature

Typical range: 0-14. For acetic acid, pKa ≈ 4.76 at 25°C.

Result Decimal Places

Primary Results

Buffer Capacity (β)

0.115 M/pH

Moles of strong acid/base per liter needed to change pH by 1 unit.

Current Buffer pH (Estimated)

4.76

Via Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA])

Intermediate Values & Checks

Total Buffer Concentration: 0.200 M
Buffer Ratio ([A⁻]/[HA]): 1.00
Max Capacity (when [HA]=[A⁻]): 0.115 M/pH

Log Entry for Lab Notebook

Understanding Buffer Capacity

Buffer capacity quantifies how well a solution resists pH change when an acid or base is added. It's crucial for maintaining stable conditions in biochemical assays, cell cultures, and analytical chemistry.

How to Use This Tool

Enter the concentrations of your weak acid (e.g., acetic acid) and its conjugate base (e.g., sodium acetate). Ensure units are consistent.
Input the pKa of the weak acid at your working temperature. (Find this in reference tables or SDS.)
Select the calculation method. Simplified is accurate for buffers where both [HA] and [A⁻] >> [H⁺] and [OH⁻]. Use Exact for very dilute buffers or extreme pH.
Click "Calculate". Review the main result, estimated pH, and intermediate values for consistency.

Worked Examples

Scenario	[HA]	[A⁻]	pKa	Buffer Capacity (β)	Notes
1: Acetate Buffer	0.10 M	0.10 M	4.76	~0.115 M/pH	Maximum capacity at equal concentrations.
2: Phosphate Buffer	0.05 M	0.15 M	7.20	~0.173 M/pH	Higher total concentration increases β.
3: Dilute Buffer	0.001 M	0.001 M	4.76	~0.00115 M/pH	Capacity scales with concentration.

Common Mistakes Checklist

Using room-temperature pKa for experiments at 37°C.
Assuming equal concentrations automatically give pH = pKa (true only if ionic strength is negligible).
Forgetting to account for dilution when preparing buffer from stock solutions.
Neglecting the buffer capacity contribution of water at very low buffer concentrations (< 1 mM).

Explore: Effect of Buffer Ratio

Adjust the ratio [A⁻]/[HA] while keeping total concentration fixed. See how buffer capacity peaks when the ratio is 1:1 (pH = pKa).

Buffer Ratio [A⁻]/[HA]: 1.00

0.1 (More Acid) 1 (Equal) 10 (More Base)

Resulting pH

4.76

Buffer Capacity (β)

0.115

% of Max Capacity

100%

Recommendation

Optimal

Frequently Asked Questions

Q: When does the simplified Henderson-Hasselbalch approximation fail?

A: It fails for very dilute buffers (< 1 mM total), buffers with pKa outside 3-11, or when the ionic strength is high (> 0.1 M). In these cases, use the "Exact" method which includes contributions from water autoionization.

Q: How does temperature affect buffer capacity?

A: Temperature changes the pKa of the weak acid. For example, Tris buffer pKa changes ≈ -0.03 per °C. Always use the pKa at your working temperature for accurate capacity estimation.

Q: What's a typical "good" buffer capacity for a lab experiment?

A: For many biochemical assays, a capacity of 0.01–0.05 M/pH is sufficient. For processes generating more acid/base (e.g., enzyme reactions), aim for 0.05–0.2 M/pH. Check your protocol.

Q: Can I use this for polyprotic buffers like phosphate?

A: This tool assumes a single pKa system. For polyprotic buffers, estimate capacity near one pKa at a time. For phosphate (pKa₂ = 7.2), enter [H₂PO₄⁻] as [HA] and [HPO₄²⁻] as [A⁻].

Q: What safety precautions are specific to buffer preparation?

A: Wear appropriate PPE (gloves, goggles). When adjusting pH with concentrated acid/base, always add small volumes with stirring. Add acid/base to water, never the reverse. Know the SDS for all components.

Formulas & Assumptions

Simplified Buffer Capacity (β): β = 2.303 × [HA] × [A⁻] / ([HA] + [A⁻])
Exact Buffer Capacity: β = 2.303 × ( [H⁺] + [OH⁻] + [HA]×[A⁻]/([HA]+[A⁻]) ) where [H⁺] and [OH⁻] are from water autoionization (Kw = 1e-14 at 25°C).
pH Estimation: pH = pKa + log₁₀([A⁻]/[HA]) (Henderson-Hasselbalch).
Assumptions: Activity coefficients ≈ 1 (ideal dilute solution). Temperature fixed at 25°C for Kw and pKa (unless you adjust pKa input). No other acid/base species present.
Rounding: Results are rounded to the selected decimal places. Intermediate calculations use double precision.

Disclaimer: For informational and educational purposes only. Estimates depend on your inputs and simplifying assumptions. This tool does not replace professional judgment, lab SOPs, or safety procedures. Always follow SDS guidance and institutional policies.

Sources & References: Common chemistry constants: R, NA, F; Standard unit conversions and lab measurement best practices; General analytical chemistry and uncertainty concepts; Intro physical chemistry relationships (ideal gas, Arrhenius, equilibrium); Laboratory safety basics and SDS principles.

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Buffer Capacity Estimator

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