Buffer Capacity Calculator

Calculate the buffer capacity (β) to determine a solution's resistance to pH change upon the addition of a strong acid or base.

Moles of Acid/Base Added (Δn)

mol

Volume of Buffer

Absolute Change in pH (|ΔpH|)

Buffer Capacity (β)

0.1000

Capacity RatingModerate Capacity

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The Shield Against Acid Rain

A buffer solution's primary job is to absorb an influx of strong acid or strong base without allowing the pH to change significantly. However, a buffer is not invincible. If you dump a massive amount of hydrochloric acid into a small beaker of buffer, the buffer will eventually be "broken," and the pH will crash.

The Buffer Capacity (β) is a quantitative measurement of exactly how much punishment a buffer can take before its pH changes by exactly 1.0 unit.

The Mechanics of Capacity

Buffer capacity is determined by two major factors:

Absolute Concentration: A buffer made with 1.0 Molar acid and base has exactly 10 times the capacity of a buffer made with 0.1 Molar acid and base, even though both have the exact same starting pH. More molecules equal more shielding.
The Acid/Base Ratio: A buffer has maximum capacity when the ratio of conjugate base to weak acid is exactly 1:1 (meaning pH = pKa). As the ratio skews further away, the buffer becomes vulnerable.

Calculating Experimental Capacity

In the laboratory, you calculate buffer capacity by performing a titration. You add a known amount of strong acid or base to your buffer and measure exactly how much the pH shifts.

The Formula

β = Δn / (ΔpH \times V)

Where:

β=

Buffer Capacity

Δn=

Moles of Acid or Base Added

ΔpH=

Absolute Change in pH

Volume of Buffer (Liters)

Note: Buffer capacity is always a positive number, regardless of whether you are adding acid (which lowers pH) or base (which raises pH).

Example Calculation

You have exactly 1.0 Liters of a biological phosphate buffer. You add 0.05 moles of NaOH (a strong base). The pH of the buffer rises from 7.20 to 7.35 (a shift of 0.15).

Moles added: $0.05$
pH change: $0.15$
Volume: $1.0$ L
Calculate: $0.05 / (0.15 \times 1.0) = 0.333$

Your buffer has a capacity of 0.333 moles per liter per pH unit.

Frequently Asked Questions

A high buffer capacity means the solution is incredibly stubborn. You have to add massive amounts of strong acid or base to force the pH to change even slightly. Your blood has a very high buffer capacity.

If the pH is exactly equal to the pKa, you have a 50/50 mix of acid and base. You are perfectly shielded against both acid attacks and base attacks. If the pH is 1 unit higher than the pKa, you have a 90/10 mix of base to acid. You have almost no acid left to neutralize an incoming base attack, so the capacity plummets.

If the buffer is perfectly at its pKa, the capacity is identical in both directions. However, if the buffer is skewed (e.g., pH > pKa), it will have a massive capacity against added strong acid, but a very weak capacity against added strong base.

The easiest way is to simply increase the concentration of your buffering agents. Instead of using a 0.05 M buffer, make a 0.5 M buffer. It will have the exact same pH, but 10 times the defensive capacity.

Donald Van Slyke formalized the math for buffer capacity in 1922. The formal calculus definition is the derivative β = db/dpH, where db is the increment of strong base added. Our formula is the standard linear approximation used in most undergraduate labs.

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