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Buffer Solution

General

Buffer Solution

A solution containing a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists large changes in pH when small amounts of acid or base are added.

Definition

A buffer solution is a mixture, usually made of a weak acid and its conjugate base (or a weak base and its conjugate acid), that resists significant changes in pH when small amounts of strong acid or strong base are added to it. This resistance happens because the weak acid component can neutralize added base, while the conjugate base component can neutralize added acid, keeping the overall hydrogen ion concentration relatively stable.

Buffers are essential wherever a stable pH environment is required despite ongoing chemical activity — in laboratory reagents, industrial processes, and especially in living organisms, where enzymes and cellular processes only function correctly within narrow pH ranges. Human blood, for instance, is buffered by the carbonic acid–bicarbonate system to stay near pH 7.4 even as metabolism constantly produces acidic waste products.

The pH of a buffer is calculated using the Henderson-Hasselbalch equation, which relates pH to the acid's pKa and the ratio of the conjugate base to the weak acid concentrations — both quantities expressed in terms of molarity. The Buffer pH Calculator, Henderson-Hasselbalch Calculator, and Buffer Capacity Calculator each handle a different part of buffer design and analysis.

Formula

pH = pKa + log10([A-] / [HA])

This is the Henderson-Hasselbalch equation. Where:

  • pKa = the negative log of the weak acid's dissociation constant (Ka)
  • [A-] = molar concentration of the conjugate base
  • [HA] = molar concentration of the weak acid

Worked Example

A buffer is made using acetic acid (pKa = 4.76) with 0.3 M acetate ion ([A-]) and 0.2 M acetic acid ([HA]).

pH = pKa + log10([A-]/[HA]) = 4.76 + log10(0.3/0.2) = 4.76 + log10(1.5) = 4.76 + 0.176 = 4.94

This buffer maintains a pH of about 4.94, close to the acid's pKa, which is exactly the range where acetic acid/acetate buffers are most effective. Try different concentration ratios with the Henderson-Hasselbalch Calculator.

Key Things to Know

  • Buffers work best near the acid's pKa: A buffer is most effective at resisting pH change within about one unit above or below the weak acid's pKa value, where both the acid and conjugate base are present in significant amounts.
  • Concentration determines buffer capacity: Higher total molarity of the acid-base pair allows the buffer to absorb larger additions of strong acid or base before its pH shifts noticeably.
  • The ratio matters more than absolute concentration: According to the Henderson-Hasselbalch equation, pH depends on the ratio of conjugate base to acid, so diluting a buffer proportionally does not change its pH, only its capacity.
  • Biological buffers keep pH within a narrow band: The bicarbonate buffer system in blood and phosphate buffers inside cells both illustrate how critical stable pH is to normal biological function.
  • Buffers eventually fail if overwhelmed: Adding enough strong acid or base to fully consume either the weak acid or conjugate base component removes the buffer's ability to resist further pH change.

Related Calculators

Buffer pH Calculator →Calculate buffer solution pH using the Henderson-Hasselbalch equation. Enter pKa and acid/conjugate base concentrations to get buffer pH and buffer capacity indicator.
Buffer Capacity Calculator →Calculate buffer capacity (β) using the Van Slyke equation. Enter pKa, total buffer concentration, and acid fraction to find pH, buffer capacity in mol/L/pH, and effective buffer range.
Henderson-Hasselbalch Calculator →Calculate pH from pKa and acid/base ratio, or find the required ratio to hit a target pH. Solve all three forms of the Henderson-Hasselbalch equation instantly.

Related Terms

pHpH (Potential of Hydrogen)MolarityMolarity (Molar Concentration)

Frequently Asked Questions

A buffer solution contains a weak acid together with its conjugate base, or a weak base together with its conjugate acid, in comparable concentrations. This pairing allows the solution to neutralize small additions of strong acid or strong base without a large shift in pH, which a solution of pure water or a strong acid alone cannot do.
The pH of a buffer is calculated using the Henderson-Hasselbalch equation, which relates pH to the acid's pKa value and the ratio of conjugate base to weak acid concentrations. The Henderson-Hasselbalch Calculator and Buffer pH Calculator both apply this formula directly from your acid and base concentrations.
Buffer capacity is the amount of acid or base a buffer solution can absorb before its pH changes significantly, and it depends on the total concentration of the weak acid and conjugate base present. The Buffer Capacity Calculator estimates this limit so you know how much strong acid or base a given buffer can handle before it stops resisting pH change effectively.
Living organisms rely on buffers, such as the bicarbonate buffer system in human blood, to keep internal pH within a very narrow range despite constantly producing acidic or basic metabolic byproducts. Blood pH is tightly maintained around 7.4, and even a shift of a few tenths outside this range can be medically dangerous.
A buffer works best within about one pH unit above or below its acid's pKa value, where the concentrations of the weak acid and conjugate base are close enough to each other to absorb additions of acid or base effectively in both directions. Outside this range, the buffer's capacity to resist pH change drops off sharply.