Buffer Composition and Action

Introduction

A buffer moderates, rather than prevents, a pH change. Its two reactive components are usually a weak acid and its conjugate base, or a weak base and its conjugate acid. To understand its action, identify which component receives an added proton and which component supplies a proton to neutralize added hydroxide.

Core explanation

Consider a mixture containing appreciable HA and A⁻. The weak-acid equilibrium is HA + H₂O ⇌ H₃O⁺ + A⁻. Adding a small amount of strong acid supplies hydronium, which A⁻ consumes: A⁻ + H₃O⁺ → HA + H₂O. Adding a small amount of strong base supplies hydroxide, which HA consumes: HA + OH⁻ → A⁻ + H₂O. Thus the strong reagent is largely converted into one member of the weak conjugate pair. Both members must be present in useful amounts for protection in both directions.

The buffer still changes composition. Acid addition lowers the A⁻/HA ratio; base addition raises it. Since acid equilibrium depends on this ratio, pH moves, but generally much less than in unbuffered water receiving the same amount per unit volume. The word “resist” should not be mistaken for “hold exactly constant.” Once one member is mostly consumed, buffer action deteriorates sharply.

An example is acetic acid mixed with a soluble acetate salt. Acetic acid supplies HA and the salt supplies much of A⁻. An ammonium/ammonia mixture works by the same proton-transfer logic: ammonia consumes added acid, while ammonium consumes added base. A weak acid alone is not normally an effective two-direction buffer, because its conjugate base concentration may be too small. A soluble salt of the weak acid alone also lacks much acid component until conditions supply it.

The buffer reaction can be represented with formal concentrations after the strong-acid or strong-base reaction. Then the weak-pair equilibrium establishes the final pH. This order—stoichiometric neutralization first, equilibrium second—avoids treating a strong reagent as though it merely shifts a weak equilibrium without reacting. The approximation assumes the added quantity is modest, the volume change is handled, and activities can be replaced by concentrations for the problem's precision.

Step-by-step reasoning

1. List the amounts of the weak acid and its conjugate base. 2. React added strong acid with the base member, or strong base with the acid member. 3. Check that both members remain after stoichiometry. 4. Use their updated ratio to interpret the final pH.

Visual explanation

Draw two reservoirs labeled HA and A⁻. An acid arrow enters the A⁻ reservoir and transfers material to HA; a base arrow enters HA and transfers material to A⁻.

Real-world analogy

Two neighboring storage tanks can absorb a delivery or withdrawal by transferring material between them. Their levels do change, but the exchange limits the abrupt swing that an empty tank would show.

Real-world example

A laboratory acetate buffer contains acetic acid and sodium acetate. A small accidental addition of acid converts some acetate to acetic acid, so its pH falls less sharply than the pH of equal-volume pure water.

Why?

Why must a buffer contain both pair members? One member is needed to consume added acid, and the other is needed to consume added base. A shortage leaves one disturbance insufficiently opposed.

Common misconception

“A buffer neutralizes unlimited acid.” Its available conjugate base is finite. Once acid has consumed most of it, additional acid makes pH fall rapidly.

Worked example

A buffer initially contains 0.100 mol HA and 0.100 mol A⁻. Add 0.010 mol HCl without appreciable volume change. The strong acid reacts with A⁻, leaving 0.090 mol A⁻ and making 0.110 mol HA. Their ratio changes from 1.00 to 0.818. By the approximate buffer relation, the pH falls by log₁₀(0.818) ≈ 0.087 pH unit. A comparable acid amount in plain water would produce a much larger shift.

Quick check

1. Which buffer member consumes added hydroxide? Answer: The weak acid HA; it donates a proton to form water and A⁻.

Exam focus

Write the neutralization reaction before substituting into a pH relation. Check whether either conjugate-pair member is exhausted; if so, the ordinary buffer approximation fails.

Advanced insight

True equilibrium constants use activities. Ionic strength and dilution can alter the activity ratio even if analytical mole ratios look unchanged. For precise buffers, temperature and solution composition therefore matter.

Summary

A useful buffer contains significant quantities of a weak conjugate acid-base pair. The base component consumes added acid, and the acid component consumes added base. This finite reaction capacity moderates pH changes.

Practice questions

1. What pair forms an ammonium buffer? Answer: NH₄⁺ and NH₃. 2. What happens to A⁻/HA after adding a modest amount of HCl? Answer: The ratio decreases because A⁻ is converted into HA. 3. Can a buffer preserve exactly the same pH after addition? Answer: Usually no. Its composition and therefore its equilibrium pH change, though the change can be small.