Is this the same α as “fraction dissociated” for pure weak acid HA in water?

Not unless the pH is tied to that solution’s equilibrium. Here pH is an independent control (bath), giving the Henderson composition ratio. In pure HA, pH is coupled to concentration and α follows from charge balance, not this fixed-pH formula alone.

Why use base-10 exponentials?

Convention in aqueous chemistry: pH and pKa are defined with log₁₀, so the ratio [HA]/[A⁻] becomes a power of ten.

Does activity coefficient γ matter?

At high ionic strength, activities deviate from concentrations and effective pKa shifts. This page uses the textbook ideal limit.

Acid Dissociation α(pH)

α = 1/(1+10^(pKa−pH)); half-equivalence at pH = pKa; curves for α and 1−α vs pH.

Markers: current pH (white) and half-equivalence at pH = pK_a (red).

Live graphs

About this model

For a single weak-acid/conjugate-base pair HA ⇌ H⁺ + A⁻ at fixed pH (fast acid–base exchange, dilute ideal solution), the fraction of the analytical pool present as A⁻ is α = [A⁻]/([HA]+[A⁻]). From Ka = a_H+ a_A− / a_HA with activities approximated by concentrations and a fixed hydrogen activity set by the bath, rearrangement gives α = 1/(1 + 10^(pKa−pH)). When pH = pKa, α = ½ — the half-equivalence composition familiar from titration curves. The simulator plots α and 1−α versus pH for an adjustable pKa and marks the current (pH, α) point plus the half-point at (pKa, ½). It does not compute pH self-consistently in a beaker of pure weak acid (that requires charge balance and water autoprotolysis) and omits ionic-strength corrections and polyprotic systems.

Who it's for: General chemistry alongside Henderson–Hasselbalch and titration labs; pairs with the buffer-solution page.

Key terms

degree of dissociation
pKa
titration
half-equivalence
conjugate base
Sigmoid

How it works

For a weak acid / conjugate base pair at fixed pH (fast exchange, ideal dilute limit), the fraction deprotonated obeys α = [A⁻]/([HA]+[A⁻]) = 1/(1 + 10^(pKa−pH)). At pH = pKa exactly half is A⁻ (α = ½) — the half-equivalence composition on a titration curve. This page plots α and 1−α vs pH for a chosen pKa; it is a thermodynamic ratio, not a kinetics of dissociation in pure acid without specifying pH.

Key equations

α = 1 / (1 + 10^(pK_a − pH))

pH = pK_a ⇒ α = ½ (half A⁻, half HA)

Frequently asked questions

Is this the same α as “fraction dissociated” for pure weak acid HA in water?: Not unless the pH is tied to that solution’s equilibrium. Here pH is an independent control (bath), giving the Henderson composition ratio. In pure HA, pH is coupled to concentration and α follows from charge balance, not this fixed-pH formula alone.
Why use base-10 exponentials?: Convention in aqueous chemistry: pH and pKa are defined with log₁₀, so the ratio [HA]/[A⁻] becomes a power of ten.
Does activity coefficient γ matter?: At high ionic strength, activities deviate from concentrations and effective pKa shifts. This page uses the textbook ideal limit.

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Acid Dissociation α(pH)

Live graphs

About this model

How it works

Key equations

Frequently asked questions

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Live graphs

About this model

How it works

Key equations

Frequently asked questions