= Conjugate base
A <conjugate base> is what remains after a <chemical acid> donates one <proton>. In the <conjugate acid-base pair> $\mathrm{HA}/\mathrm A^-$, $\mathrm{HA}$ is the <conjugate acid> of $\mathrm A^-$; accepting a <proton> reverses the conversion. For example, <acetic acid> $\mathrm{CH_3COOH}$ loses a <proton> to give its <conjugate base> $\mathrm{CH_3COO^-}$. Partners differ by one <proton> and one unit of <electric charge>; they need not always be neutral and negative. <Hydronium>/<water> and <water>/<hydroxide> are also <conjugate acid-base pairs>.
At a prescribed <pH>, the ideal dilute <acid dissociation constant> determines the partition of a <monoprotic acid> inventory between its <chemical species>:
$$
\alpha_{\mathrm A^-}=\frac{K_a}{K_a+10^{-\mathrm{pH}}},\qquad
\alpha_{\mathrm{HA}}=\frac{10^{-\mathrm{pH}}}{K_a+10^{-\mathrm{pH}}}.
$$
These <dimensionless> fractions sum to one. They are the <weak acid dissociation fraction> and its complement when the only acid-bearing <chemical species> are $\mathrm{HA}$ and $\mathrm A^-$. Equal fractions occur at $\mathrm{pH}=\mathrm pK_a$ in this ideal approximation. For nonideal <chemical solutions>, equality of <thermodynamic activities> need not mean equality of <concentrations>.
\Image[/weak-acid-equilibrium-species.png]
{title=A synthetic <monoprotic acid> partitions between <conjugate acid> and <conjugate base> as imposed <pH> changes}
{id=weak-acid-equilibrium-species}
For stipulated <pKa> $5$, the <conjugate base> fractions at <pH> $4$, $5$ and $6$ are $1/11$, $1/2$ and $10/11$. Here an external reservoir sets <pH>; its <ions> must supply the additional <electric charge> balance. The diagram omits those <ions> and <water> populations. It does not represent three pure $\mathrm{HA}$ samples at identical <analytical concentration> with freely chosen <pH>. In the latter experiment, the <weak acid mass and charge balances> determine <pH> as well as the partition.
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