= Indole-3-acetate
<Indole-3-acetate> is the <conjugate base> of <indole-3-acetic acid>, formed by loss of a proton from its <carboxylic acid> group. ChEBI records <molecular formula> $\mathrm{C_{10}H_8NO_2^-}$ and charge $-1$. The uncharged acid and its anion have related chemical roles but are distinct protonation forms.<Indole-3-acetate in ChEBI>[CHEBI:30854, Definition, Formula, Net Charge and conjugate-base relation]{ref}
Write the acid as $\mathrm{HA}$ and <indole-3-acetate> as $\mathrm{A^-}$:
$$
\mathrm{HA\rightleftharpoons H^++A^-}.
$$
For a dilute two-form model, the <Henderson-Hasselbalch equation> gives $r=[\mathrm{A^-}]/[\mathrm{HA}]=10^{\mathrm{pH}-\mathrm pK_a}$, and the anion fraction is $r/(1+r)$. At <pH> equal to $\mathrm pK_a$, the two fractions are equal. One pH unit above it gives $10/11\approx90.9\%$ anion, while one unit below gives $1/11\approx9.1\%$. This is a symbolic calculation relative to the <acid dissociation constant>, not a claim of a newly measured numerical $\mathrm pK_a$.
The fraction does not specify the total amount. A hypothetical100-nanomole pool in the90.9percent case contains about90.9 nanomoles of <indole-3-acetate> and9.1 nanomoles of acid. Specifying <pH> alone would not determine that pool size, a transport rate or a growth response to <auxin>.
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