Phosphoric acid is tribasic with three pKa values of 2.14, 6.86, and 12.4,

respectively. The ionic form of the acid that predominates at pH 3.2 is:

PO43-

HPO42-

H2PO4-

H3PO4-

Phosphoric acid (H3PO4), a tribasic acid, predominates as H2PO4- at pH 3.2. This occurs because pH 3.2 falls between pKa1 (2.14) and pKa2 (6.86), where the monohydrogen phosphate ion holds the most concentration.

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Ionic Speciation Rule

The predominant species is the one where pH lies between its flanking pKa values. For polyprotic acids like phosphoric acid, the first dissociation (H3PO4 ⇌ H2PO4- + H+) dominates below pKa1, shifts to H2PO4- between pKa1 and pKa2, then HPO42- between pKa2 and pKa3, and PO43- above pKa3.

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Option Breakdown

• H3PO4: Fully protonated form prevails below pKa1 (pH < 2.14), as the first proton remains mostly attached. At pH 3.2 (> pKa1), significant deprotonation to H2PO4- occurs.

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• H2PO4-: Correct choice, since pH 3.2 > pKa1 (2.14) but < pKa2 (6.86). The first proton is largely dissociated, but the second stays bound.

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• HPO42-: Dihydrogen phosphate form appears between pKa2 (6.86) and pKa3 (12.4). At pH 3.2 (far below pKa2), this is minimal.

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• PO43-: Fully deprotonated, dominant only above pKa3 (pH > 12.4). Negligible at pH 3.2.

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