Phosphoric Acid Has Three Dissociable H

Figure 2.14 shows the titration curve for phosphoric acid, H3PO4. This substance is a polyprotic acid, meaning it has more than one dissociable proton. Indeed, it has three, and thus three equivalents of OH" are required to neutralize it, as Figure 2.14 shows. Note that the three dissociable H+ are lost in discrete steps, each dissociation showing a characteristic pKa. Note that pKj occurs at pH = 2.15, and the concentrations of the acid H3PO4 and the conjugate base H2PO4" are equal. As the next dissociation is approached, H2PO4"

Calcium Sulfate Titration Curve

FIGURE 2.13 • The titration curves of several weak electrolytes: acetic acid, imidazole, and ammonium. Note that the shape of these different curves is identical. Only their position along the pH scale is displaced, in accordance with their respective affinities for H+ ions, as reflected in their differing pKa values.

is treated as the acid and HPO42~ is its conjugate base. Their concentrations are equal at pH 7.20, so pK2 = 7.20. (Note that at this point, 1.5 equivalents of OH~ have been added.) As more OH~ is added, the last dissociable hydrogen is titrated, and pK3 occurs at pH = 12.4, where [HPO42~] = [PO43~].

A biologically important point is revealed by the basic shape of the titration curves of weak electrolytes: in the region of the pKa, pH remains relatively unaffected as increments of OH~ (or H+) are added. The weak acid and its conjugate base are acting as a buffer.

Titrate H3po4

■ HMHiiiHtHilHHiiiilMlt MMHHiMMiiilHlMlHHfe MtHiiiiHHt'MHliiiiHHfe l.li U.O S.G r '; i.vnti. f> ■ L' .1 ■.-m

FIGURE 2.14 • The titration curve for phosphoric acid. The chemical formulas show the prevailing ionic species present at various pH values. Phosphoric acid (H3PO4) has three titratable hydrogens and therefore three midpoints are seen: at pH 2.15 (pK1), pH 7.20 (pK2), and pH 12.4 (pK3).

■ HMHiiiHtHilHHiiiilMlt MMHHiMMiiilHlMlHHfe MtHiiiiHHt'MHliiiiHHfe l.li U.O S.G r '; i.vnti. f> ■ L' .1 ■.-m

FIGURE 2.14 • The titration curve for phosphoric acid. The chemical formulas show the prevailing ionic species present at various pH values. Phosphoric acid (H3PO4) has three titratable hydrogens and therefore three midpoints are seen: at pH 2.15 (pK1), pH 7.20 (pK2), and pH 12.4 (pK3).

Curve Amino Acid

FIGURE 2.15 • A buffer system consists of a weak acid, HA, and its conjugate base, A . The pH varies only slightly in the region of the titration curve where [HA] = [A ]. The unshaded box denotes this area of greatest buffering capacity. Buffer action: when HA and A are both available in sufficient concentration, the solution can absorb input of either H+ or OH , and pH is maintained essentially constant.

FIGURE 2.15 • A buffer system consists of a weak acid, HA, and its conjugate base, A . The pH varies only slightly in the region of the titration curve where [HA] = [A ]. The unshaded box denotes this area of greatest buffering capacity. Buffer action: when HA and A are both available in sufficient concentration, the solution can absorb input of either H+ or OH , and pH is maintained essentially constant.

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  • lisa jung
    What species is present on titration?
    8 years ago
  • sandra johnson
    How to titration curve acetic acid?
    8 years ago
  • Allan Findlay
    How to neutralize phosphoric acid?
    8 years ago
  • Tobold Grubb
    How phosphoric acid interacting with basic amino acid?
    1 year ago

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