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Constants library

9 values, each with its units, its uncertainty, and where it came from.

Chemistry 9

Acid Dissociation Constant of Acetic Acid measured

Ka(CH3COOH)=0.0000175K_{\mathrm{a}}(\mathrm{CH_3COOH}) = 0.0000175

dimensionlessThe acid dissociation constant of acetic acid at 25 °C, Kₐ = 1.75 × 10⁻⁵, the reference weak acid of every textbook and every buffer.

pKₐ of Acetic Acid measured

pKa(CH3COOH)=4.756\mathrm{p}K_{\mathrm{a}}(\mathrm{CH_3COOH}) = 4.756

dimensionlessThe pKₐ of acetic acid at 25 °C, 4.756 — the pH at which acetic acid and acetate are present in exactly equal amounts.

First Dissociation Constant of Phosphoric Acid measured

Ka1(H3PO4)=0.00711K_{\mathrm{a1}}(\mathrm{H_3PO_4}) = 0.00711

dimensionlessThe first acid dissociation constant of phosphoric acid at 25 °C, Kₐ₁ = 7.11 × 10⁻³, a moderately strong first proton on a triprotic acid.

First pKₐ of Phosphoric Acid measured

pKa1(H3PO4)=2.148\mathrm{p}K_{\mathrm{a1}}(\mathrm{H_3PO_4}) = 2.148

dimensionlessThe first pKₐ of phosphoric acid at 25 °C, 2.148 — the centre of the low-pH buffering region used in HPLC mobile phases.

Second Dissociation Constant of Phosphoric Acid measured

Ka2(H3PO4)=6.32×10−8K_{\mathrm{a2}}(\mathrm{H_3PO_4}) = 6.32 \times 10^{-8}

dimensionlessThe second acid dissociation constant of phosphoric acid at 25 °C, Kₐ₂ = 6.32 × 10⁻⁸, the step that buffers cells and biological media.

Second pKₐ of Phosphoric Acid measured

pKa2(H3PO4)=7.199\mathrm{p}K_{\mathrm{a2}}(\mathrm{H_3PO_4}) = 7.199

dimensionlessThe second pKₐ of phosphoric acid at 25 °C, 7.199 — almost exactly physiological pH, which is why phosphate buffers biology.

Third Dissociation Constant of Phosphoric Acid measured

Ka3(H3PO4)=4.5×10−13K_{\mathrm{a3}}(\mathrm{H_3PO_4}) = 4.5 \times 10^{-13}

dimensionlessThe third acid dissociation constant of phosphoric acid at 25 °C, Kₐ₃ = 4.5 × 10⁻¹³, a proton so tightly held it needs strong alkali to remove.

Third pKₐ of Phosphoric Acid measured

pKa3(H3PO4)=12.35\mathrm{p}K_{\mathrm{a3}}(\mathrm{H_3PO_4}) = 12.35

dimensionlessThe third pKₐ of phosphoric acid at 25 °C, 12.35 — the pH above which free orthophosphate finally becomes the dominant species.

Base Dissociation Constant of Ammonia measured

Kb(NH3)=0.0000177K_{\mathrm{b}}(\mathrm{NH_3}) = 0.0000177

dimensionlessThe base dissociation constant of ammonia at 25 °C, K_b = 1.77 × 10⁻⁵, making it the textbook weak base and the mirror of acetic acid.