formula.expert

Constants library

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

Chemistry 17

Ionic Product of Water (Kw at 25 °C) measured

Kw=1×10−14K_{\mathrm{w}} = 1 \times 10^{-14}

dimensionlessThe autoionisation constant of pure water at 25 °C, [H⁺][OH⁻] = 1.0 × 10⁻¹⁴, the equilibrium behind the whole 0–14 pH scale.

pKw of Water at 25 °C measured

pKw=13.995\mathrm{p}K_{\mathrm{w}} = 13.995

dimensionlessThe negative logarithm of water's ionic product at 25 °C, pKw = 13.995, universally rounded to 14.00 for the pH + pOH identity.

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 Carbonic Acid measured

Ka1(H2CO3)=4.45×10−7K_{\mathrm{a1}}(\mathrm{H_2CO_3}) = 4.45 \times 10^{-7}

dimensionlessThe first acid dissociation constant of carbonic acid at 25 °C, Kₐ₁ = 4.45 × 10⁻⁷, governing the CO₂-bicarbonate equilibrium in natural water.

First pKₐ of Carbonic Acid measured

pKa1(H2CO3)=6.352\mathrm{p}K_{\mathrm{a1}}(\mathrm{H_2CO_3}) = 6.352

dimensionlessThe first pKₐ of carbonic acid at 25 °C, 6.352 — the pH at which dissolved CO₂ and bicarbonate are present in equal concentrations.

Second Dissociation Constant of Carbonic Acid measured

Ka2(H2CO3)=4.69×10−11K_{\mathrm{a2}}(\mathrm{H_2CO_3}) = 4.69 \times 10^{-11}

dimensionlessThe second acid dissociation constant of carbonic acid at 25 °C, Kₐ₂ = 4.69 × 10⁻¹¹, the bicarbonate-to-carbonate step that drives scaling.

Second pKₐ of Carbonic Acid measured

pKa2(H2CO3)=10.329\mathrm{p}K_{\mathrm{a2}}(\mathrm{H_2CO_3}) = 10.329

dimensionlessThe second pKₐ of carbonic acid at 25 °C, 10.329 — the pH at which bicarbonate and carbonate ions are present in equal concentrations.

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.

Solubility Product of Calcium Carbonate measured

Ksp(CaCO3)=3.36×10−9K_{\mathrm{sp}}(\mathrm{CaCO_3}) = 3.36 \times 10^{-9}

dimensionlessThe solubility product of calcite at 25 °C, K_sp = 3.36 × 10⁻⁹ — the number that decides whether a water scales or corrodes.

Solubility Product of Calcium Sulfate measured

Ksp(CaSO4)=0.0000493K_{\mathrm{sp}}(\mathrm{CaSO_4}) = 0.0000493

dimensionlessThe solubility product of anhydrous calcium sulfate at 25 °C, K_sp = 4.93 × 10⁻⁵ — the gypsum scale that acid cleaning cannot remove.