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16.11: Acid Dissociation Constants

  • Page ID
    220808
  • The following table provides pKa and Ka values for selected weak acids. All values are from Martell, A. E.; Smith, R. M. Critical Stability Constants, Vols. 1–4. Plenum Press: New York, 1976. Unless otherwise stated, values are for 25 oC and for zero ionic strength. Those values in brackets are considered less reliable.

    Weak acids are arranged alphabetically by the names of the neutral compounds from which they are derived. In some cases—such as acetic acid—the compound is the weak acid. In other cases—such as for the ammonium ion—the neutral compound is the conjugate base. Chemical formulas or structural formulas are shown for the fully protonated weak acid. Successive acid dissociation constants are provided for polyprotic weak acids; where there is ambiguity, the specific acidic proton is identified.

    To find the Kb value for a conjugate weak base, recall that

    \[K_\text{a} \times K_\text{b} = K_\text{w} \nonumber\]

    for a conjugate weak acid, HA, and its conjugate weak base, A.

    compound conjugate acid pKa Ka
    acetic acid \(\ce{CH3COOH}\) 4.757 \(1.75 \times 10^{-5}\)
    adipic acid
    adipic.png

    4.42

    5.42

    \(3.8 \times 10^{-5}\)

    \(3.8 \times 10^{-6}\)

    alanine alanine.png

    2.348 (\(\ce{COOH}\))

    9.867 (\(\ce{NH3}\))

    \(4.49 \times 10^{-3}\)

    \(1.36 \times 10^{-10}\)

    aminobenzene aminobenzene.png 4.601 \(2.51 \times 10^{-5}\)
    4-aminobenzene sulfonic acid aminobenzenesulfonic.png 3.232 \(5.86 \times 10^{-4}\)
    2-aminobenzoic acid
    2aminobenzoic.png

    2.08 (\(\ce{COOH}\))

    4.96 (\(\ce{NH3}\))

    \(8.3 \times 10^{-3}\)

    \(1.1 \times 10^{-5}\)

    2-aminophenol (\(T = 20 \text{°C}\)) 2aminophenol.png

    4.78 (\(\ce{NH3}\))

    9.97 (OH)

    \(1.7 \times 10^{-5}\)

    \(1.05 \times 10^{-10}\)

    ammonia \(\ce{NH4+}\) 9.244 \(5.70 \times 10^{-10}\)
    arginine
    arginine.png

    1.823 (COOH)

    8.991 (\(\ce{NH3}\))

    [12.48] (\(\ce{NH2}\))

    \(1.50 \times 10^{-2}\)

    \(1.02 \times 10^{-9}\)

    [\(3.3 \times 10^{-13}\)]

    arsenic acid \(\ce{H3AsO4}\)

    2.24

    6.96

    11.50

    \(5.8 \times 10^{-3}\)

    \(1.1 \times 10^{-7}\)

    \(3.2 \times 10^{-12}\)

    asparagine (\(\mu = 0.1 \text{ M}\)) asparagine.png

    2.14 (COOH)

    8.72 (\(\ce{NH3}\))

    \(7.2 \times 10^{-3}\)

    \(1.9 \times 10^{-9}\)

    aspartic acid aspartic.png

    1.990 (\(\alpha\)-COOH)

    3.900 (\(\beta\)-COOH)

    10.002 (\(\ce{NH3}\))

    \(1.02 \times 10^{-2}\)

    \(1.26 \times 10^{-4}\)

    \(9.95 \times 10^{-11}\)

    benzoic acid benzoic.png 4.202 \(6.28 \times 10^{-5}\)
    benzylamine benzylamine.png 9.35 \(4.5 \times 10^{-10}\)
    boric acid (\(pK_\text{a2}, pK_\text{a3: } T = 20 \text{°C}\)) \(\ce{H3BO3}\)

    9.236

    [12.74]

    [13.80]

    \(5.81 \times 10^{-10}\)

    [\(1.82 \times 10^{-13}\)]

    [\(1.58 \times 10^{-14}\)]

    carbonic acid \(\ce{H2CO3}\)

    6.352

    10.329

    \(4.45 \times 10^{-7}\)

    \(4.69 \times 10^{-11}\)

    catechol catechol.png

    9.40

    12.8

    \(4.0 \times 10^{-10}\)

    \(1.6 \times 10^{-13}\)

    chloracetic acid \(\ce{ClCH2COOH}\) 2.865 \(1.36 \times 10^{-3}\)
    chromic acid (\(pK_\text{a1: } T = 20 \text{°C}\)) \(\ce{H2CrO4}\)

    –0.2

    6.51

    1.6

    \(3.1 \times 10^{-7}\)

    citric acid citric.png

    3.128 (COOH)

    4.761 (COOH)

    6.396 (COOH)

    \(7.45 \times 10^{-4}\)

    \(1.73 \times 10^{-5}\)

    \(4.02 \times 10^{-7}\)

    cupferron (\(\mu = 0.1 \text{ M}\)) cupferron.png 4.16 \(6.9 \times 10^{-5}\)
    cysteine cysteine.png

    [1.71] (COOH)

    8.36 (SH)

    10.77 (\(\ce{NH3}\))

    [\(1.9 \times 10^{-2}\)]

    \(4.4 \times 10^{-9}\)

    \(1.7 \times 10^{-11}\)

    dichloracetic acid \(\ce{Cl2CHCOOH}\) 1.30 \(5.0 \times 10^{-2}\)
    diethylamine \(\ce{(CH3CH2)2NH2+}\) 10.933 \(1.17 \times 10^{-11}\)
    dimethylamine \(\ce{(CH3)2NH2+}\) 10.774 \(1.68 \times 10^{-11}\)
    dimethylglyoxime dimethylglyoxime.png

    10.66

    12.0

    \(2.2 \times 10^{-11}\)

    \(1. \times 10^{-12}\)

    ethylamine \(\ce{CH3CH2NH3+}\) 10.636 \(2.31 \times 10^{-11}\)
    ethylenediamine \(\ce{+H3NCH2CH2NH3+}\)

    6.848

    9.928

    \(1.42 \times 10^{-7}\)

    \(1.18 \times 10^{-10}\)

    ethylenediaminetetracetic acid

    (EDTA) (\(\mu = 0.1 \text{ M}\))

    EDTA.png

    0.0 (COOH)

    1.5 (COOH)

    2.0 (COOH)

    2.66 (COOH)

    6.16 (NH)

    10.24 (NH)

    1.0

    \(3.2 \times 10^{-2}\)

    \(1.0 \times 10^{-2}\)

    \(2.2 \times 10^{-3}\)

    \(6.9 \times 10^{-7}\)

    \(5.8 \times 10^{-11}\)

    formic acid \(\ce{HCOOH}\) 3.745 \(1.80 \times 10^{-4}\)
    fumaric acid fumaric.png

    3.053

    4.494

    \(8.85 \times 10^{-4}\)

    \(3.21 \times 10^{-5}\)

    glutamic acid
    glutamic.png

    2.33 (\(\alpha\)-COOH)

    4.42 (\(\lambda\)-COOH)

    9.95 (\(\ce{NH3}\))

    \(5.9 \times 10^{-3}\)

    \(3.8\times 10^{-5}\)

    \(1.12 \times 10^{-10}\)

    glutamine glutamine.png

    2.17 (COOH)

    9.01 (\(\ce{NH3}\))

    \(6.8 \times 10^{-3}\)

    \(9.8 \times 10^{-10}\)

    glycine glycine.png

    2.350 (COOH)

    9.778 (\(\ce{NH3}\))

    \(4.47 \times 10^{-3}\)

    \(1.67 \times 10^{-10}\)

    glycolic acid \(\ce{HOOCH2COOH}\)

    3.881 (COOH)

    \(1.48 \times 10^{-4}\)
    histidine (\(\mu = 0.1 \text{ M}\)) histidine.png

    1.7 (COOH)

    6.02 (NH)

    9.08 (\(\ce{NH3}\))

    \(2. \times 10^{-2}\)

    \(9.5 \times 10^{-7}\)

    \(8.3 \times 10^{-10}\)

    hydrogen cyanide \(\ce{HCN}\) 9.21 \(6.2 \times 10^{-10}\)
    hydrogen fluroride \(\ce{HF}\) 3.17 \(6.8 \times 10^{-4}\)
    hydrogen peroxide \(\ce{H2O2}\) 11.65 \(2.2 \times 10^{-12}\)
    hydrogen sulfide \(\ce{H2S}\)

    7.02

    13.9

    \(9.5 \times 10^{-8}\)

    \(1.3 \times 10^{-14}\)

    hydrogen thiocyanate \(\ce{HSCN}\) 0.9 \(1.3 \times 10^{-1}\)
    8-hydroxyquinoline hydroxyquinoline.png

    4.9 (NH)

    9.81 (OH)

    \(1.2 \times 10^{-5}\)

    \(1.6 \times 10^{-10}\)

    hydroxylamine \(\ce{HONH3+}\) 5.96 \(1.1 \times 10^{-6}\)
    hypobromous acid \(\ce{HOBr}\) 8.63 \(2.3 \times 10^{-9}\)
    hypochlorous acid \(\ce{HOCl}\) 7.53 \(3.0\times 10^{-8}\)
    hypoiodous acid \(\ce{HOI}\) 10.64 \(2.3 \times 10^{-11}\)
    iodic acid \(\ce{HIO3}\) 0.77 \(1.7 \times 10^{-1}\)
    isoleucine
    isoleucine.png

    2.319 (COOH)

    9.754 (\(\ce{NH3}\))

    \(4.8 \times 10^{-3}\)

    \(1.76 \times 10^{-10}\)

    leucine leucine.png

    2.329 (COOH)

    9.747 (\(\ce{NH3}\))

    \(4.69 \times 10^{-3}\)

    \(1.79 \times 10^{-10}\)

    lysine (\(\mu = 0.1 \text{ M}\)) lysine.png

    2.04 (COOH)

    9.08 (\(\alpha \text{-} \ce{NH3}\))

    10.69 (\(\epsilon \text{-} \ce{NH3}\))

    \(9.1 \times 10^{-3}\)

    \(8.3 \times 10^{-10}\)

    \(2.0 \times 10^{-11}\)

    maleic acid maleic.png

    1.910

    6.332

    \(1.23 \times 10^{-2}\)

    \(4.66 \times 10^{-7}\)

    malic acid malic.png

    3.459 (COOH)

    5.097 (COOH)

    \(3.48 \times 10^{-4}\)

    \(8.00 \times 10^{-6}\)

    malonic acid \(\ce{HOOCCH2COOH}\)

    2.847

    5.696

    \(1.42 \times 10^{-3}\)

    \(2.01 \times 10^{-6}\)

    methionine (\(\mu = 0.1 \text{ M}\)) methionine.png

    2.20 (COOH)

    9.05 (\(\ce{NH3}\))

    \(6.3 \times 10^{-3}\)

    \(8.9 \times 10^{-10}\)

    methylamine \(\ce{CH3NH3+}\) 10.64 \(2.3 \times 10^{-11}\)
    2-methylanaline methylaniline2.png 4.447 \(3.57 \times 10^{-5}\)
    4-methylanaline methylaniline4.png 5.084 \(8.24 \times 10^{-6}\)
    2-methylphenol methylphenol2.png 10.28 \(5.2 \times 10^{-11}\)
    4-methylphenol methylphenol4.png 10.26 \(5.5 \times 10^{-11}\)

    nitrilotriacetic acid

    (\(T = 20 \text{°C}), pK_\text{a1: } \mu = 0.1 \text{ M}\))

    nitriloacetic.png

    1.1 (COOH)

    1.650 (COOH)

    2.940 (COOH)

    10.334 (\(\ce{NH3}\))

    \(8. \times 10^{-2}\)

    \(2.24 \times 10^{-2}\)

    \(1.15 \times 10^{-3}\)

    \(4.63 \times 10^{-11}\)

    2-nitrobenzoic acid nitrobenzoic2.png 2.179 \(6.62 \times 10^{-3}\)
    3-nitrobenzoic acid nitrobenzoic3.png 3.449 \(3.56 \times 10^{-4}\)
    4-nitrobenzoic acid nitrobenzoic4.png 3.442 \(3.61 \times 10^{-4}\)
    2-nitrophenol nitrophenol2.png 7.21 \(6.2 \times 10^{-8}\)
    3-nitrophenol
    nitrophenol3.png
    8.39 \(4.1 \times 10^{-9}\)
    4-nitrophenol nitrophenol4.png 7.15 \(7.1 \times 10^{-8}\)
    nitrous acid \(\ce{HNO2}\) 3.15 \(7.1 \times 10^{-4}\)
    oxalic acid \(\ce{H2C2O4}\)

    1.252

    4.266

    \(5.60 \times 10^{-2}\)

    \(5.42 \times 10^{-5}\)

    1,10-phenanthroline phenanthroline.png 4.86 \(1.38 \times 10^{-5}\)
    phenol phenol.png 9.98 \(1.05 \times 10^{-10}\)
    phenylalanine phenylalanine.png

    2.20 (COOH)

    9.31 (\(\ce{NH3}\))

    \(6.3 \times 10^{-3}\)

    \(4.9 \times 10^{-10}\)

    phosphoric acid \(\ce{H3PO4}\)

    2.148

    7.199

    12.35

    \(7.11 \times 10^{-3}\)

    \(6.32 \times 10^{-8}\)

    \(4.5 \times 10^{-13}\)

    phthalic acid phthalic.png

    2.950

    5.408

    \(1.12 \times 10^{-3}\)

    \(3.91 \times 10^{-6}\)

    piperdine piperdine.png 11.123 \(7.53 \times 10^{-12}\)
    proline proline.png

    1.952 (COOH)

    10.650 (NH)

    \(1.12 \times 10^{-2}\)

    \(2.29 \times 10^{-11}\)

    propanoic acid \(\ce{CH3CH2COOH}\)

    4.874

    \(1.34 \times 10^{-5}\)
    propylamine \(\ce{CH3CH2CH2NH3+}\) 10.566 \(2.72 \times 10^{-11}\)
    pyridine pyridine.png 5.229 \(5.90 \times 10^{-6}\)
    resorcinol resorcinal.png

    9.30

    11.06

    \(5.0 \times 10^{-10}\)

    \(8.7 \times 10^{-12}\)

    salicylic acid salicylic.png

    2.97 (COOH)

    13.74 (OH)

    \(1.1 \times 10^{-3}\)

    \(1.8 \times 10^{-14}\)

    serine serine.png

    2.187 (COOH)

    9.209 (\(\ce{NH3}\))

    \(6.50 \times 10^{-3}\)

    \(6.18 \times 10^{-10}\)

    succinic acid succinic.png

    4.207

    5.636

    \(6.21 \times 10^{-5}\)

    \(2.31 \times 10^{-6}\)

    sulfuric acid \(\ce{H2SO4}\)

    strong

    1.99

    \(1.0 \times 10^{-2}\)

    sulfurous acid \(\ce{H2SO3}\)

    1.91

    7.18

    \(1.2 \times 10^{-2}\)

    \(6.6 \times 10^{-8}\)

    D-tartaric acid tartaric.png

    3.036 (COOH)

    4.366 (COOH)

    \(9.20 \times 10^{-4}\)

    \(4.31 \times 10^{-5}\)

    threonine threoline.png

    2.088 (COOH)

    9.100 (\(\ce{NH3}\))

    \(8.17 \times 10^{-3}\)

    \(7.94 \times 10^{-10}\)

    thiosulfuric acid \(\ce{H2S2O3}\)

    0.6

    1.6

    \(3. \times 10^{-1}\)

    \(3. \times 10^{-2}\)

    trichloracetic acid (\(\mu = 0.1 \text{ M}\)) \(\ce{Cl3CCOOH}\) 0.66 \(2.2 \times 10^{-1}\)
    triethanolamine \(\ce{(HOCH2CH2)3NH+}\) 7.762 \(1.73 \times 10^{-8}\)
    triethylamine \(\ce{(CH3CH2)3NH+}\) 10.715 \(1.93 \times 10^{-11}\)
    trimethylamine \(\ce{(CH3)3NH+}\) 9.800 \(1.58 \times 10^{-10}\)
    tris(hydroxymethyl)amino methane (TRIS or THAM) \(\ce{(HOCH2)3CNH3+}\) 8.075 \(8.41 \times 10^{-9}\)
    tryptophan (\(\mu = 0.1 \text{ M}\))
    tryptophan.png

    2.35 (COOH)

    9.33 (\(\ce{NH3}\))

    \(4.5 \times 10^{-3}\)

    \(4.7 \times 10^{-10}\)

    tyrosine (\(pK_\text{a1: } \mu = 0.1 \text{ M}\)) tyrosine.png

    2.17 (COOH)

    9.19 (\(\ce{NH3}\))

    10.47 (OH)

    \(6.8 \times 10^{-3}\)

    \(6.5 \times 10^{-10}\)

    \(3.4 \times 10^{-11}\)

    valine valine.png

    2.286 (COOH)

    9.718 (\(\ce{NH3}\))

    \(5.18 \times 10^{-3}\)

    \(1.91 \times 10^{-10}\)

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