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E5: Acid Dissociation Constants of Organics

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 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

\[K_a \times K_b = K_w\]

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

Compound

Conjugate Acid

pKa

Ka

acetic acid

CH3COOH

4.757

1.75×10–5

adipic acid

A11adipicacid.jpg

4.42

5.42

3.8×10–5

3.8×10–6

alanine

A11alanine.jpg

2.348 (COOH)

9.867 (NH3)

4.49×10–3

1.36×10–10

aminobenzene

A11aminobenzene.jpg

4.601

2.51×10–5

4-aminobenzene sulfonic acid

A114-aminobenzenesulfonicacid.jpg

3.232

5.86×10–4

2-aminobenozic acid

A112aminobenzoicacid.jpg

2.08 (COOH)

4.96 (NH3)

8.3×10–3

1.1×10–5

2-aminophenol (T = 20 oC)

A112aminophenol.jpg

4.78 (NH3)

9.97 (OH)

1.7×10–5

1.05×10–10

ammonia

NH4+

9.244

5.70×10–10

arginine A11arginine.jpg

1.823 (COOH)

8.991 (NH3)

[12.48] (NH2)

1.50×10–2

1.02×10–9

[3.3×10–13]

arsenic acid

H3AsO4

2.24
6.96
11.50
5.8×10–3
1.1×10–7
3.2×10–12
asparagine (μ = 0.1 M) A11asparagine.jpg 2.14 (COOH)
8.72 (NH3)
7.2×10–3
1.9×10–9
asparatic acid A11asparticacid.jpg 1.990 (α-COOH)
3.900 (β-COOH)
10.002 (NH3)
1.02×10–2
1.26×10–4
9.95×10–11
benzoic acid A11benzoicacid.jpg 4.202 6.28×10–5
benzylamine A11benzylamine.jpg 9.35 4.5×10–10
boric acid (pKa2, pKa3:T = 20 oC) H3BO3 9.236
[12.74]
[13.80]
5.81×10–10
[1.82×10–13]
[1.58×10–14]
carbonic acid H2CO3 6.352
10.329
4.45×10–7
4.69×10–11
catechol A11catechol.jpg 9.40
12.8
4.0×10–10
1.6×10–13
chloroacetic acid ClCH2COOH 2.865 1.36×10–3
chromic acid (pKa1:T = 20 oC) H2CrO4 -0.2
6.51
1.6
3.1×10–7
citric acid A11citricacid.jpg 3.128 (COOH)
4.761 (COOH)
6.396 (COOH)
7.45×10–4
1.73×10–5
4.02×10–7
cupferron (μ = 0.1 M) A11cupferrron.jpg 4.16 6.9×10–5
cysteine A11cysteine.jpg [1.71] (COOH)
8.36 (SH)
10.77 (NH3)
[1.9×10–2]
4.4×10–9
1.7×10–11
dichloracetic acid Cl2CHCOOH 1.30 5.0×10–2
diethylamine (CH3CH2)2NH2+ 10.933 1.17×10–11
dimethylamine (CH3)2NH2+ 10.774 1.68×10–11
dimethylglyoxime A11dimethylglyoxime.jpg 10.66
12.0
2.2×10–11
1.×10–12
ethylamine CH3CH2NH3+ 10.636 2.31×10–11
ethylenediamine +H3NCH2CH2NH3+ 6.848
9.928
1.42×10–7
1.18×10–10
ethylenediaminetetraacetic acid
(EDTA) (μ = 0.1 M)
A11EDTA.jpg 0.0 (COOH)
1.5 (COOH)
2.0 (COOH)
2.66 (COOH)
6.16 (NH)
10.24 (NH)
1.0
3.2×10–2
1.0×10–2
2.2×10–3
6.9×10–7
5.8×10–11
formic acid HCOOH 3.745 1.80×10–4
fumaric acid A11fumaricacid.jpg 3.053
4.494
8.85×10–4
3.21×10–5
glutamic acid A11glutamicacid.jpg 2.33 (α-COOH)
4.42 (λ-COOH)
9.95 (NH3)
5.9×10–3
3.8×10–5
1.12×10–10
glutamine (μ = 0.1 M) A11glutamine.jpg 2.17 (COOH)
9.01 (NH3)
6.8×10–3
9.8×10–10
glycine +H3NCH2COOH A11glycine.jpg 2.350 (COOH)
9.778 (NH3)
4.47×10–3
1.67×10–10
glycolic acid HOOCH2COOH 3.831 (COOH) 1.48×10–4
histidine (μ = 0.1 M) A11histidine.jpg 1.7 (COOH)
6.02 (NH)
9.08 (NH3)
2.×10–2
9.5×10–7
8.3×10–10
hydrogen cyanide HCN 9.21 6.2×10–10
hydrogen fluoride HF 3.17 6.8×10–4
hydrogen peroxide H2O2 11.65 2.2×10–12
hydrogen sulfide H2S 7.02
13.9
9.5×10–8
1.3×10–14
hydrogen thiocyanate HSCN 0.9 1.3×10–1
8-hydroxyquinoline A118-hydroxyquinoline.jpg 4.91 (NH)
9.81 (OH)
1.2×10–5
1.6×10–10
hydroxylamine HONH3+ 5.96 1.1×10–6
hypobromous acid HOBr 8.63 2.3×10–9
hypochlorous acid HOCl 7.53 3.0×10–8
hypoiodous acid HOI 10.64 2.3×10–11
iodic acid HIO3 0.77 1.7×10–1
isoleucine A11isoleucine.jpg 2.319 (COOH)
9.754 (NH3)
4.80×10–3
1.76×10–10
leucine A11leucine.jpg 2.329 (COOH)
9.747 (NH3)
4.69×10–3
1.79×10–10
lysine (μ = 0.1 M) A11lysine.jpg 2.04 (COOH)
9.08 (α-NH3)
10.69 (ε-NH3)
9.1×10–3
8.3×10–10
2.0×10–11
maleic acid A11maleicacid.jpg 1.910
6.332
9.1×10–3
9.1×10–3
malic acid A11malicacid.jpg 3.459 (COOH)
5.097 (COOH)
9.1×10–3
9.1×10–3
malonic acid HOOCCH2COOH 2.847
5.696
9.1×10–3
9.1×10–3
methionine (μ = 0.1 M) A11methionine.jpg 2.20 (COOH)
9.05 (NH3)
9.1×10–3
9.1×10–3
methylamine CH3NH3+ 10.64 9.1×10–3
2-methylanaline A112-methylanaline.jpg 4.447 9.1×10–3
4-methylanaline A114-methylanaline.jpg 5.084 9.1×10–3
2-methylphenol A112-methylphenol.jpg 10.28 9.1×10–3
4-methylphenol A114-methylphenol.jpg 10.26 9.1×10–3
nitrilotriacetic acid (T = 20 oC)
(pKa1: μ = 0.1 m)
A11nitrilotriaceticacid.jpg 1.1 (COOH)
1.650 (COOH)
2.940 (COOH)
10.334 (NH3)
9.1×10–3
9.1×10–3
9.1×10–3
9.1×10–3
2-nitrobenzoic acid A112-nitrobenzoicacid.jpg 2.179 9.1×10–3
3-nitrobenzoic acid A113-nitrobenzoicacid.jpg 3.449 9.1×10–3
4-nitrobenzoic acid A114-nitrobenzoicacid.jpg 3.442 3.61×10–4
2-nitrophenol A112-nitrophenol.jpg 7.21 6.2×10–8
3-nitrophenol A113-nitrophenol.jpg 8.39 4.1×10–9
4-nitrophenol A114-nitrophenol.jpg 7.15 7.1×10–8
nitrous acid HNO2 3.15 7.1×10–4
oxalic acid H2C2O4 1.252
4.266
5.60×10–2
5.42×10–5
1,10-phenanthroline A111,10-phenanthroline.jpg 4.86 1.38×10–5
phenol A11phenol.jpg 9.98 1.05×10–10
phenylalanine A11phenylalanine.jpg 2.20 (COOH)
9.31 (NH3)
6.3×10–3
4.9×10–10
phosphoric acid H3PO4 2.148
7.199
12.35
7.11×10–3
6.32×10–8
4.5×10–13
phthalic acid A11phthalicacid.jpg 2.950
5.408
1.12×10–3
3.91×10–6
piperdine A11piperdine.jpg 11.123 7.53×10–12
proline A11proline.jpg 1.952 (COOH)
10.640 (NH)
1.12×10–2
2.29×10–11
propanoic acid CH3CH2COOH 4.874 1.34×10–5
propylamine CH3CH2CH2NH3+ 10.566 2.72×10–11
pryidine A11pryidine.jpg 5.229 5.90×10–6
resorcinol A11resorcinol.jpg 9.30
11.06
5.0×10–10
8.7×10–12
salicylic acid A11salicylicacid.jpg 2.97 (COOH)
13.74 (OH)
1.1×10–3
1.8×10–14
serine A11serine.jpg 2.187 (COOH)
9.209 (NH3)
6.50×10–3
6.18×10–10
succinic acid A11succinicacid.jpg 4.207
5.636
6.21×10–5
2.31×10–6
sulfuric acid H2SO4 strong
1.99

1.0×10–2
sulfurous acid H2SO3 1.91
7.18
1.2×10–2
6.6×10–8

D-tartaric acid

A11d-tartaricacid.jpg 3.036 (COOH)
4.366 (COOH)
9.20×10–4
4.31×10–5
threonine A11threonine.jpg 2.088 (COOH)
9.100 (NH3)
8.17×10–3
7.94×10–10
thiosulfuric acid H2S2O3 0.6
1.6
3.×10–1
3.×10–2
trichloroacetic acid (μ = 0.1 M) Cl3CCOOH 0.66 2.2×10–1
triethanolamine (HOCH2CH2)3NH+ 7.762 1.73×10–8
triethylamine (CH3CH2)3NH+ 10.715 1.93×10–11
trimethylamine (CH3)3NH+ 9.800 1.58×10–10
tris(hydroxymethyl)amino methane (TRIS or THAM) (HOCH2)3CNH3+ 8.075 8.41×10–9
tryptophan (μ = 0.1 M) A11tryptophan.jpg 2.35 (COOH)
9.33 (NH3)
4.5×10–3
4.7×10–10
tryosine (pKa1: μ = 0.1 M) A11tryosine.jpg 2.17 (COOH)
9.19 (NH3)
10.47 (OH)
6.8×10–3
6.5×10–10
3.4×10–11
valine A11valine.jpg 2.286 (COOH)
9.718 (NH3)
5.18×10–3
1.91×10–10