 |
3.9.7 Acidity constants
Acidity constants in water at 25°C
The acidity constant Ka of an
acid-base equilibrium A
 B + H+ is defined as the limiting value at infinite
dilution of [B][H+]/[A] and pKa is an abbreviation
for −log Ka. The values in the following table relate
to aqueous solutions at 25 °C and are from R. A. Robinson and R. H. Stokes
(1970) Electrolyte Solutions, 2nd revised edn, 517–535, 561,
Butterworth, London.
|
Name |
Formula |
pKa1 |
Formula |
pKa2 |
pKa3 |
|
|
|
|
|
|
|
|
Oxalic .
. . . .
. . |
H2C2O4 |
1.271 |
HC2O4− |
4.266 |
|
|
Phosphoric .
. . . .
. |
H3PO4 |
2.148 |
H2PO4− |
7.198 |
|
|
|
|
|
HPO4− |
|
12.375 |
|
Glycine
. . . .
. . |
+H3N.CH2.CO2H |
2.350 |
+H3N.CH2.CO2− |
9.780 |
|
|
p-Aminobenzoic .
. . . . |
+H3N.C6H4.CO2H |
2.413 |
+H3N.C6H4.CO2− |
4.853 |
|
|
Cyanoacetic
. . . .
. |
CH2CN.CO2H |
2.469 |
|
|
|
|
Malonic
. . . .
. . |
CH2.(CO2H)2 |
2.847 |
(CO2H)CH2.CO2− |
5.696 |
|
|
Chloroacetic
. . . .
. |
CH2Cl.CO2H |
2.865 |
|
|
|
|
Bromoacetic
. . . .
. |
CH2Br.CO2H |
2.901 |
|
|
|
|
o-Phthalic
. . . .
. |
C6H4.(CO2H)2 |
2.950 |
(CO2H)C6H4.CO2− |
5.408 |
|
|
o-Hydroxybenzoic
. . . |
HO.C6H4.CO2H |
2.996 |
|
|
|
|
Tartaric .
. . . .
. |
C2H4O2.(CO2H)2 |
3.033 |
(CO2H)C2H4O2.CO2− |
4.366 |
|
|
Citric .
. . . .
. . |
C3H5O.(CO2H)3 |
3.128 |
(CO2H)2C3H5O.CO2− |
4.761 |
|
|
|
|
|
CO2HC3H5O.(CO2−)2 |
|
6.396 |
|
Iodoacetic
. . . .
. |
CH2I.CO2H |
3.174 |
|
|
|
|
2,6-Dinitrophenol
. . . . |
(O2N)2.C6H4.OH |
3.725 |
|
|
|
|
Formic
. . . .
. . |
H.CO2H |
3.752 |
|
|
|
|
Glycolic .
. . . .
. |
CH2OH.CO2H |
3.831 |
|
|
|
|
Lactic
. . . .
. . |
CH3.CHOH.CO2H |
3.860 |
|
|
|
|
2,4-Dinitrophenol
. . . . |
(O2N)2.C6H4.OH |
4.09 |
|
|
|
|
Benzoic .
. . . .
. |
C6H5.CO2H |
4.203 |
|
|
|
|
Succinic .
. . . . |
C2H4.(CO2H)2 |
4.207 |
(CO2H)C2H4
.CO2− |
5.638 |
|
|
Anilinium .
. . . . |
C6H5.NH+3 |
4.596 |
|
|
|
|
Acetic .
. . . .
. |
CH3.CO2H |
4.756 |
|
|
|
|
n-Butyric
. . . .
. . |
C3H7.CO2H |
4.820 |
|
|
|
|
n-Propionic
. . . .
. |
C2H5.CO2H |
4.874 |
|
|
|
|
2,4-Dinitrophenol.
. . . |
(O2N)2.C6H4.OH |
4.959 |
|
|
|
|
2,5-Dinitrophenol.
. . . |
(O2N)2.C6H4.OH |
5.231 |
|
|
|
|
3,4-Dinitrophenol.
. . . |
(O2N)2.C6H4.OH |
5.422 |
|
|
|
|
Hydroxylammonium .
. . |
NH3OH+ |
5.96 |
|
|
|
|
Carbonic .
. . . .
. |
H2CO3 |
6.352 |
HCO3− |
10.329 |
|
3,5-Dinitrophenol
. . . |
(O2N)2.C6H4.OH |
6.692 |
|
|
|
|
p-Nitrophenol
. . . . |
O2N.C6H4.OH |
7.156 |
|
|
|
|
o-Nitrophenol
. . .
. |
O2N.C6H4.OH |
7.230 |
|
|
|
|
Triethanolammonium
. . . |
(C2H4OH)3.NH+ |
7.762 |
|
|
|
|
Tris-hydroxymethyl-methyl-
ammonium .
. . . |
(CH2OH)3.CNH3+ |
8.069 |
|
|
|
|
m-Nitrophenol
. . . .
. |
O2N.C6H4.OH |
8.355 |
|
|
|
|
Diethanolammonium
. . . . |
(C2H4OH)2.NH2+ |
8.883 |
|
|
|
|
Phenol-sulphonate
. . . . |
HO.C6H4.SO3− |
9.053 |
|
|
|
|
Hydrogen cyanide
. . . . |
HCN |
9.218 |
|
|
|
|
Boric .
. . . .
. . |
H3BO3 |
9.234 |
|
|
|
|
Ammonium .
. . . .
. |
NH+4 |
9.245 |
|
|
|
|
Ethanol-ammonium .
. . . . |
C2H4OH.NH3+ |
9.499 |
|
|
|
|
Trimethyl-ammonium
. . . |
(CH3)3.NH+ |
9.800 |
|
|
|
|
Phenol .
. . . .
. . |
C6H5OH |
10.020 |
|
|
|
|
m-Cresol .
. . . .
. . |
CH3.C6H4.OH |
10.098 |
|
|
|
|
p-Cresol
. . . .
. . . |
CH3.C6H4.OH |
10.276 |
|
|
|
|
o-Cresol
. . . .
. . . |
CH3.C6H4.OH |
10.330 |
|
|
|
|
Methyl-ammonium
. . . . |
CH3.NH3+ |
10.624 |
|
|
|
|
Ethyl-ammonium .
. . . . |
C2H5.NH3+ |
10.631 |
|
|
|
|
Triethyl-ammonium
. . . . |
(C2H5)3.NH+ |
10.715 |
|
|
|
|
Dimethyl-ammonium
. . . |
(CH3)2.NH2+ |
10.774 |
|
|
|
|
Diethyl-ammonium
. . . . |
(C2H5)2.NH2+ |
10.933 |
|
|
|
|
Piperidinium
. . . .
. . |
C5H10NH2+ |
11.123 |
|
|
|
|
|
|
|
|
|
|
****
References
Extensive tables of acidity constants will also be found
in:
Kortüm, Vogel and Andrussow (1961) Dissociation Constants of
Organic Acids in Aqueous Solution,
Butterworth,
London.
Perrin (1965, 1972) Dissociation Constants of Organic Bases in
Aqueous Solution, Supplement, Butterworth,
London.
Perrin (1969) Dissociation Constants of
Inorganic Acids and Bases in Aqueous Solution, Butterworth, London.
Serjeant and Demsey (1979) Ionization Constants of Organic Acids in Aqueous
Solution, IUPAC Chemical
Data Series, No. 23,
Pergamon.
Perrin (1982) Ionization Constants of Inorganic Acids and
Bases in Aqueous Solution, IUPAC Chemical
Data
Series, No. 29, Pergamon.
A.K. Covington
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