The pH of Solutions of Weak Acids

Submitted by ChemPRIME Staff on Thu, 12/16/2010 - 14:52

Text below taken from Table of Some Acid and Base Constants

The pHA logarithmic measure of the concentration of hydrogen (hydronium) ion; pH = -log10([H+]) or pH = -log10([H3O+]). Scale

Substance pH [H3O+] [OH-]   pOH Strength
Battery acid   0 1 10-14 14 Strongly acidic
Stomach acid
Lemon juice
  1 10-1 10-13 13
  2 10-2 10-12 12
  3 10-3 10-11 11 Weakly acidic
Soda water   4 10-4 10-10 10
Black coffee   5 10-5 10-9   9 Barely acidic
  6 10-6 10-8   8
Pure water   7 10-7 10-7   7 Neutral
Seawater   8 10-8 10-6   6 Barely basic
Baking soda   9 10-9 10-5   5
Toilette soapA salt of a fatty acid produced by the saponification of fat. 10 10-10 10-4   4 Mildly basic
Laundry water 11 10-11 10-3   3
Household ammonia 12 10-12 10-2   2 Very basic
13 10-13 10-1   1
Drain cleaner 14 10-14 1   0

This table shows the relation of [H3O+] to [OH-], pH and pOH. The table also provides examples of substances at each pH. Simple household items such coffee, baking soda, lemon juice, and household cleaners cover a broad range of the pH spectrum. This table comes from CoreChem:pH and pOH.

IonizationA process in which an atom, molecule, or negative ion loses an electron; a process in which a covalent molecule reacts with a solvent to form positive and negative ions; for example, a weak acid reacting with water to form its conjugate base (an anion) and a hydrogen (hydronium) ion. Constants in Decreasing Order of Acid Strength at 25°C.

Acid Ka Base Kb
HClO4 131.8 ClO4– 7.587 × 10–17
H2SeO4 very large HSeO–4 very small
H2SO4 very large HSO–4 very small
HCl very large Cl– very small
HNO3 27.5 NO3– 3.64 × 10–16
N2H2+6 7.69 N2H+5 1.3 × 10–15
H3O+ 1 H2O 1 × 10–14
H2C2O4 5.5 × 10–2 HC2O–4 1.8 × 10–13
H3PO3 2.4 × 10–2 H2PO–3 4.2 × 10–13
HSeO–4 2.2 × 10–2 SeO2–4 4.5 × 10–13
H2SO3 1.7 × 10–2 HSO–3 5.9 × 10–13
HSO–4 1.1 × 10–2 SO2–4 9.1 × 10–13
H3PO4 7.2 × 10–3 H2PO–4 1.4 × 10–12
[Fe(H2O)6]3+ 6.76 × 10–3 [Fe(H2O)5(OH)]2+ 1.48 × 10–12
H3AsO4 6.17 × 10–3 H2AsO–4 1.62 × 10–12
H2SeO3 2.5 × 10–3 HSeO–3 4.0 × 10–12
H2TeO3 1.9 × 10–3 HTeO–3 5.3 × 10–12
H3C6H5O7 1.4 × 10–3 H2C6H5O–7 7.1 × 10–12
HNO2 7.11 × 10–4 NO–2 1.41 × 10–11
HF 6.8 × 10–4 F– 1.5 × 10–11
HCOOH 3.0 × 10–4 HCOO– 3.3 × 10–11
C3H6O3 1.48 × 10–4 C3H5O–3 6.76 × 10–11
HC2O–4 1.4 × 10–4 C2O2–4 7.1 × 10–11
C6H5COOH 1.2 × 10–4 C6H5COO– 8.3 × 10–11
H2C6H5O–7 4.5 × 10–5 HC6H5O2–7 2.2 × 10–10
C6H5NH+3 2.6 × 10–5 C6H5NH2 3.9 × 10–10
CH3COOH 1.8 × 10–5 CH3COO– 5.6 × 10–10
CH3CH2COOH 1.32 × 10–5 CH3CH2COO– 7.58 × 10–10
[Al(H2O)6]3+ 1.0 × 10–5 [Al(H2O)5(OH)]2+ 1.0 × 10–9
HN3 1.0 × 10–5 N–3 1.0 × 10–9
[Al(H2O)6]3+ 7.2 × 10–6 [Al(H2O)5(OH)]2+ 1.4 × 10–9
C5H5NH+ 6.3 × 10–6 C5H5N 1.6 × 10–9
HC6H5O2–7 1.5 × 10–6 C6H5O3–7 6.7 × 10–9
NH3OH + 1.1 × 10–6 NH2OH 9.3 × 10–9
(CH2)2(NH2)2H2+2 4.8 × 10–7 (CH2)2(NH2)2H+ 2.1 × 10–8
H2CO3 4.3 × 10–7 HCO–3 2.3 × 10–8
H2PO–3 2.9 × 10–7 HPO2–3 3.4 × 10–8
H2AsO–4 1.17 × 10–7 HAsO2–4 8.55 × 10–8
H2S 1 × 10–7 HS– 1 × 10–7
HClO 6.8 × 10–8 ClO– 1.5 × 10–7
H2PO–4 6.3 × 10–8 HPO2–4 1.6 × 10–7
HSO–3 6.3 × 10–8 SO2–3 1.6 × 10–7
[Cu(H2O)6]2+ 4.57 × 10–8 [Cu(H2O)5(OH)]+ 2.19 × 10–7
[Pb(H2O)6]2+ 1.38 × 10–8 [Pb(H2O)5(OH)]+ 7.25 × 10–7
N2H+5 8.3 × 10–9 N2H4 1.2 × 10–6
HSeO–3 1.6 × 10–9 SeO2–3 6.3 × 10–6
B(OH)3(H2O) 5.8 × 10–10 B(OH)–4 1.7 × 10–5
NH4+ 5.6 × 10–10 NH3 1.8 × 10–5
HTeO–3 4.6 × 10–10 TeO2–3 2.2 × 10–5
HCN 3.3 × 10–10 CN– 3.0 × 10–5
[Fe(H2O)6]2+ 2.63 × 10–10 [Fe(H2O)5(OH)]+ 3.80 × 10–5
HC6H5O 1.7 × 10–10 C6H5O– 5.9 × 10–5
(CH3)3NH+ 1.6 × 10–10 (CH3)3N 6.2 × 10–5
[Ni(H2O)6]2+ 1.38 × 10–10 [Ni(H2O)5(OH)]+ 7.25 × 10–5
(CH2)2(NH2)2H+ 1.3 × 10–10 (CH2)2(NH2)2 7.8 × 10–5
HCO–3 4.7 × 10–11 CO2–3 2.1 × 10–4
CH3NH+3 2.0 × 10–11 CH3NH2 5.0 × 10–4
(CH3)2NH+2 1.7 × 10–11 (CH3)2NH 5.8 × 10–4
HAsO2–4 3.09 × 10–12 AsO3–4 3.23 × 10–3
H2O2 2.1 × 10–12 H3O+ 4.8 × 10–3
HPO2–4 4.6 × 10–13 PO3–4 2.2 × 10–2
H2O 1.0 × 10–14 OH– 1.0
HS– 1 × 10–19 S2– 1 × 105
C2H5OH very small C2H5O– very large
NH3 very small NH2– very large
H2 very small H– very large
CH4 very small CH3– very large


  • Taken from Hogfelt, E. Perrin, D. D. Stability Constants of MetalAn element characterized by a glossy surface, high thermal and electrical conductivity, malleability, and ductility. Ion Complexes, 1st ed. Oxford; New Pergamon, 1979-1982. International Union of Pure and Applied Chemistry, Commission on EquilibriumA state in which no net change is occurring, that is, in which the concentrations of reactants and products remain constant; chemical equilibrium is characterized by forward and reverse reactions occurring at the same rate.. ISBN: 0080209580

Acid Constants at 25°C.

Acid Formula and Ionization Equation Ka
Acetic CH3COOH + H2O \rightleftharpoons H3O+ + CH3COO– 1.8 × 10–5
Aluminum ion Al(H2O)63+ + H2O \rightleftharpoons H3O+ + Al(H2O)5OH2+ 7.2 × 10–6
Ammonium ion NH4+ + H2O \rightleftharpoons H3O+ + NH3 5.6 × 10–10
Arsenic H3AsO4 + H2O \rightleftharpoons H3O+ + H2AsO–4

H2AsO–4 + H2O \rightleftharpoons H3O+ + HAsO2–4

HAsO2–4 + H2O \rightleftharpoons H3O+ + AsO3–4

K1 = 6.17 × 10–3

K2 = 1.17 × 10–7

K3 = 3.09 × 10–12

Benzoic C6H5COOH + H2O \rightleftharpoons H3O+ + C6H5COO– 1.2 × 10–4
Boric B(OH)3(H2O) + H2O \rightleftharpoons H3O+ + B(OH)–4 5.8 × 10–10
Carbonic H2CO3 + H2O \rightleftharpoons H3O+ + HCO–3

HCO–3 + H2O \rightleftharpoons H3O+ + CO2–3

K1 = 4.3 × 10–7

K2 = 4.7 × 10–11

Citric H3C6H5O7 + H2O \rightleftharpoons H3O+ + H2C6H5O–7

H2C6H5O–7 + H2O \rightleftharpoons H3O+ + HC6H5O2–7

HC6H5O2–7 + H2O \rightleftharpoons H3O+ + C6H5O3–7

K1 = 1.4 × 10–3

K2 = 4.5 × 10–5

K3 = 1.5 × 10–6

Formic HCOOH + H2O \rightleftharpoons H3O+ + HCOO– 3.0 × 10–4
Hexaaquaaluminum ion 1.0 × 10–5
Hexaaquacopper (II) ion 4.57 × 10–8
Hexaaquairon (II) ion 2.63 × 10–10
Hexaaquairon (III) ion 6.76 × 10–3
Hexaaqualead (II) ion 1.38 × 10–8
Hexaaquanickel (II) ion 1.38 × 10–10
Hydrazoic HN3 + H2O \rightleftharpoons H3O+ + N–3 1.0 × 10–5
Hydrochloric HCl + H2O \rightarrow H3O+ + Cl– very large
Hydrocyanic HCN + H2O \rightleftharpoons H3O+ + CN– 3.3 × 10–10
Hydrofluoric HF + H2O \rightleftharpoons H3O+ + F– 6.8 × 10–4
Hydrogen peroxide H2O2 + H2O \rightleftharpoons H3O+ + HO–2 2.1 × 10–12
Hydrosulfuric H2S + H2O \rightleftharpoons H3O+ + HS–

HS– + H2O \rightleftharpoons H3O+ + S2–

K1 = 1 × 10–7

K2 = 1 × 10–19

Hypochlorous HOCl + H2O \rightleftharpoons H3O+ + OCl– 6.8 × 10–8
Nitric HNO3 + H2O \rightleftharpoons H3O+ + NO3– 27.5
Lactic 1.48 × 10–4
Nitrous 7.11 × 10–4
Oxalic H2C2O4 + H2O \rightleftharpoons H3O+ + HC2O–4

HC2O–4 + H2O \rightleftharpoons H3O+ + C2O2–4

K1 = 5.5 × 10–2


K2 = 1.4 × 10–4

Perchloric 131.8
Phenol HC6H5O + H2O \rightleftharpoons H3O+ + C6H5O– 1.7 × 10–10
Phosphoric H3PO4 + H2O \rightleftharpoons H3O+ + H2PO–4

H2PO–4 + H2O \rightleftharpoons H3O+ + HPO2–4

HPO2–4 + H2O \rightleftharpoons H3O+ + PO3–4

K1 = 7.2 × 10–3

K2 = 6.3 × 10–8

K3 = 4.6 × 10–13

Phosphorous H3PO3 + H2O \rightleftharpoons H3O+ + H2PO–3

H2PO–3 + H2O \rightleftharpoons H3O+ + HPO2–3

K1 = 2.4 × 10–2

K2 = 2.9 × 10–7

Propanoic 1.32 × 10–5
Selenic H2SeO4 + H2O \rightleftharpoons H3O+ + HSeO–4

HSeO–4 + H2O \rightleftharpoons H3O+ + SeO2–4

K1 = very large

K2 = 2.2 × 10–2

Selenous H2SeO3 + H2O \rightleftharpoons H3O+ + HSeO–3

HSeO–3 + H2O \rightleftharpoons H3O+ + SeO2–3

K1 = 2.5 × 10–3

K2 = 1.6 × 10–9

Sulfuric H2SO4 + H2O \rightleftharpoons H3O+ + HSO–4

HSO–4 + H2O \rightleftharpoons H3O+ + SO2–4

K1 = very large

K2 = 1.1 × 10–2

Sulfurous H2SO3 + H2O \rightleftharpoons H3O+ + HSO–3

HSO–3 + H2O \rightleftharpoons H3O+ + SO2–3

K1 = 1.7 × 10–2

K2 = 6.3 × 10–8

Tellurous H2TeO3 + H2O \rightleftharpoons H3O+ + HTeO–3

HTeO–3 + H2O \rightleftharpoons H3O+ + TeO2–3

K1 = 1.9 × 10–3

K2 = 4.6 × 10–10



  • Taken from Hogfelt, E. Perrin, D. D. Stability Constants of Metal Ion Complexes, 1st ed. Oxford; New Pergamon, 1979-1982. International Union of Pure and Applied Chemistry, Commission on Equilibrium. ISBN: 0080209580

Base Constants at 25°C.

Base Formula and Ionization Equation Kb
Amide ion NH2– + H2O \rightarrow NH3 + OH– large
Ammonia NH3 + H2O \rightleftharpoons NH+4 + OH– 1.77 × 10–5
Aniline C6H5NH2 + H2O \rightleftharpoons C6H5NH+3 + OH– 3.9 × 10–10
Carbonate ion CO32– + H2O \rightleftharpoons HCO–3 + OH– 2.1 × 10–4
Dimethylamine (CH3)2NH + H2O \rightleftharpoons (CH3)2NH+2 + OH– 5.8 × 10–4
Ethylenediamine (CH2)2(NH2)2 + H2O \rightleftharpoons (CH2)2(NH2)2H+ + OH–

(CH2)2(NH2)2H+ + H2O \rightleftharpoons (CH2)2(NH2)2H2+2 + OH–

K1 = 7.8 × 10–5

K2 = 2.1 × 10–8

Hydrazine N2H4 + H2O \rightleftharpoons N2H+5 + OH–

N2H+5 + H2O \rightleftharpoons N2H2+6 + OH–

K1 = 1.2 × 10–6

K2 = 1.3 × 10–15

Hydride ion H– + H2O \rightarrow H2 + OH– large
Hydroxylamine NH2OH + H2O \rightleftharpoons NH3OH + + OH– 9.3 × 10–9
Methylamine CH3NH2 + H2O \rightleftharpoons CH3NH+3 + OH– 5.0 × 10–4
Phosphate ion PO43– + H2O \rightleftharpoons HPO2-4 + OH– 5.9 × 10–3
Pyridine C5H5N + H2O \rightleftharpoons C5H5NH+ + OH– 1.6 × 10–9
Trimethylamine (CH3)3N + H2O \rightleftharpoons (CH3)3NH+ + OH– 6.2 × 10–5



  • Taken from Hogfelt, E. Perrin, D. D. Stability Constants of Metal Ion Complexes, 1st ed. Oxford; New Pergamon, 1979-1982. International Union of Pure and Applied Chemistry, Commission on Equilibrium. ISBN: 0080209580

The first of the preceding two tables comes from CoreChem:The pH of Solutions of Weak Acids and contains acid constants(Ka) for a select set of acids.


This constant is defined by the expression:

K_{a}=\frac{\text{ }\!\![\!\!\text{ H}_{\text{3}}\text{O}^{\text{+}}\text{ }\!\!]\!\!\text{ }\!\![\!\!\text{ A}^{-}\text{ }\!\!]\!\!\text{ }}{\text{ }\!\![\!\!\text{ HA }\!\!]\!\!\text{ }}

The bottom table comes from CoreChem:The pH of Solutions of Weak Bases and contains base constants (Kb for a select set of bases.


This constant is defined by the expression:

K_{b}=\frac{\text{ }\!\![\!\!\text{ BH}^{\text{+}}\text{ }\!\!]\!\!\text{ }\!\![\!\!\text{ OH}^{-}\text{ }\!\!]\!\!\text{ }}{\text{ }\!\![\!\!\text{ B }\!\!]\!\!\text{ }}

From these tables, the Kb of the conjugate baseThe base formed when an acid releases a hydrogen ion (proton). for the acids given, and the Ka for the conjugate acidThe acid formed when a base accepts a hydrogen ion (proton). for the bases given can be found, using the relation given below for a conjugate acid-base pair:

      K_{a}\text{(HA) }\times \text{ }K_{b}\text{(A}^{-}\text{)}=K_{w}