Is HNO2 a weak acid in water

Strong and weak acids

The degree of dissociation of an acid HA, i.e. how much H+ arises, is determined solely by the equilibrium constant (acid constant KS.) certainly:1

(1)HA = H+ + A-with KS. = [H+] [A-] / [HA]

Strength Acids dissociate completely in water, whereas dissociation occurs weaknesses Acids is incomplete. A classification based on the acid constant or the pKS.Value is therefore obvious.

Let C be the total amount of acidT ≡ [HA]T (which is nevertheless the initial concentration). In the equilibrium state, this is made up of two parts, the undissociated and the dissociated acid:

Strong and weak acids differ in the following respects (roughly simplified):

Strong acidWeak acidity
Equilibrium constant:KS. ≫ 1KS. ≤ 1
pKS. = -log KS.pKS. < 0pKS. > 0
[H+] = 10-pH[H+] ≈ CT[H+] ≪ CT
undissociated acid:[HA] ≈ 0[HA] ≈ CT
dissociated acid:[A-] ≈ CT[A-] ≪ CT

From what pKS.-The value of an acid as strong or weak is not so strictly defined. In the literature one also finds fine subdivisions into very strong, strong, weak and very weak acids. But the principle remains the same.

Our subdivision is based on the way the individual acids are used in hydrochemistry programs (PhreeqC or aqion) and the thermodynamic databases used, and there are only two groups:

strong acids:pKS. < 0(negative pKS.-Values)
weak acids:pKS. > 0(positive pKS.-Values)

Multi-protonic acids. In the case of N-protonic acids (H.NA) replaces KS. the acid constant K1 the 1st stage of dissociation. The proportion of undissociated acid as a function of pH can be calculated as follows (see appendix):

(3)undissociated part = \ (\ dfrac {1} {1 + K_1 / x} \)with x = [H+] = 10-pH

The following diagram shows the undissociated fraction of some strong and weak acids. The small circle symbols mark the corresponding pK1-Value. As expected: In the entire, practically relevant pH range (pH> 0 or -1), the strong acids are completely dissociated.

Group 1: Strong acids with pKS. < 0

Strong acids have a capital KS.Values ​​(or negative pKS.-Values). Typical examples are:

Hydriodic acidHIpKS. = -10
Hydrobromic acidHBrpKS. = -9
hydrochloric acidHClpKS. = -6
Sulfuric acid (1st dissociation stage)H2SO4pKS. = -3
Selenic acid (1st dissociation stage)H2SeO4pKS. = -3
nitric acidENT3pKS. = -1.32
Chromic acidH2CrO4pKS. = -0.86

The numerical treatment of this strong acids is particularly easy. Because HI, HBr, HCl, H2SO4 and ENT3 do not appear in undissociated form, they are as species in the thermodynamic database wateq4f [W] also not included.23

pH calculations for strong acids are here.

Group 2: acids with pKS. > 0 ("weak acids")

In the aqion acid constants used (or pKS.Values) can be read directly from the log K values ​​of the thermodynamic database. A selection of these is here (Standard Terms 25 and 1 atm):

Reaction equationlog KS.pKS.Ref.
HSeO4- = H+ + SeO4-2-1.661.66[W]
HSO4- = H+ + SO4-2-1.9881.988[W]
H3PO4 = H+ + H2PO4--2.1472.147 4[M]
Fe+3 + H2O = H+ + FeOH+2-2.192.19[W]
H3AsO4 = H+ + H2AsO4--2.32.3[W]
H3Citrate = H+ + H2Citrates--3.1283.128[M]
H2SeO3 = H+ + HSeO3--33[W]
HF = H+ + F--3.183.18[W]
ENT2 = H+ + NO2--3.223.22[E, L]
HFormate = H+ + Formats--3.7533.753[M]
H2Se = H+ + HSe--3.83.8[W]
HLactate = H+ + Lactates--3.8633.863[E, L]
H2MoO4 = H+ + HMoO4--3.8653.865[M]
HMoO4- = H+ + MoO4-2-4.2904.290[M]
HAcetate = H+ + Acetates--4.7574.757[M]
H2Citrates- = H+ + HCitrate-2-4.7614.761[M]
Al+3 + H2O = H+ + AlOH+2-5.05.0[W]
H2CO3* = H+ + HCO3--6.3516.351 5[W]
HCitrate-2 = H+ + Citrate-3-6.3966.396[M]
HCrO4- = H+ + CrO4-2-6.5096.509[M]
H2S = H+ + HS--6.9946.994[W]
H2AsO4- = H+ + HAsO4-2-7.167.16[W]
H2PO4- = H+ + HPO4-2-7.2077.207[W]
HSeO3- = H+ + SeO3-2-8.58.5[W]
H3AsO3 = H+ + H2AsO3--9.159.15[W]
H3BO3 = H+ + H2BO3--9.249.24[W]
NH4+ = H+ + NH3-9.2529.252[W]
H4SiO4 = H+ + H3SiO4--9.839.83[W]
HCO3- = H+ + CO3-2-10.32910.329[W]
HAsO4-2 = H+ + AsO4-3-11.6511.65[W]
HPO4-2 = H+ + PO4-3-12.34612.346[W]
HS- = H+ + S-2-12.91812.918[W]
H3SiO4- = H+ + H2SiO4-2-13.1713.17[W]

The acids are classified according to their strength. As already noted, the strong acids With pKS. < 0 (HI, HBr, HCl, H2SO4, ENT3). The table also lacks a large number of organic acids whose pKS.-Values ​​can be found here.

The pH of the acids depends on their concentration. For 1, 10 and 100 mM Calculated pH values ​​are given here (inorganic acids) and here (organic acids). These acids are available in the Reac module for pH calculation and chemical dosing.

Difference between weak and dilute acid

A weak and a dilute acid are as different as apples and pears. The first is based on the acid constant KS. (as a thermodynamic property that no one can change), while the second is based on the amount of acid or concentration CT is based in the water:

weak acidstrong acidKS. smallKS. large
dilute acidconcentrated acidC.T smallC.T large

You can't make a strong acid out of a weak acid, but you can change the degree of dilution (or concentration) as you like:

Acid starchDegree of dilution
determined by:Acid constant KS.Amount of acid CT
Relations:weak ↔ strong aciddilute ↔ concentrated acid
KS. small ↔ KS. largeC.T small ↔ CT large
(pKS. positive ↔ pKS.) negative
compares:two different acidsDilution of the same acid
describes:Release of H+Dilution of H+
Category:fundamental propertyadjustable parameter
(Can not do anything about it)(can be changed)

The corresponding assignment in the pK-CT-Parameter space looks like this:

Appendix - Proportion of undissociated acid

An N-protonic acid H is givenNA, which is represented by N equilibrium constants K1 to KN is characterized. The sum over all species gives the total concentration:

(A1)C.T ≡ [HNA]T = [HNA] + [HN-1A.-] +… + [A-N]

The proportion of undissociated species corresponds to the distribution coefficient a0:

(A2)undissociated part:a0 = [HNA] / CT

its pH dependence (expressed here by x = [H+] = 10-pH) is given by:6

(A3)\ (a_0 (x) \, = \, \ left (1+ \ dfrac {K_1} {x} + \ dfrac {K_1K_2} {x ^ 2} + \ dfrac {K_1 \ cdots K_N} {x ^ N} \ right) ^ {- 1} \ approx \, \ left (1+ \ dfrac {K_1} {x} \ right) ^ {- 1} \)


[E]Database EQ3 / 6 taken from: T.J. Wolery: EQ3 / 6, A Software Package for Geochemical Modeling of Aqueous Systems: Package Overview and Installation Guide (Version 7.0), Lawrence Livermore National Laboratory UCRL-MA-110662 PT I, Sep 1992
[L]Database llnl taken from: ‘’ prepared by Jim Johnson at Lawrence Livermore National Laboratory, in Geochemist’s Workbench format. Converted to Phreeqc format by Greg Anderson with help from David Parkhurst (llnl.dat 4023 2010-02-09 21: 02: 42Z dlpark)
[M]Database minteq taken from: J.D. Allison, D.S. Brown, K.J. Novo-Gradac: MINTEQA2 / PRODEFA2, A Geochemical Assessment Model for Environmental Systems, Version 3.0, User’s Manual, EPA / 600 / 3-91 / 021, March 1991
[W]Database wateq4f taken from: J.W. Ball and D.K. Nordstrom: WATEQ4F - User’s manual with revised thermodynamic data base and test cases for calculating speciation of major, trace and redox elements in natural waters, U.S.G.S. Open-File Report 90-129, 1991


[last modified: 2018-03-04]