Strong Acids and Weak Acids

Classification Scheme based on pKa

In water, “strong acids” dissociate completely into ions, while “weak acids” dissociate very slightly. In other words, the stronger the acid, the higher the H+ concentration at equilibrium. A classification based on pKa values (acidic strength) seems natural.

The dissociation of an acid HA is determined by its acidity constant Ka:

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

Let us denote the total amount of the acid by CT ≡ [HA]T (which is de facto the acid’s initial concentration before it dissolves in water). In the equilibrium state, the total concentration splits into its undissociated and dissociated parts:

(2) CT  =  [HA] + [A-]

Strong and weak acids then differ as follows (strongly simplified):

    Strong Acid Weak Acid
acidity constant:   Ka ≫ 1 Ka ≤ 1
pKa = -lg Ka   pKa < 0 pKa > 0
[H+] = 10-pH   [H+]  ≈  CT [H+]  ≪  CT
undissociated acid:   [HA]  ≈  0 [HA]  ≈  CT
dissociated acid:   [A-]  ≈  CT [A-]  ≪  CT

In literature, there is no sharp border line between what we call a strong acid and what we call a weak acid. More refined classification schemes distinguish even between very strong acids, strong acids, weak acids, very weak acids etc. Concerning aqion, however, we prefer the simple division into two groups:

  • Group 1:   acids with pKa < 0   (strong acids)1
  • Group 2:   acids with pKa > 0   (weak acids)

Polyprotic Acids. The idea remains valid even for N-protic acids, HNA. The acidity constant Ka should only be replaced by the 1st dissociation constant K1. The mathematical description is quite simple (see Appendix):

(3)   undissociated fraction  =    with  x = [H+] = 10-pH

The diagram below displays the undissociated fraction of some common acids (based on A1). The small circles mark the corresponding pK1 values. As expected, strong acids are completely dissolved in real-world applications (pH > 0).

undissociated fraction of strong and weak acids

Group 1:  Strong Acids with pKa < 0

Strong acids used in aqion are:

hydroiodic acid HI pKa = -10
hydrobromic acid HBr pKa = -9
hydrochloric acid HCl pKa = -6
sulfuric acid (1st dissociation step) H2SO4 pKa = -3
selenic acid (1st dissociation step) H2SeO4 pKa = -3
nitric acid HNO3 pKa = -1.32
chromic acid H2CrO4 pKa = -0.86

Note: Due to the fact that HI, HBr, HCl, H2SO4, H2SeO4, and HNO3 virtually do not exist in non-dissociate form their first dissociation step is not explicitly contained in the thermodynamic database.2

Example calculations with strong acids are presented here.

Group 2:  Acids with pKa > 0  (Weak Acids)

In contrast to the strong acids with negative pKa values, acids with pKa > 0 are explicitly defined in the thermodynamic database of aqion by their log K values. Here are some examples listed in the order of decreasing strength (valid for standard conditions at 25 and 1 atm):

Reaction Formula log K pKa Ref.
HSeO4- = H+ + SeO4-2 -1.66 1.66 [W]
HSO4- = H+ + SO4-2 -1.988 1.988 [W]
H3PO4 = H+ + H2PO4- -2.147 2.147 3 [M]
Fe+3 + H2O = H+ + FeOH+2 -2.19 2.19 [W]
H3AsO4 = H+ + H2AsO4- -2.3 2.3 [W]
H3Citrate = H+ + H2Citrate- -3.128 3.128 [M]
H2SeO3 = H+ + HSeO3- -3 3 [W]
HF = H+ + F- -3.18 3.18 [W]
HNO2 = H+ + NO2- -3.22 3.22 [E,L]
HFormate = H+ + Formate- -3.753 3.753 [M]
H2Se = H+ + HSe- -3.8 3.8 [W]
HLactate = H+ + Lactate- -3.863 3.863 [E,L]
H2MoO4 = H+ + HMoO4- -3.865 3.865 [M]
HMoO4- = H+ + MoO4-2 -4.290 4.290 [M]
HAcetate = H+ + Acetate- -4.757 4.757 [M]
H2Citrate- = H+ + HCitrate-2 -4.761 4.761 [M]
Al+3 + H2O = H+ + AlOH+2 -5.0 5.0 [W]
H2CO3* = H+ + HCO3- -6.351 6.351 4 [W]
HCitrate-2 = H+ + Citrate-3 -6.396 6.396 [M]
HCrO4- = H+ + CrO4-2 -6.509 6.509 [M]
H2S = H+ + HS- -6.994 6.994 [W]
H2AsO4- = H+ + HAsO4-2 -7.16 7.16 [W]
H2PO4- = H+ + HPO4-2 -7.207 7.207 [W]
HSeO3- = H+ + SeO3-2 -8.5 8.5 [W]
H3AsO3 = H+ + H2AsO3- -9.15 9.15 [W]
H3BO3 = H+ + H2BO3- -9.24 9.24 [W]
NH4+ = H+ + NH3 -9.252 9.252 [W]
H4SiO4 = H+ + H3SiO4- -9.83 9.83 [W]
HCO3- = H+ + CO3-2 -10.329 10.329 [W]
HAsO4-2 = H+ + AsO4-3 -11.65 11.65 [W]
HPO4-2 = H+ + PO4-3 -12.346 12.346 [W]
HS- = H+ + S-2 -12.918 12.918 [W]
H3SiO4- = H+ + H2SiO4-2 -13.17 13.17 [W]

As mentioned above, the strong acids with pKa < 0 (e.g. HI, HBr, HCl, H2SO4, HNO3) are not present in this table (and database). In addition, acidity constants of 60 organic acids are presented here.

All these acids are available as inorganic and organic reactants in the reaction tool (pH calculator). Calculated pH values for 1, 10, and 100 mM are shown in this table.

Appendix – Undissociated Fraction of Acid

Given is an N-protic acid HNA characterized by N acidity constants K1 to KN. The sum over all acid species defines the total concentration (mass balance):

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

The fraction of the undissociated species is expressed by the ionization fraction a0:

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

Its pH dependence is given by:5

(A3)  

where x is an abbreviation for [H+] = 10-pH.

References

[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: ‘thermo.com.V8.R6.230’ 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

Remarks

  1. Other classifications (Wikipedia) prefer the condition pKa < -2 because measurable pKa values range between -2 and 12. 

  2. Nonetheless, all strong-acid calculations are de facto exact (see examples here, here, here, and here). 

  3. The thermodynamic database wateq4f contains only the 2nd and 3rd dissociation step of phosphoric acid. The missing species “H2PO4” was extra implemented into aqion (in addition to the already present species H2PO4-, HPO4-2 and PO4-3). 

  4. It is common in hydrochemistry to use the composite carbonic acid, H2CO3* = CO2(aq) + H2CO3 instead of the true carbonic acid, H2CO3. In the thermodynamic database wateq4f (and in aqion) the composite carbonic acid is abbreviated by CO2

  5. Acid-Base Systems – Mathematical Background of Simple Closed-Form Expressions (pdf link

[last modified: 2017-05-28]