Acid strength is the tendency of an acid, symbolised by the chemical formula , to dissociate into a proton, , and an anion, . The dissociation or ionization of a strong acid in solution is effectively complete, except in its most concentrated solutions.
Examples of are hydrochloric acid (), perchloric acid (), nitric acid () and sulfuric acid ().
A weak acid is only partially dissociated, or is partly ionized in water with both the undissociated acid and its dissociation products being present, in solution, in equilibrium with each other.
Acetic acid () is an example of a weak acid. The strength of a weak acid is quantified by its acid dissociation constant, value.
The strength of a weak organic acid may depend on substituent effects. The strength of an inorganic acid is dependent on the oxidation state for the atom to which the proton may be attached. Acid strength is solvent-dependent. For example, hydrogen chloride is a strong acid in aqueous solution, but is a weak acid when dissolved in glacial acetic acid.
While the value measures the tendency of an acidic solute to transfer a proton to a standard solvent (most commonly water or DMSO), the tendency of an acidic solvent to transfer a proton to a reference solute (most commonly a weak aniline base) is measured by its Hammett acidity function, the value. Although these two concepts of acid strength often amount to the same general tendency of a substance to donate a proton, the and values are measures of distinct properties and may occasionally diverge. For instance, hydrogen fluoride, whether dissolved in water () or DMSO (), has values indicating that it undergoes incomplete dissociation in these solvents, making it a weak acid. However, as the rigorously dried, neat acidic medium, hydrogen fluoride has an value of –15, making it a more strongly protonating medium than 100% sulfuric acid and thus, by definition, a superacid. (To prevent ambiguity, in the rest of this article, "strong acid" will, unless otherwise stated, refer to an acid that is strong as measured by its value (). This usage is consistent with the common parlance of most practicing .)
When the acidic medium in question is a dilute aqueous solution, the is approximately equal to the pH value, which is a negative logarithm of the concentration of aqueous in solution. The pH of a simple solution of an acid in water is determined by both and the acid concentration. For weak acid solutions, it depends on the degree of dissociation, which may be determined by an equilibrium calculation. For concentrated solutions of acids, especially strong acids for which pH < 0, the value is a better measure of acidity than the pH.
Any acid with a value which is less than about −2 behaves as a strong acid. This results from the very high buffer capacity of solutions with a pH value of 1 or less and is known as the leveling effect.
The following are strong acids in aqueous and dimethyl sulfoxide solution. As mentioned above, because the dissociation is so strongly favored, the concentrations of and thus the values of cannot be measured experimentally. The values in the following table are average values from as many as 8 different theoretical calculations.
+Estimated p Ka values |
−2.0 ± 0.6 |
−6.8 ± 0.8 |
−10.9 ± 1 |
−14 ± 2 |
−15 ± 2 |
The following can be used as protonators in organic chemistry
, such as p-toluenesulfonic acid (tosylic acid) are a class of strong organic . Some sulfonic acids can be isolated as solids. Polystyrene functionalized into polystyrene sulfonate is an example of a substance that is a solid strong acid.
For a more rigorous treatment of acid strength see acid dissociation constant. This includes acids such as the dibasic acid succinic acid, for which the simple method of calculating the pH of a solution, shown above, cannot be used.
An important example of a solvent which is more basic than water is dimethyl sulfoxide, DMSO, . A compound which is a weak acid in water may become a strong acid in DMSO. Acetic acid is an example of such a substance. An extensive bibliography of values in solution in DMSO and other solvents can be found at Acidity–Basicity Data in Nonaqueous Solvents.
are strong acids even in solvents of low dielectric constant. Examples of superacids are fluoroantimonic acid and magic acid. Some superacids can be crystallised. They can also quantitatively stabilize .
Lewis acids reacting with Lewis bases in gas phase and non-aqueous solvents have been classified in the ECW model, and it has been shown that there is no one order of acid strengths. The relative acceptor strength of Lewis acids toward a series of bases, versus other Lewis acids, can be illustrated by ECW model.Laurence, C. and Gal, J-F. Lewis Basicity and Affinity Scales, Data and Measurement, (Wiley 2010) pp 50-51 The plots shown in this paper used older parameters. Improved E&C parameters are listed in ECW model. It has been shown that to define the order of Lewis acid strength at least two properties must be considered. For the qualitative HSAB theory the two properties are hardness and strength while for the quantitative ECW model the two properties are electrostatic and covalent.
2.86 |
4.0 |
4.5 |
4.5 |
−8† |
−1 |
2.0 |
7.53 |
|
|