Acid strength is the tendency of an
acid, symbolised by the
chemical formula
In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbol ...
HA, to dissociate into a
proton,
H+, and an
anion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
,
A-. The
dissociation
Dissociation, in the wide sense of the word, is an act of disuniting or separating a complex object into parts. Dissociation may also refer to:
* Dissociation (chemistry), general process in which molecules or ionic compounds (complexes, or salts ...
of a strong acid in solution is effectively complete, except in its most concentrated solutions.
:
HA -> H+ + A-
Examples of
strong acid
Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a proton, H+, and an anion, A-. The dissociation of a strong acid in solution is effectively complete, except in its most concentrated solutions ...
s are
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
(HCl),
perchloric acid
Perchloric acid is a mineral acid with the formula H Cl O4. Usually found as an aqueous solution, this colorless compound is a stronger acid than sulfuric acid, nitric acid and hydrochloric acid. It is a powerful oxidizer when hot, but aqueous s ...
(HClO4),
nitric acid
Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...
(HNO3) and
sulfuric acid (H2SO4).
A weak acid is only partially dissociated, with both the undissociated acid and its dissociation products being present, in solution, in
equilibrium with each other.
:
HA <=> H+ + A-
Acetic acid (
CH3COOH) 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
In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemist ...
acid is dependent on the
oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
for the atom to which the proton may be attached. Acid strength is solvent-dependent. For example,
hydrogen chloride
The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride ga ...
is a strong acid in aqueous solution, but is a weak acid when dissolved in
glacial acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...
.
Measures of acid strength
The usual measure of the strength of an acid is its
acid dissociation constant (
), which can be
determined experimentally by
titration
Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte (a substance to be analyzed). A reagent, termed the ''titrant'' ...
methods. Stronger acids have a larger
and a smaller logarithmic constant (
) than weaker acids. The stronger an acid is, the more easily it loses a proton,
H+. Two key factors that contribute to the ease of
deprotonation
Deprotonation (or dehydronation) is the removal (transfer) of a proton (or hydron, or hydrogen cation), (H+) from a Brønsted–Lowry acid in an acid–base reaction.Henry Jakubowski, Biochemistry Online Chapter 2A3, https://employees.csbsju. ...
are the
polarity of the
H-A bond and the size of atom A, which determine the strength of the
H-A bond. Acid strengths also depend on the stability of the conjugate base.
While the
value measures the tendency of an acidic solute to transfer a proton to a standard solvent (most commonly water or
DMSO
Dimethyl sulfoxide (DMSO) is an organosulfur compound with the formula ( CH3)2. This colorless liquid is the sulfoxide most widely used commercially. It is an important polar aprotic solvent that dissolves both polar and nonpolar compounds ...
), the tendency of an acidic solvent to transfer a proton to a reference solute (most commonly a weak
aniline
Aniline is an organic compound with the formula C6 H5 NH2. Consisting of a phenyl group attached to an amino group, aniline is the simplest aromatic amine. It is an industrially significant commodity chemical, as well as a versatile starti ...
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 (
= 3.2) or DMSO (
= 15), 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 (
< –1.74). This usage is consistent with the common parlance of most practicing
chemists
A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties. Chemists carefully describe th ...
.)
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
H+ 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.
Strong acids
![Strong_acid](https://upload.wikimedia.org/wikipedia/commons/e/e2/Strong_acid.png)
A ''strong acid'' is an acid that dissociates according to the reaction
:
HA + S <=> SH+ + A-
where S represents a solvent molecule, such as a molecule of water or
dimethyl sulfoxide (DMSO), to such an extent that the concentration of the undissociated species
HA is too low to be measured. For practical purposes a strong acid can be said to be completely dissociated. An example of a strong acid is
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
:
HCl -> H+ + Cl- (in aqueous solution)
Any acid with a
value which is less than about -2 is classed as a strong acid. This results from the very high
buffer capacity
A buffer solution (more precisely, pH buffer or hydrogen ion buffer) is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. Its pH changes very little when a small amount of strong acid or base is ...
of solutions with a
pH value
In chemistry, pH (), historically denoting "potential of hydrogen" (or "power of hydrogen"), is a scale used to specify the acidity or basicity of an aqueous solution. Acidic solutions (solutions with higher concentrations of ions) are ...
of 1 or less and is known as the
leveling effect
Leveling effect or solvent leveling refers to the effect of solvent on the properties of acids and bases. The strength of a strong acid is limited ("leveled") by the basicity of the solvent. Similarly the strength of a strong base is leveled by ...
.
The following are strong acids in aqueous and dimethyl sulfoxide solution. The values of
, cannot be measured experimentally. The values in the following table are average values from as many as 8 different theoretical calculations.
:
Also, in water
*
Nitric acid
Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...
HNO3 = −1.6
*
Sulfuric acid H2SO4 (first dissociation only,
≈ −3)
The following can be used as protonators in
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, ...
*
Fluoroantimonic acid
Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony pentafluoride, containing various cations and anions (the simplest being and ). This substance is a superacid that can be over a billion times stronger than 100% pure sulfuri ...
H bF6/chem>
* Magic acid H SO3SbF5/chem>
* Carborane superacid
Carborane acids (X, Y, Z = H, Alk, F, Cl, Br, CF3) are a class of superacids, some of which are estimated to be at least one million times stronger than 100% pure sulfuric acid in terms of their Hammett acidity function values (''H''0 ≤ –18) ...
H HB11Cl11/chem>
* Fluorosulfuric acid H SO3/chem>( = −6.4)[
Sulfonic acids, such as ]p-toluenesulfonic acid
''p''-Toluenesulfonic acid (PTSA or ''p''TsOH) or tosylic acid (TsOH) is an organic compound with the formula CH3 C6H4 SO3H. It is a white extremely hygroscopic solid that is soluble in water, alcohols, and other polar organic solvents. The CH3C ...
(tosylic acid) are a class of strong organic oxyacid
An oxyacid, oxoacid, or ternary acid is an acid that contains oxygen. Specifically, it is a compound that contains hydrogen, oxygen, and at least one other element, with at least one hydrogen atom bonded to oxygen that can dissociate to produce ...
s. Some sulfonic acids can be isolated as solids. Polystyrene functionalized into polystyrene sulfonate
Polystyrene sulfonates are a group of medications used to treat high blood potassium. Effects generally take hours to days. They are also used to remove potassium, calcium, and sodium from solutions in technical applications.
Common side effect ...
is an example of a substance that is a solid strong acid.
Weak acids
A weak acid is a substance that partially dissociates when it is dissolved in a solvent. In solution there is an equilibrium between the acid, HA, and the products of dissociation.
:
The solvent (e.g. water) is omitted from this expression when its concentration is effectively unchanged by the process of acid dissociation. The strength of a weak acid can be quantified in terms of a dissociation constant, , defined as follows, where '' /chem>'' signifies the concentration of a chemical moiety, X.
:
When a numerical value of is known it can be used to determine the extent of dissociation in a solution with a given concentration of the acid, , by applying the law of conservation of mass
In physics and chemistry, the law of conservation of mass or principle of mass conservation states that for any system closed to all transfers of matter and energy, the mass of the system must remain constant over time, as the system's mass can ...
.
:
where '''' is the value of the analytical concentration
Molar concentration (also called molarity, amount concentration or substance concentration) is a measure of the concentration of a chemical species, in particular of a solute in a solution, in terms of amount of substance per unit volume of solut ...
of the acid. When all the quantities in this equation are treated as numbers, ionic charges are not shown and this becomes a quadratic equation
In algebra, a quadratic equation () is any equation that can be rearranged in standard form as
ax^2 + bx + c = 0\,,
where represents an unknown value, and , , and represent known numbers, where . (If and then the equation is linear, not q ...
in the value of the hydrogen ion concentration value, /chem>.
:
This equation shows that the pH of a solution of a weak acid depends on both its value and its concentration. Typical examples of weak acids include acetic acid and phosphorous acid
Phosphorous acid (or phosphonic acid (singular)) is the compound described by the formula H3PO3. This acid is diprotic (readily ionizes two protons), not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate in th ...
. An acid such as oxalic acid (HOOC-COOH) is said to be dibasic because it can lose two protons and react with two molecules of a simple base. Phosphoric acid
Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...
(H3PO4) is tribasic.
For a more rigorous treatment of acid strength see acid dissociation constant. This includes acids such as the dibasic acid succinic acid
Succinic acid () is a dicarboxylic acid with the chemical formula (CH2)2(CO2H)2. The name derives from Latin ''succinum'', meaning amber. In living organisms, succinic acid takes the form of an anion, succinate, which has multiple biological ro ...
, for which the simple method of calculating the pH of a solution, shown above, cannot be used.
Experimental determination
The experimental determination of a value is commonly performed by means of a titration
Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte (a substance to be analyzed). A reagent, termed the ''titrant'' ...
. A typical procedure would be as follows. A quantity of strong acid is added to a solution containing the acid or a salt of the acid, to the point where the compound is fully protonated. The solution is then titrated with a strong base
:HA + OH- -> A- + H2O
until only the deprotonated species, A-, remains in solution. At each point in the titration pH is measured using a glass electrode
A glass electrode is a type of ion-selective electrode made of a doped glass membrane that is sensitive to a specific ion. The most common application of ion-selective glass electrodes is for the measurement of pH. The pH electrode is an exampl ...
and a pH meter
A pH meter is a scientific instrument that measures the hydrogen-ion activity in water-based solutions, indicating its acidity or alkalinity expressed as pH. The pH meter measures the difference in electrical potential between a pH elect ...
. The equilibrium constant is found by fitting calculated pH values to the observed values, using the method of least squares.
Conjugate acid/base pair
It is sometimes stated that "the conjugate of a weak acid is a strong base". Such a statement is incorrect. For example, acetic acid is a weak acid which has a = 1.75 x 10−5. Its conjugate base is the acetate ion with ''K''b = 10−14/''K''a = 5.7 x 10−10 (from the relationship ''K''a × ''K''b = 10−14), which certainly does not correspond to a strong base. The conjugate of a weak acid is often a weak base and ''vice versa''.
Acids in non-aqueous solvents
The strength of an acid varies from solvent to solvent. An acid which is strong in water may be weak in a less basic solvent, and an acid which is weak in water may be strong in a more basic solvent. According to Brønsted–Lowry acid–base theory
The Brønsted–Lowry theory (also called proton theory of acids and bases) is an acid–base reaction theory which was proposed independently by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923. The fundamental concept of this the ...
, the solvent S can accept a proton.
:HA + S<=> A- + HS+
For example, hydrochloric acid is a weak acid in solution in pure acetic acid, HO2CCH3, which is more acidic than water.
:HO2CCH3 + HCl <=> (HO)2CCH3+ + Cl-
The extent of ionization of the hydrohalic acids decreases in the order HI > HBr > HCl. Acetic acid is said to be a differentiating solvent for the three acids, while water is not.
An important example of a solvent which is more basic than water is dimethyl sulfoxide, DMSO, (CH3)2SO. 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 a
Acidity–Basicity Data in Nonaqueous Solvents
Superacids are strong acids even in solvents of low dielectric constant. Examples of superacids are fluoroantimonic acid
Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony pentafluoride, containing various cations and anions (the simplest being and ). This substance is a superacid that can be over a billion times stronger than 100% pure sulfuri ...
and magic acid. Some superacids can be crystallised. They can also quantitatively stabilize carbocations.
Lewis acids
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any spe ...
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 C-B plots.[ 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
HSAB concept is a jargon for "hard and soft (Lewis) acids and bases". HSAB is widely used in chemistry for explaining stability of compounds, reaction mechanisms and pathways. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemic ...
the two properties are hardness and strength while for the quantitative ECW model the two properties are electrostatic and covalent.
Factors determining acid strength
The inductive effect
In organic carboxylic acids, an electronegative substituent can pull electron density out of an acidic bond through the inductive effect
In chemistry, the inductive effect in a molecule is a local change in the electron density due to electron-withdrawing or electron-donating groups elsewhere in the molecule, resulting in a permanent dipole in a bond.
It is present in a σ (sigma ...
, resulting in a smaller value. The effect decreases, the further the electronegative element is from the carboxylate group, as illustrated by the following series of halogenated
In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polymers, ...
butanoic acids.
Effect of oxidation state
In a set of oxoacid
An oxyacid, oxoacid, or ternary acid is an acid that contains oxygen. Specifically, it is a compound that contains hydrogen, oxygen, and at least one other element, with at least one hydrogen atom bonded to oxygen that can dissociate to produce ...
s of an element, values decrease with the oxidation state of the element. The oxoacids of chlorine illustrate this trend.
† theoretical
References
{{reflist, 30em
External links
* Titration of acid
- freeware for data analysis
and simulation of potentiometric titration curves
Acids