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In chemistry, neutralization or neutralisation (see
spelling differences
Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography, the two most notable variations being British and American ...
) is a
chemical reaction
A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
in which
acid
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
and a
base react quantitatively with each other. In a reaction in water, neutralization results in there being no excess of hydrogen or hydroxide ions present in the solution. The
pH of the neutralized solution depends on the acid strength of the reactants.
Meaning of "neutralization"
In the context of a
chemical reaction
A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
the term neutralization is used for a reaction between an
acid
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
and a
base or
alkali
In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...
. Historically, this reaction was represented as
:acid + base (alkali) → salt + water
For example:
:HCl + NaOH → NaCl + H
2O
The statement is still valid as long as it is understood that in an aqueous solution the substances involved are subject to
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 ...
, which changes the ionization state of the substances. The arrow sign, →, is used because the reaction is complete, that is, neutralization is a quantitative reaction. A more general definition is based on
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 theory ...
.
:AH + B → A + BH
Electrical charges are omitted from generic expressions such as this, as each species A, AH, B, or BH may or may not carry an electrical charge. Neutralization of
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...
provides a specific example. Two partial neutralization reactions are possible in this instance.
:H
2SO
4 + OH
− → + H
2O
: + OH
− → + H
2O
: Overall: H
2SO
4 + 2 OH
− → + 2 H
2O
After an acid AH has been neutralized there are no molecules of the acid (or hydrogen ions produced by dissociation of the molecule) left in solution.
When an acid is neutralized the amount of base added to it must be equal the amount of acid present initially. This amount of base is said to be the
equivalent
Equivalence or Equivalent may refer to:
Arts and entertainment
*Album-equivalent unit, a measurement unit in the music industry
* Equivalence class (music)
*'' Equivalent VIII'', or ''The Bricks'', a minimalist sculpture by Carl Andre
*''Equiva ...
amount. In a titration of an acid with a base, the point of neutralization can also be called the
equivalence point
The equivalence point, or stoichiometric point, of a chemical reaction is the point at which chemically equivalent quantities of reactants have been mixed. For an acid-base reaction the equivalence point is where the moles of acid and the moles of ...
. The quantitative nature of the neutralization reaction is most conveniently expressed in terms of the
concentration
In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', ''molar concentration'', ''number concentration'', an ...
s of acid and alkali. At the equivalence point:
:volume (acid) × concentration (H
+ ions from dissociation) = volume (base) × concentration (OH
− ions)
In general, for an acid AH
''n'' at concentration ''c''
1 reacting with a base B(OH)
''m'' at concentration ''c''
2 the volumes are related by:
: ''n'' ''v''
1 ''c''
1 = ''m'' ''v''
2 ''c''
2
An example of a base being neutralized by an acid is as follows.
:Ba(OH)
2 + 2 H
+ → Ba
2+ + 2 H
2O
The same equation relating the concentrations of acid and base applies. The concept of neutralization is not limited to reactions in solution. For example, the reaction of
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
with acid such as sulfuric acid is also a neutralization reaction.
:
a,MgO
3 + H
2SO
4 → (Ca
2+, Mg
2+) + + CO
2 + H
2O
Such reactions are important in
soil chemistry
Soil chemistry is the study of the chemical characteristics of soil. Soil chemistry is affected by mineral composition, organic matter and environmental factors. In the early 1850s a consulting chemist to the Royal Agricultural Society in England, ...
.
Strong acids and strong bases
A
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. ...
is one that is fully
dissociated
Dissociation in chemistry is a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into other things such as atoms, ions, or radicals, usually in a reversible manner. For instance, when an acid ...
in aqueous solution. For example,
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, is a strong acid.
:HCl → H
+ + Cl
−
A
strong base is one that is fully
dissociated
Dissociation in chemistry is a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into other things such as atoms, ions, or radicals, usually in a reversible manner. For instance, when an acid ...
in aqueous solution. For example,
sodium hydroxide
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly caustic base and alkali ...
, NaOH, is a strong base.
:NaOH → Na
+ + OH
−
Therefore, when a strong acid reacts with a strong base the neutralization reaction can be written as
:H
+ + OH
− → H
2O
For example, in the reaction between hydrochloric acid and sodium hydroxide the sodium and chloride ions, Na
+ and Cl
− take no part in the reaction. The reaction is consistent with the Brønsted–Lowry definition because in reality the hydrogen ion exists as the
hydronium ion
In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation , the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid is d ...
, so that the neutralization reaction may be written as
:H
3O
+ + OH
− → H
2O + H
2O
When a strong acid is neutralized by a strong base there are no excess hydrogen ions left in the solution. The solution is said to be
neutral
Neutral or neutrality may refer to:
Mathematics and natural science Biology
* Neutral organisms, in ecology, those that obey the unified neutral theory of biodiversity
Chemistry and physics
* Neutralization (chemistry), a chemical reaction in ...
as it is neither acidic nor alkaline. The
pH of such a solution is close to a value of 7; the exact pH value is dependent on the temperature of the solution.
Neutralization is an
exothermic reaction. The standard
enthalpy change
Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant p ...
for the reaction is −57.30 kJ/mol.
Quantitative treatment
The term ''fully dissociated'' is applied to a solute when the concentration of the products of its dissociation are below the
detection limit
The limit of detection (LOD or LoD) is the lowest signal, or the lowest corresponding quantity to be determined (or extracted) from the signal, that can be observed with a sufficient degree of confidence or statistical significance. However, the ...
s, that is, when the products' concentrations are too low to measured. Quantitatively, this is expressed as , or in some texts . This means that the value of the dissociation constant cannot be obtained from experimental measurements. The value can, however, be estimated theoretically. For example the value of has been estimated for
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 ...
in aqueous solution at room temperature.
A chemical compound may behave as a strong acid in solution when its concentration is low and as a weak acid when its concentration is very high.
Sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...
is an example of such a compound.
Weak acids and strong bases
A
weak acid
Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a hydron (chemistry), proton, H+, and an anion, A-. The Dissociation (chemistry), dissociation of a strong acid in solution is effectively comple ...
HA is one that does not dissociate fully when it is dissolved in water. Instead an
equilibrium mixture is formed:
:HA + H
2O H
3O
+ + A
−
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 ...
is an example of a weak acid. The pH of the neutralized solution resulting from
:HA + OH
− → H
2O + A
−
is not close to 7, as with a strong acid, but depends on the
acid dissociation constant
In chemistry, an acid dissociation constant (also known as acidity constant, or acid-ionization constant; denoted ) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction
:HA ...
, ''K''
a, of the acid. The pH at the end-point or equivalence point in a titration may be calculated as follows. At the end-point the acid is completely neutralized so the analytical hydrogen ion concentration, ''T''
H, is zero and the concentration of the conjugate base, A
−, is equal to the analytical or formal concentration ''T''
A of the acid:
−">−= ''T''
A. When a solution of an acid, HA, is at
equilibrium, by definition the concentrations are related by the expression
:
−">−H
+] = ''K''
a Ap''K''
a = −
log
Log most often refers to:
* Trunk (botany), the stem and main wooden axis of a tree, called logs when cut
** Logging, cutting down trees for logs
** Firewood, logs used for fuel
** Lumber or timber, converted from wood logs
* Logarithm, in mathe ...
''K''
a
The solvent (e.g. water) is omitted from the defining expression on the assumption that its concentration is very much greater than the concentration of dissolved acid,
2O">2O≫ ''T''
A. The equation for mass-balance in hydrogen ions can then be written as
:''T''
H =
+">++
−">−H
+]/''K''
a −
![weak acid titrations](https://upload.wikimedia.org/wikipedia/commons/f/f5/Weak_acid_titrations.png)
where ''K''
w represents the
Self-ionization of water, self-dissociation constant of water. Since ''K''
w =
+">+OH
−], the term is equal to
−">H− the concentration of hydroxide ions. At neutralization, ''T''
H is zero. After multiplying both sides of the equation by
+">+ it becomes
:
+">+sup>2 + ''T''
A +">+sup>2/''K''
a − ''K''
w = 0
and, after rearrangement and taking logarithms,
:pH = p''K''
w + log (1 + )
With a dilute solution of the weak acid, the term 1 + is equal to to a good approximation. If p''K''
w = 14,
:pH = 7 + (p''K''
a + log ''T''
A)/2
This equation explains the following facts:
*The pH at the end-point depends mainly on the strength of the acid, p''K''
a.
*The pH at the end-point is greater than 7 and increases with increasing concentration of the acid, ''T''
A, as seen in the figure.
In 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'' ...
of a weak acid with a strong base the pH rises more steeply as the end-point is approached. At the end-point, the slope of the curve of pH with respect to amount of titrant is a maximum. Since the end-point occurs at pH greater than 7, the most suitable
indicator
Indicator may refer to:
Biology
* Environmental indicator of environmental health (pressures, conditions and responses)
* Ecological indicator of ecosystem health (ecological processes)
* Health indicator, which is used to describe the health ...
to use is one, like
phenolphthalein
Phenolphthalein ( ) is a chemical compound with the formula C20 H14 O4 and is often written as "HIn", "HPh", "phph" or simply "Ph" in shorthand notation. Phenolphthalein is often used as an indicator in acid–base titrations. For this applica ...
, that changes color at high pH.
Weak bases and strong acids
The situation is analogous to that of weak acids and strong bases.
:B + H
3O
+ BH
+ + H
2O
Amine
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituen ...
s are examples of weak bases. The pH of the neutralized solution depends on the
acid dissociation constant
In chemistry, an acid dissociation constant (also known as acidity constant, or acid-ionization constant; denoted ) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction
:HA ...
of the protonated base, p''K''
a, or, equivalently, on the base association constant, p''K''
b. The most suitable indicator to use for this type of titration is one, such as
methyl orange
Methyl orange is a pH indicator frequently used in titration because of its clear and distinct color variance at different pH values. Methyl orange shows red color in acidic medium and yellow color in basic medium. Because it changes color at the ...
, that changes color at low pH.
Weak acids and weak bases
When a weak acid reacts with an equivalent amount of a weak base,
:HA + B A
− + BH
+
complete neutralization does not always occur. The concentrations of the species in equilibrium with each other will depend on the
equilibrium constant
The equilibrium constant of a chemical reaction is the value of its reaction quotient at chemical equilibrium, a state approached by a dynamic chemical system after sufficient time has elapsed at which its composition has no measurable tendency ...
, ''K'', for the reaction, which is defined as follows:
:
−">−BH
+] = ''K''
AB].
The neutralization reaction can be considered as the difference of the following two acid dissociation reactions
:HA H
+ + A
− ''K''
a,A =
−">−H
+]/
A:BH
+ B + H
+ ''K''
a,B =
H
+]/
+">H+
with the
dissociation constant
In chemistry, biochemistry, and pharmacology, a dissociation constant (K_D) is a specific type of equilibrium constant that measures the propensity of a larger object to separate (dissociate) reversibly into smaller components, as when a complex fa ...
s ''K''
a,A and ''K''
a,B of the acids HA and BH
+, respectively. Inspection of the reaction quotients shows that
''K'' = .
A weak acid cannot always be neutralized by a weak base, and vice versa. However, for the neutralization of
benzoic acid
Benzoic acid is a white (or colorless) solid organic compound with the formula , whose structure consists of a benzene ring () with a carboxyl () substituent. It is the simplest aromatic carboxylic acid. The name is derived from gum benzoin, wh ...
(''K''
a,A = 6.5 × 10
−5) with
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
(''K''
a,B = 5.6 × 10
−10 for
ammonium
The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary a ...
), ''K'' = 1.2 × 10
5 >> 1, and more than 99% of the benzoic acid is converted to benzoate.
Applications
Chemical
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 are used for analyzing acids or bases to determine the unknown
concentration
In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', ''molar concentration'', ''number concentration'', an ...
. Either 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 ...
or a
pH indicator
A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually or spectroscopically by changes in absorption and/or emission properties. Hence, ...
which shows the point of neutralization by a distinct color change can be employed. Simple
stoichiometric
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...
calculations with the known volume of the unknown and the known volume and
molarity
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 solu ...
of the added chemical gives the molarity of the unknown.
In
wastewater treatment
Wastewater treatment is a process used to remove contaminants from wastewater and convert it into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environme ...
, chemical neutralization methods are often applied to reduce the damage that an effluent may cause upon release to the environment. For pH control, popular chemicals include
calcium carbonate,
calcium oxide
Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, Caustic (substance), caustic, alkaline, crystalline solid at room temperature. The broadly used term "''lime (material), lime''" co ...
,
magnesium hydroxide
Magnesium hydroxide is the inorganic compound with the chemical formula Mg(OH)2. It occurs in nature as the mineral brucite. It is a white solid with low solubility in water (). Magnesium hydroxide is a common component of antacids, such as milk ...
, and
sodium bicarbonate. The selection of an appropriate neutralization chemical depends on the particular application.
There are many uses of neutralization reactions that are acid-alkali reactions. A very common use is
antacid
An antacid is a substance which neutralizes stomach acidity and is used to relieve heartburn, indigestion or an upset stomach. Some antacids have been used in the treatment of constipation and diarrhea. Marketed antacids contain salts of alu ...
tablets. These are designed to neutralize excess
gastric acid
Gastric acid, gastric juice, or stomach acid is a digestive fluid formed within the stomach lining. With a pH between 1 and 3, gastric acid plays a key role in digestion of proteins by activating digestive enzymes, which together break down the ...
in the stomach (
HCl HCL may refer to:
Science and medicine
* Hairy cell leukemia, an uncommon and slowly progressing B cell leukemia
* Harvard Cyclotron Laboratory, from 1961 to 2002, a proton accelerator used for research and development
* Hollow-cathode lamp, a spe ...
) that may be causing discomfort in the stomach or lower esophagus. This can also be remedied by the ingestion of
sodium bicarbonate (NaHCO
3). Sodium bicarbonate is also commonly used to neutralise acid spills in laboratories, as well as
acid burns.
In chemical synthesis of nanomaterials, the heat of neutralization reaction can be used to facilitate the chemical reduction of metal precursors.
Also in the digestive tract, neutralization reactions are used when food is moved from the stomach to the intestines. In order for the nutrients to be absorbed through the intestinal wall, an alkaline environment is needed, so the pancreas produce an antacid bicarbonate to cause this transformation to occur.
Another common use, though perhaps not as widely known, is in fertilizers and control of
soil pH
Soil pH is a measure of the acidity or basicity (alkalinity) of a soil. Soil pH is a key characteristic that can be used to make informative analysis both qualitative and quantitatively regarding soil characteristics. pH is defined as the neg ...
.
Slaked lime
Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has ma ...
(
calcium hydroxide
Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has m ...
) or
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
(
calcium carbonate) may be worked into soil that is too acidic for plant growth. Fertilizers that improve plant growth are made by neutralizing
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...
(H
2SO
4) or
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 ...
(HNO
3) with
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
gas (NH
3), making
ammonium sulfate
Ammonium sulfate (American English and international scientific usage; ammonium sulphate in British English); (NH4)2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen a ...
or
ammonium nitrate
Ammonium nitrate is a chemical compound with the chemical formula . It is a white crystalline salt consisting of ions of ammonium and nitrate. It is highly soluble in water and hygroscopic as a solid, although it does not form hydrates. It is ...
. These are salts utilized in the fertilizer.
Industrially, a by-product of the burning of
coal
Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen.
Coal is formed when dea ...
,
sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...
gas, may combine with water vapor in the air to eventually produce sulfuric acid, which falls as acid rain. To prevent the sulfur dioxide from being released, a device known as a scrubber gleans the gas from smoke stacks. This device first blows calcium carbonate into the combustion chamber where it decomposes into
calcium oxide
Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, Caustic (substance), caustic, alkaline, crystalline solid at room temperature. The broadly used term "''lime (material), lime''" co ...
(lime) and carbon dioxide. This lime then reacts with the sulfur dioxide produced forming
calcium sulfite
Calcium sulfite, or calcium sulphite, is a chemical compound, the calcium salt of sulfite with the formula CaSO3·x(H2O). Two crystalline forms are known, the hemihydrate and the tetrahydrate, respectively CaSO3·½(H2O) and CaSO3·4(H2O). All fo ...
. A suspension of lime is then injected into the mixture to produce a slurry, which removes the calcium sulfite and any remaining unreacted sulfur dioxide.
References
Further reading
Neutralization is covered in most general chemistry textbooks. Detailed treatments may be found in textbooks on analytical chemistry such as
* Chapters 14, 15 and 16
Applications
*
*
*
*Metcalf & Eddy. ''Wastewater Engineering, Treatment and Reuse''. 4th ed. New York: McGraw-Hill, 2003. 526-532.
{{Authority control
Acid–base chemistry
Chemical reactions
Water treatment