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In chemistry, pH (), also referred to as ''acidity'', historically denotes "potential of
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
" (or "power of hydrogen"). It is a scale used to specify the
acidity 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 ...
or
basicity In chemistry, there are three definitions in common use of the word base, known as Arrhenius bases, Brønsted bases, and Lewis bases. All definitions agree that bases are substances that react with acids, as originally proposed by G.-F. ...
of an aqueous solution. Acidic solutions (solutions with higher concentrations of hydrogen ( )
ions 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 conven ...
) are measured to have lower pH values than basic or alkaline solutions. The pH scale is logarithmic and inversely indicates the activity of
hydrogen ions A hydrogen ion is created when a hydrogen atom loses or gains an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particle ...
( in the solution.Bates, Roger G. ''Determination of pH: theory and practice''. Wiley, 1973. : \ce = - \log(a_\ce) \thickapprox -\log( ce where +is the equilibrium
molar 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 sol ...
(mol/L) of H+ in the solution. At 25  °C (77 °F), solutions with a pH less than 7 are acidic, and solutions with a pH greater than 7 are basic. Solutions with a pH of 7 at 25  °C are neutral (i.e. have the same concentration of H+ ions as OH ions, i.e. the same as pure water). The neutral value of the pH depends on the temperature and is lower than 7 if the temperature increases above 25 °C. The pH value can be less than 0 for very concentrated
strong acids 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 ...
or greater than 14 for very concentrated strong bases. The pH scale is traceable to a set of standard solutions whose pH is established by international agreement. Primary pH standard values are determined using a concentration cell with transference by measuring the potential difference between a hydrogen electrode and a standard electrode such as the
silver chloride electrode A silver chloride electrode is a type of reference electrode, commonly used in electrochemical measurements. For environmental reasons it has widely replaced the saturated calomel electrode. For example, it is usually the internal reference el ...
. The pH of aqueous solutions can be measured with 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 ...
or a color-changing
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 ...
. Measurements of pH are important in chemistry, agronomy, medicine, water treatment, and many other applications.


History

In 1909, the
Danish Danish may refer to: * Something of, from, or related to the country of Denmark People * A national or citizen of Denmark, also called a "Dane," see Demographics of Denmark * Culture of Denmark * Danish people or Danes, people with a Danish a ...
chemist
Søren Peter Lauritz Sørensen Søren (, ) or Sören (, ) is a Scandinavian given name that is sometimes Anglicized as Soren. The name is derived from that of the 4th-century Christian saint Severin of Cologne,Portal Rheinische Geschichte"Severin (circa 330-400), Heiliger und B ...
introduced the concept of pH at the
Carlsberg Laboratory The Carlsberg Research Laboratory is a private scientific research center in Copenhagen, Denmark under the Carlsberg Group. It was founded in 1875 by J. C. Jacobsen, the founder of the Carlsberg brewery, with the purpose of advancing biochemic ...
Two other publications appeared in 1909, one in French and one in Danish., originally using the notation "pH•", with H• as a subscript to the lowercase p. The concept was later revised in 1924 to the modern pH to accommodate definitions and measurements in terms of electrochemical cells.
For the sign ''p'', I propose the name 'hydrogen ion exponent' and the symbol pH•. Then, for the hydrogen ion exponent (pH•) of a solution, the negative value of the Briggsian logarithm of the related hydrogen ion normality factor is to be understood.
Sørensen did not explain why he used the letter p, and the exact meaning of the letter is still disputed. Sørensen described a way of measuring pH using ''potential'' differences, and it represents the negative ''power'' of 10 in the concentration of hydrogen ions. The letter ''p'' could stand for the French ''puissance,'' German ''Potenz,'' or Danish ''potens'', all meaning "power", or it could mean "potential". All of these words start with the letter ''p'' in French,
German German(s) may refer to: * Germany (of or related to) ** Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ge ...
, and
Danish Danish may refer to: * Something of, from, or related to the country of Denmark People * A national or citizen of Denmark, also called a "Dane," see Demographics of Denmark * Culture of Denmark * Danish people or Danes, people with a Danish a ...
, which where the languages Sørensen published in (Carlsberg Laboratory was French-speaking, German was the dominant language of scientific publishing, and Sørensen was Danish). He also used the letter ''q'' in much the same way elsewhere in the paper, and he might have arbitrarily labelled the test solution "p" and the reference solution "q"; these letters are often paired. Some literature sources suggest that "pH" stands for the
Latin term __NOTOC__ This is a list of Wikipedia articles of Latin phrases and their translation into English. ''To view all phrases on a single, lengthy document, see: List of Latin phrases (full)'' The list also is divided alphabetically into twenty page ...
''pondus hydrogenii'' (quantity of hydrogen) or ''potentia hydrogenii'' (power of hydrogen), although this is not supported by Sørensen's writings. In modern chemistry, the p stands for "the negative decimal logarithm of", and is used in the term p''K''a for acid dissociation constants, so pH is "the negative decimal logarithm of H+ ion concentration", while pOH is "the negative decimal logarithm of OH- ion concentration". Bacteriologist Alice Catherine Evans, who influenced
dairying A dairy is a business enterprise established for the harvesting or processing (or both) of animal milk – mostly from cows or buffaloes, but also from goats, sheep, horses, or camels – for human consumption. A dairy is typically located on ...
and food safety, credited William Mansfield Clark and colleagues, including herself, with developing pH measuring methods in the 1910s, which had a wide influence on laboratory and industrial use thereafter. In her memoir, she does not mention how much, or how little, Clark and colleagues knew about Sørensen's work a few years prior. She said:
In these studies f bacterial metabolismDr. Clark's attention was directed to the effect of acid on the growth of bacteria. He found that it is the intensity of the acid in terms of hydrogen-ion concentration that affects their growth. But existing methods of measuring acidity determined the quantity, not the intensity, of the acid. Next, with his collaborators, Dr. Clark developed accurate methods for measuring hydrogen-ion concentration. These methods replaced the inaccurate titration method of determining the acid content in use in biologic laboratories throughout the world. Also they were found to be applicable in many industrial and other processes in which they came into wide usage.
The first
electronic Electronic may refer to: *Electronics, the science of how to control electric energy in semiconductor * ''Electronics'' (magazine), a defunct American trade journal *Electronic storage, the storage of data using an electronic device *Electronic co ...
method for measuring pH was invented by
Arnold Orville Beckman Arnold Orville Beckman (April 10, 1900 – May 18, 2004) was an American chemist, inventor, investor, and philanthropist. While a professor at California Institute of Technology, he founded Beckman Instruments based on his 1934 invention of th ...
, a professor at the
California Institute of Technology The California Institute of Technology (branded as Caltech or CIT)The university itself only spells its short form as "Caltech"; the institution considers other spellings such a"Cal Tech" and "CalTech" incorrect. The institute is also occasional ...
in 1934. It was in response to a request from the local citrus grower Sunkist, which wanted a better method for quickly testing the pH of lemons they were picking from their nearby orchards.


Definition


pH

The pH of a solution is defined as the decimal
logarithm In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a number  to the base  is the exponent to which must be raised, to produce . For example, since , the ''logarithm base'' 10 of ...
of the reciprocal of the
hydrogen ion A hydrogen ion is created when a hydrogen atom loses or gains an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particle ...
activity, ''a''H+. Mathematically, pH is expressed as: : \ce = - \log_(a_\ce) = \log_\left(\frac 1 \right) For example, for a solution with a hydrogen ion activity of 5×10−6 (i.e., the concentration of hydrogen ions in
moles Moles can refer to: * Moles de Xert, a mountain range in the Baix Maestrat comarca, Valencian Community, Spain * The Moles (Australian band) *The Moles, alter ego of Scottish band Simon Dupree and the Big Sound People *Abraham Moles, French engin ...
per litre), the pH of the solution can be calculated as follows: \ce = - \log_(5*10^-6) = 5.3 The concept of pH was developed because ion-selective electrodes, which are used to measure pH, respond to activity. The electrode potential, ''E'', follows the
Nernst equation In electrochemistry, the Nernst equation is a chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction ( half-cell or full cell reaction) from the standard electrode potential, absolute tempe ...
for the hydrogen ion, which can be expressed as: : E = E^0 + \frac \ln(a_\ce)=E^0 - \frac \ce where ''E'' is a measured potential, ''E''0 is the standard electrode potential, ''R'' is the
gas constant The molar gas constant (also known as the gas constant, universal gas constant, or ideal gas constant) is denoted by the symbol or . It is the molar equivalent to the Boltzmann constant, expressed in units of energy per temperature increment per ...
, ''T'' is the temperature in
kelvins The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phy ...
, ''F'' is the
Faraday constant In physical chemistry, the Faraday constant, denoted by the symbol and sometimes stylized as ℱ, is the electric charge per mole of elementary charges. It is named after the English scientist Michael Faraday. Since the 2019 redefinition of ...
. For , the number of electrons transferred is one. The electrode potential is proportional to pH when pH is defined in terms of activity. The precise measurement of pH is presented in International Standard ISO 31-8 as follows: A
galvanic cell A galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is generated from spontaneous Oxidation-Reduction reactions. A common apparatus ...
is set up to measure the electromotive force (e.m.f.) between a reference electrode and an electrode sensitive to the hydrogen ion activity when they are both immersed in the same aqueous solution. The reference electrode may be a
silver chloride electrode A silver chloride electrode is a type of reference electrode, commonly used in electrochemical measurements. For environmental reasons it has widely replaced the saturated calomel electrode. For example, it is usually the internal reference el ...
or a
calomel electrode The saturated calomel electrode (SCE) is a reference electrode based on the reaction between elemental Mercury (element), mercury and mercury(I) chloride. It has been widely replaced by the silver chloride electrode, however the calomel electrode ...
, and the hydrogen-ion selective electrode is a
standard hydrogen electrode The standard hydrogen electrode (abbreviated SHE), is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials. Its absolute electrode potential is estimated to be at 25 °C, but to form a basis ...
. : Reference electrode , concentrated solution of KCl , , test solution , H2 , Pt Firstly, the cell is filled with a solution of known hydrogen ion activity and the electromotive force, ''E''S, is measured. Then the electromotive force, ''E''X, of the same cell containing the solution of unknown pH is measured. : \ce = \ce+\frac The difference between the two measured electromotive force values is proportional to pH. This method of calibration avoids the need to know the
standard electrode potential In electrochemistry, standard electrode potential E^\ominus, or E^\ominus_, is a measure of the reducing power of any element or compound. The IUPAC "Gold Book" defines it as: ''"the value of the standard emf (electromotive force) of a cell in wh ...
. The proportionality constant, 1/''z'', is ideally equal to \frac\ , the "Nernstian slope". In practice, 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 ...
is used instead of the cumbersome hydrogen electrode. A combined glass electrode has an in-built reference electrode. It is calibrated against
buffer solutions 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 known hydrogen ion () activity proposed by the International Union of Pure and Applied Chemistry (
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
). Two or more buffer solutions are used in order to accommodate the fact that the "slope" may differ slightly from ideal. To calibrate the electrode, it is first immersed in a standard solution, and the reading on 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 ...
is adjusted to be equal to the standard buffer's value. The reading from a second standard buffer solution is then adjusted using the "slope" control to be equal to the pH for that solution. Further details, are given in the
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
recommendations. When more than two buffer solutions are used the electrode is calibrated by fitting observed pH values to a straight line with respect to standard buffer values. Commercial standard buffer solutions usually come with information on the value at 25 °C and a correction factor to be applied for other temperatures. The pH scale is logarithmic and therefore pH is a dimensionless quantity. The pH scale ranges from 0 to 14, with a pH of 7 indicating neutrality, values less than 7 indicating acidity, and values greater than 7 indicating basicity. The pH scale is based on the hydrogen ion concentration, with each pH value representing a tenfold difference in hydrogen ion concentration.


p

This was the original definition of Sørensen in 1909, which was superseded in favor of pH in 1924. is the concentration of hydrogen ions, denoted [] in modern chemistry. More correctly, the thermodynamic activity of in dilute solution should be replaced by []/c0, where the standard state concentration c0 = 1 mol/L. This ratio is a pure number whose logarithm can be defined. It is possible to measure the concentration of hydrogen ions directly using an electrode calibrated in terms of hydrogen ion concentrations. One common method is to titrate a solution of known concentration of a strong acid with a solution of known concentration of strong base in the presence of a relatively high concentration of background electrolyte. By knowing the concentrations of the acid and base, the concentration of hydrogen ions can be calculated and the measured potential can be correlated with concentrations. The calibration is usually carried out using a
Gran plot A Gran plot (also known as Gran titration or the Gran method) is a common means of standardizing a titrate or titrant by estimating the ''equivalence volume'' or '' end point'' in a strong acid-strong base titration or in a potentiometric titrat ...
. This procedure makes the activity of hydrogen ions equal to the numerical value of concentration. The glass electrode (and other ion selective electrodes) should be calibrated in a medium similar to the one being investigated. For instance, if one wishes to measure the pH of a seawater sample, the electrode should be calibrated in a solution resembling seawater in its chemical composition. The difference between p and pH is quite small, and it has been stated that pH = p + 0.04. However, it is common practice to use the term "pH" for both types of measurement.


pOH

pOH is sometimes used as a measure of the concentration of hydroxide ions, . pOH values are derived from pH measurements. The concentration of hydroxide ions in water is related to the concentration of hydrogen ions by : ce= \frac where ''K''W is the self-ionization constant of water. Taking
logarithms In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a number  to the base  is the exponent to which must be raised, to produce . For example, since , the ''logarithm base'' 10 of ...
: \ce = \ceK_\ce - \ce So, at room temperature, pOH ≈ 14 − pH. However this relationship is not strictly valid in other circumstances, such as in measurements of
soil alkalinity Alkali, or Alkaline, soils are clay soils with high pH (greater than 8.5), a poor soil structure and a low infiltration capacity. Often they have a hard calcareous layer at 0.5 to 1 metre depth. Alkali soils owe their unfavorable physico- ...
.


Measurement


pH Indicators

pH can be measured using indicators, which change color depending on the pH of the solution they are in. By comparing the color of a test solution to a standard color chart, the pH can be estimated to the nearest whole number. For more precise measurements, the color can be measured using a colorimeter or spectrophotometer.
Universal indicator __FORCETOC__ A universal indicator is a pH indicator made of a solution of several compounds that exhibits several smooth colour changes over a wide range pH values to indicate the acidity or alkalinity of solutions. Although there are several ...
is a mixture of several indicators that can provide a continuous color change over a range of pH values, typically from about pH 2 to pH 10. Universal indicator paper is made from absorbent paper that has been impregnated with universal indicator. An alternative method of measuring pH is using an electronic
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 ...
, which directly measures the voltage difference between a pH-sensitive electrode and a reference electrode.


Non-aqueous solutions

pH values can be measured in non-aqueous solutions, but they are based on a different scale from aqueous pH values, because the standard states used for calculating hydrogen ion concentrations ( activities) are different. The hydrogen ion activity, ''aH+'', is defined as: : a_\ce = \exp\left (\frac\right ) where ''μ''H+ is the
chemical potential In thermodynamics, the chemical potential of a species is the energy that can be absorbed or released due to a change of the particle number of the given species, e.g. in a chemical reaction or phase transition. The chemical potential of a species ...
of the hydrogen ion, \mu^_\ce is its chemical potential in the chosen standard state, ''R'' is the
gas constant The molar gas constant (also known as the gas constant, universal gas constant, or ideal gas constant) is denoted by the symbol or . It is the molar equivalent to the Boltzmann constant, expressed in units of energy per temperature increment per ...
and ''T'' is the
thermodynamic temperature Thermodynamic temperature is a quantity defined in thermodynamics as distinct from kinetic theory or statistical mechanics. Historically, thermodynamic temperature was defined by Kelvin in terms of a macroscopic relation between thermodynamic ...
. Therefore, pH values on the different scales cannot be compared directly because of differences in the solvated proton ions, such as lyonium ions, which require an intersolvent scale which involves the transfer activity coefficient of hydronium/lyonium ion. pH is an example of an acidity function, but there are others that can be defined. For example, the Hammett acidity function, ''H''0, has been developed in connection with
superacids In chemistry, a superacid (according to the classical definition) is an acid with an acidity greater than that of 100% pure sulfuric acid (), which has a Hammett acidity function (''H''0) of −12. According to the modern definition, a superacid ...
.


Unified absolute pH scale

In 2010, a new approach to measuring pH was proposed, called the "unified absolute pH scale". This approach allows for a common reference standard to be used across different solutions, regardless of their pH range. The unified absolute pH scale is based on the absolute chemical potential of the proton, as defined by the Lewis acid–base theory. This scale is applicable to liquids, gases, and even solids. The advantages of the unified absolute pH scale include consistency, accuracy, and applicability to a wide range of sample types. It is precise and versatile because it serves as a common reference standard for pH measurements. However, implementation efforts, compatibility with existing data, complexity, and potential costs are some challenges.


Extremes of pH measurements

The measurement of pH can become difficult at extremely acidic or alkaline conditions, such as below pH 2.5 (ca. 0.003  mol/dm3 acid) or above pH 10.5 (above ca. 0.0003 mol/dm3 alkaline). This is due to the breakdown of the
Nernst equation In electrochemistry, the Nernst equation is a chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction ( half-cell or full cell reaction) from the standard electrode potential, absolute tempe ...
in this conditions when using a glass electrode. There are several factors contribute to this problem. Firstly, liquid junction potentials may not be independent of pH. Secondly, the high ionic strength of concentrated solutions can affect the electrode potentials. At high pH the glass electrode may be affected by "alkaline error", because the electrode becomes sensitive to the concentration of cations such as and in the solution. To overcome these problems, specially constructed electrodes are available. Runoff from mines or mine tailings can produce some extremely low pH values.


Applications

The pH scale ranges from 0 to 14, with 7 being neutral. Pure water has a pH of 7 at 25°C, meaning it is neutral. When an acid is dissolved in water, the pH will be less than 7, while a base, or alkali, will have a pH greater than 7. A strong acid, such as
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 ...
, at concentration 1 mol dm−3 has a pH of 0, while a strong alkali like sodium hydroxide, at the same concentration, has a pH of 14. Since pH is a logarithmic scale, a difference of one in pH is equivalent to a tenfold difference in hydrogen ion concentration. It's important to note that neutrality isn't exactly 7 at 25°C, although it's a good approximation in most cases. Neutrality occurs when the concentration of hydrogen ions ([]) equals the concentration of hydroxide ions ([]), or when their activities are equal. Since self-ionization of water holds the product of these concentration [] × [] = Kw, it can be seen that at neutrality [] = [] = , or pH = pKw/2. pKw is approximately 14 but depends on ionic strength and temperature, and so the pH of neutrality does also. Pure water and a solution of Sodium chloride, NaCl in pure water are both neutral, since Self-ionization of water, dissociation of water produces equal numbers of both ions. However the pH of the neutral NaCl solution will be slightly different from that of neutral pure water because the hydrogen and hydroxide ions' activity is dependent on ionic strength, so Kw varies with ionic strength. When pure water is exposed to air, it becomes mildly acidic. This is because water absorbs
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
from the air, which is then slowly converted into
bicarbonate In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula . Bicarbonate serves a crucial biochem ...
and hydrogen ions (essentially creating carbonic acid). :


pH in Soil


Classification of soil pH ranges

The United States Department of Agriculture Natural Resources Conservation Service, formerly Soil Conservation Service classifies soil pH ranges as follows: In Europe, topsoil pH is influenced by soil parent material, erosional effects, climate and vegetation. A recent map of topsoil pH in Europe shows the alkaline soils in Mediterranean, Hungary, East Romania, North France. Scandinavian countries, Portugal, Poland and North Germany have more acid soils.


pH in nature

Plants contain pH-dependent
pigments A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compoun ...
that can be used as
pH indicators 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. Henc ...
, such as those found in
hibiscus ''Hibiscus'' is a genus of flowering plants in the mallow family, Malvaceae. The genus is quite large, comprising several hundred species that are native to warm temperate, subtropical and tropical regions throughout the world. Member species ...
,
red cabbage The red cabbage (purple-leaved varieties of ''Brassica oleracea'' Capitata Group) is a kind of cabbage, also known as Blaukraut after preparation. Its leaves are colored dark red/purple. However, the plant changes its color according to the pH ...
( anthocyanin), and grapes ( red wine).
Citrus ''Citrus'' is a genus of flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, lemons, grapefruits, pomelos, and limes. The genus ''Citrus'' is native to ...
fruits have acidic jiuice primarily due to the presence of
citric acid Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in ...
, while other carboxylic acids can be found in various living systems. For instance, muscle activity results in the production of
lactic acid Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as nat ...
. The
protonation In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid ...
state of
phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...
derivatives, including ATP, is pH-dependent.
Hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyt ...
, an oxygen-transport enzyme, is also affected by pH in a phenomenon known as the
Root effect The Root effect is a physiological phenomenon that occurs in fish hemoglobin, named after its discoverer R. W. Root. It is the phenomenon where an increased proton or carbon dioxide concentration (lower pH) lowers hemoglobin's affinity and carry ...
.


pH in Seawater

The pH of
seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appr ...
is typically limited to a range between 7.4 and 8.5. It plays an important role in the ocean's
carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major componen ...
, and there is evidence of ongoing
ocean acidification Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14. The root cause of ocean acidification is carbon dioxid ...
caused by
carbon dioxide emissions Greenhouse gas emissions from human activities strengthen the greenhouse effect, contributing to climate change. Most is carbon dioxide from burning fossil fuels: coal, oil, and natural gas. The largest emitters include coal in China and la ...
. However, pH measurement is complicated by the
chemical properties A chemical property is any of a material's properties that becomes evident during, or after, a chemical reaction; that is, any quality that can be established only by changing a substance's chemical identity.William L. Masterton, Cecile N. Hurley, ...
of seawater, and several distinct pH scales exist in
chemical oceanography Marine chemistry, also known as ocean chemistry or chemical oceanography, is influenced by plate tectonics and seafloor spreading, turbidity currents, sediments, pH levels, atmospheric constituents, metamorphic activity, and ecology. The fiel ...
.Zeebe, R. E. and Wolf-Gladrow, D. (2001) ''CO2 in seawater: equilibrium, kinetics, isotopes'', Elsevier Science B.V., Amsterdam, Netherlands As part of its
operational definition An operational definition specifies concrete, replicable procedures designed to represent a construct. In the words of American psychologist S.S. Stevens (1935), "An operation is the performance which we execute in order to make known a concept." F ...
of the pH scale, the
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
defines a series of
buffer solutions 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 ...
across a range of pH values (often denoted with
National Bureau of Standards The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sci ...
(NBS) or
National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical s ...
(NIST) designation). These solutions have a relatively low ionic strength (≈0.1) compared to that of seawater (≈0.7), and, as a consequence, are not recommended for use in characterizing the pH of seawater, since the ionic strength differences cause changes in
electrode potential In electrochemistry, electrode potential is the electromotive force of a galvanic cell built from a standard reference electrode and another electrode to be characterized. By convention, the reference electrode is the standard hydrogen electrode ( ...
. To resolve this problem, an alternative series of buffers based on
artificial seawater Artificial seawater (abbreviated ASW) is a mixture of dissolved mineral salts (and sometimes vitamins) that simulates seawater. Artificial seawater is primarily used in marine biology and in marine and reef aquaria, and allows the easy preparation ...
was developed. This new series resolves the problem of ionic strength differences between samples and the buffers, and the new pH scale is referred to as the 'total scale', often denoted as pHT. The total scale was defined using a medium containing
sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
ions. These ions experience
protonation In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid ...
, + , such that the total scale includes the effect of both
protons A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mas ...
(free hydrogen ions) and hydrogen sulfate ions: : []T = []F + [] An alternative scale, the 'free scale', often denoted 'pHF', omits this consideration and focuses solely on []F, in principle making it a simpler representation of hydrogen ion concentration. Only []T can be determined, therefore []F must be estimated using the [] and the stability constant of , : : []F = []T − [] = []T ( 1 + [] / K )−1 However, it is difficult to estimate K in seawater, limiting the utility of the otherwise more straightforward free scale. Another scale, known as the 'seawater scale', often denoted 'pHSWS', takes account of a further protonation relationship between hydrogen ions and fluoride ions, + ⇌ HF. Resulting in the following expression for []SWS: : []SWS = []F + [] + [HF] However, the advantage of considering this additional complexity is dependent upon the abundance of fluoride in the medium. In seawater, for instance, sulfate ions occur at much greater concentrations (>400 times) than those of fluoride. As a consequence, for most practical purposes, the difference between the total and seawater scales is very small. The following three equations summarize the three scales of pH: : pHF = −log []F : pHT = −log([]F + []) = −log[]T : pHSWS = −log(]F + [] + [HF]) = −log[v]SWS In practical terms, the three seawater pH scales differ in their pH values up to 0.10, differences that are much larger than the accuracy of pH measurements typically required, in particular, in relation to the ocean's carbonate system. Since it omits consideration of sulfate and fluoride ions, the free scale is significantly different from both the total and seawater scales. Because of the relative unimportance of the fluoride ion, the total and seawater scales differ only very slightly.


Living systems

: In living organisms, the pH of various
body fluids Body fluids, bodily fluids, or biofluids, sometimes body liquids, are liquids within the human body. In lean healthy adult men, the total body water is about 60% (60–67%) of the total body weight; it is usually slightly lower in women (52-55%). ...
, cellular compartments, and organs is tightly regulated to maintain a state of acid-base balance known as
acid–base homeostasis Acid–base homeostasis is the homeostatic regulation of the pH of the body's extracellular fluid (ECF). The proper balance between the acids and bases (i.e. the pH) in the ECF is crucial for the normal physiology of the body—and for cellu ...
.
Acidosis Acidosis is a process causing increased acidity in the blood and other body tissues (i.e., an increase in hydrogen ion concentration). If not further qualified, it usually refers to acidity of the blood plasma. The term ''acidemia'' describes ...
, defined by a pH below 7.35, is the most common disorder of acid–base homeostasis and occurs when there is an excess of acid in the body. In contrast,
alkalosis Alkalosis is the result of a process reducing hydrogen ion concentration of arterial blood plasma (alkalemia). In contrast to acidemia (serum pH 7.35 or lower), alkalemia occurs when the serum pH is higher than normal (7.45 or higher). Alkalosis ...
is characterized by excessively high blood pH. Blood pH is usually slightly basic, with a pH of 7.365, referred to as physiological pH in biology and medicine.
Plaque Plaque may refer to: Commemorations or awards * Commemorative plaque, a plate or tablet fixed to a wall to mark an event, person, etc. * Memorial Plaque (medallion), issued to next-of-kin of dead British military personnel after World War I * Pl ...
formation in teeth can create a local acidic environment that results in
tooth decay Tooth decay, also known as cavities or caries, is the breakdown of teeth due to acids produced by bacteria. The cavities may be a number of different colors from yellow to black. Symptoms may include pain and difficulty with eating. Complicatio ...
through demineralization. Enzymes and other proteins have an optimal pH range for function and can become inactivated or denatured outside this range.


pH Calculations

When calculating the pH of a solution containing acids and/or bases, a chemical speciation calculation is used to determine the concentration of all chemical species present in the solution. The complexity of the procedure depends on the nature of the solution. Strong acids and bases are compounds that are almost completely dissociated in water, which simplifies the calculation. However, for weak acids, 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 ...
must be solved, and for weak bases, a cubic equation is required. In general, a set of non-linear
simultaneous equations In mathematics, a set of simultaneous equations, also known as a system of equations or an equation system, is a finite set of equations for which common solutions are sought. An equation system is usually classified in the same manner as single e ...
must be solved. Water itself is a weak acid and a weak base, so its dissociation must be takent into account at high pH and low solute concentration (see amphoterism). It dissociates according to the equilibrium : with a dissociation constant, ' defined as : K_w = \ce where +stands for the concentration of the aqueous
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 ...
and Hrepresents the concentration of the hydroxide ion. This equilibrium needs to be taken into account at high pH and when the solute concentration is extremely low.


Strong acids and bases

Strong acids 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 ...
and bases are compounds that are essentially fully dissociated in water. This means that in an acidic solution, the concentration of hydrogen ions (H+) can be considered equal to the concentration of the acid. Similarly, in a basic solution, the concentration of hydroxide ions (OH-) can be considered equal to the concentration of the base. The pH of a solution is defined as the negative logarithm of the concentration of H+, and the pOH is defined as the negative logarithm of the concentration of OH-. For example, the pH of a 0.01M solution of hydrochloric acid (HCl) is equal to 2 (pH = −log10(0.01)), while the pOH of a 0.01M solution of sodium hydroxide (NaOH) is equal to 2 (pOH = −log10(0.01)), which corresponds to a pH of about 12. However, self-ionization of water must also be considered when concentrations of a strong acid or base is very low or high. For instance, a 5×10−8M solution of HCl would be expected to have a pH of 7.3 based on the above procedure, which is incorrect as it is acidic and should have a pH of less than 7. In such cases, the system can be treated as a mixture of the acid or base and water, which is an
amphoteric In chemistry, an amphoteric compound () is a molecule or ion that can react both as an acid and as a base. What exactly this can mean depends on which definitions of acids and bases are being used. One type of amphoteric species are amphipro ...
substance. By accounting for the self-ionization of water, the true pH of the solution can be calculated. For example, a 5×10−8M solution of HCl would have a pH of 6.89 when treated as a mixture of HCl and water. The self-ionization equilibrium of solutions of sodium hydroxide at higher concentrations must also be considered.


Weak acids and bases

A
weak 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 ...
or the conjugate acid of a weak base can be treated using the same formalism. * Acid HA: * Base A: First, an acid dissociation constant is defined as follows. Electrical charges are omitted from subsequent equations for the sake of generality : K_a = \frac \ce\ce and its value is assumed to have been determined by experiment. This being so, there are three unknown concentrations, A +and to determine by calculation. Two additional equations are needed. One way to provide them is to apply the law of mass conservation in terms of the two "reagents" H and A. : C_\ce = \ce + \ce : C_\ce = \ce + \ce C stands for
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 ...
. In some texts, one mass balance equation is replaced by an equation of charge balance. This is satisfactory for simple cases like this one, but is more difficult to apply to more complicated cases as those below. Together with the equation defining Ka, there are now three equations in three unknowns. When an acid is dissolved in water CA = CH = Ca, the concentration of the acid, so = After some further algebraic manipulation an equation in the hydrogen ion concentration may be obtained. : ce H2 + K_a ce H- K_a C_a = 0 Solution of this
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 ...
gives the hydrogen ion concentration and hence p or, more loosely, pH. This procedure is illustrated in an ICE table which can also be used to calculate the pH when some additional (strong) acid or alkaline has been added to the system, that is, when CA ≠ CH. For example, what is the pH of a 0.01M solution 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, ...
, pKa = 4.19? * Step 1: K_a = 10^ = 6.46\times10^ * Step 2: Set up the quadratic equation. ce2 + 6.46\times 10^ ce- 6.46\times 10^ = 0 * Step 3: Solve the quadratic equation. ce= 7.74\times 10^;\quad \mathrm = 3.11 For alkaline solutions, an additional term is added to the mass-balance equation for hydrogen. Since the addition of hydroxide reduces the hydrogen ion concentration, and the hydroxide ion concentration is constrained by the self-ionization equilibrium to be equal to \frac\ce, the resulting equation is: : C_\ce = \frac\ce


General method

Some systems, such as with polyprotic acids, are amenable to spreadsheet calculations. With three or more reagents or when many complexes are formed with general formulae such as ApBqHr, the following general method can be used to calculate the pH of a solution. For example, with three reagents, each equilibrium is characterized by an equilibrium constant, β. : ce_p\ce_q\ce_r=\beta_ ce A ce B ce H Next, write down the mass-balance equations for each reagent: : \begin C_\ce &= ce A+ \Sigma p \beta_ ce Ap ce Bq ce H \\ C_\ce &= ce B+ \Sigma q \beta_ ce Ap ce Bq ce Hr \\ C_\ce &= ce H+ \Sigma r \beta_ ce Ap ce Bq ce Hr - K_w ce H \end Note that there are no approximations involved in these equations, except that each stability constant is defined as a quotient of concentrations, not activities. Much more complicated expressions are required if activities are to be used. There are three non-linear
simultaneous equations In mathematics, a set of simultaneous equations, also known as a system of equations or an equation system, is a finite set of equations for which common solutions are sought. An equation system is usually classified in the same manner as single e ...
in the three unknowns, and Because the equations are non-linear and their concentrations may range over many powers of 10, the solution of these equations is not straightforward. However, many computer programs are available which can be used to perform these calculations. There may be more than three reagents. The calculation of hydrogen ion concentrations, using this approach, is a key element in the determination of equilibrium constants by
potentiometric titration Potentiometric titration is a technique similar to direct titration of a redox reaction. It is a useful means of characterizing an acid. No indicator is used; instead the potential is measured across the analyte, typically an electrolyte solution. ...
.


See also

*
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, ...
*
Arterial blood gas An arterial blood gas (ABG) test, or arterial blood gas analysis (ABGA) measures the amounts of arterial gases, such as oxygen and carbon dioxide. An ABG test requires that a small volume of blood be drawn from the radial artery with a syringe an ...
*
Chemical equilibrium In a chemical reaction, chemical equilibrium is the state in which both the Reagent, reactants and Product (chemistry), products are present in concentrations which have no further tendency to change with time, so that there is no observable chan ...
* p * p''K''a


References


External links

{{DEFAULTSORT:pH Acid–base chemistry Equilibrium chemistry Units of measurement Water quality indicators Logarithmic scales of measurement General chemistry