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In chemistry, pH (), historically denoting "potential of hydrogen" (or "power of hydrogen"), is a scale used to specify the
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 ...
ity or basicity of an
aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be ...
. Acidic solutions (solutions with higher concentrations of
ion 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 conv ...
s) are measured to have lower pH values than basic 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 ...
ne solutions. The pH scale is
logarithmic Logarithmic can refer to: * Logarithm, a transcendental function in mathematics * Logarithmic scale, the use of the logarithmic function to describe measurements * Logarithmic spiral, * Logarithmic growth * Logarithmic distribution, a discrete pro ...
and inversely indicates the concentration 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) = -\log( ce\ce M) where M = mol dm−3. At 25 
°C The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The d ...
(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 this temperature are neutral (i.e. have the same concentration of H+ ions as OH ions, i.e.
pure water Purified water is water that has been mechanically filtered or processed to remove impurities and make it suitable for use. Distilled water was, formerly, the most common form of purified water, but, in recent years, water is more frequently puri ...
). The neutral value of the pH depends on the temperaturebeing 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 hydron (chemistry), proton, H+, and an anion, A-. The Dissociation (chemistry), dissociation of a strong acid in solution is effectively comple ...
, 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 elect ...
. The pH of aqueous solutions can be measured with a glass electrode and a pH meter, 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 o ...
. Measurements of pH are important in chemistry,
agronomy Agronomy is the science and technology of producing and using plants by agriculture for food, fuel, fiber, chemicals, recreation, or land conservation. Agronomy has come to include research of plant genetics, plant physiology, meteorology, and s ...
, medicine, water treatment, and many other applications.


History

The concept of pH was first introduced by 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 ance ...
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 ...
at the Carlsberg Laboratory in 1909 Two other publications appeared in 1909, one in French and one in Danish. and was revised to the modern pH in 1924 to accommodate definitions and measurements in terms of electrochemical cells. In the first papers, the notation had H as a subscript to the lowercase ''p'', thus: pH•.
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.
The exact meaning of the letter ''p'' in "pH" is disputed, as Sørensen did not explain why he used it. Sørensen describes 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'', meaning "power", or it could mean "potential". All the words for these 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 ance ...
—all 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. He might also have labelled the test solution "p" and the reference solution "q" arbitrarily; these letters are often paired. Some literature sources state that the "pH" stands for the Latin term ''pondus hydrogenii'' (quantity of hydrogen) or ''potentia hydrogenii'' (power of hydrogen), although this is not supported by Sørensen's writings. Currently in chemistry, the p stands for " decimal logarithm of", and is also used in the term p''K''a, used for
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 ...
s and pOH, the equivalent for
hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...
ions. Bacteriologist Alice C. Evans, famed for her work's influence on dairying and
food safety Food safety (or food hygiene) is used as a scientific method/discipline describing handling, preparation, and storage of food in ways that prevent food-borne illness. The occurrence of two or more cases of a similar illness resulting from t ...
, credited William Mansfield Clark and colleagues (of whom she was one) 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, a professor at
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 local citrus grower Sunkist that wanted a better method for quickly testing the pH of lemons they were picking from their nearby orchards.


Definition and measurement


pH

pH is defined as the decimal logarithm 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+, in a solution. :\ce = - \log_(a_\ce) = \log_\left(\frac 1 \right) For example, for a solution with a hydrogen ion activity of 5×10−6 (at that level, this is essentially the number of moles of hydrogen ions per litre of solution) the argument of the logarithm is 1/(5×10−6) = 2×105; thus such a solution has a pH of log10(2×105) = 5.3. Consider the following example: a quantity of 107 moles of pure water at 25 °C (pH = 7), or 180 metric tonnes (18×107 g), contains close to 18 milligrams of
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 ...
hydrogen ions. Note that pH depends on temperature. For instance at 0 °C the pH of pure water is about 7.47. At 25 °C it is 7.00, and at 100 °C it is 6.14. This definition was adopted because ion-selective electrodes, which are used to measure pH, respond to activity. Ideally, 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 tempera ...
, which for the hydrogen ion can be written 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, ''T'' is the temperature in kelvins, ''F'' is the Faraday constant. For the number of electrons transferred is one. It follows that the electrode potential is proportional to pH when pH is defined in terms of activity. Precise measurement of pH is presented in International Standard
ISO 31-8 ISO 31-8 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to ''physical chemistry and molecular physics''. Quantities and units Notes In the tables of quantities and their units, the I ...
as follows: A galvanic cell is set up to measure the
electromotive force In electromagnetism and electronics, electromotive force (also electromotance, abbreviated emf, denoted \mathcal or ) is an energy transfer to an electric circuit per unit of electric charge, measured in volts. Devices called electrical ''transd ...
(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 elect ...
or a
calomel electrode The saturated calomel electrode (SCE) is a reference electrode based on the reaction between elemental mercury and mercury(I) chloride. It has been widely replaced by the silver chloride electrode, however the calomel electrode has a reputation o ...
. The hydrogen-ion selective electrode is a standard hydrogen electrode. :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. The proportionality constant, 1/''z'', is ideally equal to \frac\ , the "Nernstian slope". To apply this process in practice, a glass electrode is used rather than the cumbersome hydrogen electrode. A combined glass electrode has an in-built reference electrode. It is calibrated against
buffer solution 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 ...
s of known hydrogen ion activity.
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 ...
(International Union of Pure and Applied Chemistry) has proposed the use of a set of buffer solutions of known activity. Two or more buffer solutions are used in order to accommodate the fact that the "slope" may differ slightly from ideal. To implement this approach to calibration, the electrode is first immersed in a standard solution and the reading on a pH meter 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 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.


P /h2>

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, which appears to have units of concentration. 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. However, it is possible to measure the concentration of hydrogen ions directly, if the electrode is calibrated in terms of hydrogen ion concentrations. One way to do this, which has been used extensively, is to
titrate 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 solution of known concentration of a strong acid with a solution of known concentration of strong alkaline in the presence of a relatively high concentration of background electrolyte. Since the concentrations of acid and alkaline are known, it is easy to calculate the concentration of hydrogen ions so that 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 ...
. Thus, the effect of using this procedure is to make activity 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, as detailed below. The difference between p and pH is quite small. It has been stated that pH = p + 0.04. It is common practice to use the term "pH" for both types of measurement.


pH indicators

Indicators may be used to measure pH, by making use of the fact that their color changes with pH. Visual comparison of the color of a test solution with a standard color chart provides a means to measure pH accurate to the nearest whole number. More precise measurements are possible if the color is measured spectrophotometrically, 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 severa ...
consists of a mixture of indicators such that there is a continuous color change from about pH 2 to pH 10. Universal indicator paper is made from absorbent paper that has been impregnated with universal indicator. Another method of measuring pH is using an electronic pH meter.


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 :\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.


Extremes of pH

Measurement of pH below about 2.5 (ca. 0.003  mol/dm3 acid) and above about 10.5 (ca. 0.0003 mol/dm3 alkaline) requires special procedures because, when using the glass electrode, the Nernst law breaks down under those conditions. Various factors contribute to this. It cannot be assumed that
liquid junction potential Liquid junction potential (shortly LJP) occurs when two solutions of electrolytes of different concentrations are in contact with each other. The more concentrated solution will have a tendency to diffuse into the comparatively less concentrated on ...
s are independent of pH. Also, extreme pH implies that the solution is concentrated, so electrode potentials are affected by
ionic strength The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such a ...
variation. 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. Specially constructed electrodes are available which partly overcome these problems. Runoff from mines or mine tailings can produce some very low pH values.


Non-aqueous solutions

Hydrogen ion concentrations (activities) can be measured in non-aqueous solvents. pH values based on these measurements belong to a different scale from aqueous pH values, because activities relate to different standard states. Hydrogen ion activity, ''aH+'', can be 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 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 w ...
. Therefore, pH values on the different scales cannot be compared directly due to different solvated proton ions such as lyonium ions, requiring an intersolvent scale which involves the transfer activity coefficient of hydronium/lyonium ion. pH is an example of an
acidity function An acidity function is a measure of the acidity of a medium or solvent system, usually expressed in terms of its ability to donate protons to (or accept protons from) a solute ( Brønsted acidity). The pH scale is by far the most commonly used ac ...
. Other acidity functions can be defined. For example, the Hammett acidity function, ''H''0, has been developed in connection with superacids.


Unified absolute pH scale

In 2010, a new "unified absolute pH scale" has been proposed that would allow various pH ranges across different solutions to use a common proton reference standard. It has been developed on the basis of the absolute chemical potential of the proton. This model uses the Lewis acid–base definition. This scale applies to liquids, gases and even solids.


Applications

Pure water is neutral. When 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 ...
is dissolved in water, the pH will be less than 7 (25 °C). When a base, or specifically an
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 ...
, is dissolved in water, the pH will be greater than 7. A solution of a strong acid, such as hydrochloric acid, at concentration 1 mol dm−3 has a pH of 0. A solution of a strong alkali, such as sodium hydroxide, at concentration 1 mol dm−3, has a pH of 14. Thus, measured pH values will lie mostly in the range 0 to 14, though negative pH values and values above 14 are entirely possible. Since pH is a logarithmic scale, a difference of one pH unit is equivalent to a tenfold difference in hydrogen ion concentration. The pH of neutrality is not exactly 7 (25 °C), although this is a good approximation in most cases. Neutrality is defined as the condition where [] = [] (or the activities are equal). Since self-ionization of water holds the product of these concentration []/M×[]/M = Kw, it can be seen that at neutrality []/M = []/M = , 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
NaCl Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g/ ...
in pure water are both neutral, since 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 The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such a ...
, so Kw varies with ionic strength. If pure water is exposed to air it becomes mildly acidic. This is because water absorbs carbon dioxide 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 biochemic ...
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 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 ne ...
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.


Measuring soil pH

Soil in the field is a heterogeneous colloidal system that comprises sand, silt, clays, microorganisms, plant roots, and myriad other living cells and decaying organic material. Soil pH is a master variable that affects myriad processes and properties of interest to soil and environmental scientists, farmers, and engineers. To quantify the concentration of the H+ in such a complex system, soil samples from a given soil horizon are brought to the laboratory where they are homogenized, sieved, and sometimes dried prior to analysis. A mass of soil (for example, 5 g field-moist to best represent field conditions) is mixed into a slurry with distilled water or 0.01 M CaCl2 (for example, 10 mL). After mixing well, the suspension is stirred vigorously and allowed to stand for 15–20 minutes, during which time, the sand and silt particles settle out and the clays and other colloids remain suspended in the overlying water, known as the aqueous phase. A pH electrode connected to a pH meter is calibrated with buffered solutions of known pH (for example, pH 4 and 7) before being inserting into the upper portion of the aqueous phase, and the pH is measured. A combination pH electrode incorporates both the H+ sensing electrode (glass electrode) and a reference electrode that provides a pH-insensitive reference voltage and a salt bridge to the hydrogen electrode. In other configurations, the glass and reference electrodes are separate and attach to the pH meter in two ports. The pH meter measures the potential (voltage) difference between the two electrodes and converts it to pH. The separate reference electrode is usually the calomel electrode, the silver-silver chloride electrode is used in the combination electrode. There are numerous uncertainties in operationally defining soil pH in the above way. Since an electrical potential difference between the glass and reference electrodes is what is measured, the activity of H+ is really being quantified, rather than concentration. The H+ activity is sometimes called the "effective H+ concentration" and is directly related to the chemical potential of the proton and its ability to do chemical and electrical work in the soil solution in equilibrium with the solid phases. Clay and organic matter particles carry negative charge on their surfaces, and H+ ions attracted to them are in equilibrium with H+ ions in the soil solution. The measured pH is quantified in the aqueous phase only, by definition, but the value obtained is affected by the presence and nature of the soil colloids and the ionic strength of the aqueous phase. Changing the water-to-soil ratio in the slurry can change the pH by disturbing the water-colloid equilibrium, particularly the ionic strength. The use of 0.01 M CaCl2 instead of water obviates this effect of water-to-soil ratio and gives a more consistent approximation of "soil pH" that relates to plant root growth, rhizosphere and microbial activity, drainage water acidity, and chemical processes in the soil. Using 0.01 M CaCl2 brings all of the soluble ions in the aqueous phase closer to the colloidal surfaces, and allows the H+ activity to be measured closer to them. Using the 0.01 M CaCl2 solution thereby allows a more consistent, quantitative estimation of H+ activity, especially if diverse soil samples are being compared in space and time.


pH in nature

pH-dependent
plant pigment Biological pigments, also known simply as pigments or biochromes, are substances produced by living organisms that have a color resulting from selective color absorption. Biological pigments include plant pigments and flower pigments. Many biol ...
s that can be used as pH indicators occur in many plants, including hibiscus,
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 p ...
(
anthocyanin Anthocyanins (), also called anthocyans, are water-soluble vacuolar pigments that, depending on their pH, may appear red, purple, blue, or black. In 1835, the German pharmacist Ludwig Clamor Marquart gave the name Anthokyan to a chemical com ...
), and grapes ( red wine). The juice of
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 nativ ...
fruits is acidic mainly because it contains
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 t ...
. Other
carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...
s occur in many living systems. For example, lactic acid is produced by muscle activity. The state of
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, i ...
of phosphate derivatives, such as ATP, is pH-dependent. The functioning of the oxygen-transport enzyme hemoglobin is affected by pH in a process 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 ...
.


Seawater

The pH of seawater 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 dioxide ...
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 field ...
.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 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 solution 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 ...
s across a range of pH values (often denoted with National Bureau of Standards (NBS) or National Institute of Standards and Technology (NIST) designation). These solutions have a relatively low
ionic strength The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such a ...
(≈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 preparatio ...
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, + , such that the total scale includes the effect of both protons (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 Fluoride (). According to this source, is a possible pronunciation in British English. is an inorganic, monatomic anion of fluorine, with the chemical formula (also written ), whose salts are typically white or colorless. Fluoride salts ty ...
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 summarise 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 values by up to 0.10 pH units, 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

: The pH of different cellular compartments,
body fluid 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%). ...
s, and organs is usually tightly regulated in a process called
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 cellula ...
. The most common disorder in acid–base
homeostasis In biology, homeostasis (British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and i ...
is
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 ...
, which means an acid overload in the body, generally defined by pH falling below 7.35.
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 the opposite condition, with blood pH being excessively high. The pH of blood is usually slightly basic with a value of pH 7.365. This value is often 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 * Pla ...
can create a local acidic environment that can result in tooth decay by demineralization. Enzymes and other proteins have an optimum pH range and can become inactivated or denatured outside this range.


Calculations of pH

The calculation of the pH of a solution containing acids and/or bases is an example of a chemical speciation calculation, that is, a mathematical procedure for calculating the concentrations of all chemical species that are present in the solution. The complexity of the procedure depends on the nature of the solution. For strong acids and bases no calculations are necessary except in extreme situations. The pH of a solution containing a weak acid requires the solution of a quadratic equation. The pH of a solution containing a weak base may require the solution of a
cubic equation In algebra, a cubic equation in one variable is an equation of the form :ax^3+bx^2+cx+d=0 in which is nonzero. The solutions of this equation are called roots of the cubic function defined by the left-hand side of the equation. If all of th ...
. The general case requires the solution of a set of
non-linear In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many other ...
simultaneous equations. A complicating factor is that water itself is a weak acid and a weak base (see
amphoterism 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 amphiprot ...
). It dissociates according to the equilibrium : with a
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 f ...
, defined as :K_w = \ce/\ce M^2 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 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 and bases are compounds that for practical purposes, are completely dissociated in water. Under normal circumstances this means that the concentration of hydrogen ions in acidic solution can be taken to be equal to the concentration of the acid. The pH is then equal to minus the logarithm of the concentration value. Hydrochloric acid (HCl) is an example of a strong acid. The pH of a 0.01M solution of HCl is equal to −log10(0.01), that is, pH = 2. Sodium hydroxide, NaOH, is an example of a strong base. The p Hvalue of a 0.01M solution of NaOH is equal to −log10(0.01), that is, p H= 2. From the definition of p Hin the pOH section above, this means that the pH is equal to about 12. For solutions of sodium hydroxide at higher concentrations the self-ionization equilibrium must be taken into account. Self-ionization must also be considered when concentrations are extremely low. Consider, for example, a solution of hydrochloric acid at a concentration of 5×10−8M. The simple procedure given above would suggest that it has a pH of 7.3. This is clearly wrong as an acid solution should have a pH of less than 7. Treating the system as a mixture of hydrochloric acid and the
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 amphiprot ...
substance water, a pH of 6.89 results.


Weak acids and bases

A weak acid 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 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 ...
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 so ...
. 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 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, wh ...
, 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 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 :C_\ce = \frac\ce In this case the resulting equation in is a cubic equation.


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 3
non-linear In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many other ...
simultaneous equations in the three unknowns, and Because the equations are non-linear, and because 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 formalism, is a key element in the
determination of equilibrium constants Equilibrium constants are determined in order to quantify chemical equilibria. When an equilibrium constant is expressed as a concentration quotient, :K=\frac it is implied that the activity quotient is constant. For this assumption to be vali ...
by potentiometric titration.


See also

* pH indicator *
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 a ...
* Chemical equilibrium * 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