In chemistry, neutralization or neutralisation is a chemical reaction in which an acid and a base react quantitatively with each other. In a reaction in water, neutralization results in there being no excess of hydrogen or hydroxide ions present in the solution; the pH of the neutralized solution depends on the acid strength of the reactants. Neutralization is used in many applications. In the context of a chemical reaction the term neutralization is used for a reaction between an acid and a base or alkali; this reaction was represented as acid + base → salt + waterFor example: HCl + NaOH → NaCl + H2OThe statement is still valid as long as it is understood that in an aqueous solution the substances involved are subject to dissociation, which changes the substances ionization state. The arrow sign, →, is used because the reaction is complete, that is, neutralization is a quantitative reaction. A more general definition is based on Brønsted–Lowry acid–base theory. AH + B → A + BHElectrical charges are omitted from generic expressions such as this, as each species A, AH, B, or BH may or may not carry an electrical charge.
Neutralization of sulphuric acid provides a specific example. Two partial neutralization reactions are possible in this instance. H2SO4 + OH− → HSO4−+ H2O HSO4− + OH− → SO42−+ H2O Overall: H2SO4 + 2OH− → SO42−+ 2H2OAfter an acid AH has been neutralized there are no molecules of the acid left in solution; when an acid is neutralized the amount of base added to it must be equal the amount of acid present initially. This amount of base is said to be the equivalent amount. In a titration of an acid with a base, the point of neutralization can be called the equivalence point; the quantitative nature of the neutralization reaction is most conveniently expressed in terms of the concentrations of acid and alkali. At the equivalence point: volume × concentration = volume × concentration In general, for an acid AHn at concentration c1 reacting with a base Bm at concentration c2 the volumes are related by: n v1 c1 = m v2 c2An example of a base being neutralized by an acid is as follows. Ba2 + 2H + → Ba2 + + 2H2OThe same equation relating the concentrations of base applies.
The concept of neutralization is not limited to reactions in solution. For example, the reaction of limestone with acid such as sulfuric acid is a neutralization reaction. CO3 + H2SO4 → + SO42− + CO2 + H2OSuch reactions are important in soil chemistry. A strong acid is one, dissociated in aqueous solution. For example, hydrochloric acid, HCl, is a strong acid. HCl → H+ + Cl−A strong base is one, dissociated in aqueous solution. For example, sodium hydroxide, NaOH, is a strong base. NaOH → Na+ + OH−Therefore, when a strong acid reacts with a strong base the neutralization reaction can be written as H+ + OH− → H2OFor example, in the reaction between hydrochloric acid and sodium hydroxide the sodium and chloride ions, Na+ and Cl− take no part in the reaction; the reaction is consistent with the Brønsted–Lowry definition because in reality the hydrogen ion exists as the hydronium ion, so that the neutralization reaction may be written as H3O+ + OH− → H2O + H2O → 2 H2OWhen a strong acid is neutralized by a strong base there are no excess hydrogen ions left in the solution.
The solution is said to be neutral as it is neither alkaline. The pH of such a solution is close to a value of 7. Neutralization is an exothermic reaction; the standard enthalpy change for the reaction H+ + OH− → H2O is −57.30 kJ/mol. A weak acid is one that does not dissociate when it is dissolved in water. Instead an equilibrium mixture is formed. AH + H2O ⇌ H3O + +; the pH of the neutralized solution is not close to 7, as with a strong acid, but depends on the acid dissociation constant of the acid. The pH at the end-point or equivalence point in a titration may be calculated. At the end-point the acid is neutralized so the analytical hydrogen ion concentration, TH, is zero and the concentration of the conjugate base, A−, is equal to the analytical concentration of the acid. Defining the acid dissociation constant, pKa, as = Ka. TH = + Ka − Kw/The term Kw/ is equal to the concentration of hydroxide ions. At neutralization, TH is zero. + Ka − Kw/ = 0 2 + KaTA2 − Kw = 0 2 = Kw/1 + KaTA log = 1/2 log Kw − 1/2 log pH = 1/2 pKw − 1/2 log In most circumstances the term 1 + TA/Ka is much larger than 1, is equal to TA/Ka to a good approximation.
PH ≈ 1/2 This equation explains the following facts: The pH at the end-point depends on the strength of the acid, pKa. The pH at the end-point depends on the concentration of the acid, TA; the pH rises more steeply at the end-point as the acid concentration increases. When a weak acid is titrated with a strong base the end-point occurs at pH greater than 7. Therefore, the most suitable indicator to use is one, like phenolphthalein, that changes color at high pH; the situation is analogous to that of strong bases. H3O+ + B ⇌ H2O + BH+The pH of the neutralized solution depends on the acid dissociation constant of the base, pKa, or, equivalently, on the base association constant, pKb; the most suitable indicator to use for this type of titr
In chemistry, bases are substances that, in aqueous solution, release hydroxide ions, are slippery to the touch, can taste bitter if an alkali, change the color of indicators, react with acids to form salts, promote certain chemical reactions, accept protons from any proton donor or contain or displaceable OH− ions. Examples of bases are the hydroxides of the alkaline earth metals; these particular substances produce hydroxide ions in aqueous solutions, are thus classified as Arrhenius bases. For a substance to be classified as an Arrhenius base, it must produce hydroxide ions in an aqueous solution. Arrhenius believed; this makes the Arrhenius model limited, as it cannot explain the basic properties of aqueous solutions of ammonia or its organic derivatives. There are bases that do not contain a hydroxide ion but react with water, resulting in an increase in the concentration of the hydroxide ion. An example of this is the reaction between water to produce ammonium and hydroxide. In this reaction ammonia is the base.
Ammonia and other bases similar to it have the ability to form a bond with a proton due to the unshared pair of electrons that they possess. In the more general Brønsted–Lowry acid–base theory, a base is a substance that can accept hydrogen cations —otherwise known as protons. In the Lewis model, a base is an electron pair donor. In water, by altering the autoionization equilibrium, bases yield solutions in which the hydrogen ion activity is lower than it is in pure water, i.e. the water has a pH higher than 7.0 at standard conditions. A soluble base is called an alkali if it releases OH − ions quantitatively. However, it is important to realize. Metal oxides and alkoxides are basic, conjugate bases of weak acids are weak bases. Bases can be thought of as the chemical opposite of acids. However, some strong acids are able to act as bases. Bases and acids are seen as opposites because the effect of an acid is to increase the hydronium concentration in water, whereas bases reduce this concentration.
A reaction between an acid and a base is called neutralization. In a neutralization reaction, an aqueous solution of a base reacts with an aqueous solution of an acid to produce a solution of water and salt in which the salt separates into its component ions. If the aqueous solution is saturated with a given salt solute, any additional such salt precipitates out of the solution; the notion of a base as a concept in chemistry was first introduced by the French chemist Guillaume François Rouelle in 1754. He noted that acids, which at that time were volatile liquids, turned into solid salts only when combined with specific substances. Rouelle considered that such a substance serves as a "base" for the salt, giving the salt a "concrete or solid form". General properties of bases include: Concentrated or strong bases are caustic on organic matter and react violently with acidic substances. Aqueous solutions or molten bases dissociate in ions and conduct electricity. Reactions with indicators: bases turn red litmus paper blue, phenolphthalein pink, keep bromothymol blue in its natural colour of blue, turn methyl orange yellow.
The pH of a basic solution at standard conditions is greater than seven. Bases are bitter in taste; the following reaction represents the general reaction between a base and water to produce a conjugate acid and a conjugate base: B + H2O ⇌ BH+ + OH−The equilibrium constant, Kb, for this reaction can be found using the following general equation: Kb = /In this equation, both the base and the strong base compete with one another for the proton. As a result, bases that react with water have small equilibrium constant values; the base is weaker. Bases react with acids to neutralize each other at a fast rate both in alcohol; when dissolved in water, the strong base sodium hydroxide ionizes into hydroxide and sodium ions: NaOH → Na+ + OH−and in water the acid hydrogen chloride forms hydronium and chloride ions: HCl + H2O → H3O+ + Cl−When the two solutions are mixed, the H3O+ and OH− ions combine to form water molecules: H3O+ + OH− → 2 H2OIf equal quantities of NaOH and HCl are dissolved, the base and the acid neutralize leaving only NaCl table salt, in solution.
Weak bases, such as baking soda or egg white, should be used to neutralize any acid spills. Neutralizing acid spills with strong bases, such as sodium hydroxide or potassium hydroxide, can cause a violent exothermic reaction, the base itself can cause just as much damage as the original acid spill. Bases are compounds that can neutralize an amount of acids. Both sodium carbonate and ammonia are bases, although neither of these substances contains OH− groups. Both compounds accept H+ when dissolved in protic solvents such as water: Na2CO3 + H2O → 2 Na+ + HCO3− + OH− NH3 + H2O → NH4+ + OH−From this, a pH, or acidity, can be calculated for aqueous solutions of bases. Bases directly act as electron-pair donors themselves: CO32− + H+ → HCO3− NH3 + H+ → NH4+A base is defined as a molecule that has the ability to accept an electron pair bond by entering another atom's valence shell through its possession of one electron pair. There are a limited number of elements that have atoms with the ability to provide a molecule with basic properties
E numbers are codes for substances that are permitted to be used as food additives for use within the European Union and EFTA. The "E" stands for "Europe". Found on food labels, their safety assessment and approval are the responsibility of the European Food Safety Authority. Having a single unified list for food additives was first agreed upon in 1962 with food colouring. In 1964, the directives for preservatives were added, 1970 for antioxidants and 1974 for the emulsifiers, stabilisers and gelling agents; the numbering scheme follows that of the International Numbering System as determined by the Codex Alimentarius committee, though only a subset of the INS additives are approved for use in the European Union as food additives. Outside the European continent plus Russia, E numbers are encountered on food labelling in other jurisdictions, including the Cooperation Council for the Arab States of the Gulf, South Africa, New Zealand and Israel, they are though still found on North American packaging on imported European products.
In some European countries, "E number" is sometimes used informally as a pejorative term for artificial food additives, products may promote themselves as "free of E numbers". This is incorrect, because many components of natural foods have assigned E numbers, e.g. vitamin C and lycopene, found in carrots. NB: Not all examples of a class fall into the given numeric range. Moreover, many chemicals in the E400–499 range, have a variety of purposes; the list shows all components that had an E-number assigned. Not all additives listed are still allowed in the EU, but are listed as they used to have an E-number. For an overview of allowed additives see information provided by the Food Standards Agency of the UK. Food Chemicals Codex List of food additives List of food additives, Codex Alimentarius Codex Alimentarius, the international foods standards, established by the Food and Agriculture Organization and the World Health Organization in 1963 See their document "Class Names and the International Numbering System for Food Additives" Joint FAO/WHO Expert Committee on Food Additives publications at the World Health Organization Food Additive Index, JECFA, Food and Agriculture Organization E-codes and ingredients search engine with details/suggestions for Muslims Current EU approved additives and their E Numbers Food Additives in the European Union Food Additives, Food Safety, website of the European Union.
Includes Lists of authorised food additives Food additives database The Food Additives and Ingredients Association, FAIA website, UK
Food additives are substances added to food to preserve flavor or enhance its taste, appearance, or other qualities. Some additives have been used for centuries. With the advent of processed foods in the second half of the twentieth century, many more additives have been introduced, of both natural and artificial origin. Food additives include substances that may be introduced to food indirectly in the manufacturing process, through packaging, or during storage or transport. To regulate these additives, inform consumers, each additive is assigned a unique number, termed as "E numbers", used in Europe for all approved additives; this numbering scheme has now been adopted and extended by the Codex Alimentarius Commission to internationally identify all additives, regardless of whether they are approved for use. E numbers are all prefixed by "E", but countries outside Europe use only the number, whether the additive is approved in Europe or not. For example, acetic acid is written as E260 on products sold in Europe, but is known as additive 260 in some countries.
Additive 103, alkannin, is not approved for use in Europe so does not have an E number, although it is approved for use in Australia and New Zealand. Since 1987, Australia has had an approved system of labelling for additives in packaged foods; each food additive has to be numbered. The numbers are the same as in Europe, but without the prefix "E"; the United States Food and Drug Administration lists these items as "generally recognized as safe". See list of food additives for a complete list of all the names. Food additives can be divided into several groups, although there is some overlap because some additives exert more than one effect. For example, salt is both a preservative as well as a flavor. Acidulents Acidulents confer sour or acid taste. Common acidulents include vinegar, citric acid, tartaric acid, malic acid, fumaric acid, lactic acid. Acidity regulators Acidity regulators are used for controlling the pH of foods for stability or to affect activity of enzymes. Anticaking agents Anticaking agents keep powders such as milk powder from sticking.
Antifoaming and foaming agents Antifoaming agents prevent foaming in foods. Foaming agents do the reverse. Antioxidants Antioxidants such as vitamin C are preservatives by inhibiting the degradation of food by oxygen. Bulking agents Bulking agents such as starch are additives that increase the bulk of a food without affecting its taste. Food coloring Colorings are added to food to replace colors lost during preparation or to make food look more attractive. Fortifying agents Vitamins and dietary supplements to increase the nutritional value Color retention agents In contrast to colorings, color retention agents are used to preserve a food's existing color. Emulsifiers Emulsifiers allow water and oils to remain mixed together in an emulsion, as in mayonnaise, ice cream, homogenized milk. Flavors Flavors are additives that give food a particular taste or smell, may be derived from natural ingredients or created artificially. Flavor enhancers Flavor enhancers enhance a food's existing flavors. A popular example is monosodium glutamate.
Some flavor enhancers have their own flavors. Flour treatment agents Flour treatment agents are added to flour to improve its color or its use in baking. Glazing agents Glazing agents provide a shiny appearance or protective coating to foods. Humectants Humectants prevent foods from drying out. Tracer gas Tracer gas allow for package integrity testing to prevent foods from being exposed to atmosphere, thus guaranteeing shelf life. Preservatives Preservatives prevent or inhibit spoilage of food due to fungi and other microorganisms. Stabilizers Stabilizers and gelling agents, like agar or pectin give foods a firmer texture. While they are not true emulsifiers, they help to stabilize emulsions. Sweeteners Sweeteners are added to foods for flavoring. Sweeteners other than sugar are added to keep the food energy low, or because they have beneficial effects regarding diabetes mellitus, tooth decay, or diarrhea. Thickeners Thickening agents are substances which, when added to the mixture, increase its viscosity without modifying its other properties.
Packaging Bisphenols and perfluoroalkyl chemicals are indirect additives used in manufacturing or packaging. In July 2018 the American Academy of Pediatrics called for more careful study of those three substances, along with nitrates and food coloring, as they might harm children during development. With the increasing use of processed foods since the 19th century, food additives are more used. Many countries regulate their use. For example, boric acid was used as a food preservative from the 1870s to the 1920s, but was banned after World War I due to its toxicity, as demonstrated in animal and human studies. During World War II, the urgent need for cheap, available food preservatives led to it being used again, but it was banned in the 1950s; such cases led to a general mistrust of food additives, an application of the precautionary principle led to the conclusion that only additives that are known to be safe should be used in foods. In the United States, this led to the adoption of the Delaney clause, an amendment to the Federal Food and Cosmetic Act of 1938, stating that no carcinogenic substances may be used as food additives.
A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed by the reception of a proton by a base—in other words, it is a base with a hydrogen ion added to it. On the other hand, a conjugate base is what is left over after an acid has donated a proton during a chemical reaction. Hence, a conjugate base is a species formed by the removal of a proton from an acid; because some acids are capable of releasing multiple protons, the conjugate base of an acid may itself be acidic. In summary, this can be represented as the following chemical reaction: Acid + Base ⇌ Conjugate Base + Conjugate Acid Johannes Nicolaus Brønsted and Martin Lowry introduced the Brønsted–Lowry theory, which proposed that any compound that can transfer a proton to any other compound is an acid, the compound that accepts the proton is a base. A proton is a nuclear particle with a unit positive electrical charge. A cation can be a conjugate acid, an anion can be a conjugate base, depending on which substance is involved and which acid–base theory is the viewpoint.
The simplest anion which can be a conjugate base is the solvated electron whose conjugate acid is the atomic hydrogen. In an acid-base reaction, an acid plus a base reacts to form a conjugate base plus a conjugate acid: Conjugates are formed when an acid loses a hydrogen proton or a base gains a hydrogen proton. Refer to the following figure: We say that the water molecule is the conjugate acid of the hydroxide ion after the latter received the hydrogen proton donated by ammonium. On the other hand, ammonia is the conjugate base for the acid ammonium after ammonium has donated a hydrogen ion towards the production of the water molecule. We can refer to OH- as a conjugate base of H2O, since the water molecule donates a proton towards the production of NH+4 in the reverse reaction, the predominating process in nature due to the strength of the base NH3 over the hydroxide ion. Based on this information, it is clear that the terms "Acid", "Base", "conjugate acid", "conjugate base" are not fixed for a certain chemical species.
The strength of a conjugate acid is directly proportional to its dissociation constant. If a conjugate acid is strong, its dissociation will have a higher equilibrium constant and the products of the reaction will be favored; the strength of a conjugate base can be seen as the tendency of the species to "pull" hydrogen protons towards itself. If a conjugate base is classified as strong, it will "hold on" to the hydrogen proton when in solution and its acid will not dissociate. On the other hand, if a species is classified as a strong acid, its conjugate base will be weak in nature. An example of this case would be the dissociation of Hydrochloric acid HCl in water. Since HCl is a strong acid, its conjugate base will be a weak conjugate base. Therefore, in this system, most H+ will be in the form of a Hydronium ion H3O+ instead of attached to a Cl anion and the conjugate base will be weaker than a water molecule. If an acid is weak, its conjugate base will be strong; when considering the fact that the Kw is equal to the product of the concentrations of H+ and OH.
A weak acid will have a low concentration of H+. The Kw divided by a low H+ concentration will result in a low OH- concentration as well. Therefore, weak acids will have weak conjugate bases, unlike the misconception that they have strong conjugate bases; the acid and conjugate base as well as the base and conjugate acid are known as conjugate pairs. When finding a conjugate acid or base, it is important to look at the reactants of the chemical equation. In this case, the reactants are the acids and bases, the acid corresponds to the conjugate base on the product side of the chemical equation. To identify the conjugate acid, look for the pair of compounds that are related; the acid–base reaction can be viewed in a before and after sense. The before is the reactant side of the after is the product side of the equation; the conjugate acid in the after side of an equation gains a hydrogen ion, so in the before side of the equation the compound that has one less hydrogen ion of the conjugate acid is the base.
The conjugate base in the after side of the equation lost a hydrogen ion, so in the before side of the equation, the compound that has one more hydrogen ion of the conjugate base is the acid. Consider the following acid–base reaction: HNO3 + H2O → H3O+ + NO−3Nitric acid is an acid because it donates a proton to the water molecule and its conjugate base is nitrate; the water molecule acts as a base because it receives the Hydrogen Proton and its conjugate acid is the hydronium ion. One use of conjugate acids and bases lies in buffering systems. In a buffer, a weak acid and its conjugate base, or a weak base and its conjugate acid, are used in order to limit the pH change during a titration process. Buffers have both non-organic chemical applications. For example, besides buffers being used in lab processes, our blood acts as a buffer to maintain pH; the most important buffer in our bloodstream is the carbonic acid-bicarbonate buffer, which prevents drastic pH changes when CO2 is introduced. This functions as such: CO 2 + H 2 O ↽ − − ⇀ H 2 CO 3 ↽
Acetic acid, systematically named ethanoic acid, is a colourless liquid organic compound with the chemical formula CH3COOH. When undiluted, it is sometimes called glacial acetic acid. Vinegar is no less than 4% acetic acid by volume, making acetic acid the main component of vinegar apart from water. Acetic acid has pungent smell. In addition to household vinegar, it is produced as a precursor to polyvinyl acetate and cellulose acetate, it is classified as a weak acid since it only dissociates in solution, but concentrated acetic acid is corrosive and can attack the skin. Acetic acid is the second simplest carboxylic acid, it consists of a methyl group attached to a carboxyl group. It is an important chemical reagent and industrial chemical, used in the production of cellulose acetate for photographic film, polyvinyl acetate for wood glue, synthetic fibres and fabrics. In households, diluted acetic acid is used in descaling agents. In the food industry, acetic acid is controlled by the food additive code E260 as an acidity regulator and as a condiment.
In biochemistry, the acetyl group, derived from acetic acid, is fundamental to all forms of life. When bound to coenzyme A, it is central to the metabolism of fats; the global demand for acetic acid is about 6.5 million metric tons per year, of which 1.5 Mt/a is met by recycling. Vinegar is dilute acetic acid produced by fermentation and subsequent oxidation of ethanol; the trivial name acetic acid is the most used and preferred IUPAC name. The systematic name ethanoic acid, a valid IUPAC name, is constructed according to the substitutive nomenclature; the name acetic acid derives from acetum, the Latin word for vinegar, is related to the word acid itself. Glacial acetic acid is a name for water-free acetic acid. Similar to the German name Eisessig, the name comes from the ice-like crystals that form below room temperature at 16.6 °C. A common symbol for acetic acid is AcOH, where Ac is the pseudoelement symbol representing the acetyl group CH3−C−. To better reflect its structure, acetic acid is written as CH3–COH, CH3−COH, CH3COOH, CH3CO2H.
In the context of acid-base reactions, the abbreviation HAc is sometimes used, where Ac in this case is a symbol for acetate. Acetate is the ion resulting from loss of H+ from acetic acid; the name acetate can refer to a salt containing this anion, or an ester of acetic acid. The hydrogen centre in the carboxyl group in carboxylic acids such as acetic acid can separate from the molecule by ionization: CH3CO2H ⇌ CH3CO2− + H+Because of this release of the proton, acetic acid has acidic character. Acetic acid is a weak monoprotic acid. In aqueous solution, it has a pKa value of 4.76. Its conjugate base is acetate. A 1.0 M solution has a pH of 2.4, indicating that 0.4% of the acetic acid molecules are dissociated. However, in dilute solution acetic acid is >90% dissociated. In solid acetic acid, the molecules form chains, individual molecules being interconnected by hydrogen bonds. In the vapour at 120 °C, dimers can be detected. Dimers occur in the liquid phase in dilute solutions in non-hydrogen-bonding solvents, a certain extent in pure acetic acid, but are disrupted by hydrogen-bonding solvents.
The dissociation enthalpy of the dimer is estimated at 65.0–66.0 kJ/mol, the dissociation entropy at 154–157 J mol−1 K−1. Other carboxylic acids engage in similar intermolecular hydrogen bonding interactions. Liquid acetic acid is a hydrophilic protic similar to ethanol and water. With a moderate relative static permittivity of 6.2, it dissolves not only polar compounds such as inorganic salts and sugars, but non-polar compounds such as oils as well as polar solutes. It is miscible with polar and non-polar solvents such as water and hexane. With higher alkanes, acetic acid is not miscible, its miscibility declines with longer n-alkanes; the solvent and miscibility properties of acetic acid make it a useful industrial chemical, for example, as a solvent in the production of dimethyl terephthalate. At physiological pHs, acetic acid is fully ionised to acetate; the acetyl group, formally derived from acetic acid, is fundamental to all forms of life. When bound to coenzyme A, it is central to the metabolism of fats.
Unlike longer-chain carboxylic acids, acetic acid does not occur in natural triglycerides. However, the artificial triglyceride triacetin is a common food additive and is found in cosmetics and topical medicines. Acetic acid is produced and excreted by acetic acid bacteria, notably the genus Acetobacter and Clostridium acetobutylicum; these bacteria are found universally in foodstuffs and soil, acetic acid is produced as fruits and other foods spoil. Acetic acid is a component of the vaginal lubrication of humans and other primates, where it appears to serve as a mild antibacterial agent. Acetic acid is produced industrially both synthetically and by bacterial fermentation. About 75% of acetic acid made for use in the chemical industry is made by the carbonylation of methanol, explained below; the biological route accounts for only a
Citric acid is a weak organic acid that has the chemical formula C6H8O7. It occurs in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in the metabolism of all aerobic organisms. More than a million tons of citric acid are manufactured every year, it is used as an acidifier, as a flavoring and chelating agent. A citrate is a derivative of citric acid. An example of the former, a salt is trisodium citrate; when part of a salt, the formula of the citrate ion is written as C6H5O3−7 or C3H5O3−3. Citric acid exists in greater than trace amounts in a variety of fruits and vegetables, most notably citrus fruits. Lemons and limes have high concentrations of the acid; the concentrations of citric acid in citrus fruits range from 0.005 mol/L for oranges and grapefruits to 0.30 mol/L in lemons and limes. Within species, these values vary depending on the cultivar and the circumstances in which the fruit was grown. Industrial-scale citric acid production first began in 1890 based on the Italian citrus fruit industry, where the juice was treated with hydrated lime to precipitate calcium citrate, isolated and converted back to the acid using diluted sulfuric acid.
In 1893, C. Wehmer discovered. However, microbial production of citric acid did not become industrially important until World War I disrupted Italian citrus exports. In 1917, American food chemist James Currie discovered certain strains of the mold Aspergillus niger could be efficient citric acid producers, the pharmaceutical company Pfizer began industrial-level production using this technique two years followed by Citrique Belge in 1929. In this production technique, still the major industrial route to citric acid used today, cultures of A. niger are fed on a sucrose or glucose-containing medium to produce citric acid. The source of sugar is corn steep liquor, hydrolyzed corn starch or other inexpensive sugary solutions. After the mold is filtered out of the resulting solution, citric acid is isolated by precipitating it with calcium hydroxide to yield calcium citrate salt, from which citric acid is regenerated by treatment with sulfuric acid, as in the direct extraction from citrus fruit juice.
In 1977, a patent was granted to Lever Brothers for the chemical synthesis of citric acid starting either from aconitic or isocitrate/alloisocitrate calcium salts under high pressure conditions. This produced citric acid in near quantitative conversion under what appeared to be a reverse non-enzymatic Krebs cycle reaction. In 2007, worldwide annual production stood at 1,600,000 tons. More than 50% of this volume was produced in China. More than 50% was used as an acidity regulator in beverages, some 20% in other food applications, 20% for detergent applications and 10% for related applications other than food, such as cosmetics, pharmaceutics and in the chemical industry. Citric acid was first isolated in 1784 by the chemist Carl Wilhelm Scheele, who crystallized it from lemon juice, it can exist either as a monohydrate. The anhydrous form crystallizes from hot water, while the monohydrate forms when citric acid is crystallized from cold water; the monohydrate can be converted to the anhydrous form at about 78 °C.
Citric acid dissolves in absolute ethanol at 15 °C. It decomposes with loss of carbon dioxide above about 175 °C. Citric acid is considered to be a tribasic acid, with pKa values, extrapolated to zero ionic strength, of 5.21, 4.28 and 2.92 at 25 °C. The pKa of the hydroxyl group has been found, by means of 13C NMR spectroscopy, to be 14.4. The speciation diagram shows that solutions of citric acid are buffer solutions between about pH 2 and pH 8. In biological systems around pH 7, the two species present are the citrate ion and mono-hydrogen citrate ion; the SSC 20X hybridization buffer is an example in common use. Tables compiled for biochemical studies are available. On the other hand, the pH of a 1 mM solution of citric acid will be about 3.2. The pH of fruit juices from citrus fruits like oranges and lemons depends on the citric acid concentration, being lower for higher acid concentration and conversely. Acid salts of citric acid can be prepared by careful adjustment of the pH before crystallizing the compound.
See, for example, sodium citrate. The citrate ion forms complexes with metallic cations; the stability constants for the formation of these complexes are quite large because of the chelate effect. It forms complexes with alkali metal cations. However, when a chelate complex is formed using all three carboxylate groups, the chelate rings have 7 and 8 members, which are less stable thermodynamically than smaller chelate rings. In consequence, the hydroxyl group can be deprotonated, forming part of a more stable 5-membered ring, as in ammonium ferric citrate, 5Fe2·2H2O. Citric acid can be esterified at one or more of the carboxylic acid functional groups on the molecule, to form any of a variety of mono-, di-, tri-, mixed esters. Citrate is an intermediate in the TCA cycle, a central metabolic pathway for animals and bacteria. Citrate synthase catalyzes the condensation of oxaloacetate with acetyl CoA to form citrate. Citrate acts as the substrate for aconitase and is converted into aconitic acid.
The cycle ends with regeneration of oxaloacetate. This series