uric acid

Heterocyclic compound of the purine type, the end product of metabolism of the purines in nucleic acids in many animals, including humans. It is excreted by reptiles and birds as the chief nitrogenous end product of protein breakdown. Small quantities are normally found in human blood; in gout, levels are abnormally high. Uric acid is used industrially in organic synthesis.

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or Krebs cycle or citric-acid cycle

Last stage of the chemical processes by which living cells obtain energy from foodstuffs. Described by Hans Adolf Krebs in 1937, the reactions of the cycle have been shown in animals, plants, microorganisms, and fungi, and it is thus a feature of cell chemistry shared by all types of life. It is a complex series of reactions beginning and ending with the compound oxaloacetate. In addition to re-forming oxaloacetate, the cycle produces carbon dioxide and the energy-rich compound ATP. The enzymes that catalyze each step are located in mitochondria in animals, in chloroplasts in plants, and in the cell membrane in microorganisms. The hydrogen atoms and electrons that are removed from intermediate compounds formed during the cycle are channeled ultimately to oxygen in animal cells or to carbon dioxide in plant cells.

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or tannic acid

Any of a group of pale yellow to light brown amorphous substances widely distributed in plants and used chiefly in tanning leather, dyeing fabric, and making ink. Their solutions are acid and have an astringent taste. They are isolated from oak bark, sumac, myrobalan (an Asian tree), and galls. Tannins give tea astringency, colour, and some flavour. Tannins are used industrially to clarify wine and beer, reduce viscosity of oil-well drilling mud, and prevent scale in boiler water; they have also had medical uses.

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or oil of vitriol

Dense, colourless, oily, corrosive liquid inorganic compound (H₂SO₄). A very strong acid, it forms ions of hydrogen or hydronium (H⁺ or H₃O⁺), hydrogen sulfate (HSO₄⁻), and sulfate (SO₄²⁻). It is also an oxidizing (see oxidation-reduction) and dehydrating agent and chars many organic materials. It is one of the most important industrial chemicals, used in various concentrations in manufacturing fertilizers, pigments, dyes, drugs, explosives, detergents, and inorganic salts and acids, in petroleum refining and metallurgical processes, and as the acid in lead-acid storage batteries. It is made industrially by dissolving sulfur trioxide (SO₃) in water, sometimes beyond the saturation point to make oleum (fuming sulfuric acid), used to make certain organic chemicals.

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White, crystalline solid organic compound used chiefly to make aspirin and other pharmaceutical products, including methyl salicylate (oil of wintergreen, for medicines and flavourings), phenyl salicylate (for sunburn creams and pill coatings), and salicylanilide (a cutaneous fungicide). Its molecular structure, with the formula C₆H₄(OH)COOH, consists of a six-membered aromatic ring (see aromatic compound) having a hydroxyl group (singlehorzbondOH) and a carboxyl group (singlehorzbondCOOH) bonded to adjacent carbon atoms; as such, it is both a phenol and a carboxylic acid. It and certain derivatives occur naturally in some plants, particularly species of Spiraea and Salix (willow). Large amounts are used in producing certain dyes.

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in full ribonucleic acid

One of the two main types of nucleic acid (the other being DNA), which functions in cellular protein synthesis in all living cells and replaces DNA as the carrier of genetic information in some viruses. Like DNA, it consists of strands of repeating nucleotides joined in chainlike fashion, but the strands are single (except in certain viruses), and it has the nucleotide uracil (U) where DNA has thymine (T). Messenger RNA (mRNA), a single strand copied from a DNA strand that acts as its template, carries the message of the genetic code from DNA (in chromosomes) to the site of protein synthesis (on ribosomes). Ribosomal RNA (rRNA), part of the building blocks of ribosomes, participates in protein synthesis. Transfer RNA (tRNA), the smallest type, has fewer than 100 nucleotide units (mRNA and rRNA contain thousands). Each nucleotide triplet on mRNA specifies which amino acid comes next on the protein being synthesized, and a tRNA molecule with that triplet's complement on its protruding end brings the specified amino acid to the site of synthesis to be linked into the protein. Various minor types of RNA also exist; at least some act as catalysts (ribozymes), a function long ascribed only to proteins.

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or folate

Organic compound essential to animal growth and health and needed by bacteria as a growth factor. Part of the vitamin B complex, folic acid is necessary for synthesis of nucleic acids and formation of the heme component of hemoglobin in red blood cells. To prevent neural tube defects in babies, it should ideally be taken by women starting at least a month before conception. Dietary folate sources include leafy and dark green vegetables, citrus fruits, cereals, beans, poultry, and egg yolks, but free folic acid is available only in supplements. Low intake leads to folic acid deficiency anemia.

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Organic compound, essential in animal metabolism. The nature of the bound form was clarified through the discovery and synthesis (1947–50) of the compound pantetheine, which contains pantothenic acid combined with the compound thioethanolamine. Pantetheine is part of two larger compounds (coenzyme A and acyl-carrier protein) that promote a large number of metabolic reactions essential for the growth and well-being of animals. A dietary deficiency severe enough to lead to clear-cut disease has not been described in humans; however, when a person is severely malnourished, deficiency of the vitamin appears to contribute to the observed weakness and mental depression.

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Colourless, crystalline, toxic carboxylic acid found in many plants, especially rhubarb, wood sorrel, and spinach. Because it forms soluble chelates with iron, some of the iron in these plants is not available nutritionally. However, this property makes it useful for removing blood and rust stains, cleaning metals other than iron, and flushing car radiators. Oxalic acid and its salts (oxalates) are used in many chemical processes.

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Any of the organic compounds making up the genetic material of living cells. Nucleic acids direct the course of protein synthesis, thereby regulating all cell activities. Their transmission from one generation to the next is the basis of heredity. The two main types, DNA and RNA, are composed of similar materials but differ in structure and function. Both are long chains of repeating nucleotides. The sequence of purines and pyrimidines (bases)—adenine (A), guanine (G), cytosine (C), and either thymine (T; in DNA) or uracil (U; in RNA)—in the nucleotides, in groups of three (triplets, or codons), constitutes the genetic code.

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Inorganic compound, colourless, fuming, highly corrosive liquid, chemical formula HNO₃. A common laboratory reagent, it is important in the manufacture of fertilizers and explosives (including nitroglycerin), as well as in organic syntheses, metallurgy, ore flotation, and reprocessing of spent nuclear fuel. A strong acid, it is toxic and can cause severe burns. It attacks most metals and is used for etching steel and photoengraving.

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or nicotinic acid or vitamin B₃

Water-soluble vitamin of the vitamin B complex, essential to growth and health in animals, including humans. It is found in the body only in combined form as a coenzyme, nicotinamide adenine dinucleotide (NAD), which is involved in the metabolism of carbohydrates and the oxidation of sugar derivatives and other substances. One of the most stable vitamins, it survives most cooking and most preserving processes. It is widely found in dietary sources, especially lean meat. Deficiency causes pellagra. It is used as a drug to reduce high cholesterol levels in the blood.

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Simplest carboxylic acid, chemical formula HCOOH. It is secreted by some insects, especially red ants (its name comes from the Latin word for ant), in their bite or sting. It has many industrial uses, in textile and leather manufacture, as an industrial solvent, and as an intermediate.

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in full lysergic acid diethylamide

Highly potent hallucinogenic drug. An organic compound, LSD can be derived from the alkaloids ergotamine and ergonovine, found in the ergot fungus, but most LSD is produced synthetically. It can block the action of the neurotransmitter serotonin and produces marked deviations from normal perceptions and behaviour lasting 8–10 hours or longer. Mood shifts, time and space distortions, and impulsive behaviour may progress to paranoia and aggression. Flashbacks to LSD-induced hallucinations can occur years later. LSD is not an approved drug, and no clinically valuable uses have been found for it.

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Carboxylic acid found in certain plant juices, in blood and muscle, and in soil. In blood it occurs in the form of its salts (lactates) when glycogen is broken down in muscle; it can be reconverted to glycogen in the liver. Stiffness and soreness after prolonged heavy exercise are due to accumulated lactic acid in the muscles. The end product of bacterial fermentation, lactic acid is the most common acidic constituent of fermented milk products (e.g., sour milk and cream, cheese, buttermilk, yogurt). It is used in other foods as a flavouring or preservative and industrially in tanning leather and dyeing wool and as a raw material or catalyst in many chemical processes.

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or ascorbic acid

Water-soluble organic compound important in animal metabolism. Most animals produce it in their bodies, but humans, other primates, and guinea pigs need it in the diet to prevent scurvy. It is essential in collagen synthesis, wound healing, blood-vessel maintenance, and immunity. Some studies have found a moderate benefit of vitamin C in reducing the duration and severity of the common cold. It works as an antioxidant in the body and is used as a preservative. It is easily destroyed by oxygen. Excellent sources are citrus fruits and fresh vegetables.

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One of the nonessential amino acids, closely related to glutamine. The two constitute a substantial fraction of the amino acids in many proteins (10–20percnt in many cases and up to 45percnt in some plant proteins). An important metabolic intermediate as well as a neurotransmitter molecule in the central nervous system, glutamic acid finds uses in medicine and biochemical research. Its sodium salt is  the food flavour enhancer monosodium glutamate (MSG).

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Most important carboxylic acid (CH₃COOH). Pure (“glacial”) acetic acid is a clear, syrupy, corrosive liquid that mixes readily with water. Vinegar is its dilute solution, from fermentation and oxidation (see oxidation-reduction) of natural products. Its salts and esters are acetates. It occurs naturally as a metabolic intermediate in body fluids and plant juices. Industrial production is either synthetic, from acetylene, or biological, from ethanol. Industrial chemicals made from it are used in printing and as plastics, photographic films, textiles, and solvents.

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Organic compound that is an important component of lipids in plants, animals, and microorganisms. Fatty acids are carboxylic acids with a long hydrocarbon chain, usually straight, as the fourth substituent group on the carboxyl (singlehorzbondCOOH) group (see functional group) that makes the molecule an acid. If the carbon-to-carbon bonds (see bonding) in that chain are all single, the fatty acid is saturated; artificial saturation is called hydrogenation. A fatty acid with one double bond is monounsaturated; one with more is polyunsaturated. These are more reactive chemically. Most unsaturated fats are liquid at room temperature, so food manufacturers hydrogenate them to make them solid (see margarine). A high level of saturated fatty acids in the diet raises blood cholesterol levels. A few fatty acids have branched chains. Others (e.g., prostaglandins) contain ring structures. Fatty acids in nature are always combined, usually with glycerol as triglycerides in fats. Oleic acid (unsaturated, with 18 carbon atoms) is almost half of human fat and is abundant in such oils as olive, palm, and peanut. Most animals, including mammals, cannot synthesize some unsaturated “essential” fatty acids; humans need linoleic, linolenic, and arachidonic acids in their diet.

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Method developed by the British geneticist Alec Jeffreys (born 1950) in 1984 for isolating and making images of sequences of DNA. The procedure consists of obtaining a sample of cells containing DNA (e.g., from skin, blood, or hair), extracting the DNA, and purifying it. The DNA is then cut by enzymes, and the resulting fragments of varying lengths undergo procedures that permit them to be analyzed. The pattern of fragments is unique for each individual. DNA fingerprinting is used to help solve crimes and determine paternity; it is also used to locate gene segments that cause genetic diseases, to map the genetic material of humans (see Human Genome Project), to engineer drought-resistant plants (see genetic engineering), and to produce biological drugs from genetically altered cells.

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or deoxyribonucleic acid

One of two types of nucleic acid (the other is RNA); a complex organic compound found in all living cells and many viruses. It is the chemical substance of genes. Its structure, with two strands wound around each other in a double helix to resemble a twisted ladder, was first described (1953) by Francis Crick and James D. Watson. Each strand is a long chain (polymer) of repeating nucleotides: adenine (A), guanine (G), cytosine (C), and thymine (T). The two strands contain complementary information: A forms hydrogen bonds (see hydrogen bonding) only with T, C only with G. When DNA is copied in the cell, the strands separate and each serves as a template for assembling a new complementary strand; this is the key to stable heredity. DNA in cells is organized into dense protein-DNA complexes (see nucleoprotein) called chromosomes. In eukaryotes these are in the nucleus, and DNA also occurs in mitochondria and chloroplasts (if any). Prokaryotes have a single circular chromosome in the cytoplasm. Some prokaryotes and a few eukaryotes have DNA outside the chromosomes in plasmids. Seealso Rosalind Franklin; genetic engineering; mutation; Maurice Wilkins.

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Colourless, crystalline organic compound (C₆H₈O₇), one of the carboxylic acids. It is present in almost all plants (especially citrus fruits) and in many animal tissues and fluids. It is one of a series of compounds involved in the physiological oxidation (see oxidation-reduction) of fats, proteins, and carbohydrates to carbon dioxide and water (see tricarboxylic acid cycle). It has a characteristic sharply sour taste and is used in many foods, confections, and soft drinks. It is added to certain foods to improve their stability in metal containers. Industrially, it is used as a water conditioner, cleaning and polishing agent, and chemical intermediate.

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Any organic compound with the general chemical formula singlehorzbondCOOH in which a carbon (C) atom is bonded to an oxygen (O) atom by a double bond to make a carbonyl group (singlehorzbondCdoublehorzbondO; see functional group) and to a hydroxyl group (singlehorzbondOH) by a single bond (see bonding). The fourth bond on the carbon links it to a hydrogen (H) atom (for formic acid), a methyl (singlehorzbondCH₃) group (for acetic acid), or another natural or synthetic monovalent group. Carboxylic acids occur widely in nature. In fatty acids, the fourth group is a hydrocarbon chain. In aromatic acids (see aromatic compound), it is a ring-structured hydrocarbon. In amino acids, it contains a nitrogen atom. Carboxylic acids participate in chemical reactions as acids, usually fairly weak. Many carboxylic acids (acetic acid, citric acid, lactic acid) are intermediates in metabolism and can be found in natural products; others (e.g., salicylic acid) are used as solvents and to prepare many chemical compounds. Important carboxylic-acid derivatives include esters, anhydrides, amides, halides (see halogen), and salts (see soap).

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One of the nonessential amino acids, found in many proteins and closely related to asparagine. It is used in medical and biochemical research, as an organic intermediate, and in various industrial applications. It is one of the two components of aspartame.

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Any of a class of organic compounds in which a carbon atom has bonds to an amino group (singlehorzbondNH₂), a carboxyl group (singlehorzbondCOOH), a hydrogen atom (singlehorzbondH), and an organic side group (called singlehorzbondR). They are therefore both carboxylic acids and amines. The physical and chemical properties unique to each result from the properties of the R group, particularly its tendency to interact with water and its charge (if any). Amino acids joined linearly by peptide bonds (see covalent bond) in a particular order make up peptides and proteins. Of over 100 natural amino acids, each with a different R group, only 20 make up the proteins of all living organisms. Humans can synthesize 10 of them (by interconversions) from each other or from other molecules of intermediary metabolism, but the other 10 (essential amino acids: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) must be consumed in the diet.

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Division of igneous rocks on the basis of their silicate mineral content, these minerals usually being the most abundant in such rocks. Rocks are described as felsic, intermediate, mafic, and ultramafic, in order of decreasing silica content, and, in general, the gradation from felsic to mafic corresponds to an increase in colour (i.e., light to dark). Due to the erroneous belief that silica present in rock magmas occurred in the form of silicic acid, high- and low-silica rocks were once known as acid and basic rocks, respectively.

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Any precipitation, including snow, that contains a heavy concentration of sulfuric and nitric acids. This form of pollution is a serious environmental problem in the large urban and industrial areas of North America, Europe, and Asia. Automobiles, certain industrial operations, and electric power plants that burn fossil fuels emit the gases sulfur dioxide and nitrogen oxide into the atmosphere, where they combine with water vapour in clouds to form sulfuric and nitric acids. The highly acidic precipitation from these clouds may contaminate lakes and streams, damaging fish and other aquatic species; damage vegetation, including agricultural crops and trees; and corrode the outsides of buildings and other structures (historic monuments are especially vulnerable). Though usually most severe around large urban and industrial areas, acid precipitation may also occur at great distances from the source of the pollutants.

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Any substance that in water solution tastes sour, changes the colour of acid-base indicators (e.g., litmus), reacts with some metals (e.g., iron) to yield hydrogen gas, reacts with bases to form salts, and promotes certain chemical reactions (e.g., acid catalysis). Acids contain one or more hydrogen atoms that, in solution, dissociate as positively charged hydrogen ions. Inorganic, or mineral, acids include sulfuric acid, nitric acid, hydrochloric acid, and phosphoric acid. Organic acids include carboxylic acids, phenols, and sulfonic acids. Broader definitions of acids cover situations in which water is not present. Seealso acid-base theory.

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Common name of acetylsalicylic acid, an organic compound introduced in 1899. The ester of salicylic acid and acetic acid, it inhibits production of prostaglandins in the body. Its analgesic, fever-reducing, and anti-inflammatory effects make it useful in treating headaches, muscle and joint aches, arthritis pain, and the symptoms of mild fevers and infections. It also has anticoagulant activity and is taken in low doses by coronary heart disease patients to prevent heart attack. Prolonged use may cause stomach bleeding and peptic ulcer, and its use in children with fever has been linked to Reye syndrome. Seealso acetaminophen; ibuprofen; NSAID.

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An acid (often represented by the generic formula HA [H⁺A⁻]) is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion activity greater than in pure water, i.e. a pH less than 7.0. That approximates the modern definition of Johannes Nicolaus Brønsted and Martin Lowry, who independently defined an acid as a compound which donates a hydrogen ion (H⁺) to another compound (called a base). Common examples include acetic acid (in vinegar) and sulfuric acid (used in car batteries). Acid/base systems are different from redox reactions in that there is no change in oxidation state.

Definitions

The word "acid" comes from the Latin acidus meaning "sour," but in chemistry the term acid has a more specific meaning. There are four common ways to define an acid:

• Arrhenius: According to this definition developed by the Swedish chemist Svante Arrhenius, an acid is a substance that increases the concentration of hydrogen ions (H⁺), which are carried as hydronium ions (H₃O⁺) when dissolved in water, while bases are substances that increase the concentration of hydroxide ions (OH^-). This definition limits acids and bases to substances that can dissolve in water. Around 1800, many French chemists, including Antoine Lavoisier, incorrectly believed that all acids contained oxygen. Indeed the modern German word for oxygen is Sauerstoff (lit. sour substance), as is the Afrikaans word for oxygen suurstof, with the same meaning. English chemists, including Sir Humphry Davy, at the same time believed all acids contained hydrogen. Arrhenius used this belief to develop this definition of acid.
• Brønsted-Lowry: According to this definition, an acid is a proton (hydrogen nucleus) donor and a base is a proton acceptor. The acid is said to be dissociated after the proton is donated. An acid and the corresponding base are referred to as conjugate acid-base pairs. Brønsted and Lowry independently formulated this definition, which includes water-insoluble substances not in the Arrhenius definition. Acids according to this definition are variously referred to as Brønsted acids, Brønsted-Lowry acids, proton acids, protic acids, or protonic acids.
• Solvent-system definition: According to this definition, an acid is a substance that, when dissolved in an autodissociating solvent, increases the concentration of the solvonium cations, such as H₃O⁺ in water, NH₄⁺ in liquid ammonia, NO⁺ in liquid N₂O₄, SbCl₂⁺ in SbCl₃, etc. Base is defined as the substance that increases the concentration of the solvate anions, respectively OH^-, NH₂^-, NO₃^-, or SbCl₄^-. This definition extends acid-base reactions to non-aqueous systems and even some aprotic systems, where no hydrogen nuclei are involved in the reactions. This definition is not absolute, a compound acting as acid in one solvent may act as a base in another.
• Lewis: According to this definition developed by Gilbert N. Lewis, an acid is an electron-pair acceptor and a base is an electron-pair donor. (These are frequently referred to as "Lewis acids" and "Lewis bases," and are electrophiles and nucleophiles, respectively, in organic chemistry; Lewis bases are also ligands in coordination chemistry.) Lewis acids include substances with no transferable protons (ie H⁺ hydrogen ions), such as iron(III) chloride, and hence the Lewis definition of an acid has wider application than the Brønsted-Lowry definition. In fact, the term Lewis acid is often used to exclude protic (Brønsted-Lowry) acids. The Lewis definition can also be explained with molecular orbital theory. In general, an acid can receive an electron pair in its lowest unoccupied orbital (LUMO) from the highest occupied orbital (HOMO) of a base. That is, the HOMO from the base and the LUMO from the acid combine to a bonding molecular orbital.

Although not the most general theory, the Brønsted-Lowry definition is the most widely used definition. The strength of an acid may be understood by this definition by the stability of hydronium and the solvated conjugate base upon dissociation. Increasing or decreasing stability of the conjugate base will increase or decrease the acidity of a compound. This concept of acidity is used frequently for organic acids such as carboxylic acid. The molecular orbital description, where the unfilled proton orbital overlaps with a lone pair, is connected to the Lewis definition.

Properties

Bronsted-Lowry acids:

• Are generally sour in taste
• Strong or concentrated acids often produce a stinging feeling on mucous membranes
• Change the color of pH indicators as follows: turn blue litmus and methyl orange red, turn phenolphthalein colorless
• React with metals to produce a metal salt and hydrogen
• React with metal carbonates to produce water, CO₂ and a salt
• React with a base to produce a salt and water
• React with a metal oxide to produce water and a salt
• Conduct electricity, depending on the degree of dissociation
• Produce solvonium ions, such as oxonium (H₃O⁺) ions in water

Acids are/can be gases, liquids, or solids. Respective examples (at 20 °C and 1 atm) are hydrogen chloride, sulfuric acid and citric acid. Solutions of acids in water are liquids, such as hydrochloric acid - an aqueous solution of hydrogen chloride. At 20 °C and 1 atm, linear carboxylic acids are liquids up to nonanoic acid (nine carbon atoms) and solids beginning from decanoic acid (ten carbon atoms). Aromatic carboxylic acids, the simplest being benzoic acid, are solids.

Strong acids and many concentrated acids, being corrosive, can be dangerous; causing severe burns for even minor contact. Generally, acid burns on the skin are treated by rinsing the affected area abundantly with running water, followed up with immediate medical attention. In the case of highly concentrated mineral acids such as sulfuric acid or nitric acid, the acid should first be wiped off, otherwise the exothermic mixing of the acid and the water could cause thermal burns. Particular acids may also be dangerous for reasons not related to their acidity. Material Safety Data Sheets (MSDS) can be consulted for detailed information on dangers and handling instructions.

Nomenclature

In the classical naming system, acids are named according to their anions. That ionic suffix is dropped and replaced with a new suffix (and sometimes prefix), according to the table below. For example, HCl has chloride as its anion, so the -ide suffix makes it take the form hydrochloric acid. In the IUPAC naming system, "aqueous" is simply added to the name of the ionic compound. Thus, for hydrogen chloride, the IUPAC name would be aqueous hydrogen chloride. The prefix "hydro-" is added only if the acid is made up of just hydrogen and one other element.

Classical naming system:

Anion Prefix	Anion Suffix	Acid Prefix	Acid Suffix	Example
per	ate	per	ic acid	perchloric acid (HClO4)
	ate		ic acid	chloric acid (HClO3)
	ite		ous acid	chlorous acid (HClO2)
hypo	ite	hypo	ous acid	hypochlorous acid (HClO)
	ide	hydro	ic acid	hydrochloric acid (HCl)

Chemical characteristics

In water the following equilibrium occurs between a weak acid (HA) and water, which acts as a base:

HA(aq) + H₂O ⇌ H₃O⁺(aq) + A^-(aq)

The acidity constant (or acid dissociation constant) is the equilibrium constant for the reaction of HA with water:

$K\_a\; =\; frac\{[mbox\{H\}\_3mbox\{O\}^+][mbox\{A\}^-]\}\{[mbox\{HA\}]\}$

Strong acids have large K_a values (i.e. the reaction equilibrium lies far to the right; the acid is almost completely dissociated to H₃O⁺ and A^-). Strong acids include the heavier hydrohalic acids: hydrochloric acid (HCl), hydrobromic acid (HBr), and hydroiodic acid (HI). (However, hydrofluoric acid, HF, is relatively weak.) For example, the K_a value for hydrochloric acid (HCl) is 10⁷.

Weak acids have small K_a values (i.e. at equilibrium significant amounts of HA and A⁻ exist together in solution; modest levels of H₃O⁺ are present; the acid is only partially dissociated). For example, the K_a value for acetic acid is 1.8 x 10^-5. Most organic acids are weak acids. Oxoacids, which tend to contain central atoms in high oxidation states surrounded by oxygen may be quite strong or weak. Nitric acid, sulfuric acid, and perchloric acid are all strong acids, whereas nitrous acid, sulfurous acid and hypochlorous acid are all weak.

Note on terms used:

• The terms "hydrogen ion" and "proton" are used interchangeably; both refer to H⁺.
• In aqueous solution, the water is protonated to form hydronium ion, H₃O⁺(aq). This is often abbreviated as H⁺(aq) even though the symbol is not chemically correct.
• The strength of an acid is measured by its acid dissociation constant (K_a) or equivalently its pK_a (pK_a= - log(K_a)).
• The pH of a solution is a measurement of the concentration of hydronium. This will depend on the concentration and nature of acids and bases in solution.

Monoprotic acids

Monoprotic acids are those acids that are able to donate one proton per molecule during the process of dissociation (sometimes called ionization) as shown below (symbolized by HA):

HA(aq) + H₂O(l) ⇌ H₃O⁺(aq) + A⁻(aq)         K_a

Common examples of monoprotic acids in mineral acids include hydrochloric acid (HCl) and nitric acid (HNO₃). On the other hand, for organic acids the term mainly indicates the presence of one carboxyl group and sometimes these acids are known as monocarboxylic acid. Examples in organic acids include formic acid (HCOOH), acetic acid (CH₃COOH) and benzoic acid (C₆H₅COOH).

Polyprotic acids

Polyprotic acids are able to donate more than one proton per acid molecule, in contrast to monoprotic acids that only donate one proton per molecule. Specific types of polyprotic acids have more specific names, such as diprotic acid (two potential protons to donate) and triprotic acid (three potential protons to donate).

A diprotic acid (here symbolized by H₂A) can undergo one or two dissociations depending on the pH. Each dissociation has its own dissociation constant, K_a1 and K_a2.

H₂A(aq) + H₂O(l) ⇌ H₃O⁺(aq) + HA⁻(aq)       K_a1

HA⁻(aq) + H₂O(l) ⇌ H₃O⁺(aq) + A²⁻(aq)       K_a2

The first dissociation constant is typically greater than the second; i.e., K_a1 > K_a2 . For example, sulfuric acid (H₂SO₄) can donate one proton to form the bisulfate anion (HSO₄⁻), for which K_a1 is very large; then it can donate a second proton to form the sulfate anion (SO₄²⁻), wherein the K_a2 is intermediate strength. The large K_a1 for the first dissociation makes sulfuric a strong acid. In a similar manner, the weak unstable carbonic acid (H₂CO₃) can lose one proton to form bicarbonate anion (HCO₃⁻) and lose a second to form carbonate anion (CO₃²⁻). Both K_a values are small, but K_a1 > K_a2 .

A triprotic acid (H₃A) can undergo one, two, or three dissociations and has three dissociation constants, where K_a1 > K_a2 > K_a3 .

H₃A(aq) + H₂O(l) ⇌ H₃O⁺(aq) + H₂A⁻(aq)        K_a1

H₂A⁻(aq) + H₂O(l) ⇌ H₃O⁺(aq) + HA²⁻(aq)       K_a2

HA²⁻(aq) + H₂O(l) ⇌ H₃O⁺(aq) + A³⁻(aq)         K_a3

An inorganic example of a triprotic acid is orthophosphoric acid (H₃PO₄), usually just called phosphoric acid. All three protons can be successively lost to yield H₂PO₄⁻, then HPO₄²⁻, and finally PO₄³⁻ , the orthophosphate ion, usually just called phosphate. An organic example of a triprotic acid is citric acid, which can successively lose three protons to finally form the citrate ion. Even though the positions of the protons on the original molecule may be equivalent, the successive K_a values will differ since it is energetically less favorable to lose a proton if the conjugate base is more negatively charged.

Neutralization

Neutralization is the reaction between an acid and a base, producing a salt and neutralized base; for example, hydrochloric acid and sodium hydroxide form sodium chloride and water:

HCl(aq) + NaOH(aq) → H₂O(l) + NaCl(aq)

Neutralization is the basis of titration, where a pH indicator shows equivalence point when the equivalent number of moles of a base have been added to an acid. It is often wrongly assumed that neutralization should result in a solution with pH 7.0, which is only the case with similar acid and base strengths during a reaction.

Neutralization with a base weaker than the acid results in a weakly acidic salt. An example is the weakly acidic ammonium chloride, which is produced from the strong acid hydrogen chloride and the weak base ammonia. Conversely, neutralizing a weak acid with a strong base gives a weakly basic salt, e.g. sodium fluoride from hydrogen fluoride and sodium hydroxide.

Weak acid/weak base equilibria

In order to lose a proton, it is necessary that the pH of the system rise above the pK_a of the protonated acid. The decreased concentration of H⁺ in that basic solution shifts the equilibrium towards the conjugate base form (the deprotonated form of the acid). In lower-pH (more acidic) solutions, there is a high enough H⁺ concentration in the solution to cause the acid to remain in its protonated form, or to protonate its conjugate base (the deprotonated form).

Solutions of weak acids and salts of their conjugate bases form buffer solutions.

Applications of acids

There are numerous uses for acids. Acids are often used to remove rust and other corrosion from metals in a process known as pickling. They may be used as an electrolyte in a wet cell battery, such as sulfuric acid in a car battery.

Strong acids, sulfuric acid in particular, are widely used in mineral processing. For example, phosphate minerals react with sulfuric acid to produce phosphoric acid for the production of phosphate fertilizers, and zinc is produced by dissolving zinc oxide into sulfuric acid, purifying the solution and electrowinning.

In the chemical industry, acids react in neutralization reactions to produce salts. For example, nitric acid reacts with ammonia to produce ammonium nitrate, a fertilizer. Additionally, carboxylic acids can be esterified with alcohols, to produce esters.

Acids are used as catalysts; for example, sulfuric acid is used in very large quantities in the alkylation process to produce gasoline. Strong acids, such as sulfuric, phosphoric and hydrochloric acids also effect dehydration and condensation reactions.

Acids are used as additives to drinks and foods, as they alter their taste and serve as preservatives. Phosphoric acid, for example, is a component of cola drinks.

Biological occurrence

In humans and many other animals, hydrochloric acid is a part of the gastric acid secreted within the stomach to help hydrolyze proteins and polysaccharides, as well as converting the inactive pro-enzyme, pepsinogen into the enzyme, pepsin. Some organisms produce acids for defense; for example, ants produce formic acid.

Common acids

Mineral acids

• Solutions of hydrogen halides, such as hydrochloric acid (HCl) and hydrobromic acid (HBr)
• Sulfuric acid (H₂SO₄)
• Nitric acid (HNO₃)
• Phosphoric acid (H₃PO₄)
• Chromic acid (H₂CrO₄)

Sulfonic acids

• Methanesulfonic acid (aka mesylic acid) (MeSO₃H)
• Ethanesulfonic acid (aka esylic acid) (EtSO₃H)
• Benzenesulfonic acid (aka besylic acid) (PhSO₃H)
• Toluenesulfonic acid (aka tosylic acid, or (C₆H₄(CH₃) (SO₃H))

Carboxylic acids

• Formic acid
• Acetic acid
• Citric acid

Vinylogous carboxylic acids

• Ascorbic acid
• Meldrum's acid

References

• Listing of strengths of common acids and bases
• Zumdahl, Chemistry, 4th Edition.

See also

Chemistry

• Acid value
• Acid salt
• Base
• Basic salt
• Binary acid
• Vitriol
• Acid-base extraction Environment
• Acid rain
• Ocean acidification

External links

• Science Aid: Acids and Bases Information for High School students
• Curtipot: Acid-Base equilibria diagrams, pH calculation and titration curves simulation and analysis - freeware
• A summary of the Properties of Acids for the beginning chemistry student
• The UN ECE Convention on Long-Range Transboundary Air Pollution