Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table, a reactive nonmetal, an oxidizing agent that forms oxides with most elements as well as with other compounds. By mass, oxygen is the third-most abundant element in the universe, after helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere. As compounds including oxides, the element makes up half of the Earth's crust. Dioxygen is used in cellular respiration and many major classes of organic molecules in living organisms contain oxygen, such as proteins, nucleic acids and fats, as do the major constituent inorganic compounds of animal shells and bone. Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms. Oxygen is continuously replenished in Earth's atmosphere by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide.
Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone absorbs ultraviolet UVB radiation and the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation. However, ozone present at the surface is a byproduct of thus a pollutant. Oxygen was isolated by Michael Sendivogius before 1604, but it is believed that the element was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, Joseph Priestley in Wiltshire, in 1774. Priority is given for Priestley because his work was published first. Priestley, called oxygen "dephlogisticated air", did not recognize it as a chemical element; the name oxygen was coined in 1777 by Antoine Lavoisier, who first recognized oxygen as a chemical element and characterized the role it plays in combustion. Common uses of oxygen include production of steel and textiles, brazing and cutting of steels and other metals, rocket propellant, oxygen therapy, life support systems in aircraft, submarines and diving.
One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle and surrounding the vessel's neck with water resulted in some water rising into the neck. Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries Leonardo da Vinci built on Philo's work by observing that a portion of air is consumed during combustion and respiration. In the late 17th century, Robert Boyle proved. English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. In one experiment, he found that placing either a mouse or a lit candle in a closed container over water caused the water to rise and replace one-fourteenth of the air's volume before extinguishing the subjects.
From this he surmised that nitroaereus is consumed in both combustion. Mayow observed that antimony increased in weight when heated, inferred that the nitroaereus must have combined with it, he thought that the lungs separate nitroaereus from air and pass it into the blood and that animal heat and muscle movement result from the reaction of nitroaereus with certain substances in the body. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract "De respiratione". Robert Hooke, Ole Borch, Mikhail Lomonosov, Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element; this may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, modified by the chemist Georg Ernst Stahl by 1731, phlogiston theory stated that all combustible materials were made of two parts.
One part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. Combustible materials that leave little residue, such as wood or coal, were thought to be made of phlogiston. Air did not play a role in phlogiston theory, nor were any initial quantitative experiments conducted to test the idea. Polish alchemist and physician Michael Sendivogius in his work De Lapide Philosophorum Tractatus duodecim e naturae fonte et manuali experientia depromti described a substance contained in air, referring to it as'cibus vitae', this substance is identical with oxygen. Sendivogius, during his experiments performed between 1598 and 1604, properly recognized that the substance is equivalent to the gaseous byproduct released by the thermal decomposition of potassium nitrate. In Bugaj’s view, the isolation of oxygen and the proper association of the substance to that part of air, required for life, lends sufficient weight to the discovery of oxygen by Sendivogius.
Mercury chloride or mercuric chloride is the chemical compound of mercury and chlorine with the formula HgCl2. It is white crystalline solid and is a laboratory reagent and a molecular compound, toxic to humans. Once used as a treatment for syphilis, it is no longer used for medicinal purposes because of mercury toxicity and the availability of superior treatments. Mercuric chloride exists not as a salt composed of discrete ions, but rather is composed of linear triatomic molecules, hence its tendency to sublime. In the crystal, each mercury atom is bonded to two close chloride ligands with Hg—Cl distance of 2.38 Å. Mercuric chloride is obtained by the action of chlorine on mercury or mercury chloride, by the addition of hydrochloric acid to a hot, concentrated solution of mercury compounds such as the nitrate: HgNO3 + 2 HCl → HgCl2 + H2O + NO2,Heating a mixture of solid mercury sulfate and sodium chloride affords volatile HgCl2, which sublimes and condenses in the form of small rhombic crystals.
Its solubility increases from 6% at 20 °C to 36% in 100 °C. In the presence of chloride ions, it dissolves to give the tetrahedral coordination complex 2−; the main application of mercuric chloride is as a catalyst for the conversion of acetylene to vinyl chloride, the precursor to polyvinylchloride: C2H2 + HCl → CH2=CHClFor this application, the mercuric chloride is supported on carbon in concentrations of about 5 weight percent. This technology has been eclipsed by the thermal cracking of 1,2-dichloroethane. Other significant applications of mercuric chloride include its use as a depolarizer in batteries and as a reagent in organic synthesis and analytical chemistry, it is being used in plant tissue culture for surface sterilisation of explants such as leaf or stem nodes. Mercuric chloride is used to form an amalgam with metals, such as aluminium. Upon treatment with an aqueous solution of mercuric chloride, aluminium strips become covered by a thin layer of the amalgam. Aluminium is protected by a thin layer of oxide, thus making it inert.
Once amalgamated, aluminium can undergo a variety of reactions. For example, upon removal of the oxide layer, the exposed aluminium will react with water generating Al3 and hydrogen gas. Halocarbons react with amalgamated aluminium in the Barbier reaction; these alkylaluminium compounds are nucleophilic and can be used in a similar fashion to the Grignard reagent. Amalgamated aluminium is used as a reducing agent in organic synthesis. Zinc is commonly amalgamated using mercuric chloride. Mercuric chloride is used to remove dithiane groups attached to a carbonyl in an umpolung reaction; this reaction exploits the high affinity of Hg2+ for anionic sulfur ligands. Mercuric chloride may be used as a stabilising agent for analytical samples. Care must be taken to ensure that detected mercuric chloride does not eclipse the signals of other components in the sample, such as is possible in gas chromatography. Mercury chloride was used as a photographic intensifier to produce positive pictures in the collodion process of the 1800s.
When applied to a negative, the mercury chloride whitens and thickens the image, thereby increasing the opacity of the shadows and creating the illusion of a positive image. For the preservation of anthropological and biological specimens during the late 19th and early 20th centuries, objects were dipped in or were painted with a "mercuric solution"; this was done to prevent the specimens' destruction by moths and mold. Objects in drawers were protected by scattering crystalline mercuric chloride over them, it finds minor use in tanning, wood was preserved by kyanizing. Mercuric chloride was one of the three chemicals used for railroad tie wood treatment between 1830 and 1856 in Europe and the United States. Limited railroad ties were treated in the United States until there were concerns over lumber shortages in the 1890s; the process was abandoned because mercuric chloride was water-soluble and not effective for the long term, as well as being poisonous. Furthermore, alternative treatment processes, such as copper sulfate, zinc chloride, creosote.
Limited kyanizing was used for some railroad ties in early 1900s. Mercuric chloride was used to disinfect wounds by Arab physicians in the Middle Ages, it continued to be used by Arab doctors into the twentieth century, until modern medicine deemed it unsafe for use. Syphilis was treated with mercuric chloride before the advent of antibiotics, it was inhaled, ingested and applied topically. Both mercuric-chloride treatment for syphilis and poisoning during the course of treatment were so common that the latter's symptoms were confused with those of syphilis; this use of "salts of white mercury" is referred to in the English-language folk song "The Unfortunate Rake". Yaws was treated with mercuric chloride before the advent of antibiotics, it was applied topically to alleviate ulcerative symptoms. Evidence of this is found in Jack London's book "The Cruise of the Snark" in the chapter entitled The Amateur M. D. In volume V of Alexandre Dumas' Celebrated Crimes, he recounts the history of Antoine François Desrues, who killed a noblewoman, Madame de Lamotte, with "corrosive sublimate".
In one publicized case in 1920, "mercury bichloride" was reported to have caused the death of 25-year-old American silent-film star Olive Thomas. While vacationing in France and staying at the Hôtel Ritz in Paris, she accidentally ingested the compound, which
Benzalkonium chloride known as BZK, BKC, BAC, alkyldimethylbenzylammonium chloride and ADBAC, is a type of cationic surfactant. It is an organic salt classified as a quaternary ammonium compound, it has three main categories of use: as a biocide, a cationic surfactant, as a phase transfer agent. ADBACs are a mixture of alkylbenzyldimethylammonium chlorides, in which the alkyl group has various even-numbered alkyl chain lengths. Depending on purity, benzalkonium chloride ranges from colourless to a pale yellow. Benzalkonium chloride is soluble in ethanol and acetone. Dissolution in water is slow. Aqueous solutions should be neutral to alkaline. Solutions foam. Concentrated solutions have a faint almond-like odour. Standard concentrates are manufactured as 50% and 80% w/w solutions, sold under trade names such as BC50, BC80, BAC50, BAC80, etc; the 50% solution is purely aqueous, while more concentrated solutions require incorporation of rheology modifiers to prevent increases in viscosity or gel formation under low temperature conditions.
Benzalkonium chloride possesses surfactant properties, dissolving the lipid phase of the tear film and increasing drug penetration, making it a useful excipient, but at the risk of causing damage to the surface of the eye°. Laundry detergents and treatments Softeners for textiles<°TFOS DEWS II Report July 2017> Benzalkonium chloride is a mainstay of phase-transfer catalysis, an important technology in the synthesis of organic compounds, including drugs. For their antimicrobial activity, benzalkonium chloride is an active ingredient in many consumer products: Pharmaceutical products such as eye and nasal drops or sprays, as a preservative Personal care products such as hand sanitizers, wet wipes, shampoos and cosmetics Skin antiseptics, such as Bactine and Dettol. Throat lozenges and mouthwashes, as a biocide Spermicidal creams Over-the-counter single-application treatments for herpes, cold-sores, fever blisters, such as RELEEV and Viroxyn Burn and ulcer treatment Spray disinfectants for hard surface sanitization Cleaners for floor and hard surfaces as a disinfectant, such as Lysol Algaecides for clearing of algae, lichens from paths, roof tiles, swimming pools, etc.
Benzalkonium chloride is used in many non-consumer processes and products, including as an active ingredient in surgical disinfection. A comprehensive list of uses includes industrial applications. An advantage of benzalkonium chloride, not shared by ethanol-based antiseptics or hydrogen peroxide antiseptic, is that it does not cause a burning sensation when applied to broken skin.. However, prolonged or repeated skin contact may cause dermatitis. Benzalkonium chloride is a used preservative in eye drops. Stronger concentrations can be caustic and cause irreversible damage to the corneal endothelium. Avoiding the use of benzalkonium chloride solutions while contact lenses are in place is discussed in the literature. Although benzalkonium chloride has been ubiquitous as a preservative in ophthalmic preparations, its ocular toxicity and irritant properties, in conjunction with consumer demand, have led pharmaceutical companies to increase production of preservative-free preparations, or to replace benzalkonium chloride with preservatives which are less harmful.
Many mass-marketed inhaler and nasal spray formulations contain benzalkonium chloride as a preservative, despite substantial evidence that it can adversely affect ciliary motion, mucociliary transport, nasal mucosal histology, human neutrophil function, leukocyte response to local inflammation. Although some studies have found no correlation between use of benzalkonium chloride in concentrations at or below 0.1% in nasal sprays and drug-induced rhinitis, others have recommended that benzalkonium chloride in nasal sprays be avoided. In the United States, nasal steroid preparations that are free of benzalkonium chloride include budesonide, triamcinolone acetonide and Beconase and Vancenase aerosol inhalers. Benzalkonium chloride is irritant to middle ear tissues at used concentrations. Inner ear toxicity has been demonstrated. Occupational exposure to benzalkonium chloride has been linked to the development of asthma. In 2011, a large clinical trial designed to evaluate the efficacy of hand sanitizers based on different active ingredients in preventing virus transmission amongst schoolchildren was re-designed to exclude sanitizers based on benzalkonium chloride due to safety concerns.
Benzalkonium chloride has been in common use as a pharmaceutical preservative and antimicrobial since the 1940s. While early studies confirmed the corrosive and irritant properties of benzalkonium chloride, investigations into the adverse effects of, disease states linked to, benzalkonium chloride have only surfaced during the past 30 years. Benzalkonium chloride is classed as a Category III antiseptic active ingredient by the United States Food and Drug Administration. Ingredients are categorised as Category III when "available data are insufficient to classify as safe and effective, further testing is required”. Benzalkonium chloride is excluded from the current United States Food and Drug Administration review of the safety and effectiveness of consumer antiseptics and topical antimicrobial over-the-counter drug products, meaning it will remain a Category III ingredient. There is acknowledgement that more data are required on its safety and effectiveness with relation to: Human pharmacokinetic studies, including information on its metabolites Studies on animal absorption, distribution and excretion Data to help define the effect of formulation on
In chemistry, an alcohol is any organic compound in which the hydroxyl functional group is bound to a carbon. The term alcohol referred to the primary alcohol ethanol, used as a drug and is the main alcohol present in alcoholic beverages. An important class of alcohols, of which methanol and ethanol are the simplest members, includes all compounds for which the general formula is CnH2n+1OH, it is these simple monoalcohols. The suffix -ol appears in the IUPAC chemical name of all substances where the hydroxyl group is the functional group with the highest priority; when a higher priority group is present in the compound, the prefix hydroxy- is used in its IUPAC name. The suffix -ol in non-IUPAC names typically indicates that the substance is an alcohol. However, many substances that contain hydroxyl functional groups have names which include neither the suffix -ol, nor the prefix hydroxy-. Alcohol distillation originated in India. During 2000 BCE, people of India used. Alcohol distillation was known to Islamic chemists as early as the eighth century.
The Arab chemist, al-Kindi, unambiguously described the distillation of wine in a treatise titled as "The Book of the chemistry of Perfume and Distillations". The Persian physician, alchemist and philosopher Rhazes is credited with the discovery of ethanol; the word "alcohol" is from a powder used as an eyeliner. Al- is the Arabic definite article, equivalent to the in English. Alcohol was used for the fine powder produced by the sublimation of the natural mineral stibnite to form antimony trisulfide Sb2S3, it was considered to be the essence or "spirit" of this mineral. It was used as an antiseptic and cosmetic; the meaning of alcohol was extended to distilled substances in general, narrowed to ethanol, when "spirits" was a synonym for hard liquor. Bartholomew Traheron, in his 1543 translation of John of Vigo, introduces the word as a term used by "barbarous" authors for "fine powder." Vigo wrote: "the barbarous auctours use alcohol, or alcofoll, for moost fine poudre."The 1657 Lexicon Chymicum, by William Johnson glosses the word as "antimonium sive stibium."
By extension, the word came to refer to any fluid obtained by distillation, including "alcohol of wine," the distilled essence of wine. Libavius in Alchymia refers to "vini alcohol vel vinum alcalisatum". Johnson glosses alcohol vini as "quando omnis superfluitas vini a vino separatur, ita ut accensum ardeat donec totum consumatur, nihilque fæcum aut phlegmatis in fundo remaneat." The word's meaning became restricted to "spirit of wine" in the 18th century and was extended to the class of substances so-called as "alcohols" in modern chemistry after 1850. The term ethanol was invented 1892, combining the word ethane with the "-ol" ending of "alcohol". IUPAC nomenclature is used in scientific publications and where precise identification of the substance is important in cases where the relative complexity of the molecule does not make such a systematic name unwieldy. In naming simple alcohols, the name of the alkane chain loses the terminal e and adds the suffix -ol, e.g. as in "ethanol" from the alkane chain name "ethane".
When necessary, the position of the hydroxyl group is indicated by a number between the alkane name and the -ol: propan-1-ol for CH3CH2CH2OH, propan-2-ol for CH3CHCH3. If a higher priority group is present the prefix hydroxy-is used, e.g. as in 1-hydroxy-2-propanone. In cases where the OH functional group is bonded to an sp2 carbon on an aromatic ring the molecule is known as a phenol, is named using the IUPAC rules for naming phenols. In other less formal contexts, an alcohol is called with the name of the corresponding alkyl group followed by the word "alcohol", e.g. methyl alcohol, ethyl alcohol. Propyl alcohol may be n-propyl alcohol or isopropyl alcohol, depending on whether the hydroxyl group is bonded to the end or middle carbon on the straight propane chain; as described under systematic naming, if another group on the molecule takes priority, the alcohol moiety is indicated using the "hydroxy-" prefix. Alcohols are classified into primary and tertiary, based upon the number of carbon atoms connected to the carbon atom that bears the hydroxyl functional group.
The primary alcohols have general formulas RCH2OH. The simplest primary alcohol is methanol, for which R=H, the next is ethanol, for which R=CH3, the methyl group. Secondary alcohols are those of the form RR'CHOH, the simplest of, 2-propanol. For the tertiary alcohols the general form is RR'R"COH; the simplest example is tert-butanol, for which each of R, R', R" is CH3. In these shorthands, R, R', R" represent substituents, alkyl or other attached organic groups. In archaic nomenclature, alcohols can be named as derivatives of methanol using "-carbinol" as the ending. For instance, 3COH can be named trimethylcarbinol. Alcohols have a long history of myriad uses. For simple mono-alcohols, the focus on this article, the following are most important industrial alcohols: methanol for the production of formaldehyde and as a fuel additive ethanol for alcoholic beverages, fuel additive, solvent 1-propanol, 1-butanol, isobutyl alcohol for use as a solvent a
A permanganate is the general name for a chemical compound containing the manganate ion. Because manganese is in the +7 oxidation state, the permanganate ion is a strong oxidizing agent; the ion has tetrahedral geometry. Permanganate solutions are purple in color and are stable in neutral or alkaline media; the exact chemical reaction is dependent upon the organic contaminants present and the oxidant utilized. For example, trichloroethene is oxidized by sodium permanganate to form carbon dioxide, manganese dioxide, sodium ions, hydronium ions, chloride ions. In an acidic solution, permanganate is reduced to the pale pink +2 oxidation state of the manganese ion. 8 H+ + MnO−4 + 5 e− → Mn2+ + 4 H2OIn a basic solution, permanganate is reduced to the green +6 oxidation state of the manganate ion, MnO2−4. MnO−4 + e− → MnO2−4In a neutral medium, however, it gets reduced to the brown +4 oxidation state of manganese dioxide MnO2. 2 H2O + MnO−4 + 3 e− → MnO2 + 4 OH− Permanganates can be produced by oxidation of manganese compounds such as manganese chloride or manganese sulfate by strong oxidizing agents, for instance, sodium hypochlorite or lead dioxide: 2 MnCl2 + 5 NaClO + 6 NaOH → 2 NaMnO4 + 9 NaCl + 3 H2O 2 MnSO4 + 5 PbO2 + 3 H2SO4 → 2 HMnO4 + 5 PbSO4 + 2 H2OIt may be produced by the disproportionation of manganates, with manganese dioxide as a side-product: 3 Na2MnO4 + 2 H2O → 2 NaMnO4 + MnO2 + 4 NaOHThey are produced commercially by electrolysis or air oxidation of alkaline solutions of manganate salts.
Permanganates are salts of permanganic acid. They have a deep purple colour, due to a charge transfer transition. Permanganate is a strong oxidizer, similar to perchlorate, it is therefore in common use in qualitative analysis. According to theory, permanganate is strong enough to oxidize water, but this does not happen to any extent. Besides this, it is stable, it is a useful reagent, though with organic compounds, not selective. Potassium permanganate is used as a disinfectant. Manganates are not stable thermally. For instance, potassium permanganate decomposes at 230 °C to potassium manganate and manganese dioxide, releasing oxygen gas: 2 KMnO4 → K2MnO4 + MnO2 + O2A permanganate can oxidize an amine to a nitro compound, an alcohol to a ketone, an aldehyde to a carboxylic acid, a terminal alkene to a carboxylic acid, oxalic acid to carbon dioxide, an alkene to a diol; this list is not exhaustive. In alkene oxidations one intermediate is a cyclic Mn species: Ammonium permanganate, NH4MnO4 Calcium permanganate, Ca2 Potassium permanganate, KMnO4 Sodium permanganate, NaMnO4 Silver permanganate, AgMnO4 Perchlorate, a similar ion with a chlorine center Chromate, isoelectronic with permanganate Pertechnetate
Thymol is a natural monoterpenoid phenol derivative of cymene, C10H14O, isomeric with carvacrol, found in oil of thyme, extracted from Thymus vulgaris and various other kinds of plants as a white crystalline substance of a pleasant aromatic odor and strong antiseptic properties. Thymol provides the distinctive, strong flavor of the culinary herb thyme produced from T. vulgaris. Thymol is only soluble in water at neutral pH, but it is soluble in alcohols and other organic solvents, it is soluble in alkaline aqueous solutions due to deprotonation of the phenol. Thymol has a refractive index of 1.5208 and an experimental dissociation exponent of 10.59±0.10. Thymol absorbs maximum UV radiation at 274 nm. Thymol is chemically related to the anesthetic propofol. Regions lacking natural sources of thymol obtain the compound via total synthesis. Thymol is produced from m-cresol and propene in the gas phase: C7H8O + C3H6 ⇌ C10H14O The bee balms Monarda fistulosa and Monarda didyma, North American wildflowers, are natural sources of thymol.
The Blackfoot Native Americans recognized these plants' strong antiseptic action, used poultices of the plants for skin infections and minor wounds. A tisane made from them was used to treat mouth and throat infections caused by dental caries and gingivitis. Thymol was first isolated by the German chemist Caspar Neumann in 1719. In 1853, the French chemist A. Lallemand determined its empirical formula. Thymol was first synthesized by the Swedish chemist Oskar Widman in 1882. An in vitro study found thymol and carvacrol to be effective in reducing the minimum inhibitory concentration of several antibiotics against zoonotic pathogens and food spoilage bacteria such as Salmonella Typhimurium SGI 1 and Streptococcus pyogenes ermB. In vitro studies have found thymol to be useful as an antifungal against food spoilage and bovine mastitis. Thymol demonstrates in vitro post-antibacterial effect against the test strains E. coli and P. aeruginosa, S. aureus and B. cereus. This antibacterial activity is caused by inhibiting growth and lactate production, by decreasing cellular glucose uptake.
Thyme essential oil is useful in preservation of food. The antibacterial properties of thymol, a major part of thyme essential oil, as well as other constituents, are in part associated with their lipophilic character, leading to accumulation in bacterial membranes and subsequent membrane-associated events, such as energy depletion; the antifungal nature of thymol against some fungi that are pathogenic to plants is due to its ability to alter the hyphal morphology and cause hyphal aggregates, resulting in reduced hyphal diameters and lyses of the hyphal wall. Thymol has been used in alcohol solutions and in dusting powders for the treatment of tinea or ringworm infections, was used in the United States to treat hookworm infections. People of the Middle East continue to use za'atar, a delicacy made with large amounts of thyme, to reduce and eliminate internal parasites, it is used as a preservative in halothane, an anaesthetic, as an antiseptic in mouthwash. When used to reduce plaque and gingivitis, thymol has been found to be more effective when used in combination with chlorhexidine than when used purely by itself.
Thymol is the active antiseptic ingredient in some toothpastes, such as Johnson & Johnson's Euthymol. Thymol has been used to control varroa mites and prevent fermentation and the growth of mold in bee colonies, methods developed by beekeeper R. O. B. Manley. Thymol is used as a degrading, non-persisting pesticide. Thymol can be used as a medical disinfectant and general purpose disinfectant. Euphrasia rostkoviana Monarda didyma Monarda fistulosa Trachyspermum ammi Origanum compactum Origanum dictamnus Origanum onites Origanum vulgare Thymus glandulosus Thymus hyemalis Thymus vulgaris Thymus zygis In 2009, the U. S. Environmental Protection Agency reviewed the research literature on the toxicology and environmental impact of thymol and concluded that "thymol has minimal potential toxicity and poses minimal risk". Studies have shown that hydrocarbon monoterpenes and thymol in particular degrade in the environment and are, low risks because of rapid dissipation and low bound residues, supporting the use of thymol as a pesticide agent that offers a safe alternative to other more persistent chemical pesticides that can be dispersed in runoff and produce subsequent contamination.
British Pharmacopoeia Japanese Pharmacopoeia Thymoquinone Nigella sativa Bromothymol Media related to Thymol at Wikimedia Commons
An acid is a molecule or ion capable of donating a hydron, or, capable of forming a covalent bond with an electron pair. The first category of acids is the proton donors or Brønsted acids. In the special case of aqueous solutions, proton donors form the hydronium ion H3O+ and are known as Arrhenius acids. Brønsted and Lowry generalized the Arrhenius theory to include non-aqueous solvents. A Brønsted or Arrhenius acid contains a hydrogen atom bonded to a chemical structure, still energetically favorable after loss of H+. Aqueous Arrhenius acids have characteristic properties which provide a practical description of an acid. Acids form aqueous solutions with a sour taste, can turn blue litmus red, react with bases and certain metals to form salts; the word acid is derived from the Latin acidus/acēre meaning sour. An aqueous solution of an acid has a pH less than 7 and is colloquially referred to as'acid', while the strict definition refers only to the solute. A lower pH means a higher acidity, thus a higher concentration of positive hydrogen ions in the solution.
Chemicals or substances having the property of an acid are said to be acidic. Common aqueous acids include hydrochloric acid, acetic acid, sulfuric acid, citric acid; as these examples show, acids can be solutions or pure substances, can be derived from acids that are solids, liquids, or gases. Strong acids and some concentrated weak acids are corrosive, but there are exceptions such as carboranes and boric acid; the second category of acids are Lewis acids. An example is boron trifluoride, whose boron atom has a vacant orbital which can form a covalent bond by sharing a lone pair of electrons on an atom in a base, for example the nitrogen atom in ammonia. Lewis considered this as a generalization of the Brønsted definition, so that an acid is a chemical species that accepts electron pairs either directly or by releasing protons into the solution, which accept electron pairs. However, hydrogen chloride, acetic acid, most other Brønsted-Lowry acids cannot form a covalent bond with an electron pair and are therefore not Lewis acids.
Conversely, many Lewis acids are not Brønsted-Lowry acids. In modern terminology, an acid is implicitly a Brønsted acid and not a Lewis acid, since chemists always refer to a Lewis acid explicitly as a Lewis acid. Modern definitions are concerned with the fundamental chemical reactions common to all acids. Most acids encountered in everyday life are aqueous solutions, or can be dissolved in water, so the Arrhenius and Brønsted-Lowry definitions are the most relevant; the Brønsted-Lowry definition is the most used definition. Hydronium ions are acids according to all three definitions. Although alcohols and amines can be Brønsted-Lowry acids, they can function as Lewis bases due to the lone pairs of electrons on their oxygen and nitrogen atoms; the Swedish chemist Svante Arrhenius attributed the properties of acidity to hydrogen ions or protons in 1884. An Arrhenius acid is a substance that, when added to water, increases the concentration of H+ ions in the water. Note that chemists write H+ and refer to the hydrogen ion when describing acid-base reactions but the free hydrogen nucleus, a proton, does not exist alone in water, it exists as the hydronium ion, H3O+.
Thus, an Arrhenius acid can be described as a substance that increases the concentration of hydronium ions when added to water. Examples include molecular substances such as acetic acid. An Arrhenius base, on the other hand, is a substance which increases the concentration of hydroxide ions when dissolved in water; this decreases the concentration of hydronium because the ions react to form H2O molecules: H3O+ + OH− ⇌ H2O + H2ODue to this equilibrium, any increase in the concentration of hydronium is accompanied by a decrease in the concentration of hydroxide. Thus, an Arrhenius acid could be said to be one that decreases hydroxide concentration, while an Arrhenius base increases it. In an acidic solution, the concentration of hydronium ions is greater than 10−7 moles per liter. Since pH is defined as the negative logarithm of the concentration of hydronium ions, acidic solutions thus have a pH of less than 7. While the Arrhenius concept is useful for describing many reactions, it is quite limited in its scope.
In 1923 chemists Johannes Nicolaus Brønsted and Thomas Martin Lowry independently recognized that acid-base reactions involve the transfer of a proton. A Brønsted-Lowry acid is a species. Brønsted-Lowry acid-base theory has several advantages over Arrhenius theory. Consider the following reactions of acetic acid, the organic acid that gives vinegar its characteristic taste: CH3COOH + H2O ⇌ CH3COO− + H3O+ CH3COOH + NH3 ⇌ CH3COO− + NH+4Both theories describe the first reaction: CH3COOH acts as an Arrhenius acid because it acts as a source of H3O+ when dissolved in water, it acts as a Brønsted acid by donating a proton to water. In the second example CH3COOH undergoes the same transformation, in this case donating a proton to ammonia, but does not relate to the Arrhenius definition of an acid because the reaction does not produce hydronium. CH3COOH is