The cochineal is a scale insect in the suborder Sternorrhyncha, from which the natural dye carmine is derived. A sessile parasite native to tropical and subtropical South America through North America, this insect lives on cacti in the genus Opuntia, feeding on plant moisture and nutrients; the insects are found on the pads of prickly pear cacti, collected by brushing them off the plants, dried. The insect produces carminic acid. Carminic acid 17-24% of dried insects' weight, can be extracted from the body and eggs mixed with aluminium or calcium salts to make carmine dye known as cochineal. Today, carmine is used as a colorant in food and in lipstick; the carmine dye was used in North America in the 15th century for coloring fabrics and became an important export good during the colonial period. After synthetic pigments and dyes such as alizarin were invented in the late 19th century, natural-dye production diminished. Health fears over artificial food additives, have renewed the popularity of cochineal dyes, the increased demand has made cultivation of the insect profitable again, with Peru being the largest exporter.
Some towns in the Mexican state of Oaxaca are still working in handmade textiles using this cochineal. Other species in the genus Dactylopius can be used to produce "cochineal extract", are difficult to distinguish from D. coccus for expert taxonomists. Cochineal is derived from the French "cochenille", derived from Spanish "cochinilla", in turn derived from Latin "coccinus" meaning "scarlet-colored", or from the Latin "coccum", meaning "berry yielding scarlet dye". See the related word kermes, the source of a similar but weaker Mediterranean dye called crimson, used to color cloth red before discovery of cochineal in the New World; some sources identify the Spanish source word for cochineal as cochinilla "wood louse"." Cochineal dye was used by the Maya peoples of North and Central America. Eleven cities conquered by Montezuma in the 15th century paid a yearly tribute of 2000 decorated cotton blankets and 40 bags of cochineal dye each. Production of cochineal is depicted in Codex Osuna. During the colonial period, the production of cochineal grew rapidly.
Produced exclusively in Oaxaca by indigenous producers, cochineal became Mexico's second-most valued export after silver. Soon after the Spanish conquest of the Aztec Empire, it began to be exported to Spain, by the 17th century was a commodity traded as far away as India; the dyestuff was consumed throughout Europe and was so prized, its price was quoted on the London and Amsterdam Commodity Exchanges. In 1777, French botanist Nicolas-Joseph Thiéry de Menonville, presenting himself as a botanizing physician, smuggled the insects and pads of the Opuntia cactus to Saint Domingue; this particular collection failed to thrive and died out, leaving the Mexican monopoly intact. After the Mexican War of Independence in 1810–1821, the Mexican monopoly on cochineal came to an end. Large-scale production of cochineal emerged in Guatemala and the Canary Islands; the demand for cochineal fell with the appearance on the market of alizarin crimson and many other artificial dyes discovered in Europe in the middle of the 19th century, causing a significant financial shock in Spain as a major industry ceased to exist.
The delicate manual labour required for the breeding of the insect could not compete with the modern methods of the new industry, less so with the lowering of production costs. The "tuna blood" dye stopped being used and trade in cochineal totally disappeared in the course of the 20th century. In recent decades, the breeding of cochineal has been done for the purposes of maintaining the tradition rather than to satisfy any sort of demand. However, the product has become commercially valuable again. One reason for its popularity is that many commercial synthetic red dyes and food colorings have been found to be carcinogenic; the carmine of antiquity contains carminic acid, was extracted from a similar insect, Kermes vermilio, which lives on Quercus coccifera oaks native to the Near East and the European side of the Mediterranean Basin. Kermes carmine was used as a dye and a laked pigment in ancient Egypt and the Near East and is one of the oldest organic pigments. Recipes for artists' use of carmine appear in many early painting and alchemical handbooks throughout the Middle Ages.
Carmine was not light-fast and was abandoned in art. Spanish influence changed the way in which Aztecs used pigments in their manuscripts. For example, Cochineal was replaced by Spanish dyes like alizarin crimson; the image of Moctezuma's death uses both indigenous and Spanish pigments, is therefore representative of the transition and influence between cultures. Cochineal insects are soft-bodied, oval-shaped scale insects
A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is applied in an aqueous solution, may require a mordant to improve the fastness of the dye on the fiber. Both dyes and pigments are colored. Dyes are soluble in water whereas pigments are insoluble; some dyes can be rendered insoluble with the addition of salt to produce a lake pigment. The majority of natural dyes are derived from plant sources: roots, bark and wood, lichens. Most dyes are synthetic, i.e. are man-made from petrochemicals. Other than pigmentation, they have a range of applications including organic dye lasers, optical media and camera sensors. Textile dyeing dates back to the Neolithic period. Throughout history, people have dyed their textiles using common, locally available materials. Scarce dyestuffs that produced brilliant and permanent colors such as the natural invertebrate dyes Tyrian purple and crimson kermes were prized luxury items in the ancient and medieval world.
Plant-based dyes such as woad, indigo and madder were important trade goods in the economies of Asia and Europe. Across Asia and Africa, patterned fabrics were produced using resist dyeing techniques to control the absorption of color in piece-dyed cloth. Dyes from the New World such as cochineal and logwood were brought to Europe by the Spanish treasure fleets, the dyestuffs of Europe were carried by colonists to America. Dyed flax fibers have been found in the Republic of Georgia in a prehistoric cave dated to 36,000 BP. Archaeological evidence shows that in India and Phoenicia, dyeing has been carried out for over 5,000 years. Early dyes were obtained from animal, vegetable or mineral sources, with no to little processing. By far the greatest source of dyes has been from the plant kingdom, notably roots, bark and wood, only few of which are used on a commercial scale; the first synthetic dye, was discovered serendipitously by William Henry Perkin in 1856. The discovery of mauveine started a surge in organic chemistry in general.
Other aniline dyes followed, such as fuchsine and induline. Many thousands of synthetic dyes have since been prepared. Dyes are classified according to their chemical properties. Acid dyes are water-soluble anionic dyes that are applied to fibers such as silk, wool and modified acrylic fibers using neutral to acid dye baths. Attachment to the fiber is attributed, at least to salt formation between anionic groups in the dyes and cationic groups in the fiber. Acid dyes are not substantive to cellulosic fibers. Most synthetic food colors fall in this category. Examples of acid dye are Acid red 88, etc.. Basic dyes are water-soluble cationic dyes that are applied to acrylic fibers, but find some use for wool and silk. Acetic acid is added to the dye bath to help the uptake of the dye onto the fiber. Basic dyes are used in the coloration of paper. Direct or substantive dyeing is carried out in a neutral or alkaline dye bath, at or near boiling point, with the addition of either sodium chloride or sodium sulfate or sodium carbonate.
Direct dyes are used on cotton, leather, wool and nylon. They are used as pH indicators and as biological stains. Mordant dyes require a mordant, which improves the fastness of the dye against water and perspiration; the choice of mordant is important as different mordants can change the final color significantly. Most natural dyes are mordant dyes and there is therefore a large literature base describing dyeing techniques; the most important mordant dyes are chrome dyes, used for wool. The mordant potassium dichromate is applied as an after-treatment, it is important to note that many mordants those in the heavy metal category, can be hazardous to health and extreme care must be taken in using them. Vat dyes are insoluble in water and incapable of dyeing fibres directly. However, reduction in alkaline liquor produces the water-soluble alkali metal salt of the dye; this form is colorless, in which case it is referred to as a Leuco dye, has an affinity for the textile fibre. Subsequent oxidation reforms the original insoluble dye.
The color of denim is due to the original vat dye. Reactive dyes utilize a chromophore attached to a substituent, capable of directly reacting with the fiber substrate; the covalent bonds that attach reactive dye to natural fibers make them among the most permanent of dyes. "Cold" reactive dyes, such as Procion MX, Cibacron F, Drimarene K, are easy to use because the dye can be applied at room temperature. Reactive dyes are by far the best choice for dyeing cotton and other cellulose fibers at home or in the art studio. Disperse dyes were developed for the dyeing of cellulose acetate, are water-insoluble; the dyes are finely ground in the presence of a dispersing agent and sold as a paste, or spray-dried and sold as a powder. Their main use is to dye polyester, but they can be used to dye nylon, cellulose triacetate, acrylic fibers. In some cases, a dyeing temperature of 130 °C is required, a pressurized dyebath is used; the fine particle size gives a large surface area that aids dissolution to allow uptake by the fiber.
The dyeing rate can be influenced by the choice of dispersing agent used during the grinding. Azoic dyeing is a technique in which an insoluble Azo dye is produced directly
Congo red is an organic compound, the sodium salt of 3,3′-bis. It is an azo dye. Congo red is water-soluble. However, the use of Congo red has long been abandoned because of its carcinogenic properties. Congo red was first synthesized in 1883 by Paul Böttiger, employed at Friedrich Bayer Company in Elberfeld, Germany, he was looking for textile dyes. The company was not interested in this bright red color, so he filed the patent under his own name and sold it to the AGFA company of Berlin. AGFA marketed the dye under the name "Congo red", a catchy name in Germany at the time of the 1884 Berlin West Africa Conference, an important event in the Colonisation of Africa; the dye was a major commercial success for AGFA. In the following years, for the same reason, other dyes were marketed using the "Congo" name: Congo rubine, Congo corinth, brilliant Congo, Congo orange, Congo brown, Congo blue. Once of economic significance, Congo red has fallen into disuse as have all benzidine-derived dyes, owing to their carcinogenic activity.
It is prepared by azo coupling of the bis derivative of benzidine with naphthionic acid. Due to a color change from blue to red at pH 3.0–5.2, Congo red can be used as a pH indicator. Since this color change is an approximate inverse of that of litmus, it can be used with litmus paper in a simple parlor trick: add a drop or two of Congo red to both an acid solution and a base solution. Dipping red litmus paper in the red solution will turn it blue, while dipping blue litmus paper in the blue solution will turn it red; this property gives Congo red a metachromatic property as a dye, both in acidic solutions and with acidophilic tissue. Congo red has a propensity to aggregate in organic solutions; the proposed mechanisms suggest hydrophobic interactions between the aromatic rings of the dye molecules, leading to a pi–pi stacking phenomenon. Although these aggregates are present under various sizes and shapes, the "ribbon-like micelles" of a few molecules seem to be the predominant form; this aggregation phenomenon is more prevalent in high Congo red concentrations, at high salinity and/or low pH.
In histology and microscopy, Congo red is used for staining in amyloidosis, for the cell walls of plants and fungi, for the outer membrane of Gram-negative bacteria. Apple-green birefringence of Congo red stained preparations under polarized light is indicative of the presence of amyloid fibrils. Additionally, Congo red is used for the diagnostics of the Shigella flexneri serotype 2a, where the dye binds the bacterium's unique lipopolysaccharide structure. Furthermore, Congo red may be used to induce expression of the type III secretion system of Shigella flexneri, bringing about the secretion of IpaB and IpaC, which form translocation pores within host cell membrane, allowing effector proteins to pass through and alter the host cell's biochemistry; the dye can be used in flow cytometry experiments for the detection of Acanthamoeba and other amoebal cysts
Gram stain or Gram staining called Gram's method, is a method of staining used to distinguish and classify bacterial species into two large groups. The name comes from the Danish bacteriologist Hans Christian Gram. Gram staining differentiates bacteria by the chemical and physical properties of their cell walls by detecting peptidoglycan, present in the cell wall of Gram-positive bacteria. Gram-negative cells contain peptidoglycan, but a small layer of it, dissolved when the alcohol is added; this is. Gram-positive bacteria retain the crystal violet dye, thus are stained violet, while the Gram-negative bacteria do not. Both Gram-positive bacteria and Gram-negative bacteria pick up the counterstain; the counterstain, however, is unseen on Gram-positive bacteria because of the darker crystal violet stain. The Gram stain is always the first step in the preliminary identification of a bacterial organism. While Gram staining is a valuable diagnostic tool in both clinical and research settings, not all bacteria can be definitively classified by this technique.
This gives rise to Gram-indeterminate groups. The method is named after its inventor, the Danish scientist Hans Christian Gram, who developed the technique while working with Carl Friedländer in the morgue of the city hospital in Berlin in 1884. Gram devised his technique not for the purpose of distinguishing one type of bacterium from another but to make bacteria more visible in stained sections of lung tissue, he published his method in 1884, included in his short report the observation that the typhus bacillus did not retain the stain. Gram staining is a bacteriological laboratory technique used to differentiate bacterial species into two large groups based on the physical properties of their cell walls. Gram staining is not used to classify archaea archaeabacteria, since these microorganisms yield varying responses that do not follow their phylogenetic groups; the Gram stain is not an infallible tool for diagnosis, identification, or phylogeny, it is of limited use in environmental microbiology.
It is used to make a preliminary morphologic identification or to establish that there are significant numbers of bacteria in a clinical specimen. It cannot identify bacteria to the species level, for most medical conditions, it should not be used as the sole method of bacterial identification. In clinical microbiology laboratories, it is used in combination with other traditional and molecular techniques to identify bacteria; some organisms are Gram-variable. In a modern environmental or molecular microbiology lab, most identification is done using genetic sequences and other molecular techniques, which are far more specific and informative than differential staining. Gram staining has been suggested to be as effective a diagnostic tool as PCR in one primary research report regarding gonococcal urethritis. Gram stains are performed on body biopsy when infection is suspected. Gram stains yield results much more than culturing, is important when infection would make an important difference in the patient's treatment and prognosis.
Gram-positive bacteria have a thick mesh-like cell wall made of peptidoglycan, as a result are stained purple by crystal violet, whereas Gram-negative bacteria have a thinner layer, so do not retain the purple stain and are counter-stained pink by safranin. There are four basic steps of the Gram stain: Applying a primary stain to a heat-fixed smear of a bacterial culture. Heat fixation kills some bacteria but is used to affix the bacteria to the slide so that they don't rinse out during the staining procedure; the addition of iodide, which binds to crystal violet and traps it in the cell Rapid decolorization with ethanol or acetone Counterstaining with safranin. Carbol fuchsin is sometimes substituted for safranin since it more intensely stains anaerobic bacteria, but it is less used as a counterstain. Crystal violet dissociates in aqueous solutions into chloride ions; these ions penetrate through the cell wall and cell membrane of both Gram-positive and Gram-negative cells. The CV+ ion interacts with negatively charged components of bacterial cells and stains the cells purple.
Iodide interacts with CV+ and forms large complexes of crystal violet and iodine within the inner and outer layers of the cell. Iodine is referred to as a mordant, but is a trapping agent that prevents the removal of the CV–I complex and, colors the cell; when a decolorizer such as alcohol or acetone is added, it interacts with the lipids of the cell membrane. A Gram-negative cell loses its outer lipopolysaccharide membrane, the inner peptidoglycan layer is left exposed; the CV–I complexes are washed from the gram-negative cell along with the outer membrane. In contrast, a Gram-positive cell becomes dehydrated from an ethanol treatment; the large CV–I complexes become trapped within the Gram-positive cell due to the multilayered nature of its peptidoglycan. The decolorization step must be timed correctly.
Tin is a chemical element with the symbol Sn and atomic number 50. It is a post-transition metal in group 14 of the periodic table of elements, it is obtained chiefly from the mineral cassiterite, which contains stannic oxide, SnO2. Tin shows a chemical similarity to both of its neighbors in group 14, germanium and lead, has two main oxidation states, +2 and the more stable +4. Tin is the 49th most abundant element and has, with 10 stable isotopes, the largest number of stable isotopes in the periodic table, thanks to its magic number of protons, it has two main allotropes: at room temperature, the stable allotrope is β-tin, a silvery-white, malleable metal, but at low temperatures it transforms into the less dense grey α-tin, which has the diamond cubic structure. Metallic tin does not oxidize in air; the first tin alloy used on a large scale was bronze, made of 1/8 tin and 7/8 copper, from as early as 3000 BC. After 600 BC, pure metallic tin was produced. Pewter, an alloy of 85–90% tin with the remainder consisting of copper and lead, was used for flatware from the Bronze Age until the 20th century.
In modern times, tin is used in many alloys, most notably tin/lead soft solders, which are 60% or more tin, in the manufacture of transparent, electrically conducting films of indium tin oxide in optoelectronic applications. Another large application for tin is corrosion-resistant tin plating of steel; because of the low toxicity of inorganic tin, tin-plated steel is used for food packaging as tin cans. However, some organotin compounds can be as toxic as cyanide. Tin is a soft, malleable and crystalline silvery-white metal; when a bar of tin is bent, a crackling sound known as the "tin cry" can be heard from the twinning of the crystals. Tin melts at low temperatures of about 232 °C, the lowest in group 14; the melting point is further lowered to 177.3 °C for 11 nm particles. Β-tin, stable at and above room temperature, is malleable. In contrast, α-tin, stable below 13.2 °C, is brittle. Α-tin has a diamond cubic crystal structure, similar to silicon or germanium. Α-tin has no metallic properties at all because its atoms form a covalent structure in which electrons cannot move freely.
It is a dull-gray powdery material with no common uses other than a few specialized semiconductor applications. These two allotropes, α-tin and β-tin, are more known as gray tin and white tin, respectively. Two more allotropes, γ and σ, exist at temperatures above 161 pressures above several GPa. In cold conditions, β-tin tends to transform spontaneously into α-tin, a phenomenon known as "tin pest". Although the α-β transformation temperature is nominally 13.2 °C, impurities lower the transition temperature well below 0 °C and, on the addition of antimony or bismuth, the transformation might not occur at all, increasing the durability of the tin. Commercial grades of tin resist transformation because of the inhibiting effect of the small amounts of bismuth, antimony and silver present as impurities. Alloying elements such as copper, bismuth and silver increase its hardness. Tin tends rather to form hard, brittle intermetallic phases, which are undesirable, it does not form wide solid solution ranges in other metals in general, few elements have appreciable solid solubility in tin.
Simple eutectic systems, occur with bismuth, lead and zinc. Tin was one of the first superconductors to be studied. Tin can be attacked by acids and alkalis. Tin can be polished and is used as a protective coat for other metals. A protective oxide layer prevents further oxidation, the same that forms on pewter and other tin alloys. Tin helps to accelerate the chemical reaction. Tin has ten stable isotopes, with atomic masses of 112, 114 through 120, 122 and 124, the greatest number of any element. Of these, the most abundant are 120Sn, 118Sn, 116Sn, while the least abundant is 115Sn; the isotopes with mass numbers have no nuclear spin, while those with odd have a spin of +1/2. Tin, with its three common isotopes 116Sn, 118Sn and 120Sn, is among the easiest elements to detect and analyze by NMR spectroscopy, its chemical shifts are referenced against SnMe4; this large number of stable isotopes is thought to be a direct result of the atomic number 50, a "magic number" in nuclear physics. Tin occurs in 29 unstable isotopes, encompassing all the remaining atomic masses from 99 to 137.
Apart from 126Sn, with a half-life of 230,000 years, all the radioisotopes have a half-life of less than a year. The radioactive 100Sn, discovered in 1994, 132Sn are one of the few nuclides with a "doubly magic" nucleus: despite being unstable, having lopsided proton–neutron ratios, they represent endpoints beyond which stability drops off rapidly. Another 30 metastable isomers have been characterized for isotopes between 111 and 131, the most stable being 121mSn with a half-life of 43.9 years. The relative differences in the abundances of tin's stable isotopes can be explained by their different modes of formation in stellar nucleosynthesis. 116Sn through 120Sn inclusive are formed in the s-process in most stars and hence they are the most common isotopes, while 122Sn and 124Sn are only formed in the r-process (rapid neutr
Iron is a chemical element with symbol Fe and atomic number 26. It is a metal, that belongs to group 8 of the periodic table, it is by mass the most common element on Earth, forming much of Earth's inner core. It is the fourth most common element in the Earth's crust. Pure iron is rare on the Earth's crust being limited to meteorites. Iron ores are quite abundant, but extracting usable metal from them requires kilns or furnaces capable of reaching 1500 °C or higher, about 500 °C higher than what is enough to smelt copper. Humans started to dominate that process in Eurasia only about 2000 BCE, iron began to displace copper alloys for tools and weapons, in some regions, only around 1200 BCE; that event is considered the transition from the Bronze Age to the Iron Age. Iron alloys, such as steel and special steels are now by far the most common industrial metals, because of their mechanical properties and their low cost. Pristine and smooth pure iron surfaces are mirror-like silvery-gray. However, iron reacts with oxygen and water to give brown to black hydrated iron oxides known as rust.
Unlike the oxides of some other metals, that form passivating layers, rust occupies more volume than the metal and thus flakes off, exposing fresh surfaces for corrosion. The body of an adult human contains about 3 to 5 grams of elemental iron in hemoglobin and myoglobin; these two proteins play essential roles in vertebrate metabolism oxygen transport by blood and oxygen storage in muscles. To maintain the necessary levels, human iron metabolism requires a minimum of iron in the diet. Iron is the metal at the active site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants and animals. Chemically, the most common oxidation states of iron are +2 and +3. Iron shares many properties of other transition metals, including the other group 8 elements and osmium. Iron forms compounds in a wide range of oxidation states, −2 to +7. Iron forms many coordination compounds. At least four allotropes of iron are known, conventionally denoted α, γ, δ, ε; the first three forms are observed at ordinary pressures.
As molten iron cools past its freezing point of 1538 °C, it crystallizes into its δ allotrope, which has a body-centered cubic crystal structure. As it cools further to 1394 °C, it changes to its γ-iron allotrope, a face-centered cubic crystal structure, or austenite. At 912 °C and below, the crystal structure again becomes the bcc α-iron allotrope; the physical properties of iron at high pressures and temperatures have been studied extensively, because of their relevance to theories about the cores of the Earth and other planets. Above 10 GPa and temperatures of a few hundred kelvin or less, α-iron changes into another hexagonal close-packed structure, known as ε-iron; the higher-temperature γ-phase changes into ε-iron, but does so at higher pressure. Some controversial experimental evidence exists for a stable β phase at pressures above 50 GPa and temperatures of at least 1500 K, it is supposed to have a double hcp structure. The inner core of the Earth is presumed to consist of an iron-nickel alloy with ε structure.
The melting and boiling points of iron, along with its enthalpy of atomization, are lower than those of the earlier 3d elements from scandium to chromium, showing the lessened contribution of the 3d electrons to metallic bonding as they are attracted more and more into the inert core by the nucleus. This same trend appears for ruthenium but not osmium; the melting point of iron is experimentally well defined for pressures less than 50 GPa. For greater pressures, published data still varies by tens of gigapascals and over a thousand kelvin. Below its Curie point of 770 °C, α-iron changes from paramagnetic to ferromagnetic: the spins of the two unpaired electrons in each atom align with the spins of its neighbors, creating an overall magnetic field; this happens because the orbitals of those two electrons do not point toward neighboring atoms in the lattice, therefore are not involved in metallic bonding. In the absence of an external source of magnetic field, the atoms get spontaneously partitioned into magnetic domains, about 10 micrometres across, such that the atoms in each domain have parallel spins, but different domains have other orientations.
Thus a macroscopic piece of iron will have a nearly zero overall magnetic field. Application of an external magnetic field causes the domains that are magnetized in the same general direction to grow at the expense of adjacent ones that point in other directions, reinforcing the external field; this effect is exploited in devices that needs to channel magnetic fields, such as electrical transformers, magnetic recording heads, electric motors. Impurities, lattice defects, or grain and particle boundaries can "pin" the domains in the new positions, so that the effect persists after the external field is removed -- thus turning the iron object into a magnet. Similar behavior is exhibited by some iron compounds, such as the fer
Tungsten, or wolfram, is a chemical element with symbol W and atomic number 74. The name tungsten comes from the former Swedish name for the tungstate mineral scheelite, tung sten or "heavy stone". Tungsten is a rare metal found on Earth exclusively combined with other elements in chemical compounds rather than alone, it was identified as a new element in 1781 and first isolated as a metal in 1783. Its important ores include scheelite; the free element is remarkable for its robustness the fact that it has the highest melting point of all the elements discovered, melting at 3422 °C. It has the highest boiling point, at 5930 °C, its density is 19.3 times that of water, comparable to that of uranium and gold, much higher than that of lead. Polycrystalline tungsten is an intrinsically hard material, making it difficult to work. However, pure single-crystalline tungsten can be cut with a hard-steel hacksaw. Tungsten's many alloys have numerous applications, including incandescent light bulb filaments, X-ray tubes, electrodes in gas tungsten arc welding and radiation shielding.
Tungsten's hardness and high density give it military applications in penetrating projectiles. Tungsten compounds are often used as industrial catalysts. Tungsten is the only metal from the third transition series, known to occur in biomolecules that are found in a few species of bacteria and archaea, it is the heaviest element known to be essential to any living organism. However, tungsten interferes with molybdenum and copper metabolism and is somewhat toxic to more familiar forms of animal life. In its raw form, tungsten is a hard steel-grey metal, brittle and hard to work. If made pure, tungsten retains its hardness, becomes malleable enough that it can be worked easily, it is worked by drawing, or extruding. Tungsten objects are commonly formed by sintering. Of all metals in pure form, tungsten has the highest melting point, lowest vapor pressure, the highest tensile strength. Although carbon remains solid at higher temperatures than tungsten, carbon sublimes at atmospheric pressure instead of melting, so it has no melting point.
Tungsten has the lowest coefficient of thermal expansion of any pure metal. The low thermal expansion and high melting point and tensile strength of tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d electrons. Alloying small quantities of tungsten with steel increases its toughness. Tungsten exists in two major crystalline forms: α and β; the former is the more stable form. The structure of the β phase is called A15 cubic. Contrary to the α phase which crystallizes in isometric grains, the β form exhibits a columnar habit; the α phase has one third of the electrical resistivity and a much lower superconducting transition temperature TC relative to the β phase: ca. 0.015 K vs. 1–4 K. The TC value can be raised by alloying tungsten with another metal; such tungsten alloys are sometimes used in low-temperature superconducting circuits. Occurring tungsten consists of four stable isotopes and one long-lived radioisotope, 180W. Theoretically, all five can decay into isotopes of element 72 by alpha emission, but only 180W has been observed to do so, with a half-life of ×1018 years.
The other occurring isotopes have not been observed to decay, constraining their half-lives to be at least 4 × 1021 years. Another 30 artificial radioisotopes of tungsten have been characterized, the most stable of which are 181W with a half-life of 121.2 days, 185W with a half-life of 75.1 days, 188W with a half-life of 69.4 days, 178W with a half-life of 21.6 days, 187W with a half-life of 23.72 h. All of the remaining radioactive isotopes have half-lives of less than 3 hours, most of these have half-lives below 8 minutes. Tungsten has 11 meta states, with the most stable being 179mW. Elemental tungsten resists attack by oxygen and alkalis; the most common formal oxidation state of tungsten is +6, but it exhibits all oxidation states from −2 to +6. Tungsten combines with oxygen to form the yellow tungstic oxide, WO3, which dissolves in aqueous alkaline solutions to form tungstate ions, WO2−4. Tungsten carbides are produced by heating powdered tungsten with carbon. W2C is resistant to chemical attack, although it reacts with chlorine to form tungsten hexachloride.
In aqueous solution, tungstate gives the heteropoly acids and polyoxometalate anions under neutral and acidic conditions. As tungstate is progressively treated with acid, it first yields the soluble, metastable "paratungstate A" anion, W7O6–24, which over time converts to the less soluble "paratungstate B" anion, H2W12O10–42. Further acidification produces the soluble metatungstate anion, H2W12O6–40, after which equilibrium is reached; the metatungstate ion exists as a symmetric cluster of twelve tungsten-oxygen octahedra known as the Keggin anion. Many other polyoxometalate anions exist as metastable species; the inclusion of a different atom such as phosphorus in place of the two central hydrogens in metatungstate produces a wide v