Neodymium

From Wikipedia, the free encyclopedia

Neodymium,  ₆₀Nd

Neodymium
Pronunciation	/ˌniːoʊˈdɪmiəm/ ​(NEE-oh-DIM-ee-əm)
Appearance	silvery white
Standard atomic weight Ar, std(Nd)	144.242(3)
Neodymium in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson – ↑ Nd ↓ U praseodymium ← neodymium → promethium
Atomic number (Z)	60
Group	group n/a
Period	period 6
Block	f-block
Element category	lanthanide
Electron configuration	[Xe] 4f4 6s2
Electrons per shell	2, 8, 18, 22, 8, 2
Physical properties
Phase at STP	solid
Melting point	1297 K ​(1024 °C, ​1875 °F)
Boiling point	3347 K ​(3074 °C, ​5565 °F)
Density (near r.t.)	7.01 g/cm3
when liquid (at m.p.)	6.89 g/cm3
Heat of fusion	7.14 kJ/mol
Heat of vaporization	289 kJ/mol
Molar heat capacity	27.45 J/(mol·K)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 1595 1774 1998 (2296) (2715) (3336)
Atomic properties
Oxidation states	+2, +3, +4 (a mildly basic oxide)
Electronegativity	Pauling scale: 1.14
Ionization energies	1st: 533.1 kJ/mol 2nd: 1040 kJ/mol 3rd: 2130 kJ/mol
Atomic radius	empirical: 181 pm
Covalent radius	201±6 pm
Spectral lines of neodymium
Other properties
Natural occurrence	primordial
Crystal structure	​double hexagonal close-packed (dhcp)
Speed of sound thin rod	2330 m/s (at 20 °C)
Thermal expansion	α, poly: 9.6 µm/(m·K) (at r.t.)
Thermal conductivity	16.5 W/(m·K)
Electrical resistivity	α, poly: 643 nΩ·m
Magnetic ordering	paramagnetic, antiferromagnetic below 20 K
Magnetic susceptibility	+5628.0·10−6 cm3/mol (287.7 K)
Young's modulus	α form: 41.4 GPa
Shear modulus	α form: 16.3 GPa
Bulk modulus	α form: 31.8 GPa
Poisson ratio	α form: 0.281
Vickers hardness	345–745 MPa
Brinell hardness	265–700 MPa
CAS Number	7440-00-8
History
Discovery	Carl Auer von Welsbach (1885)
Main isotopes of neodymium
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct 142Nd 27.2% stable 143Nd 12.2% stable 144Nd 23.8% 2.29×1015 y α 140Ce 145Nd 8.3% stable 146Nd 17.2% stable 148Nd 5.8% stable 150Nd 5.6% 6.7×1018 y β−β− 150Sm

Neodymium is a chemical element with the symbol Nd and atomic number 60. Neodymium belongs to the lanthanide series and is a rare-earth element. It is a hard, slightly malleable silvery metal, that quickly tarnishes in air and moisture. When oxidized, neodymium reacts quickly to produce pink, purple/blue and yellow compounds in the +2, +3 and +4 oxidation state. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite. Neodymium is not found naturally in metallic form or unmixed with other lanthanides, and it is usually refined for general use. Although neodymium is classed as a rare-earth element, it is fairly common, no rarer than cobalt, nickel, or copper, and is widely distributed in the Earth's crust. Most of the world's commercial neodymium is mined in China.

Neodymium compounds were first commercially used as glass dyes in 1927, and they remain a popular additive in glasses. The color of neodymium compounds is due to the Nd³⁺ ion and is often a reddish-purple but it changes with the type of lighting, due to the interaction of the sharp light absorption bands of neodymium with ambient light enriched with the sharp visible emission bands of mercury, trivalent europium or terbium. Some neodymium-doped glasses are used in lasers that emit infrared with wavelengths between 1047 and 1062 nanometers. These have been used in extremely-high-power applications, such as experiments in inertial confinement fusion.

Neodymium is also used with various other substrate crystals, such as yttrium aluminium garnet in the Nd:YAG laser. This laser usually emits infrared at a wavelength of about 1064 nanometers. The Nd:YAG laser is one of the most commonly used solid-state lasers.

Another important use of neodymium is as a component in the alloys used to make high-strength neodymium magnets—powerful permanent magnets. These magnets are widely used in such products as microphones, professional loudspeakers, in-ear headphones, high performance hobby DC electric motors, and computer hard disks, where low magnet mass (or volume) or strong magnetic fields are required. Larger neodymium magnets are used in high-power-versus-weight electric motors (for example in hybrid cars) and generators (for example aircraft and wind turbine electric generators).

Characteristics

Physical properties

Neodymium, a rare-earth metal, was present in the classical mischmetal at a concentration of about 18%. Metallic neodymium has a bright, silvery metallic luster, but as one of the more reactive lanthanide rare-earth metals, it quickly oxidizes in ordinary air. The oxide layer that forms then peels off, exposing the metal to further oxidation. Thus, a centimeter-sized sample of neodymium completely oxidizes within a year.

Neodymium commonly exists in two allotropic forms, with a transformation from a double hexagonal to a body-centered cubic structure taking place at about 863 °C.

Chemical properties

Neodymium metal tarnishes slowly in air and it burns readily at about 150 °C to form neodymium(III) oxide:

4 Nd + 3 O₂ → 2 Nd₂O₃

Neodymium is a quite electropositive element, and it reacts slowly with cold water, but quite quickly with hot water to form neodymium(III) hydroxide:

2 Nd (s) + 6 H₂O (l) → 2 Nd(OH)₃ (aq) + 3 H₂ (g)

Neodymium metal reacts vigorously with all the halogens:

2 Nd (s) + 3 F₂ (g) → 2 NdF₃ (s) [a violet substance]

2 Nd (s) + 3 Cl₂ (g) → 2 NdCl₃ (s) [a mauve substance]

2 Nd (s) + 3 Br₂ (g) → 2 NdBr₃ (s) [a violet substance]

2 Nd (s) + 3 I₂ (g) → 2 NdI₃ (s) [a green substance]

Neodymium dissolves readily in dilute sulfuric acid to form solutions that contain the lilac Nd(III) ion. These exist as a [Nd(OH₂)₉]³⁺ complexes:

2 Nd (s) + 3 H₂SO₄ (aq) → 2 Nd³⁺ (aq) + 3 SO²⁻
₄ (aq) + 3 H₂ (g)

Compounds

Neodymium compounds include

• halides: neodymium(III) fluoride (NdF₃); neodymium(III) chloride (NdCl₃); neodymium(III) bromide (NdBr₃); neodymium(III) iodide (NdI₃)
• oxides: neodymium(III) oxide (Nd₂O₃)
• sulfides: neodymium(II) sulfide (NdS), neodymium(III) sulfide (Nd₂S₃)
• nitrides: neodymium(III) nitride (NdN)
• hydroxide: neodymium(III) hydroxide (Nd(OH)₃)
• phosphide: neodymium phosphide (NdP)
• carbide: neodymium carbide (NdC₂)
• nitrate: neodymium(III) nitrate (Nd(NO₃)₃)
• sulfate: neodymium(III) sulfate (Nd₂(SO₄)₃)

Neodymium(III)-sulfate

Some neodymium compounds have colors that vary based upon the type of lighting.

• 

  Neodymium compounds in fluorescent tube light—from left to right, the sulfate, nitrate, and chloride
  
• 

  Neodymium compounds in compact fluorescent lamp light
  
• 

  Neodymium compounds in normal daylight
  

Isotopes

Naturally occurring neodymium is a mixture of five stable isotopes, ¹⁴²Nd, ¹⁴³Nd, ¹⁴⁵Nd, ¹⁴⁶Nd and ¹⁴⁸Nd, with ¹⁴²Nd being the most abundant (27.2% of the natural abundance), and two radioisotopes, ¹⁴⁴Nd and ¹⁵⁰Nd. In all, 31 radioisotopes of neodymium have been detected as of 2010, with the most stable radioisotopes being the naturally occurring ones: ¹⁴⁴Nd (alpha decay with a half-life (t_1/2) of 2.29×10¹⁵ years) and ¹⁵⁰Nd (double beta decay, t_1/2 = 7×10¹⁸ years, approximately). All of the remaining radioactive isotopes have half-lives that are shorter than eleven days, and the majority of these have half-lives that are shorter than 70 seconds. Neodymium also has 13 known meta states, with the most stable one being ^139mNd (t_1/2 = 5.5 hours), ^135mNd (t_1/2 = 5.5 minutes) and ^133m1Nd (t_1/2 ~70 seconds). 

The primary decay modes before the most abundant stable isotope, ¹⁴²Nd, are electron capture and positron decay, and the primary mode after is beta minus decay. The primary decay products before ¹⁴²Nd are element Pr (praseodymium) isotopes and the primary products after are element Pm (promethium) isotopes.

History

Carl Auer von Welsbach (1858–1929), the discoverer of neodymium

 

Neodymium was discovered by Baron Carl Auer von Welsbach, an Austrian chemist, in Vienna in 1885. He separated neodymium, as well as the element praseodymium, from a material known as didymium by means of fractional crystallization of the double ammonium nitrate tetrahydrates from nitric acid, while following the separation by spectroscopic analysis; however, it was not isolated in relatively pure form until 1925. The name neodymium is derived from the Greek words neos (νέος), new, and didymos (διδύμος), twin.

Double nitrate crystallization was the means of commercial neodymium purification until the 1950s. Lindsay Chemical Division was the first to commercialize large-scale ion-exchange purification of neodymium. Starting in the 1950s, high purity (above 99%) neodymium was primarily obtained through an ion exchange process from monazite, a mineral rich in rare-earth elements. The metal itself is obtained through electrolysis of its halide salts. Currently, most neodymium is extracted from bastnäsite, (Ce,La,Nd,Pr)CO₃F, and purified by solvent extraction. Ion-exchange purification is reserved for preparing the highest purities (typically more than 99.99%). The evolving technology, and improved purity of commercially available neodymium oxide, was reflected in the appearance of neodymium glass that resides in collections today. Early neodymium glasses made in the 1930s have a more reddish or orange tinge than modern versions which are more cleanly purple, due to the difficulties in removing the last traces of praseodymium in the era when manufacturing relied upon fractional crystallization technology.

Occurrence and production

Bastnäsite

 

Neodymium is rarely found in nature as a free element, but rather it occurs in ores such as monazite and bastnäsite (these are mineral group names rather than single mineral names) that contain small amounts of all rare-earth metals. In these minerals neodymium is rarely dominant (as in the case of lanthanum), with cerium being the most abundant lanthanide; some exceptions include monazite-(Nd) and kozoite-(Nd). The main mining areas are in China, the United States, Brazil, India, Sri Lanka, and Australia. The reserves of neodymium are estimated at about eight million tonnes. Although it belongs to the rare-earth metals, neodymium is not rare at all. Its abundance in the Earth's crust is about 38 mg/kg, which is the second highest among rare-earth elements, following cerium. The world's production of neodymium was about 7,000 tonnes in 2004. The bulk of current production is from China. As of January 2010 the Chinese government has imposed strategic material controls on the element, raising some concerns in consuming countries and causing skyrocketing prices of neodymium and other rare-earth metals. As of late 2011, 99% pure neodymium was traded in world markets for US$300 to US$350 per kilogram, down from the mid-2011 peak of US$500/kg. The price of neodymium oxide fell from $200/kg in 2011 to $40 in 2015, largely due to illegal production in China which circumvented government restrictions. The uncertainty of pricing and availability have caused companies (particularly Japanese ones) to create permanent magnets and associated electric motors with fewer rare-earth metals; however, so far they have been unable to eliminate the need for neodymium.

Neodymium is typically 10–18% of the rare-earth content of commercial deposits of the light rare-earth-element minerals bastnäsite and monazite. With neodymium compounds being the most strongly colored for the trivalent lanthanides, it can occasionally dominate the coloration of rare-earth minerals when competing chromophores are absent. It usually gives a pink coloration. Outstanding examples of this include monazite crystals from the tin deposits in Llallagua, Bolivia; ancylite from Mont Saint-Hilaire, Quebec, Canada; or lanthanite from the Saucon Valley, Pennsylvania, United States. As with neodymium glasses, such minerals change their colors under the differing lighting conditions. The absorption bands of neodymium interact with the visible emission spectrum of mercury vapor, with the unfiltered shortwave UV light causing neodymium-containing minerals to reflect a distinctive green color. This can be observed with monazite-containing sands or bastnäsite-containing ore.

Applications

• Neodymium has an unusually large specific heat capacity at liquid-helium temperatures, so is useful in cryocoolers.
• Probably because of similarities to Ca²⁺, Nd³⁺ has been reported to promote plant growth. Rare-earth-element compounds are frequently used in China as fertilizer.
• Samarium–neodymium dating is useful for determining the age relationships of rocks and meteorites.
• Neodymium isotopes recorded in marine sediments are used to reconstruct changes in past ocean circulation.

Magnets

Neodymium magnet on a mu-metal bracket from a hard drive

 

Neodymium magnets (actually an alloy, Nd₂Fe₁₄B) are the strongest permanent magnets known. A neodymium magnet of a few grams can lift a thousand times its own weight. These magnets are cheaper, lighter, and stronger than samarium–cobalt magnets. However, they are not superior in every aspect, as neodymium-based magnets lose their magnetism at lower temperatures and tend to rust, while samarium–cobalt magnets do not. 

Neodymium magnets appear in products such as microphones, professional loudspeakers, in-ear headphones, guitar and bass guitar pick-ups, and computer hard disks where low mass, small volume, or strong magnetic fields are required. Neodymium magnet electric motors have also been responsible for the development of purely electrical model aircraft within the first decade of the 21st century, to the point that these are displacing internal combustion–powered models internationally. Likewise, due to this high magnetic capacity per weight, neodymium is used in the electric motors of hybrid and electric automobiles, and in the electricity generators of some designs of commercial wind turbines (only wind turbines with "permanent magnet" generators use neodymium). For example, drive electric motors of each Toyota Prius require one kilogram (2.2 pounds) of neodymium per vehicle.

Neodymium doped lasers

Neodymium ions in various types of ionic crystals, and also in glasses, act as a laser gain medium, typically emitting 1064 nm light from a particular atomic transition in the neodymium ion, after being "pumped" into excitation from an external source

 

Neodymium doped glass slabs used in extremely powerful lasers for inertial confinement fusion.

 

Nd:YAG laser rod

 

Certain transparent materials with a small concentration of neodymium ions can be used in lasers as gain media for infrared wavelengths (1054–1064 nm), e.g. Nd:YAG (yttrium aluminium garnet), Nd:YLF (yttrium lithium fluoride), Nd:YVO₄ (yttrium orthovanadate), and Nd:glass. Neodymium-doped crystals (typically Nd:YVO₄) generate high-powered infrared laser beams which are converted to green laser light in commercial DPSS hand-held lasers and laser pointers. 

The current laser at the UK Atomic Weapons Establishment (AWE), the HELEN (High Energy Laser Embodying Neodymium) 1-terawatt neodymium-glass laser, can access the midpoints of pressure and temperature regions and is used to acquire data for modeling on how density, temperature, and pressure interact inside warheads. HELEN can create plasmas of around 10⁶ K, from which opacity and transmission of radiation are measured.

Neodymium glass solid-state lasers are used in extremely high power (terawatt scale), high energy (megajoules) multiple beam systems for inertial confinement fusion. Nd:glass lasers are usually frequency tripled to the third harmonic at 351 nm in laser fusion devices.

Neodymium glass for other applications

A neodymium glass light bulb, with the base and inner coating removed, under two different types of light: fluorescent on the left, and incandescent on the right.

 

Neodymium-colored glass

 

Neodymium glass (Nd:glass) is produced by the inclusion of neodymium oxide (Nd₂O₃) in the glass melt. Usually in daylight or incandescent light neodymium glass appears lavender, but it appears pale blue under fluorescent lighting. Neodymium may be used to color glass in delicate shades ranging from pure violet through wine-red and warm gray. 

The first commercial use of purified neodymium was in glass coloration, starting with experiments by Leo Moser in November 1927. The resulting "Alexandrite" glass remains a signature color of the Moser glassworks to this day. Neodymium glass was widely emulated in the early 1930s by American glasshouses, most notably Heisey, Fostoria ("wisteria"), Cambridge ("heatherbloom"), and Steuben ("wisteria"), and elsewhere (e.g. Lalique, in France, or Murano). Tiffin's "twilight" remained in production from about 1950 to 1980. Current sources include glassmakers in the Czech Republic, the United States, and China. 

The sharp absorption bands of neodymium cause the glass color to change under different lighting conditions, being reddish-purple under daylight or yellow incandescent light, but blue under white fluorescent lighting, or greenish under trichromatic lighting. This color-change phenomenon is highly prized by collectors. In combination with gold or selenium, beautiful red colors result. Since neodymium coloration depends upon "forbidden" f-f transitions deep within the atom, there is relatively little influence on the color from the chemical environment, so the color is impervious to the thermal history of the glass. However, for the best color, iron-containing impurities need to be minimized in the silica used to make the glass. The same forbidden nature of the f-f transitions makes rare-earth colorants less intense than those provided by most d-transition elements, so more has to be used in a glass to achieve the desired color intensity. The original Moser recipe used about 5% of neodymium oxide in the glass melt, a sufficient quantity such that Moser referred to these as being "rare-earth doped" glasses. Being a strong base, that level of neodymium would have affected the melting properties of the glass, and the lime content of the glass might have had to be adjusted accordingly.

Light transmitted through neodymium glasses shows unusually sharp absorption bands; the glass is used in astronomical work to produce sharp bands by which spectral lines may be calibrated. Another application is the creation of selective astronomical filters to reduce the effect of light pollution from sodium and fluorescent lighting while passing other colours, especially dark red hydrogen-alpha emission from nebulae. Neodymium is also used to remove the green color caused by iron contaminants from glass. 

Neodymium is a component of "didymium" (referring to mixture of salts of neodymium and praseodymium) used for coloring glass to make welder's and glass-blower's goggles; the sharp absorption bands obliterate the strong sodium emission at 589 nm. The similar absorption of the yellow mercury emission line at 578 nm is the principal cause of the blue color observed for neodymium glass under traditional white-fluorescent lighting. 

Neodymium and didymium glass are used in color-enhancing filters in indoor photography, particularly in filtering out the yellow hues from incandescent lighting.

Similarly, neodymium glass is becoming widely used more directly in incandescent light bulbs. These lamps contain neodymium in the glass to filter out yellow light, resulting in a whiter light which is more like sunlight.

The use of neodymium in automobile rear-view mirrors, to reduce the glare at night, has been patented.

Similar to its use in glasses, neodymium salts are used as a colorant for enamels.

Precautions

Neodymium

Hazards
GHS pictograms
GHS signal word	Warning
GHS hazard statements	H315, H319, H335
GHS precautionary statements	P261, P305+351+338
NFPA 704	0 2 0

Neodymium metal dust is combustible and therefore an explosion hazard. Neodymium compounds, as with all rare-earth metals, are of low to moderate toxicity; however, its toxicity has not been thoroughly investigated. Neodymium dust and salts are very irritating to the eyes and mucous membranes, and moderately irritating to skin. Breathing the dust can cause lung embolisms, and accumulated exposure damages the liver. Neodymium also acts as an anticoagulant, especially when given intravenously.

Neodymium magnets have been tested for medical uses such as magnetic braces and bone repair, but biocompatibility issues have prevented widespread application. Commercially available magnets made from neodymium are exceptionally strong, and can attract each other from large distances. If not handled carefully, they come together very quickly and forcefully, causing injuries. For example, there is at least one documented case of a person losing a fingertip when two magnets he was using snapped together from 50 cm away.

Another risk of these powerful magnets is that if more than one magnet is ingested, they can pinch soft tissues in the gastrointestinal tract. This has led to at least 1,700 emergency room visits and necessitated the recall of the Buckyballs line of toys, which were construction sets of small neodymium magnets.

Plasticizer

From Wikipedia, the free encyclopedia

Plasticizers (UK: plasticisers) or dispersants are additives that increase the plasticity or decrease the viscosity of a material. These are the substances which are added in order to alter their physical properties. These are either liquids with low volatility or solids. They decrease the attraction between polymer chains to make them more flexible. Over the last 60 years more than 30,000 different substances have been evaluated for their plasticizing properties. Of these, only a small number – approximately 50 – are today in commercial use. The dominant applications are for plastics, especially polyvinyl chloride (PVC). The properties of other materials may also be modified when blended with plasticizers including concrete, clays, and related products. According to 2014 data, the total global market for plasticizers was 8.4 million metric tonnes  including 1.3 million metric tonnes in Europe. 

 

Shares of global plasticizer consumption in 2014 (8 million metric tons)

For plastics

Plasticizers for plastics are additives, most commonly phthalate esters in PVC applications. Almost 90% of plasticizers are used in PVC, giving this material improved flexibility and durability. The majority is used in films and cables. It was commonly thought that plasticizers work by embedding themselves between the chains of polymers, spacing them apart (increasing the "free volume"), or swelling them and thus significantly lowering the glass transition temperature for the plastic and making it softer; however it was later shown that the free volume explanation could not account for all of the effects of plasticization. For plastics such as PVC, the more plasticizer added, the lower their cold flex temperature will be. Plastic items containing plasticizers can exhibit improved flexibility and durability. Plasticizers can become available for exposure due to migration and abrasion of the plastic since they are not bound to the polymer matrix. The "new car smell" is often attributed to plasticizers or their degradation products. However, multiple studies on the makeup of the smell do not find phthalates in appreciable amounts, likely due to their extremely low volatility and vapor pressure.

Plasticizers make it possible to achieve improved compound processing characteristics, while also providing flexibility in the end-use product. Ester plasticizers are selected based upon cost-performance evaluation. The rubber compounder must evaluate ester plasticizers for compatibility, processibility, permanence and other performance properties. The wide variety of ester chemistries that are in production include sebacates, adipates, terephthalates, dibenzoates, gluterates, phthalates, azelates, and other specialty blends. This broad product line provides an array of performance benefits required for the many elastomer applications such as tubing and hose products, flooring, wall-coverings, seals and gaskets, belts, wire and cable, and print rolls. Low to high polarity esters provide utility in a wide range of elastomers including nitrile, polychloroprene, EPDM, chlorinated polyethylene, and epichlorohydrin. Plasticizer-elastomer interaction is governed by many factors such as solubility parameter, molecular weight, and chemical structure. Compatibility and performance attributes are key factors in developing a rubber formulation for a particular application.

Plasticizers also function as softeners, extenders, and lubricants, and play a significant role in rubber manufacturing.

Antiplasticizers

Antiplasticizers exhibit effects that are similar, but sometimes opposite, to those of plasticizers on polymer systems. The effect of plasticizers on elastic modulus is dependent on both temperature and plasticizer concentration. Below a certain concentration, referred to as the crossover concentration, a plasticizer can increase the modulus of a material. The material's glass transition temperature will decrease however, at all concentrations. In addition to a crossover concentration a crossover temperature exists. Below the crossover temperature the plasticizer will also increase the modulus. Antiplasticizers are any small molecule or oligomer additive which increases the modulus while decreasing the glass transition temperature. 

Bis(2-ethylhexyl) phthalate is a common plasticizer.

Ester plasticizers

Plasticizers used in PVC and other plastics are often based on esters of polycarboxylic acids with linear or branched aliphatic alcohols of moderate chain length. These compounds are selected on the basis of many critieria including low toxicity, compatibility with the host material, nonvolatility, and expense. Phthalate esters of straight-chain and branched-chain alkyl alcohols meet these specifications and are common plasticizers. Ortho-phthalate esters have traditionally been the most dominant plasticizers, but regulatory concerns have led to the move away from classified substances to non-classified which includes high molecular weight ortho-phthalates and other plasticisers, especially in Europe.

For concrete

In the concrete technology, plasticizers and superplasticizers are also called high range water reducers. When added to concrete mixtures, they confer a number of properties including improve workability and strength. Unless the mix is "starved" of water, the strength of concrete is inversely proportional to the amount of water added, i.e., the water-cement (w/c) ratio. In order to produce stronger concrete, less water is added (without "starving" the mix), which makes the concrete mixture less workable and difficult to mix, necessitating the use of plasticizers, water reducers, superplasticizers, or dispersants.

Plasticizers are also often used when pozzolanic ash is added to concrete to improve strength. This method of mix proportioning is especially popular when producing high-strength concrete and fiber-reinforced concrete. 

Adding 1-2% plasticizer per unit weight of cement is usually sufficient. Adding an excessive amount of plasticizer will result in excessive segregation of concrete and is not advisable. Depending on the particular chemical used, use of too much plasticizer may result in a retarding effect.

Plasticizers are commonly manufactured from lignosulfonates, a by-product from the paper industry. Superplasticizers have generally been manufactured from sulfonated naphthalene condensate or sulfonated melamine formaldehyde, although newer products based on polycarboxylic ethers are now available. Traditional lignosulfonate-based plasticisers, naphthalene and melamine sulfonate-based superplasticisers disperse the flocculated cement particles through a mechanism of electrostatic repulsion. In normal plasticisers, the active substances are adsorbed on to the cement particles, giving them a negative charge, which leads to repulsion between particles. Lignin, naphthalene, and melamine sulfonate superplasticisers are organic polymers. The long molecules wrap themselves around the cement particles, giving them a highly negative charge so that they repel each other. 

Polycarboxylate ether superplasticizer (PCE) or just polycarboxylate (PC), work differently from sulfonate-based superplasticizers, giving cement dispersion by steric stabilisation, instead of electrostatic repulsion. This form of dispersion is more powerful in its effect and gives improved workability retention to the cementitious mix.

For gypsum wallboard production

Plasticizers can be added to wallboard stucco mixtures to improve workability. In order to reduce the energy consumed drying wallboard, less water is added, which makes the gypsum mixture very unworkable and difficult to mix, necessitating the use of plasticizers, water reducers, or dispersants. Some studies also show that too much lignosulfonate dispersant could result in a set-retarding effect. Data showed that amorphous crystal formations occurred that detracted from the mechanical needle-like crystal interaction in the core, preventing a stronger core. The sugars, chelating agents in lignosulfonates such as aldonic acids and extractive compounds are mainly responsible for set retardation. These low range water reducing dispersants are commonly manufactured from lignosulfonates, a by-product from the paper industry. 

High range superplasticizers (dispersants) have generally been manufactured from sulfonated naphthalene condensate, although polycarboxylic ethers represent more modern alternatives. Both of these high range water reducers are used at 1/2 to 1/3 of the lignosulfonate types.

Traditional lignosulfonate and naphthalene sulfonate-based plasticisers disperse the flocculated gypsum particles through a mechanism of electrostatic repulsion. In normal plasticisers, the active substances are adsorbed on to the gypsum particles, giving them a negative charge, which leads to repulsion between particles. Lignin and naphthalene sulfonate plasticizers are organic polymers. The long molecules wrap themselves around the gypsum particles, giving them a highly negative charge so that they repel each other.

Migration of plasticizers out of their host plastics leads to loss of flexibility, embrittlement, and cracking. This decades-old plastic lamp cord crumbles when flexed, due to loss of the plasticizers.

Plasticizers for energetic materials

Energetic material pyrotechnic compositions, especially solid rocket propellants and smokeless powders for guns, often employ plasticizers to improve physical properties of the propellant binder or of the overall propellant, to provide a secondary fuel, and ideally, to improve specific energy yield (e.g. specific impulse, energy yield per gram of propellant, or similar indices) of the propellant. An energetic plasticizer improves the physical properties of an energetic material while also increasing its specific energy yield. Energetic plasticizers are usually preferred to non-energetic plasticizers, especially for solid rocket propellants. Energetic plasticizers reduce the required mass of propellant, enabling a rocket vehicle to carry more payload or reach higher velocities than would otherwise be the case. However, safety or cost considerations may demand that non-energetic plasticizers be used, even in rocket propellants. The solid rocket propellant used to fuel the Space Shuttle solid rocket booster employs HTPB, a synthetic rubber, as a non-energetic secondary fuel.

Effect on health

Substantial concerns have been expressed over the safety of some plasticizers, especially because some low molecular weight ortho-phthalates have been classified as potential endocrine disruptors with some developmental toxicity reported.

Compounds used as plasticizers

Dicarboxylic/tricarboxylic ester-based plasticizers

• Phthalate-based plasticizers are used in situations where good resistance to water and oils is required. Some common phthalate plasticizers are:
  • Bis(2-ethylhexyl) phthalate (DEHP), used in construction materials and medical devices
  • Bis(2-propylheptyl) phthalate (DPHP), used in cables, wires and roofing materials
  • Diisononyl phthalate (DINP), used in flooring materials, found in garden hoses, shoes, toys, and building materials
  • Di-n-butyl phthalate (DnBP, DBP), used for cellulose plastics, food wraps, adhesives, perfumes, and cosmetics - about a third of nail polishes, glosses, enamels, and hardeners contain it, together with some shampoos, sunscreens, skin emollients, and insect repellents
  • Butyl benzyl phthalate (BBzP) is found in vinyl tiles, traffic cones, food conveyor belts, artificial leather, and plastic foams
  • Diisodecyl phthalate (DIDP), used for insulation of wires and cables, car undercoating, shoes, carpets, pool liners
  • Dioctyl phthalate (DOP or DnOP), used in flooring materials, carpets, notebook covers, and high explosives, such as Semtex. Together with DEHP it was the most common plasticizers.
  • Diisooctyl phthalate (DIOP), all-purpose plasticizer for polyvinyl chloride, polyvinyl acetate, rubbers, cellulose plastics, and polyurethane
  • Diethyl phthalate (DEP)
  • Diisobutyl phthalate (DIBP)
  • Di-n-hexyl phthalate, used in flooring materials, tool handles, and automobile parts

Trimellitates

• Trimellitates are used in automobile interiors and other applications where resistance to high temperature is required. They have extremely low volatility.
  • Trimethyl trimellitate (TMTM)
  • Tri-(2-ethylhexyl) trimellitate (TEHTM)(TOTM)
  • Tri-(n-octyl,n-decyl) trimellitate (ATM)
  • Tri-(heptyl,nonyl) trimellitate (LTM)
  • n-octyl trimellitate (OTM)

Adipates, sebacates, maleates

• Adipate-based plasticizers are used for low-temperature or resistance to ultraviolet light. Some examples are:
  • Bis(2-ethylhexyl)adipate (DEHA)
  • Dimethyl adipate (DMAD)
  • Monomethyl adipate (MMAD)
  • Dioctyl adipate (DOA)
  • Dibutyl sebacate (DBS)
  • Dibutyl maleate (DBM)
  • Diisobutyl maleate (DIBM)

Other plasticizers

• Azelates
• Benzoates
• Terephthalates such as dioctyl terephthalate/DEHT (Eastman Chemical Company Trademark: Eastman 168).
• 1,2-Cyclohexane dicarboxylic acid diisononyl ester (BASF trademark: Hexamoll DINCH).
• Alkyl sulphonic acid phenyl ester (ASE).
• Sulfonamides
  • N-ethyl toluene sulfonamide (o/p ETSA), ortho and para isomers
  • N-(2-hydroxypropyl) benzene sulfonamide (HP BSA)
  • N-(n-butyl) benzene sulfonamide (BBSA-NBBS)
• Organophosphates
  • Tricresyl phosphate (TCP)
  • Tributyl phosphate (TBP)
• Glycols and polyethers
  • Triethylene glycol dihexanoate (3G6, 3GH)
  • Tetraethylene glycol diheptanoate (4G7)
• Polymeric plasticizers
• Polybutene

Bio-based plasticizers

Plasticizers with better biodegradability and presumably lower environmental toxicity are being developed. Some such plasticizers are:

• Acetylated monoglycerides; these can be used as food additives
• Alkyl citrates, used in food packagings, medical products, cosmetics, and children's toys
  • Triethyl citrate (TEC)
  • Acetyl triethyl citrate (ATEC), higher boiling point and lower volatility than TEC
  • Tributyl citrate (TBC)
  • Acetyl tributyl citrate (ATBC), compatible with PVC and vinyl chloride copolymers
  • Trioctyl citrate (TOC), also used for gums and controlled release medicines
  • Acetyl trioctyl citrate (ATOC), also used for printing ink
  • Trihexyl citrate (THC), compatible with PVC, also used for controlled release medicines
  • Acetyl trihexyl citrate (ATHC), compatible with PVC
  • Butyryl trihexyl citrate (BTHC, trihexyl o-butyryl citrate), compatible with PVC
  • Trimethyl citrate (TMC), compatible with PVC
• Methyl ricinoleate
• Green plasticizers
  • Epoxidized soybean oil (ESBO)
  • Epoxidized vegetable oils
  • Epoxidized esters of soybean oil

Plasticizers for energetic materials

Here are some energetic plasticizers used in rocket propellants and smokeless powders:

• Nitroglycerine (NG, aka "nitro", glyceryl trinitrate)
• Butanetriol trinitrate (BTTN)
• Dinitrotoluene (DNT)
• Trimethylolethane trinitrate (TMETN, aka Metriol trinitrate, METN)
• Diethylene glycol dinitrate (DEGDN, less commonly DEGN)
• Triethylene glycol dinitrate (TEGDN, less commonly TEGN)
• Bis(2,2-dinitropropyl)formal (BDNPF)
• Bis(2,2-dinitropropyl)acetal (BDNPA)
• 2,2,2-Trinitroethyl 2-nitroxyethyl ether (TNEN)

Due to the secondary alcohol groups, NG and BTTN have relatively low thermal stability. TMETN, DEGDN, BDNPF, and BDNPA have relatively low energies. NG and DEGN have relatively high vapor pressure.

Reduction potential

From Wikipedia, the free encyclopedia

Redox potential (also known as oxidation / reduction potential, ORP, pe, ε, or ${\displaystyle E_{h}}$) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised, respectively. Redox potential is measured in volts (V), or millivolts (mV). Each species has its own intrinsic redox potential; for example, the more positive the reduction potential (reduction potential is more often used due to general formalism in electrochemistry), the greater the species' affinity for electrons and tendency to be reduced. ORP can reflect the antimicrobial potential of the water.

Measurement and interpretation

In aqueous solutions, redox potential is a measure of the tendency of the solution to either gain or lose electrons when it is subjected to change by introduction of a new species. A solution with a higher (more positive) reduction potential than the new species will have a tendency to gain electrons from the new species (i.e. to be reduced by oxidizing the new species) and a solution with a lower (more negative) reduction potential will have a tendency to lose electrons to the new species (i.e. to be oxidized by reducing the new species). Because the absolute potentials are next to impossible to accurately measure, reduction potentials are defined relative to a reference electrode. Reduction potentials of aqueous solutions are determined by measuring the potential difference between an inert sensing electrode in contact with the solution and a stable reference electrode connected to the solution by a salt bridge.

The sensing electrode acts as a platform for electron transfer to or from the reference half cell. It is typically platinum, although gold and graphite can be used as well. The reference half cell consists of a redox standard of known potential. The standard hydrogen electrode (SHE) is the reference from which all standard redox potentials are determined and has been assigned an arbitrary half cell potential of 0.0 mV. However, it is fragile and impractical for routine laboratory use. Therefore, other more stable reference electrodes such as silver chloride and saturated calomel (SCE) are commonly used because of their more reliable performance. 

Although measurement of the redox potential in aqueous solutions is relatively straightforward, many factors limit its interpretation, such as effects of solution temperature and pH, irreversible reactions, slow electrode kinetics, non-equilibrium, presence of multiple redox couples, electrode poisoning, small exchange currents and inert redox couples. Consequently, practical measurements seldom correlate with calculated values. Nevertheless, reduction potential measurement has proven useful as an analytical tool in monitoring changes in a system rather than determining their absolute value (e.g. process control and titrations).

Explanation

Similar to the concentration of hydrogen ion determines the acidity or pH of an aqueous solution, the tendency of electron transfer between a chemical species and an electrode determines the redox potential of an electrode couple. Like pH, redox potential represents how easily electrons are transferred to or from species in solution. Redox potential characterises the ability under the specific condition of a chemical species to lose or gain electrons instead of the amount of electrons available for oxidation or reduction. 

In fact, it is possible to define pe, the negative logarithm of electron concentration (-log[e]) in a solution, which will be directly proportional to the redox potential. Sometimes pe is used as a unit of reduction potential instead of ${\displaystyle E_{h}}$, for example in environmental chemistry. If we normalize pe of hydrogen to zero, we will have the relation pe=16.9 ${\displaystyle E_{h}}$ at room temperature. This point of view is useful for understanding redox potential, although the transfer of electrons, rather than the absolute concentration of free electrons in thermal equilibrium, is how one usually thinks of redox potential. Theoretically, however, the two approaches are equivalent. 

Conversely, one could define a potential corresponding to pH as a potential difference between a solute and pH neutral water, separated by porous membrane (that is permeable to hydrogen ions). Such potential differences actually do occur from differences in acidity on biological membranes. This potential (where pH neutral water is set to 0 V) is analogous with redox potential (where standardized hydrogen solution is set to 0 V), but instead of hydrogen ions, electrons are transferred across in the redox case. Both pH and redox potentials are properties of solutions, not of elements or chemical compounds per se, and depend on concentrations, temperature etc.

Standard reduction potential

The standard reduction potential (${\displaystyle E_{0}}$) is measured under standard conditions: 25 °C, a 1 activity for each ion participating in the reaction, a partial pressure of 1 bar for each gas that is part of the reaction, and metals in their pure state. The standard reduction potential is defined relative to a standard hydrogen electrode (SHE) reference electrode, which is arbitrarily given a potential of 0.00 V. However, because these can also be referred to as "redox potentials", the terms "reduction potentials" and "oxidation potentials" are preferred by the IUPAC. The two may be explicitly distinguished in symbols as ${\displaystyle E_{0}^{r}}$ and ${\displaystyle E_{0}^{o}}$.

Half cells

The relative reactivities of different half cells can be compared to predict the direction of electron flow. A higher ${\displaystyle E_{0}}$ means there is a greater tendency for reduction to occur, while a lower one means there is a greater tendency for oxidation to occur. 

Any system or environment that accepts electrons from a normal hydrogen electrode is a half cell that is defined as having a positive redox potential; any system donating electrons to the hydrogen electrode is defined as having a negative redox potential. ${\displaystyle E_{h}}$ is measured in millivolts (mV). A high positive ${\displaystyle E_{h}}$ indicates an environment that favors oxidation reaction such as free oxygen. A low negative ${\displaystyle E_{h}}$ indicates a strong reducing environment, such as free metals. 

Sometimes when electrolysis is carried out in an aqueous solution, water, rather than the solute, is oxidized or reduced. For example, if an aqueous solution of NaCl is electrolyzed, water may be reduced at the cathode to produce H_2(g) and OH⁻ ions, instead of Na⁺ being reduced to Na_(s), as occurs in the absence of water. It is the reduction potential of each species present that will determine which species will be oxidized or reduced. 

Absolute reduction potentials can be determined if we find the actual potential between electrode and electrolyte for any one reaction. Surface polarization interferes with measurements, but various sources give an estimated potential for the standard hydrogen electrode of 4.4 V to 4.6 V (the electrolyte being positive.) 

Half-cell equations can be combined if one is reversed to an oxidation in a manner that cancels out the electrons to obtain an equation without electrons in it.

Nernst equation

The ${\displaystyle E_{h}}$ and pH of a solution are related. For a half cell equation, conventionally written as reduction (electrons on the left side):

${\displaystyle aA+bB+n[e^{-}]+h[{{\ce {H+}}}]=cC+dD}$

The half cell standard potential ${\displaystyle E_{0}}$ is given by:

${\displaystyle E_{0}({\textrm {volts}})=-{\frac {\Delta G^{\ominus }}{nF}}}$

where ${\displaystyle \Delta G^{\ominus }}$ is the standard Gibbs free energy change, n is the number of electrons involved, and F is Faraday's constant. The Nernst equation relates pH and ${\displaystyle E_{h}}$:

${\displaystyle E_{h}=E_{0}+{\frac {0.05916}{n}}\log \left({\frac {\{A\}^{a}\{B\}^{b}}{\{C\}^{c}\{D\}^{d}}}\right)-{\frac {0.05916h}{n}}{\text{pH}}}$

where curly brackets indicate activities and exponents are shown in the conventional manner. This equation is the equation of a straight line for ${\displaystyle E_{h}}$ as a function of pH with a slope of ${\displaystyle -0.05916h/n}$ volt (pH has no units.) This equation predicts lower ${\displaystyle E_{h}}$ at higher pH values. This is observed for reduction of O₂ to OH⁻ and for reduction of H⁺ to H₂. If H⁺ were on the opposite side of the equation from H⁺, the slope of the line would be reversed (higher ${\displaystyle E_{h}}$ at higher pH). An example of that would be the formation of magnetite (Fe₃O₄) from HFeO⁻
_2 (aq):

3 HFeO⁻
₂ + H⁺ = Fe₃O₄ + 2 H₂O + 2 [[e⁻]]

where E_h = −1.1819 − 0.0885 log([HFeO⁻
₂]³) + 0.0296 pH. Note that the slope of the line is −1/2 the −0.05916 value above, since h/n = −1/2.

Biochemistry

Many enzymatic reactions are oxidation-reduction reactions in which one compound is oxidized and another compound is reduced. The ability of an organism to carry out oxidation-reduction reactions depends on the oxidation-reduction state of the environment, or its reduction potential (${\displaystyle E_{h}}$). 

Strictly aerobic microorganisms are generally active at positive ${\displaystyle E_{h}}$ values, whereas strict anaerobes are generally active at negative ${\displaystyle E_{h}}$ values. Redox affects the solubility of nutrients, especially metal ions.

There are organisms that can adjust their metabolism to their environment, such as facultative anaerobes. Facultative anaerobes can be active at positive Eh values, and at negative Eh values in the presence of oxygen bearing inorganic compounds, such as nitrates and sulfates.

Environmental chemistry

In the field of environmental chemistry, the reduction potential is used to determine if oxidizing or reducing conditions are prevalent in water or soil, and to predict the states of different chemical species in the water, such as dissolved metals. pe values in water range from -12 to 25; the levels where the water itself becomes reduced or oxidized, respectively.

The reduction potentials in natural systems often lie comparatively near one of the boundaries of the stability region of water. Aerated surface water, rivers, lakes, oceans, rainwater and acid mine water, usually have oxidizing conditions (positive potentials). In places with limitations in air supply, such as submerged soils, swamps and marine sediments, reducing conditions (negative potentials) are the norm. Intermediate values are rare and usually a temporary condition found in systems moving to higher or lower pe values.

In environmental situations, it is common to have complex non-equilibrium conditions between a large number of species, meaning that it is often not possible to make accurate and precise measurements of the reduction potential. However, it is usually possible to obtain an approximate value and define the conditions as being in the oxidizing or reducing regime.

In the soil there are two main redox constituents: 1) anorganic redox systems (mainly ox/red compounds of Fe and Mn) and measurement in water extracts; 2) natural soil samples with all microbial and root components and measurement by direct method [Husson O. et al.: Practical improvements in soil redox potential (${\displaystyle E_{h}}$) measurement for characterisation of soil properties. Application for comparison of conventional and conservation agriculture cropping systems. Anal. Chim. Acta 906 (2016): 98-109].

Water quality

Oxidation reduction potential (ORP) can be used for water system monitoring with the benefit of a single-value measure of the disinfection potential, showing the activity of the disinfectant rather than the applied dose. For example, E. coli, Salmonella, Listeria and other pathogens have survival times of under 30 s when the ORP is above 665 mV, compared against >300 s when it is below 485 mV.

A study was conducted comparing traditional parts per million chlorination reading and ORP in Hennepin County, Minnesota. The results of this study argue for the inclusion of ORP above 650mV in local health codes.

Geology

Eh-pH (Pourbaix) diagrams are commonly used in mining and geology for assessment of the stability fields of minerals and dissolved species. Under the conditions where a mineral (solid) phase is predicted to be the most stable form of an element, these diagrams show that mineral. As the predicted results are all from thermodynamic (at equilibrium state) evaluations, these diagrams should be used with caution. Although the formation of a mineral or its dissolution may be predicted to occur under a set of conditions, the process may practically be negligible because its rate is too slow. Consequently, kinetic evaluations at the same time are necessary. Nevertheless, the equilibrium conditions can be used to evaluate the direction of spontaneous changes and the magnitude of the driving force behind them.