A solvent is a substance that dissolves a solute, resulting in a solution. A solvent is a liquid but can be a solid, a gas, or a supercritical fluid; the quantity of solute that can dissolve in a specific volume of solvent varies with temperature. Common uses for organic solvents are in dry cleaning, as paint thinners, as nail polish removers and glue solvents, in spot removers, in detergents and in perfumes. Water is a solvent for the most common solvent used by living things. Solvents find various applications in chemical, pharmaceutical and gas industries, including in chemical syntheses and purification processes; when one substance is dissolved into another, a solution is formed. This is opposed to the situation. In a solution, all of the ingredients are uniformly distributed at a molecular level and no residue remains. A solvent-solute mixture consists of a single phase with all solute molecules occurring as solvates, as opposed to separate continuous phases as in suspensions and other types of non-solution mixtures.
The ability of one compound to be dissolved in another is known as solubility. In addition to mixing, the substances in a solution interact with each other at the molecular level; when something is dissolved, molecules of the solvent arrange around molecules of the solute. Heat transfer is involved and entropy is increased making the solution more thermodynamically stable than the solute and solvent separately; this arrangement is mediated by the respective chemical properties of the solvent and solute, such as hydrogen bonding, dipole moment and polarizability. Solvation does not cause a chemical chemical configuration changes in the solute. However, solvation resembles a coordination complex formation reaction with considerable energetics and is thus far from a neutral process. Solvents can be broadly classified into two categories: non-polar. A special case is mercury; the dielectric constant of the solvent provides a rough measure of a solvent's polarity. The strong polarity of water is indicated by its high dielectric constant of 88.
Solvents with a dielectric constant of less than 15 are considered to be nonpolar. The dielectric constant measures the solvent's tendency to cancel the field strength of the electric field of a charged particle immersed in it; this reduction is compared to the field strength of the charged particle in a vacuum. Heuristically, the dielectric constant of a solvent can be thought of as its ability to reduce the solute's effective internal charge; the dielectric constant of a solvent is an acceptable predictor of the solvent's ability to dissolve common ionic compounds, such as salts. Dielectric constants are not the only measure of polarity; because solvents are used by chemists to carry out chemical reactions or observe chemical and biological phenomena, more specific measures of polarity are required. Most of these measures are sensitive to chemical structure; the Grunwald–Winstein mY scale measures polarity in terms of solvent influence on buildup of positive charge of a solute during a chemical reaction.
Kosower's Z scale measures polarity in terms of the influence of the solvent on UV-absorption maxima of a salt pyridinium iodide or the pyridinium zwitterion. Donor number and donor acceptor scale measures polarity in terms of how a solvent interacts with specific substances, like a strong Lewis acid or a strong Lewis base; the Hildebrand parameter is the square root of cohesive energy density. It can not accommodate complex chemistry. Reichardt's dye, a solvatochromic dye that changes color in response to polarity, gives a scale of ET values. ET is the transition energy between the ground state and the lowest excited state in kcal/mol, identifies the dye. Another correlated scale can be defined with Nile red; the polarity, dipole moment and hydrogen bonding of a solvent determines what type of compounds it is able to dissolve and with what other solvents or liquid compounds it is miscible. Polar solvents dissolve polar compounds best and non-polar solvents dissolve non-polar compounds best: "like dissolves like".
Polar compounds like sugars or ionic compounds, like inorganic salts dissolve only in polar solvents like water, while non-polar compounds like oils or waxes dissolve only in non-polar organic solvents like hexane. Water and hexane are not miscible with each other and will separate into two layers after being shaken well. Polarity can be separated to different contributions. For example, the Kamlet-Taft parameters are dipolarity/polarizability, hydrogen-bonding acidity and hydrogen-bonding basicity; these can be calculated from the wavelength shifts of 3–6 different solvatochromic dyes in the solvent including Reichardt's dye and diethylnitroaniline. Another option, Hansen's parameters, separate the cohesive energy density into dispersion and hydrogen bonding contributions. Solvents with a dielectric constant (more relative
Enzymes are macromolecular biological catalysts. Enzymes accelerate chemical reactions; the molecules upon which enzymes may act are called substrates and the enzyme converts the substrates into different molecules known as products. All metabolic processes in the cell need enzyme catalysis in order to occur at rates fast enough to sustain life. Metabolic pathways depend upon enzymes to catalyze individual steps; the study of enzymes is called enzymology and a new field of pseudoenzyme analysis has grown up, recognising that during evolution, some enzymes have lost the ability to carry out biological catalysis, reflected in their amino acid sequences and unusual'pseudocatalytic' properties. Enzymes are known to catalyze more than 5,000 biochemical reaction types. Most enzymes are proteins; the latter are called ribozymes. Enzymes' specificity comes from their unique three-dimensional structures. Like all catalysts, enzymes increase the reaction rate by lowering its activation energy; some enzymes can make their conversion of substrate to product occur many millions of times faster.
An extreme example is orotidine 5'-phosphate decarboxylase, which allows a reaction that would otherwise take millions of years to occur in milliseconds. Chemically, enzymes are like any catalyst and are not consumed in chemical reactions, nor do they alter the equilibrium of a reaction. Enzymes differ from most other catalysts by being much more specific. Enzyme activity can be affected by other molecules: inhibitors are molecules that decrease enzyme activity, activators are molecules that increase activity. Many therapeutic drugs and poisons are enzyme inhibitors. An enzyme's activity decreases markedly outside its optimal temperature and pH, many enzymes are denatured when exposed to excessive heat, losing their structure and catalytic properties; some enzymes are used commercially, in the synthesis of antibiotics. Some household products use enzymes to speed up chemical reactions: enzymes in biological washing powders break down protein, starch or fat stains on clothes, enzymes in meat tenderizer break down proteins into smaller molecules, making the meat easier to chew.
By the late 17th and early 18th centuries, the digestion of meat by stomach secretions and the conversion of starch to sugars by plant extracts and saliva were known but the mechanisms by which these occurred had not been identified. French chemist Anselme Payen was the first to discover an enzyme, diastase, in 1833. A few decades when studying the fermentation of sugar to alcohol by yeast, Louis Pasteur concluded that this fermentation was caused by a vital force contained within the yeast cells called "ferments", which were thought to function only within living organisms, he wrote that "alcoholic fermentation is an act correlated with the life and organization of the yeast cells, not with the death or putrefaction of the cells."In 1877, German physiologist Wilhelm Kühne first used the term enzyme, which comes from Greek ἔνζυμον, "leavened" or "in yeast", to describe this process. The word enzyme was used to refer to nonliving substances such as pepsin, the word ferment was used to refer to chemical activity produced by living organisms.
Eduard Buchner submitted his first paper on the study of yeast extracts in 1897. In a series of experiments at the University of Berlin, he found that sugar was fermented by yeast extracts when there were no living yeast cells in the mixture, he named the enzyme that brought about the fermentation of sucrose "zymase". In 1907, he received the Nobel Prize in Chemistry for "his discovery of cell-free fermentation". Following Buchner's example, enzymes are named according to the reaction they carry out: the suffix -ase is combined with the name of the substrate or to the type of reaction; the biochemical identity of enzymes was still unknown in the early 1900s. Many scientists observed that enzymatic activity was associated with proteins, but others argued that proteins were carriers for the true enzymes and that proteins per se were incapable of catalysis. In 1926, James B. Sumner crystallized it; the conclusion that pure proteins can be enzymes was definitively demonstrated by John Howard Northrop and Wendell Meredith Stanley, who worked on the digestive enzymes pepsin and chymotrypsin.
These three scientists were awarded the 1946 Nobel Prize in Chemistry. The discovery that enzymes could be crystallized allowed their structures to be solved by x-ray crystallography; this was first done for lysozyme, an enzyme found in tears and egg whites that digests the coating of some bacteria. This high-resolution structure of lysozyme marked the beginning of the field of structural biology and the effort to understand how enzymes work at an atomic level of detail. An enzyme's name is derived from its substrate or the chemical reaction it catalyzes, with the word ending in -ase. Examples are alcohol dehydrogenase and DNA polymerase. Different enzymes that catalyze the same chemical reaction are called isozymes; the International Union of Biochemistry and Molecular Biology have developed a nomenclature for enzymes, the EC numbers. The first number broadly classifies the enzyme based on its mechanism; the top-level classification is: EC 1, Oxidoreductases: catalyze oxidation/reducti
Mycobacterium vaccae is a nonpathogenic species of the Mycobacteriaceae family of bacteria that lives in soil. Its name originates from the Latin word, since it was first cultured from cow dung in Austria. Research areas being pursued with regard to killed Mycobacterium vaccae vaccine include immunotherapy for allergic asthma, depression, psoriasis, dermatitis and tuberculosis. A research group at Henry Wellcome Laboratories for Integrative Neuroscience and Endocrinology, University of Bristol, England, UK has shown that Mycobacterium vaccae stimulated a newly discovered group of neurons, increased levels of serotonin and decreased levels of anxiety in mice. Other researchers fed live Mycobacterium vaccae to mice measured their ability to navigate a maze compared to control mice not fed the bacteria. "Mice that were fed live M. vaccae navigated the maze twice as fast and with less demonstrated anxiety behaviors as control mice", according to Dorothy Matthews, who conducted the research with Susan Jenks at the Sage Colleges, New York, USA.
Mycobacterium vaccae is in the same genus as Mycobacterium tuberculosis, the bacterium which causes tuberculosis. Numerous trials have indicated that exposure to oral and injectable products derived from M. vaccae bacteria can have positive effects in treating tuberculosis. Although a 2002 review of selected clinical trials failed to find any consistent benefit of certain dosage regimens of injectable Mycobacterium products in people with tuberculosis, a more recent meta-analysis of 54 clinical studies of M. vaccae products for tuberculosis showed treatment resulted in improved sputum conversion and radiological assessment. Medical researchers at Kharkiv National Medical University, Ukraine have reported two clinical trials with oral formulations of Immunitor Inc's killed Mycobacterium vaccae oral vaccine and An Hui Longcom's killed Mycobacterium vaccae oral vaccine in treating tuberculosis, including drug resistant TB; the research team reported greater success with the Immunitor vaccine than the An Hui Longcom vaccine.
A team of researchers at the Genetics and Microbiology Department of the Autonomous University of Barcelona, Spain discovered that Mycobacterium vaccae changes from its "smooth" type to its "rough" type at thirty degrees Celsius. They discovered that the "smooth" type of Mycobacterium vaccae has a substance on the outside of its cell wall which interferes with the production of Th-1 cytokines, responsible for some kinds of T-helper cell immune response; the team found that the spleen cells of mice inoculated with "rough" Mycobacterium vaccae produced more Th-1 cytokines than those inoculated with "smooth" Mycobacterium vaccae. The researchers say this may explain why different vaccines made from Mycobacterium vaccae vary in their effectiveness in increasing immune response to other organisms during clinical trials. Bacteria and depression -- Bad is good Treatment of conditions of the central nervous system using mycobacteria - Patent 20030170275 U. S. Patent 10258550 - Compositions Derived From Mycobacterium Vaccae and Methods for Their Use Can bacteria make you smarter?
Google translated article from Spanish Type strain of Mycobacterium vaccae at BacDive - the Bacterial Diversity Metadatabase
Redox is a chemical reaction in which the oxidation states of atoms are changed. Any such reaction involves both a reduction process and a complementary oxidation process, two key concepts involved with electron transfer processes. Redox reactions include all chemical reactions; the chemical species from which the electron is stripped is said to have been oxidized, while the chemical species to which the electron is added is said to have been reduced. It can be explained in simple terms: Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion. Reduction is a decrease in oxidation state by a molecule, atom, or ion; as an example, during the combustion of wood, oxygen from the air is reduced, gaining electrons from carbon, oxidized. Although oxidation reactions are associated with the formation of oxides from oxygen molecules, oxygen is not included in such reactions, as other chemical species can serve the same function; the reaction can occur slowly, as with the formation of rust, or more in the case of fire.
There are simple redox processes, such as the oxidation of carbon to yield carbon dioxide or the reduction of carbon by hydrogen to yield methane, more complex processes such as the oxidation of glucose in the human body. "Redox" is a portmanteau of the words "reduction" and "oxidation". The word oxidation implied reaction with oxygen to form an oxide, since dioxygen was the first recognized oxidizing agent; the term was expanded to encompass oxygen-like substances that accomplished parallel chemical reactions. The meaning was generalized to include all processes involving loss of electrons; the word reduction referred to the loss in weight upon heating a metallic ore such as a metal oxide to extract the metal. In other words, ore was "reduced" to metal. Antoine Lavoisier showed. Scientists realized that the metal atom gains electrons in this process; the meaning of reduction became generalized to include all processes involving a gain of electrons. Though "reduction" seems counter-intuitive when speaking of the gain of electrons, it might help to think of reduction as the loss of oxygen, its historical meaning.
Since electrons are negatively charged, it is helpful to think of this as reduction in electrical charge. The electrochemist John Bockris has used the words electronation and deelectronation to describe reduction and oxidation processes when they occur at electrodes; these words are analogous to protonation and deprotonation, but they have not been adopted by chemists worldwide. The term "hydrogenation" could be used instead of reduction, since hydrogen is the reducing agent in a large number of reactions in organic chemistry and biochemistry. But, unlike oxidation, generalized beyond its root element, hydrogenation has maintained its specific connection to reactions that add hydrogen to another substance; the word "redox" was first used in 1928. The processes of oxidation and reduction occur and cannot happen independently of one another, similar to the acid–base reaction; the oxidation alone and the reduction alone are each called a half-reaction, because two half-reactions always occur together to form a whole reaction.
When writing half-reactions, the gained or lost electrons are included explicitly in order that the half-reaction be balanced with respect to electric charge. Though sufficient for many purposes, these general descriptions are not correct. Although oxidation and reduction properly refer to a change in oxidation state — the actual transfer of electrons may never occur; the oxidation state of an atom is the fictitious charge that an atom would have if all bonds between atoms of different elements were 100% ionic. Thus, oxidation is best defined as an increase in oxidation state, reduction as a decrease in oxidation state. In practice, the transfer of electrons will always cause a change in oxidation state, but there are many reactions that are classed as "redox" though no electron transfer occurs. In redox processes, the reductant transfers electrons to the oxidant. Thus, in the reaction, the reductant or reducing agent loses electrons and is oxidized, the oxidant or oxidizing agent gains electrons and is reduced.
The pair of an oxidizing and reducing agent that are involved in a particular reaction is called a redox pair. A redox couple is a reducing species and its corresponding oxidizing form, e.g. Fe2+/Fe3+ Substances that have the ability to oxidize other substances are said to be oxidative or oxidizing and are known as oxidizing agents, oxidants, or oxidizers; that is, the oxidant removes electrons from another substance, is thus itself reduced. And, because it "accepts" electrons, the oxidizing agent is called an electron acceptor. Oxygen is the quintessential oxidizer. Oxidants are chemical substances with elements in high oxidation states, or else electronegative elements that can gain extra electrons by oxidizing another substance. Substances that have the ability to reduce other substances are said to be reductive or reducing and are known as
International Standard Serial Number
An International Standard Serial Number is an eight-digit serial number used to uniquely identify a serial publication, such as a magazine. The ISSN is helpful in distinguishing between serials with the same title. ISSN are used in ordering, interlibrary loans, other practices in connection with serial literature; the ISSN system was first drafted as an International Organization for Standardization international standard in 1971 and published as ISO 3297 in 1975. ISO subcommittee TC 46/SC 9 is responsible for maintaining the standard; when a serial with the same content is published in more than one media type, a different ISSN is assigned to each media type. For example, many serials are published both in electronic media; the ISSN system refers to these types as electronic ISSN, respectively. Conversely, as defined in ISO 3297:2007, every serial in the ISSN system is assigned a linking ISSN the same as the ISSN assigned to the serial in its first published medium, which links together all ISSNs assigned to the serial in every medium.
The format of the ISSN is an eight digit code, divided by a hyphen into two four-digit numbers. As an integer number, it can be represented by the first seven digits; the last code digit, which may be 0-9 or an X, is a check digit. Formally, the general form of the ISSN code can be expressed as follows: NNNN-NNNC where N is in the set, a digit character, C is in; the ISSN of the journal Hearing Research, for example, is 0378-5955, where the final 5 is the check digit, C=5. To calculate the check digit, the following algorithm may be used: Calculate the sum of the first seven digits of the ISSN multiplied by its position in the number, counting from the right—that is, 8, 7, 6, 5, 4, 3, 2, respectively: 0 ⋅ 8 + 3 ⋅ 7 + 7 ⋅ 6 + 8 ⋅ 5 + 5 ⋅ 4 + 9 ⋅ 3 + 5 ⋅ 2 = 0 + 21 + 42 + 40 + 20 + 27 + 10 = 160 The modulus 11 of this sum is calculated. For calculations, an upper case X in the check digit position indicates a check digit of 10. To confirm the check digit, calculate the sum of all eight digits of the ISSN multiplied by its position in the number, counting from the right.
The modulus 11 of the sum must be 0. There is an online ISSN checker. ISSN codes are assigned by a network of ISSN National Centres located at national libraries and coordinated by the ISSN International Centre based in Paris; the International Centre is an intergovernmental organization created in 1974 through an agreement between UNESCO and the French government. The International Centre maintains a database of all ISSNs assigned worldwide, the ISDS Register otherwise known as the ISSN Register. At the end of 2016, the ISSN Register contained records for 1,943,572 items. ISSN and ISBN codes are similar in concept. An ISBN might be assigned for particular issues of a serial, in addition to the ISSN code for the serial as a whole. An ISSN, unlike the ISBN code, is an anonymous identifier associated with a serial title, containing no information as to the publisher or its location. For this reason a new ISSN is assigned to a serial each time it undergoes a major title change. Since the ISSN applies to an entire serial a new identifier, the Serial Item and Contribution Identifier, was built on top of it to allow references to specific volumes, articles, or other identifiable components.
Separate ISSNs are needed for serials in different media. Thus, the print and electronic media versions of a serial need separate ISSNs. A CD-ROM version and a web version of a serial require different ISSNs since two different media are involved. However, the same ISSN can be used for different file formats of the same online serial; this "media-oriented identification" of serials made sense in the 1970s. In the 1990s and onward, with personal computers, better screens, the Web, it makes sense to consider only content, independent of media; this "content-oriented identification" of serials was a repressed demand during a decade, but no ISSN update or initiative occurred. A natural extension for ISSN, the unique-identification of the articles in the serials, was the main demand application. An alternative serials' contents model arrived with the indecs Content Model and its application, the digital object identifier, as ISSN-independent initiative, consolidated in the 2000s. Only in 2007, ISSN-L was defined in the
Phenol is an aromatic organic compound with the molecular formula C6H5OH. It is a white crystalline solid, volatile; the molecule consists of a phenyl group bonded to a hydroxy group. It requires careful handling due to its propensity for causing chemical burns. Phenol was first extracted from coal tar, it is an important industrial commodity as a precursor to useful compounds. It is used to synthesize plastics and related materials. Phenol and its chemical derivatives are essential for production of polycarbonates, Bakelite, detergents, herbicides such as phenoxy herbicides, numerous pharmaceutical drugs. Phenol is an organic compound appreciably soluble in water, with about 84.2 g dissolving in 1000 mL. Homogeneous mixtures of phenol and water at phenol to water mass ratios of ~2.6 and higher are possible. The sodium salt of phenol, sodium phenoxide, is far more water-soluble. Phenol is weakly acidic and at high pHs gives the phenolate anion C6H5O−: PhOH ⇌ PhO− + H+ Compared to aliphatic alcohols, phenol is about 1 million times more acidic, although it is still considered a weak acid.
It reacts with aqueous NaOH to lose H+, giving the salt sodium phenoxide, whereas most alcohols react only partially. One explanation for the increased acidity over alcohols is resonance stabilization of the phenoxide anion by the aromatic ring. In this way, the negative charge on oxygen is delocalized on to the ortho and para carbon atoms through the pi system. An alternative explanation involves the sigma framework, postulating that the dominant effect is the induction from the more electronegative sp2 hybridised carbons. In support of the second explanation, the pKa of the enol of acetone in water is 10.9, making it only less acidic than phenol. Thus, the greater number of resonance structures available to phenoxide compared to acetone enolate seems to contribute little to its stabilization. However, the situation changes. A recent in silico comparison of the gas phase acidities of the vinylogues of phenol and cyclohexanol in conformations that allow for or exclude resonance stabilization leads to the inference that about 1⁄3 of the increased acidity of phenol is attributable to inductive effects, with resonance accounting for the remaining difference.
The phenoxide anion has a similar nucleophilicity to free amines, with the further advantage that its conjugate acid does not become deactivated as a nucleophile in moderately acidic conditions. Phenolate esters are more stable toward hydrolysis than acid anhydrides and acyl halides but are sufficiently reactive under mild conditions to facilitate the formation of amide bonds. Phenol exhibits keto-enol tautomerism with its unstable keto tautomer cyclohexadienone, but only a tiny fraction of phenol exists as the keto form; the equilibrium constant for enolisation is 10−13, which means only one in every ten trillion molecules is in the keto form at any moment. The small amount of stabilisation gained by exchanging a C=C bond for a C=O bond is more than offset by the large destabilisation resulting from the loss of aromaticity. Phenol therefore exists entirely in the enol form. Phenoxides are enolates stabilised by aromaticity. Under normal circumstances, phenoxide is more reactive at the oxygen position, but the oxygen position is a "hard" nucleophile whereas the alpha-carbon positions tend to be "soft".
Phenol is reactive toward electrophilic aromatic substitution as the oxygen atom's pi electrons donate electron density into the ring. By this general approach, many groups can be appended to the ring, via halogenation, acylation and other processes. However, phenol's ring is so activated—second only to aniline—that bromination or chlorination of phenol leads to substitution on all carbon atoms ortho and para to the hydroxy group, not only on one carbon. Phenol reacts with dilute nitric acid at room temperature to give a mixture of 2-nitrophenol and 4-nitrophenol while with concentrated nitric acid, more nitro groups get substituted on the ring to give 2,4,6-trinitrophenol, known as picric acid. Aqueous solutions of phenol are weakly acidic and turn blue litmus to red. Phenol is neutralized by sodium hydroxide forming sodium phenate or phenolate, but being weaker than carbonic acid, it cannot be neutralized by sodium bicarbonate or sodium carbonate to liberate carbon dioxide. C6H5OH + NaOH → C6H5ONa + H2OWhen a mixture of phenol and benzoyl chloride are shaken in presence of dilute sodium hydroxide solution, phenyl benzoate is formed.
This is an example of the Schotten-Baumann reaction: C6H5OH + C6H5COCl → C6H5OCOC6H5 + HClPhenol is reduced to benzene when it is distilled with zinc dust, or when phenol vapour is passed over granules of zinc at 400 °C: C6H5OH + Zn → C6H6 + ZnOWhen phenol is reacted with diazomethane in the presence of boron trifluoride, anisole is obtained as the main product and nitrogen gas as a byproduct. C6H5OH + CH2N2 → C6H5OCH3 + N2When phenol reacts with iron chloride solution, an intense violet-purple solution is formed; because of phenol's commercial importance, many methods have been developed for its production. The dominant current route, accounting for 95% of production, is the cumene process, which uses benzene and propene as feedstock and involves the partial oxidation of cumene vi
Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria, fungi. The process of biodegradation can be divided into three stages: biodeterioration and assimilation. Biodeterioration is a surface-level degradation that modifies the mechanical and chemical properties of the material; this stage occurs when the material is exposed to abiotic factors in the outdoor environment and allows for further degradation by weakening the material's structure. Some abiotic factors that influence these initial changes are compression, light and chemicals in the environment. While biodeterioration occurs as the first stage of biodegradation, it can in some cases be parallel to biofragmentation. Biofragmentation of a polymer is the lytic process in which bonds within a polymer are cleaved, generating oligomers and monomers in its place; the steps taken to fragment these materials differ based on the presence of oxygen in the system. The breakdown of materials by microorganisms when oxygen is present is aerobic digestion, the breakdown of materials when is oxygen is not present is anaerobic digestion.
The main difference between these processes is that anaerobic reactions produce methane, while aerobic reactions do not. In addition, aerobic digestion occurs more than anaerobic digestion, while anaerobic digestion does a better job reducing the volume and mass of the material. Due to anaerobic digestion's ability to reduce the volume and mass of waste materials and produce a natural gas, anaerobic digestion technology is used for waste management systems and as a source of local, renewable energy; the resulting products from biofragmentation are integrated into microbial cells, this is the assimilation stage. Some of the products from fragmentation are transported within the cell by membrane carriers. However, others still have to undergo biotransformation reactions to yield products that can be transported inside the cell. Once inside the cell, the products enter catabolic pathways that either lead to the production of adenosine triphosphate or elements of the cells structure. In practice all chemical compounds and materials are subject to biodegradation processes.
The significance, however, is in the relative rates of such processes, such as days, years or centuries. A number of factors determine the rate. Factors include light, water and temperature; the degradation rate of many organic compounds is limited by their bioavailability, the rate at which a substance is absorbed into a system or made available at the site of physiological activity, as compounds must be released into solution before organisms can degrade them. The rate of biodegradation can be measured in a number of ways. Respirometry tests can be used for aerobic microbes. First one places a solid waste sample in a container with microorganisms and soil, aerates the mixture. Over the course of several days, microorganisms digest the sample bit by bit and produce carbon dioxide – the resulting amount of CO2 serves as an indicator of degradation. Biodegradability can be measured by anaerobic microbes and the amount of methane or alloy that they are able to produce. It’s important to note factors that effect biodegradation rates during product testing to ensure that the results produced are accurate and reliable.
Several materials will test as being biodegradable under optimal conditions in a lab for approval but these results may not reflect real world outcomes where factors are more variable. For example, a material may have tested as biodegrading at a high rate in the lab may not degrade at a high rate in a landfill because landfills lack light and microbial activity that are necessary for degradation to occur. Thus, it is important that there are standards for plastic biodegradable products, which have a large impact on the environment; the development and use of accurate standard test methods can help ensure that all plastics that are being produced and commercialized will biodegrade in natural environments. One test, developed for this purpose is DINV 54900; the term Biodegradable Plastics refers to a material that maintains its mechanical strength during practical use but break down into low-weight compounds and non-toxic byproducts after their use. This breakdown is made possible through an attack of microorganisms on the material, a non-water soluble polymer.
Such materials can be obtained through chemical synthesis, fermentation by microorganisms, from chemically modified natural products. Plastics biodegrade at variable rates. PVC-based plumbing is selected for handling sewage; some packaging materials on the other hand are being developed that would degrade upon exposure to the environment. Examples of synthetic polymers that biodegrade include polycaprolactone, other polyesters and aromatic-aliphatic esters, due to their ester bonds being susceptible to attack by water. A prominent example is poly-3-hydroxybutyrate, the renewably derived polylactic acid, the synthetic polycaprolactone. Others are the cellulose-based cellulose celluloid. Under low oxygen conditions plastics break down more slowly; the breakdown process can be accelerated in specially designed compost heap. Starch-based plastics will degrade within two to four months in a home compost bin, while polylactic acid is undecomposed, requiring higher temperatures. Polycaprolactone and polycaprolactone-starch composites decompose slower, but the starch content accelerate