1.
PH
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In chemistry, pH is a numeric scale used to specify the acidity or basicity of an aqueous solution. It is approximately the negative of the base 10 logarithm of the concentration, measured in units of moles per liter. More precisely it is the negative of the logarithm to base 10 of the activity of the hydrogen ion, solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic. Pure water is neutral, at pH7, being neither an acid nor a base, contrary to popular belief, the pH value can be less than 0 or greater than 14 for very strong acids and bases respectively. The pH scale is traceable to a set of standard solutions whose pH is established by international agreement, the pH of aqueous solutions can be measured with a glass electrode and a pH meter, or an indicator. In the first papers, the notation had the H as a subscript to the p, as so. The exact meaning of the p in pH is disputed, but according to the Carlsberg Foundation and it has also been suggested that the p stands for the German Potenz, others refer to French puissance. Another suggestion is that the p stands for the Latin terms pondus hydrogenii, potentia hydrogenii and it is also suggested that Sørensen used the letters p and q simply to label the test solution and the reference solution. Currently in chemistry, the p stands for decimal cologarithm of, PH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, aH+, in a solution. P H = − log 10 = log 10 For example and this definition was adopted because ion-selective electrodes, which are used to measure pH, respond to activity. For H+ number of electrons transferred is one and it follows that electrode potential is proportional to pH when pH is defined in terms of activity. The reference electrode may be a silver chloride electrode or a calomel electrode, the hydrogen-ion selective electrode is a standard hydrogen electrode. Reference electrode | concentrated solution of KCl || test solution | H2 | Pt Firstly, the cell is filled with a solution of hydrogen ion activity. Then the emf, EX, of the cell containing the solution of unknown pH is measured. PH = pH + E S − E X z The difference between the two measured emf values is proportional to pH and this method of calibration avoids the need to know the standard electrode potential. The proportionality constant, 1/z is ideally equal to 12.303 R T / F the Nernstian slope, to apply this process in practice, a glass electrode is used rather than the cumbersome hydrogen electrode. A combined glass electrode has a reference electrode. It is calibrated against buffer solutions of hydrogen ion activity
2.
Manganese
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Manganese is a chemical element with symbol Mn and atomic number 25. It is not found as an element in nature, it is often found in minerals in combination with iron. Manganese is a metal with important industrial metal alloy uses, particularly in stainless steels, by the mid-18th century, Swedish chemist Carl Wilhelm Scheele had used pyrolusite to produce chlorine. Scheele and others were aware that pyrolusite contained a new element, johan Gottlieb Gahn was the first to isolate an impure sample of manganese metal in 1774, which he did by reducing the dioxide with carbon. Manganese phosphating is used for rust and corrosion prevention on steel, ionized manganese is used industrially as pigments of various colors, which depend on the oxidation state of the ions. The permanganates of alkali and alkaline earth metals are powerful oxidizers, Manganese dioxide is used as the cathode material in zinc-carbon and alkaline batteries. In biology, manganese ions function as cofactors for a variety of enzymes with many functions. Manganese enzymes are essential in detoxification of superoxide free radicals in organisms that must deal with elemental oxygen. Manganese also functions in the complex of photosynthetic plants. The element is a trace mineral for all known living organisms but is a neurotoxin. In larger amounts, and apparently with far greater effectiveness through inhalation, Manganese is a silvery-gray metal that resembles iron. It is hard and very brittle, difficult to fuse, Manganese metal and its common ions are paramagnetic. Manganese tarnishes slowly in air and oxidizes like iron in water containing dissolved oxygen, naturally occurring manganese is composed of one stable isotope, 55Mn. Eighteen radioisotopes have been isolated and described, the most stable being 53Mn with a half-life of 3.7 million years, 54Mn with a half-life of 312.3 days, and 52Mn with a half-life of 5.591 days. All of the radioactive isotopes have half-lives of less than three hours, and the majority of less than one minute. Manganese also has three meta states, Manganese is part of the iron group of elements, which are thought to be synthesized in large stars shortly before the supernova explosion. 53Mn decays to 53Cr with a half-life of 3.7 million years, because of its relatively short half-life, 53Mn is relatively rare, produced by cosmic rays impact on iron. Manganese isotopic contents are combined with chromium isotopic contents and have found application in isotope geology
3.
Oxygen
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Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the group on the periodic table and is a highly reactive nonmetal. By mass, oxygen is the third-most abundant element in the universe, after hydrogen, at standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. This is an important part of the atmosphere and diatomic oxygen gas constitutes 20. 8% of the Earths atmosphere, additionally, as oxides the element makes up almost half of the Earths crust. Most of the mass of living organisms is oxygen as a component of water, conversely, oxygen is continuously replenished by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide. Oxygen is too reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone, strongly absorbs ultraviolet UVB radiation, but ozone is a pollutant near the surface where it is a by-product of smog. At low earth orbit altitudes, sufficient atomic oxygen is present to cause corrosion of spacecraft, the name oxygen was coined in 1777 by Antoine Lavoisier, whose experiments with oxygen helped to discredit the then-popular phlogiston theory of combustion and corrosion. One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle, Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries later Leonardo da Vinci built on Philos work by observing that a portion of air is consumed during combustion and respiration, Oxygen was discovered by the Polish alchemist Sendivogius, who considered it the philosophers stone. In the late 17th century, Robert Boyle proved that air is necessary for combustion, English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. From this he surmised that nitroaereus is consumed in both respiration and combustion, Mayow observed that antimony increased in weight when heated, and inferred that the nitroaereus must have combined with it. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract De respiratione. Robert Hooke, Ole Borch, Mikhail Lomonosov, and Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element. This may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, which was then the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, one part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. The fact that a substance like wood gains overall weight in burning was hidden by the buoyancy of the combustion products
4.
Chromate and dichromate
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Chromate salts contain the chromate anion, CrO2−4. Dichromate salts contain the anion, Cr 2O2−7. They are oxoanions of chromium in the oxidation state +6 and they are moderately strong oxidizing agents. In an aqueous solution, chromate and dichromate ions can be interconvertible, chromates react with hydrogen peroxide giving products in which peroxide, O2−2, replaces one or more oxygen atoms. In acid solution the unstable blue peroxo complex Chromium oxide peroxide, CrO2, is formed, addition of pyridine results in the formation of the more stable complex CrO2py. In aqueous solution, chromate and dichromate anions exist in a chemical equilibrium,2 CrO2−4 +2 H+ ⇌ Cr 2O2−7 + H2O The predominance diagram shows that the position of the equilibrium depends on both pH and the analytical concentration of chromium. The chromate ion is the predominant species in solutions. Further condensation reactions can occur in acidic solution with the formation of trichromates, Cr 3O2−10. All polyoxyanions of chromium have structures made up of tetrahedral CrO4 units sharing corners, the chromate ion is a weak acid. HCrO−4 ⇌ CrO2−4 + H+, pKa ≈5.9 The hydrogen chromate ion,2 HCrO−4 ⇌ Cr 2O2−7 + H2O This equilibrium does not involve a change in hydrogen ion concentration, so should be independent of pH. The red line on the diagram is not quite horizontal due to the simultaneous equilibrium with the chromate ion. The hydrogenchromate ion may be protonated, with the formation of molecular chromic acid, H2CrO4, reported values vary between about −0.8 to 1.6. The dichromate ion is a weaker base than the chromate ion. HCr 2O−7 ⇌ Cr 2O2−7 + H+, pK =1.8 The pK value for this shows that it can be ignored at pH >4. The chromate and dichromate ions are fairly strong oxidizing agents, commonly three electrons are added to a chromium atom, reducing it to oxidation state +3. In acid solution the aquated Cr3+ ion is produced, Cr 2O2−7 +14 H3O+ +6 e− →2 Cr3+ +7 H2O ε0 =1.33 V In alkaline solution chromium hydroxide is produced. The redox potential shows that chromates are weaker oxidizing agent in alkaline solution than in acid solution. CrO2−4 +4 H 2O +3 e− → Cr 3 +5 OH− ε0 = −0.13 V Approximately 136,000 tonnes of hexavalent chromium, chromates and dichromates are used in chrome plating to protect metals from corrosion and to improve paint adhesion
5.
Conjugate acid
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A conjugate acid, within the Brønsted–Lowry acid–base theory, is a species formed by the reception of a proton by a base—in other words, it is a base with a hydrogen ion added to it. On the other hand, a base is merely what is left after an acid has donated a proton in a chemical reaction. Hence, a base is a species formed by the removal of a proton from an acid. A proton is a particle with a unit positive electrical charge, it is represented by the symbol H+ because it constitutes the nucleus of a hydrogen atom, that is. A cation can be an acid, and an anion can be a conjugate base, depending on which substance is involved. In an acid-base reaction, an acid plus a base reacts to form a conjugate base plus a conjugate acid, refer to the following figure, We say that the water molecule is the conjugate acid of the hydroxide ion after the latter received the hydrogen proton donated by ammonium. On the other hand, ammonia is the base for the acid ammonium after ammonium has donated a hydrogen ion towards the production of the water molecule. The strength of an acid is directly proportional to its dissociation constant. If a conjugate acid is strong, its dissociation will have an equilibrium constant. The strength of a base can be seen as the tendency of the species to pull hydrogen protons towards itself. If a conjugate base is classified as strong, it will hold on to the hydrogen proton when in solution, if a chemical species is classified as a weak acid, its conjugate base will be strong in nature. This can be observed in ammonias reaction with water, the reaction proceeds until most of the ammonia has been transformed to ammonium. This shift to the right in the equilibrium of the reaction means that ammonium does not dissociate easily in water. On the other hand, if a species is classified as a strong acid, an example of this case would be the dissociation of Hydrochloric acid HCl in water. Since HCl is an acid, its conjugate base will be a weak conjugate base. Therefore, in system, most H+ will be in the form of a Hydronium ion H 3O+ instead of attached to a Cl anion. To summarize, the stronger the acid or base, the weaker the conjugate, the acid and conjugate base as well as the base and conjugate acid are known as conjugate pairs. When finding a conjugate acid or base, it is important to look at the reactants of the chemical equation, to identify the conjugate acid, look for the pair of compounds that are related
6.
Charge-transfer complex
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The resulting electrostatic attraction provides a stabilizing force for the molecular complex. The source molecule from which the charge is transferred is called the electron donor, the nature of the attraction in a charge-transfer complex is not a stable chemical bond, and is thus much weaker than covalent forces. Many such complexes can undergo a transition into an excited electronic state. The excitation energy of this occurs very frequently in the visible region of the electromagnetic spectrum. These optical absorption bands are referred to as charge-transfer bands. Optical spectroscopy is a technique to characterize charge-transfer bands. Charge-transfer complexes exist in many types of molecules, inorganic as well as organic, and in solids, liquids, a well-known example is the complex formed by iodine when combined with starch, which exhibits an intense blue charge-transfer band. In inorganic chemistry, most charge-transfer complexes involve electron transfer between metal atoms and ligands, if the transfer occurs from the MO with ligand-like character to the metal-like one, the complex is called a ligand-to-metal charge-transfer complex. If the electronic charge shifts from the MO with metal-like character to the ligand-like one, thus, a MLCT results in oxidation of the metal center, whereas a LMCT results in the reduction of the metal center. Resonance Raman spectroscopy is also a technique to assign and characterize charge-transfer bands in these complexes. Charge-transfer complexes are formed by association of molecules or molecular subgroups, one acting as an electron donor. The association does not constitute a strong covalent bond and is subject to significant temperature, concentration, the formation of the dative state is an electronic transition giving rise to the colorful absorption bands. The intensity of charge-transfer bands in the spectrum is strongly dependent upon the degree of this association reaction. The methods were first described for the association of iodine dissolved in aromatic hydrocarbons, the procedure is called the Benesi-Hildebrand method, named after the authors of the study. The absorption wavelength of charge-transfer bands, i. e. the charge-transfer transition energy, is characteristic of the type of donor and acceptor entities. The electron donating power of a molecule is measured by its ionization potential. The electron accepting power of the electron acceptor is determined by its electron affinity, the overall energy balance is the energy gained in a spontaneous charge transfer. Charge-transfer complexes are identified by Color, The color of CT complexes is reflective of the energy balance resulting from the transfer of electronic charge from donor to acceptor
7.
Manganese(II) chloride
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Manganese chloride describes a series of compounds with the formula MnCl2x, where the value of x can be 0,2, or 4. The tetrahydrate is the most common form of chloride and is the tetrahydrate with the formula MnCl2·4H2O. The anhydrous form and a dihydrate MnCl2·2H2O are also known, like many Mn species, these salts are pink, with the paleness of the color being characteristic of transition metal complexes with high spin d5 configurations. Manganese chloride is produced by treating manganese oxide with concentrated hydrochloric acid, mnO2 +4 HCl → MnCl2 +2 H2O + Cl2 This reaction was once used for the manufacture of chlorine. By carefully neutralizing the solution with MnCO3, one can selectively precipitate iron salts. The tetrahydrate consists of octahedral cis-Mn4Cl2 molecules, the trans isomer, which is metastable, is also known. The dihydrate MnCl22 is a coordination polymer, each Mn center is coordinated to four doubly bridging chloride ligands. The octahedron is completed by a pair of mutually trans aquo ligands, the hydrates dissolve in water to give mildly acidic solutions with a pH of around 4. These solutions consist of the aquo complex 2+. It is a weak Lewis acid, reacting with chloride ions to produce a series of solids containing the following ions −, 2−, both − and 2− are polymeric. Upon treatment with typical organic ligands, manganese undergoes oxidation by air to give Mn complexes, triphenylphosphine forms a labile 2,1 adduct, MnCl2 +2 Ph3P → Anhydrous manganese chloride serves as a starting point for the synthesis of a variety of manganese compounds. For example, manganocene is prepared by reaction of MnCl2 with a solution of sodium cyclopentadienide in THF, MnCl2 +2 NaC5H5 → Mn2 +2 NaCl Aqueous solutions of manganese chloride are used in 31P-NMR to determine the size and lamellarity of phospholipid vesicles. Only phospholipids located in the outermost monolayer exposed to Mn2+ experience this broadening, the effect is negligle for multilamellar vesicles, but for large unilamellar vesicles, a ~50% reduction in signal intensity is observed. Manganese chloride is used in the production of dry cell batteries. It is the precursor to the antiknock compound methylcyclopentadienyl manganese tricarbonyl, manganism, or manganese poisoning, can be caused by long-term exposure to manganese dust or fumes. National Pollutant Inventory, Manganese and compounds Fact Sheet
8.
Acid
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An acid is a molecule or ion capable of donating a hydron, or, alternatively, capable of forming a covalent bond with an electron pair. The first category of acids is the donors or Brønsted acids. In the special case of solutions, proton donors form the hydronium ion H3O+ and are known as Arrhenius acids. Brønsted and Lowry generalized the Arrhenius theory to include non-aqueous solvents, acids form aqueous solutions with a sour taste, can turn blue litmus red, and react with bases and certain metals to form salts. The word acid is derived from the Latin acidus/acēre meaning sour, an aqueous solution of an acid has a pH less than 7 and is colloquially also referred to as acid, while the strict definition refers only to the solute. A lower pH means a higher acidity, and thus a higher concentration of hydrogen ions in the solution. Chemicals or substances having the property of an acid are said to be acidic, common aqueous acids include hydrochloric acid, acetic acid, sulfuric acid, and citric acid. As these examples show, acids can be solutions or pure substances, strong acids and some concentrated weak acids are corrosive, but there are exceptions such as carboranes and boric acid. The second category of acids are Lewis acids, which form a covalent bond with an electron pair, however, hydrogen chloride, acetic acid, and most other Brønsted-Lowry acids cannot form a covalent bond with an electron pair and are therefore not Lewis acids. Conversely, many Lewis acids are not Arrhenius or Brønsted-Lowry acids, in modern terminology, an acid is implicitly a Brønsted acid and not a Lewis acid, since chemists almost always refer to a Lewis acid explicitly as a Lewis acid. Modern definitions are concerned with the chemical reactions common to all acids. Most acids encountered in life are aqueous solutions, or can be dissolved in water, so the Arrhenius. The Brønsted-Lowry definition is the most widely used definition, unless otherwise specified, hydronium ions are acids according to all three definitions. Interestingly, although alcohols and amines can be Brønsted-Lowry acids, they can function as Lewis bases due to the lone pairs of electrons on their oxygen and nitrogen atoms. The Swedish chemist Svante Arrhenius attributed the properties of acidity to hydrogen ions or protons in 1884, an Arrhenius acid is a substance that, when added to water, increases the concentration of H+ ions in the water. Thus, an Arrhenius acid can also be described as a substance that increases the concentration of ions when added to water. Examples include molecular substances such as HCl and acetic acid, an Arrhenius base, on the other hand, is a substance which increases the concentration of hydroxide ions when dissolved in water. Thus, an Arrhenius acid could also be said to be one that decreases hydroxide concentration, in an acidic solution, the concentration of hydronium ions is greater than 10−7 moles per liter
9.
Salt (chemistry)
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In chemistry, a salt is an ionic compound that results from the neutralization reaction of an acid and a base. Salts are composed of related numbers of cations and anions so that the product is electrically neutral and these component ions can be inorganic, such as chloride, or organic, such as acetate, and can be monatomic, such as fluoride, or polyatomic, such as sulfate. There are several varieties of salts, salts that hydrolyze to produce hydroxide ions when dissolved in water are alkali salts, whilst those that hydrolyze to produce hydronium ions in water are acidic salts. Neutral salts are those salts that are neither acidic nor basic, zwitterions contain an anionic centre and a cationic centre in the same molecule, but are not considered to be salts. Examples of zwitterions include amino acids, many metabolites, peptides, usually, non-dissolved salts at standard conditions for temperature and pressure are solid, but there are exceptions. Molten salts and solutions containing dissolved salts are called electrolytes, as they are able to conduct electricity. As observed in the cytoplasm of cells, in blood, urine, plant saps and mineral waters, therefore, their salt content is given for the respective ions. Salts can appear to be clear and transparent, opaque, and even metallic, in many cases, the apparent opacity or transparency are only related to the difference in size of the individual monocrystals. Since light reflects from the boundaries, larger crystals tend to be transparent. The color of the salt is due to the electronic structure in the d-orbitals of transition elements or in the conjugated organic dye framework. Different salts can elicit all five basic tastes, e. g. salty, sweet, sour, bitter, and umami or savory. Salts of strong acids and strong bases are non-volatile and odorless and that slow, partial decomposition is usually accelerated by the presence of water, since hydrolysis is the other half of the reversible reaction equation of formation of weak salts. Many ionic compounds can be dissolved in water or other similar solvents, the exact combination of ions involved makes each compound have a unique solubility in any solvent. The solubility is dependent on how well each ion interacts with the solvent, for example, all salts of sodium, potassium and ammonium are soluble in water, as are all nitrates and many sulfates – barium sulfate, calcium sulfate and lead sulfate are examples of exceptions. However, ions that bind tightly to each other and form highly stable lattices are less soluble, for example, most carbonate salts are not soluble in water, such as lead carbonate and barium carbonate. Some soluble carbonate salts are, sodium carbonate, potassium carbonate, solid salts do not conduct electricity. Moreover, solutions of salts also conduct electricity, the name of a salt starts with the name of the cation followed by the name of the anion. Salts are often referred to only by the name of the cation or by the name of the anion. g
10.
Lead dioxide
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Lead oxide, commonly called lead dioxide or plumbic oxide or anhydrous plumbic acid is a chemical compound with the formula PbO2. It is an oxide where lead is in an oxidation state +4 and it is of an intermediate bond type, displaying both ionic and covalent properties. It is an odorless dark-brown crystalline powder which is insoluble in water. It exists in two crystalline forms, the alpha phase has orthorhombic symmetry, it has been first synthesized in 1941 and identified in nature as a rare mineral scrutinyite in 1988. The more common, tetragonal beta phase was first identified as the mineral plattnerite around 1845, Lead dioxide is a strong oxidizing agent which is used in the manufacture of matches, pyrotechnics, dyes and other chemicals. It also has important applications in electrochemistry, in particular in the positive plates of lead acid batteries. Lead dioxide is an odorless dark-brown crystalline powder which is insoluble in water. It has two polymorphs, alpha and beta, which occur naturally as rare minerals scrutinyite and plattnerite. Whereas the beta form had been identified in 1845, α-PbO2 was first identified in 1946 and found as a naturally occurring mineral 1988. The alpha form has symmetry, space group Pbcn, Pearson symbol oP12, lattice constants a =0.497 nm, b =0.596 nm, c =0.544 nm. The lead atoms are 6 coordinate and this contrasts with the α- form where the octahedra are linked by adjacent edges to give zig-zag chains. In addition, Pb2O3 can be obtained by decomposing PbO2 at 580–620 °C under oxygen pressure of 1.4 kbar, therefore, thermal decomposition of lead dioxide is a common industrial way of producing various lead oxides. Lead dioxide is a compound with prevalent acidic properties. Deficiency of oxygen results in the characteristic metallic conductivity of lead dioxide, with a resistivity as low as 10−4 Ohm·cm, like metals, lead dioxide has a characteristic electrode potential, and in electrolytes it can be polarized both anodically and cathodically. Lead dioxide electrodes have an action, that is both the lead and oxygen ions take part in the electrochemical reactions. By this way lead 2+ is oxidized to lead 4+ and Cl2 vapors rise from the hypochlorite solution, by the decomposition of NaClO to NaOH, stoichiometric amounts of PbO2 react with NaOH to form the di-sodium hexa hydroxo plumbate ion, soluble in water. An alternative synthesis method is electrochemical, lead dioxide forms on pure lead, in dilute sulfuric acid and this procedure is used for large-scale industrial production of PbO2 anodes. Lead and copper electrodes are immersed in sulfuric acid flowing at a rate of 5–10 L/min, the electrodeposition is carried out galvanostatically, by applying a current of about 100 A/m2 for about 30 minutes
