Lewis acids and bases
A Lewis acid is a chemical species that contains an empty orbital, capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base is any species that has a filled orbital containing an electron pair, not involved in bonding but may form a dative bond with a Lewis acid to form a Lewis adduct. For example, NH3 is a Lewis base. Trimethylborane is a Lewis acid. In a Lewis adduct, the Lewis acid and base share an electron pair furnished by the Lewis base, forming a dative bond. In the context of a specific chemical reaction between NH3 and Me3B, the lone pair from NH3 will form a dative bond with the empty orbital of Me3B to form an adduct NH3•BMe3; the terminology refers to the contributions of Gilbert N. Lewis; the terms nucleophile and electrophile are more or less interchangeable with Lewis base and Lewis acid, respectively. However, these terms their abstract noun forms nucleophilicity and electrophilicity, emphasize the kinetic aspect of reactivity, while the Lewis basicity and Lewis acidity emphasize the thermodynamic aspect of Lewis adduct formation.
In many cases, the interaction between the Lewis base and Lewis acid in a complex is indicated by an arrow indicating the Lewis base donating electrons toward the Lewis acid using the notation of a dative bond—for example, Me3B←NH3. Some sources indicate the Lewis base with a pair of dots, which allows consistent representation of the transition from the base itself to the complex with the acid: Me3B +:NH3 → Me3B:NH3A center dot may be used to represent a Lewis adduct, such as Me3B•NH3. Another example is boron trifluoride diethyl etherate, BF3•Et2O. Although there have been attempts to use computational and experimental energetic criteria to distinguish dative bonding from non-dative covalent bonds, for the most part, the distinction makes note of the source of the electron pair, dative bonds, once formed, behave as other covalent bonds do, though they have considerable polar character. Moreover, in some cases, the use of the dative bond arrow is just a notational convenience for avoiding the drawing of formal charges.
In general, the donor–acceptor bond is viewed as somewhere along a continuum between idealized covalent bonding and ionic bonding. Classically, the term "Lewis acid" is restricted to trigonal planar species with an empty p orbital, such as BR3 where R can be an organic substituent or a halide. For the purposes of discussion complex compounds such as Et3Al2Cl3 and AlCl3 are treated as trigonal planar Lewis acids. Metal ions such as Na+, Mg2+, Ce3+, which are invariably complexed with additional ligands, are sources of coordinatively unsaturated derivatives that form Lewis adducts upon reaction with a Lewis base. Other reactions might be referred to as "acid-catalyzed" reactions; some compounds, such as H2O, are both Lewis acids and Lewis bases, because they can either accept a pair of electrons or donate a pair of electrons, depending upon the reaction. Lewis acids are diverse. Simplest are those, but more common are those. Examples of Lewis acids based on the general definition of electron pair acceptor include: the proton and acidic compounds onium ions, such as NH4+ and H3O+ high oxidation state transition metal cations, e.g. Fe3+.
Again, the description of a Lewis acid is used loosely. For example, in solution, bare protons do not exist; some of the most studied examples of such Lewis acids are the boron trihalides and organoboranes, but other compounds exhibit this behavior: BF3 + F− → BF4−In this adduct, all four fluoride centres are equivalent. BF3 + OMe2 → BF3OMe2Both BF4− and BF3OMe2 are Lewis base adducts of boron trifluoride. In many cases, the adducts violate the octet rule, such as the triiodide anion: I2 + I− → I3−The variability of the colors of iodine solutions reflects the variable abilities of the solvent to form adducts with the Lewis acid I2. In some cases, the Lewis acid is capable of binding two Lewis base, a famous example being the formation of hexafluorosilicate: SiF4 + 2 F− → SiF62− Most compounds considered to be Lewis acids require an activation step prior to formation of the adduct with the Lewis base. Well known cases are the aluminium trihalides, which are viewed as Lewis acids. Aluminium trihalides, unlike the boron trihalides, do not exist in the form AlX3, but as aggregates and polymers that must be degraded by the Lewis base.
A simpler case is the formation of adducts of borane. Monomeric BH3 does not exist appreciably, so the adducts of borane are generated by degradation of diborane: B2H6 + 2 H− → 2 BH4−In this case, an intermediate B2H7− can be isolated. Many metal complexes serve as Lewis acids, but only after dissociating a more weakly bound Lewis base water. 2+ + 6 NH3 → 2+ + 6 H2O The proton is one of the strongest but is one of the most complicated Lewis acids. It is convention to ignore the fact that a proton is solvated (bound to solvent
Solid acids are acids that do not dissolve in the reaction medium. They are used in heterogeneous catalysts. Most of the acids solid in state are organic acids that includes oxalic acid,tartaric acid,citric acid,maleic acid,etc Examples include oxides, which function as Lewis acids including silico-aluminates, sulfated zirconia. Many transition metal oxides are acidic, including titania and niobia; such acids are used in cracking. Many solid Brønsted acids are employed industrially, including sulfonated polystyrene, solid phosphoric acid, niobic acid, heteropolyoxometallates. Solid acids are used in catalysis in many industrial chemical processes, from large-scale catalytic cracking in petroleum refining to the synthesis of various fine chemicals. One large scale application is alkylation, e.g. the combination of benzene and ethylene to give ethylbenzene. Another application is the rearrangement of cyclohexanone oxime to caprolactam. Many alkylamines are prepared by amination of alcohols, catalyzed by solid acids.
Solid acids can be used as electrolytes in fuel cells
Blood plasma is a yellowish liquid component of blood that holds the blood cells in whole blood in suspension. In other words, it is the liquid part of the blood that carries cells and proteins throughout the body, it makes up about 55% of the body's total blood volume. It is the intravascular fluid part of extracellular fluid, it is water, contains dissolved proteins, clotting factors, hormones, carbon dioxide and oxygen. It plays a vital role in an intravascular osmotic effect that keeps electrolyte concentration balanced and protects the body from infection and other blood disorders. Blood plasma is separated from the blood by spinning a tube of fresh blood containing an anticoagulant in a centrifuge until the blood cells fall to the bottom of the tube; the blood plasma is poured or drawn off. Blood plasma has a density of 1025 kg/m3, or 1.025 g/ml. Blood serum is blood plasma without clotting factors. Plasmapheresis is a medical therapy that involves blood plasma extraction and reintegration.
Fresh frozen plasma is on the WHO Model List of Essential Medicines, the most important medications needed in a basic health system. It is of critical importance in the treatment of many types of trauma which result in blood loss, is therefore kept stocked universally in all medical facilities capable of treating trauma or that pose a risk of patient blood loss such as surgical suite facilities. Blood plasma volume may be expanded by or drained to extravascular fluid when there are changes in Starling forces across capillary walls. For example, when blood pressure drops in circulatory shock, Starling forces drive fluid into the interstitium, causing third spacing. Standing still for a prolonged period will cause an increase in transcapillary hydrostatic pressure; as a result 12% of blood plasma volume will cross into the extravascular compartment. This causes an increase in hematocrit, serum total protein, blood viscosity and, as a result of increased concentration of coagulation factors, it causes orthostatic hypercoagulability.
Plasma was well-known when described by William Harvey in de Mortu Cordis in 1628, but knowledge of it extends as far back as Vesalius.. The discovery of fibrinogen by William Henson in ca 1770 made it easier to study plasma, as ordinarily, upon coming in contact with a foreign surface – something other than vascular endothelium – clotting factors become activated and clotting proceeds trapping RBCs etc in the plasma and preventing separation of plasma from the blood. Adding citrate and other anticoagulants is a recent advance. Note that, upon formation of a clot, the remaining clear fluid is Serum, plasma without the clotting factors; the use of blood plasma as a substitute for whole blood and for transfusion purposes was proposed in March 1918, in the correspondence columns of the British Medical Journal, by Gordon R. Ward. "Dried plasmas" in powder or strips of material format were developed and first used in World War II. Prior to the United States' involvement in the war, liquid plasma and whole blood were used.
The "Blood for Britain" program during the early 1940s was quite successful based on Charles Drew's contribution. A large project began in August 1940 to collect blood in New York City hospitals for the export of plasma to Britain. Drew was appointed medical supervisor of the "Plasma for Britain" project, his notable contribution at this time was to transform the test tube methods of many blood researchers into the first successful mass production techniques. The decision was made to develop a dried plasma package for the armed forces as it would reduce breakage and make the transportation and storage much simpler; the resulting dried. One bottle contained enough distilled water to reconstitute the dried plasma contained within the other bottle. In about three minutes, the plasma could stay fresh for around four hours; the Blood for Britain program operated for five months, with total collections of 15,000 people donating blood, with over 5,500 vials of blood plasma. Following the "Plasma for Britain" invention, Drew was named director of the Red Cross blood bank and assistant director of the National Research Council, in charge of blood collection for the United States Army and Navy.
Drew argued against the armed forces directive that blood/plasma was to be separated by the race of the donor. Drew insisted that there was no racial difference in human blood and that the policy would lead to needless deaths as soldiers and sailors were required to wait for "same race" blood. By the end of the war the American Red Cross had provided enough blood for over six million plasma packages. Most of the surplus plasma was returned to the United States for civilian use. Serum albumin replaced dried plasma for combat use during the Korean War. Plasma as a blood product prepared from blood donations is used in blood transfusions as fresh frozen plasma or plasma Frozen Within 24 Hours After Phlebotomy; when donating whole blood or packed red blood cell transfusions, O- is the most desirable and is considered a "universal donor," since it has neither A nor B antigens and can be safely transfused to most recipients. Type AB+ is the "universal recipient" type for PRBC donations. However, for plasma the situation is somewhat reverse
Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In the context of food production, it may more broadly refer to any process in which the activity of microorganisms brings about a desirable change to a foodstuff or beverage; the science of fermentation is known as zymology. In microorganisms, fermentation is the primary means of producing ATP by the degradation of organic nutrients anaerobically. Humans have used fermentation to produce beverages since the Neolithic age. For example, fermentation is used for preservation in a process that produces lactic acid found in such sour foods as pickled cucumbers and yogurt, as well as for producing alcoholic beverages such as wine and beer. Fermentation occurs within the gastrointestinal tracts including humans. Below are some definitions of fermentation, they range from general usages to more scientific definitions.
Preservation methods for food via microorganisms. Any process that produces alcoholic beverages or acidic dairy products. Any large-scale microbial process occurring with or without air. Any energy-releasing metabolic process that takes place only under anaerobic conditions. Any metabolic process that releases energy from a sugar or other organic molecule, does not require oxygen or an electron transport system, uses an organic molecule as the final electron acceptor. Along with photosynthesis and aerobic respiration, fermentation is a way of extracting energy from molecules, but it is the only one common to all bacteria and eukaryotes, it is therefore considered the oldest metabolic pathway, suitable for an environment that does not yet have oxygen. Yeast, a form of fungus, occurs in any environment capable of supporting microbes, from the skins of fruits to the guts of insects and mammals and the deep ocean, they harvest sugar-rich materials to produce ethanol and carbon dioxide; the basic mechanism for fermentation remains present in all cells of higher organisms.
Mammalian muscle carries out the fermentation that occurs during periods of intense exercise where oxygen supply becomes limited, resulting in the creation of lactic acid. In invertebrates, fermentation produces succinate and alanine. Fermentative bacteria play an essential role in the production of methane in habitats ranging from the rumens of cattle to sewage digesters and freshwater sediments, they produce hydrogen, carbon dioxide and acetate and carboxylic acids. Acetogenic bacteria oxidize the acids, obtaining more acetate and either formate. Methanogens convert acetate to methane. Fermentation reacts NADH with an organic electron acceptor; this is pyruvate formed from sugar through glycolysis. The reaction produces NAD+ and an organic product, typical examples being ethanol, lactic acid, carbon dioxide, hydrogen gas. However, more exotic compounds can be produced by fermentation, such as butyric acetone. Fermentation products contain chemical energy, but are considered waste products, since they cannot be metabolized further without the use of oxygen.
Fermentation occurs in an anaerobic environment. In the presence of O2, NADH, pyruvate are used to generate ATP in respiration; this is called oxidative phosphorylation, it generates much more ATP than glycolysis alone. For that reason, fermentation is utilized when oxygen is available; however in the presence of abundant oxygen, some strains of yeast such as Saccharomyces cerevisiae prefer fermentation to aerobic respiration as long as there is an adequate supply of sugars. Some fermentation processes involve obligate anaerobes. Although yeast carries out the fermentation in the production of ethanol in beers and other alcoholic drinks, this is not the only possible agent: bacteria carry out the fermentation in the production of xanthan gum. In ethanol fermentation, one glucose molecule is converted into two ethanol molecules and two carbon dioxide molecules, it is used to make bread dough rise: the carbon dioxide forms bubbles, expanding the dough into a foam. The ethanol is the intoxicating agent in alcoholic beverages such as wine and liquor.
Fermentation of feedstocks, including sugarcane and sugar beets, produces ethanol, added to gasoline. In some species of fish, including goldfish and carp, it provides energy; the figure illustrates the process. Before fermentation, a glucose molecule breaks down into two pyruvate molecules; the energy from this exothermic reaction is used to bind inorganic phosphates to ATP and convert NAD+ to NADH. The pyruvates break down into two acetaldehyde molecules and give off two carbon dioxide molecules as a waste product; the acetaldehyde is reduced into ethanol using the energy and hydrogen from NADH, the NADH is oxidized into NAD+ so that the cycle may repeat. The reaction is catalysed by the enzymes pyruvate alcohol dehydrogenase. Homolactic fermentation is the simplest type of fermentation; the pyruvate from glycolysis undergoes a simple redox reaction. It is unique because it is one of the only respiration processes to not produce a gas as a byproduct. Overall, one molecule of glucose is converted to two molecules of lactic ac
Citric acid is a weak organic acid that has the chemical formula C6H8O7. It occurs in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in the metabolism of all aerobic organisms. More than a million tons of citric acid are manufactured every year, it is used as an acidifier, as a flavoring and chelating agent. A citrate is a derivative of citric acid. An example of the former, a salt is trisodium citrate; when part of a salt, the formula of the citrate ion is written as C6H5O3−7 or C3H5O3−3. Citric acid exists in greater than trace amounts in a variety of fruits and vegetables, most notably citrus fruits. Lemons and limes have high concentrations of the acid; the concentrations of citric acid in citrus fruits range from 0.005 mol/L for oranges and grapefruits to 0.30 mol/L in lemons and limes. Within species, these values vary depending on the cultivar and the circumstances in which the fruit was grown. Industrial-scale citric acid production first began in 1890 based on the Italian citrus fruit industry, where the juice was treated with hydrated lime to precipitate calcium citrate, isolated and converted back to the acid using diluted sulfuric acid.
In 1893, C. Wehmer discovered. However, microbial production of citric acid did not become industrially important until World War I disrupted Italian citrus exports. In 1917, American food chemist James Currie discovered certain strains of the mold Aspergillus niger could be efficient citric acid producers, the pharmaceutical company Pfizer began industrial-level production using this technique two years followed by Citrique Belge in 1929. In this production technique, still the major industrial route to citric acid used today, cultures of A. niger are fed on a sucrose or glucose-containing medium to produce citric acid. The source of sugar is corn steep liquor, hydrolyzed corn starch or other inexpensive sugary solutions. After the mold is filtered out of the resulting solution, citric acid is isolated by precipitating it with calcium hydroxide to yield calcium citrate salt, from which citric acid is regenerated by treatment with sulfuric acid, as in the direct extraction from citrus fruit juice.
In 1977, a patent was granted to Lever Brothers for the chemical synthesis of citric acid starting either from aconitic or isocitrate/alloisocitrate calcium salts under high pressure conditions. This produced citric acid in near quantitative conversion under what appeared to be a reverse non-enzymatic Krebs cycle reaction. In 2007, worldwide annual production stood at 1,600,000 tons. More than 50% of this volume was produced in China. More than 50% was used as an acidity regulator in beverages, some 20% in other food applications, 20% for detergent applications and 10% for related applications other than food, such as cosmetics, pharmaceutics and in the chemical industry. Citric acid was first isolated in 1784 by the chemist Carl Wilhelm Scheele, who crystallized it from lemon juice, it can exist either as a monohydrate. The anhydrous form crystallizes from hot water, while the monohydrate forms when citric acid is crystallized from cold water; the monohydrate can be converted to the anhydrous form at about 78 °C.
Citric acid dissolves in absolute ethanol at 15 °C. It decomposes with loss of carbon dioxide above about 175 °C. Citric acid is considered to be a tribasic acid, with pKa values, extrapolated to zero ionic strength, of 5.21, 4.28 and 2.92 at 25 °C. The pKa of the hydroxyl group has been found, by means of 13C NMR spectroscopy, to be 14.4. The speciation diagram shows that solutions of citric acid are buffer solutions between about pH 2 and pH 8. In biological systems around pH 7, the two species present are the citrate ion and mono-hydrogen citrate ion; the SSC 20X hybridization buffer is an example in common use. Tables compiled for biochemical studies are available. On the other hand, the pH of a 1 mM solution of citric acid will be about 3.2. The pH of fruit juices from citrus fruits like oranges and lemons depends on the citric acid concentration, being lower for higher acid concentration and conversely. Acid salts of citric acid can be prepared by careful adjustment of the pH before crystallizing the compound.
See, for example, sodium citrate. The citrate ion forms complexes with metallic cations; the stability constants for the formation of these complexes are quite large because of the chelate effect. It forms complexes with alkali metal cations. However, when a chelate complex is formed using all three carboxylate groups, the chelate rings have 7 and 8 members, which are less stable thermodynamically than smaller chelate rings. In consequence, the hydroxyl group can be deprotonated, forming part of a more stable 5-membered ring, as in ammonium ferric citrate, 5Fe2·2H2O. Citric acid can be esterified at one or more of the carboxylic acid functional groups on the molecule, to form any of a variety of mono-, di-, tri-, mixed esters. Citrate is an intermediate in the TCA cycle, a central metabolic pathway for animals and bacteria. Citrate synthase catalyzes the condensation of oxaloacetate with acetyl CoA to form citrate. Citrate acts as the substrate for aconitase and is converted into aconitic acid.
The cycle ends with regeneration of oxaloacetate. This series
Chiral Lewis acid
Chiral Lewis acids are a type of Lewis acid catalyst that effects the chirality of the substrate as it reacts with it. In such reactions the synthesis favors the formation of a specific diastereomer; the method is an enantioselective asymmetric synthesis reaction. Since they affect chirality, they produce optically active products from optically inactive or mixed starting materials; this type of preferential formation of one enantiomer or diastereomer over the other is formally known as an asymmetric induction. In this kind of Lewis acid; the electron-accepting atom is a metal, such as indium, lithium, titanium, or boron. The chiral-altering ligands employed for synthesizing these acids most have multiple Lewis basic sites that allow the formation of a ring structure involving the metal atom. Achiral Lewis acids have been used for decades to promote the synthesis of racemic mixtures in a myriad different reactions. Starting in the 1960s chemists have use the chiral acids to induce the enantioselective reactions.
Common reaction types include Diels-Alder reactions, the ene reaction, cycloaddition reactions, hydrocyanation of aldehydes, most notably, Sharpless expoxidations. The enantioselectivity of CLAs derives from their ability to perturb the free energy barrier along the reaction coordinate pathway that leads to either the R- or S- enantiomer. Ground state diastereomers and enantiomers are of equal energy in the ground state, when reacted with an achiral lewis acid, their diastereomeric intermediates, transition states, products are of equal energy; this leads to the production of racemic mixtures of products. However, when a CLA is used in the same reaction, the energetic barrier of formation of one diastereomer is less than that of another – the reaction is under kinetic control. If the difference in the energy barriers between the diastereomeric transition states are of sufficient magnitude a high enantiomeric excess of one isomer should be observed. Diels-Alder reactions occur between an alkene.
This cycloaddition process allows for the stereoselective formation of cyclohexene rings capable of possessing as many as four contiguous stereogenic centers. Diels-Alder reactions can lead to formation of a variety of structural stereoisomers; the molecular orbital theory considers that endo transition state, instead of the exo transition state, is favored. Augmented secondary orbital interactions have been postulated as the source of enhanced endo diastereoselection; the addition of a CLA selectively activates one component of the reaction while providing a stereodefined environment that permits unique enantioselectivity. Koga and coworkers disclosed the first practical example of a catalytic enantioselective Diels-Alder reaction promoted by a CLA - menthoxyaluminum dichloride - derived from menthol and ethylaluminum dichloride. A decade Elias James Corey introduced an effective aluminum-diamine controller for Diels-Alder reaction. Formation of the active catalyst is achieved by treatment of the bis with trimethylaluminum.
The proposed tetracoordinate aluminum prevent the imide acting as a chelating Lewis base, while enhance the α-vinyl proton of the dienphile and the benzylic proton of the catalyst. The X-ray structure of the catalyst showed a stereodefined environment. In 1993, Wulff and coworkers found a complex derived from diethylaluminium chloride and a “vaulted” biaryl ligand below catalyzed the enantioselective Diels-Alder reaction between cyclopentadiene and methacrolein; the chiral ligand is recovered quantitatively by silica gel chromatography. Hisashi Yamamoto and coworkers have developed a practical Diels-Alder catalyst for aldehyde dienophiles; the chiral borane complex is effective in catalyzing a number of aldehyde Diels-Alder reactions. NMR spectroscopic experiments indicated close proximity of the aryl ring. Pi stacking between the aryl group and aldehyde was suggested as an organizational feature which imparted high enantioselectivity to the cycloaddition. Yamamoto and co-wokers have introduced a conceptually interesting series of catalysts that incorporate an acidic proton into the active catalyst.
This kind of what so called Bronsted acid-assisted chiral Lewis acid catalyzes a number of diene-aldehyde cycloaddition reactions. In the aldol reaction, the diastereoselectivity of the product is dictated by the geometry of the enolate according to the Zimmerman-Traxler model; the model predicts that the Z enolate will give syn products and that E enolates will give anti products. Chiral Lewis acids allow products that defy the Zimmerman-Traxler model, allows for control of absolute stereochemistry. Kobayashi and Horibe demonstrated this in the synthesis of dihydroxythioester derivatives, using a tin-based chiral Lewis acid; the transition structures for reactions with both the R and S catalyst enantiomers are shown below. The Baylis-Hillman reaction is a route for C-C bond formation between an alpha, beta-unsaturated carbonyl and an aldehyde, which requires a nucleophilic catalyst a tertiary amine, for a Michael-type addition and elimination; the stereoselectivity of these reactions is poor.
Chen et al. demonstrated an enantioselective chiral Lewis acid-catalyzed reaction. Lanthanum was used in this case. A chiral amine may be used to achieve stereoselectivity; the product obtained by the reaction using the chiral catalyst was obtained in good yield with excellent enantioselectivity. Chiral lewis acids have proven useful in the ene reaction. When
In chemistry, pH is a scale used to specify how acidic or basic a water-based solution is. Acidic solutions have a lower pH, while basic solutions have a higher pH. At room temperature, pure water is neither acidic nor basic and has a pH of 7; the pH scale is logarithmic and approximates the negative of the base 10 logarithm of the molar concentration of hydrogen ions in a solution. More it is the negative of the base 10 logarithm of the activity of the hydrogen ion. At 25 °C, solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic; the neutral value of the pH depends on the temperature, being lower than 7 if the temperature increases. Contrary to popular belief, the pH value can be less than 0 or greater than 14 for strong acids and bases respectively; the pH scale is traceable to a set of standard solutions whose pH is established by international agreement. Primary pH standard values are determined using a concentration cell with transference, by measuring the potential difference between a hydrogen electrode and a standard electrode such as the silver chloride electrode.
The pH of aqueous solutions can be measured with a glass electrode and a pH meter, or a color-changing indicator. Measurements of pH are important in chemistry, medicine, water treatment, many other applications; the concept of pH was first introduced by the Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg Laboratory in 1909 and revised to the modern pH in 1924 to accommodate definitions and measurements in terms of electrochemical cells. In the first papers, the notation had the "H" as a subscript to the lowercase "p", as so: pH; the exact meaning of the "p" in "pH" is disputed, but according to the Carlsberg Foundation, pH stands for "power of hydrogen". It has 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, or potential hydrogen, it is suggested that Sørensen used the letters "p" and "q" to label the test solution and the reference solution.
In chemistry, the p stands for "decimal cologarithm of", is used in the term pKa, used for acid dissociation constants. Bacteriologist Alice C. Evans, famed for her work's influence on dairying and food safety, credited William Mansfield Clark and colleagues with developing pH measuring methods in the 1910s, which had a wide influence on laboratory and industrial use thereafter. In her memoir, she does not mention how much, or how little and colleagues knew about Sørensen's work a few years prior, she said: In these studies Dr. Clark's attention was directed to the effect of acid on the growth of bacteria, he found that it is the intensity of the acid in terms of hydrogen-ion concentration that affects their growth. But existing methods of measuring acidity determined not the intensity, of the acid. Next, with his collaborators, Dr. Clark developed accurate methods for measuring hydrogen-ion concentration; these methods replaced the inaccurate titration method of determining acid content in use in biologic laboratories throughout the world.
They were found to be applicable in many industrial and other processes in which they came into wide usage. The first electronic method for measuring pH was invented by Arnold Orville Beckman, a professor at California Institute of Technology in 1934, it was in response to local citrus grower Sunkist that wanted a better method for testing the pH of lemons they were picking from their nearby orchards. PH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, aH+, in a solution. PH = − log 10 = log 10 For example, for a solution with a hydrogen ion activity of 5×10−6 we get 1/ = 2×105, thus such a solution has a pH of log10 = 5.3. For a commonplace example based on the facts that the masses of a mole of water, a mole of hydrogen ions, a mole of hydroxide ions are 18 g, 1 g, 17 g, a quantity of 107 moles of pure water, or 180 tonnes, contains close to 1 g of dissociated hydrogen ions and 17 g of hydroxide ions. Note that pH depends on temperature. For instance at 0 °C the pH of pure water is 7.47.
At 25 °C it's 7.00, at 100 °C it's 6.14. This definition was adopted because ion-selective electrodes, which are used to measure pH, respond to activity. Ideally, electrode potential, E, follows the Nernst equation, for the hydrogen ion can be written as E = E 0 + R T F ln = E 0 − 2.303 R T F pH where E is a measured potential, E0 is the standard electrode potential, R is the gas const