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Encyclopedia > Acid dissociation constant

An acid dissociation constant, denoted by Ka, is an equilibrium constant for the dissociation of a weak acid. According to the Brønsted-Lowry theory of acids and bases, an acid is a proton donor (HA, where H represents an acidic hydrogen atom), and a base is a proton acceptor. In aqueous solution, water can function as a base, as in the following general example. In chemistry, the equilibrium constant is a quantity characterizing a chemical equilibrium in a chemical reaction. ... A weak acid is an acid that does not fully ionize in solution; that is, if the acid was represented by the general formula HA, then in aqueous solution a significant amount of undissolved HA still remains. ... In chemistry, the BrÃ¸nsted-Lowry system defines acids and alkalis. ... For other uses, see Acid (disambiguation). ... Acids and bases: Acid-base extraction Acid-base reaction Acid dissociation constant Acidity function Buffer solutions pH Proton affinity Self-ionization of water Acids: Lewis acids Mineral acids Organic acids Strong acids Superacids Weak acids Bases: Lewis bases Organic bases Strong bases Superbases Non-nucleophilic bases Weak bases edit In... For other uses, see Proton (disambiguation). ... Depiction of a hydrogen atom showing the diameter as about twice the Bohr model radius. ... The first solvation shell of a sodium ion dissolved in water An aqueous solution is a solution in which the solvent is water. ...

HA + H2O $rightleftharpoons$ A- + H3O+

Acid dissociation constants are also known as the acidity constant or the acid-ionization constant. The term is also used for pKa, which is equal to the negative decimal logarithm of Ka (cologarithm of Ka). Look up logarithm in Wiktionary, the free dictionary. ...

Concepts in
Chemical Equilibria
Acid dissociation constant
Binding constant
Chemical equilibrium
Chemical stability
Dissociation constant
Distribution coefficient
Distribution ratio
Equilibrium constant
Equilibrium unfolding
Equilibrium stage
Liquid-liquid extraction
Phase diagram
Phase rule
Reaction quotient
Relative volatility
Solubility equilibrium
Stability constant
Thermodynamic equilibrium
Theoretical plate
Vapor-liquid equilibrium
edit

A burette, an apparatus for carrying out acid-base titration, is an important part of equilibrium chemistry. ... The binding constant is a special case of the equilibrium constant K. The equilibrium state of molecular binding, i. ... A burette, an apparatus for carrying out acid-base titration, is an important part of equilibrium chemistry. ... In chemistry and biochemistry, a dissociation constant or an ionization constant is a specific type of equilibrium constant used for reversible reactions or processes. ... A partition coefficient is a measure of differential solubility of a compound in two solvents. ... Relative volatility is a measure comparing the vapor pressures of the components in a liquid mixture of chemicals. ... In chemistry, the equilibrium constant is a quantity characterizing a chemical equilibrium in a chemical reaction. ... In biochemistry, equilibrium unfolding is the process of unfolding a protein or RNA molecule by gradually changing its solution conditions, i. ... A theoretical plate in separation processes is a hypothetical zone in which two phases establish an equilibrium, also referred to as an equilibrium stage or a theoretical tray. ... Liquid-liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their solution preferences for two different immiscible liquids, usually water and an organic solvent. ... In physical chemistry, mineralogy, and materials science, a phase diagram is a type of graph used to show the equilibrium conditions between the thermodynamically-distinct phases. ... It has been suggested that this article or section be merged with Gibbs phase rule. ... In a chemical reaction with certain initial concentrations of reactants and products, it is useful to know if the reaction will shift to the right (increasing the concentrations of the products) or if it will shift to the left (increasing the concentrations of the reactants). ... Relative volatility is a measure comparing the vapor pressures of the components in a liquid mixture of chemicals. ... It has been suggested that this article or section be merged with chemical equilibrium. ... Stability constants, formation constants, binding constants, association constants and dissociation constants are all types of equilibrium constant. ... In thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, and chemical equilibrium. ... A theoretical plate in separation processes is a hypothetical zone in which two phases establish an equilibrium, also referred to as an equilibrium stage or a theoretical tray. ... Vapor-liquid equilibrium, abbreviated as VLE by some, is a condition where a liquid and its vapor (gas phase) are in equilibrium with each other, a condition or state where the rate of evaporation (liquid changing to vapor) equals the rate of condensation (vapor changing to liquid) on a molecular...

### Monoprotic acids

When an acid, HA, dissolves in water (TOR en NeliZ zijn Koningen, Marit niet), some molecules of the acid 'dissociate' to form hydronium ions and the conjugate base, (A-), of the acid. Hydronium is the common name for the cation H3O+. Nomenclature According to IUPAC ion nomenclature, it should be referred to as oxonium. ... In chemistry, the BrÃ¸nsted-Lowry system defines acids and alkalis. ...

$H A rightleftharpoons H^{+} + A^{-}$

It is understood that H + stands for the hydronium ion and that each species in this equilibrium may solvate to a greater or lesser extent. The acid dissociation constant is defined as In chemistry, the equilibrium constant is a quantity characterizing a chemical equilibrium in a chemical reaction. ...

$K_a = frac{[mbox{H}^+][mbox{A}^- ]} {[mbox{HA}]}$

where the square brackets are usually taken to signify concentration. H2O is omitted from these expressions because in dilute solution the concentration of water may be assumed to be constant. Values of Ka vary over many orders of magnitude, so it is common to take the negative logarithm to base ten of the value. For other uses, see Concentration (disambiguation). ... Look up logarithm in Wiktionary, the free dictionary. ...

$operatorname{p}K_a = -log_{10} K_a$

It is easier to compare the strengths of different acids by comparing pKa values as they vary over a much smaller range.

### Polyprotic acids

A polyprotic acid is one that has more than one proton to dissociate. Typical examples are malonic acid, which has two ionizable protons, and phosphoric acid, which has three. The constant for dissociation of the first proton may be denoted as pKa1 and the constants for dissociation of successive protons as pKa2, etc. A polyprotic acid is an acid that can dissociate to release more than one proton (hydrogen ion, H+). The first proton is the easiest to dissociate from the polyprotic acid, and each subsequent proton is less likely to dissociate. ... Malonic acid (IUPAC systematic name: propanedioic acid) is a dicarboxylic acid with structure CH2(COOH)2. ... This article is about orthophosphoric acid. ...

It is generally true that successive pK values increase (Pauling's first rule).[1] For example, for a diprotic acid, H2A, the two equilibria are

$H_2 A rightleftharpoons HA^{-} + H^{+}$
$HA^{-} rightleftharpoons A^{2-} + H^{+}$

it can be seen that the second proton is removed from a negatively charged species. Since the proton carries a positive charge extra work is needed to remove it. Therefore pKa2 > pKa1. There are a few exceptions to this rule which occur when there is a major structural change such as in the sequence

 $VO_2^{+}(aq) rightleftharpoons H_3 VO_4 + H^+$ pKa1 = 4.2 $H_3 VO_4 rightleftharpoons H_2 VO_4^{-} + H^{+}$ pKa2 = 2.60 $H_2 VO_4^{-} rightleftharpoons HVO_4^{2-} + H^{+}$ pKa3 = 7.92 $HVO_4^{2-} rightleftharpoons VO_4^{3-} + H^{+}$ pKa4 = 13.27

The pKs of vanadic acid, H3VO4, follow Pauling's rule just like phosphoric acid (values below). All species in this series are tetrahedral, but $VO_2 left( H_2 O right)_4 ^{+}$ is octahedral and pKa2 < pKa1. [2] This article is about orthophosphoric acid. ...

### Bases

Historically the equilibrium constant Kb for a base was defined as the dissociation constant of BH, the acid conjugate to the base, B. Ionic charges on B and/or BH are omitted here because the base may be a neutral species such as ammonia or a charged species such as the ethanoate ion (acetate). For other uses, see Ammonia (disambiguation). ... Acetate, or ethanoate, is the anion of a salt or ester of acetic acid. ...

$B + H_2 O rightleftharpoons BH + OH^{-}$

Using similar reasoning to that used before

$K_b = frac{[mbox{BH}][mbox{OH}^-]} {[mbox{B}]}$
$operatorname{p}K_b = -log_{10} K_b,$

Now, the concentration of the hydroxide ion is related to the concentration of the hydronium by $K_w = left[ H^{+} right]left[ OH^{-} right]$, therefore Hydroxide is a polyatomic ion consisting of oxygen and hydrogen: OHâˆ’ It has a charge of âˆ’1. ...

$[mbox{OH}^-] = frac{K_w}{[mbox{H}^+]}$;

Kw is the constant for the self-ionization of water. Substituting the expression for $left[ OH^{-} right]$ into the expression for Kb The self-ionization of water is the chemical reaction in which two water molecules react to produce a hydronium (H3O+) and a hydroxide ion (OH-): The reaction is also known as the autoionization or autodissociation of water. ...

$K_b = K_w frac{ left[ mbox{BH} right] }{ left[ mbox{B} right] left[ mbox{H}^{+} right] } = frac{K_w}{K_a}$

It follows that

pKb = pKwpKa

In water at 25 °C pKw is 14 so then pKb = 14 − pKa.

In effect there is no need to define pKb separately from pKa, but it is done because pKb values can be found in literature.

### Protonation constants

The protonation constant for a substance S, is given by

$S + H^{+} rightleftharpoons SH$
$K_{assoc} = frac{[mbox{SH}]} {[mbox{S}][mbox{H}^+]}$

Again, ionic charges are omitted from S and SH. Kassoc is an association constant. In chemistry, the equilibrium constant is a quantity characterizing a chemical equilibrium in a chemical reaction. ...

• For any substance $log_{10} left( K_{assoc} right) = pK_a$

### Temperature dependence

All equilibrium constants vary with temperature according the van 't Hoff equation The van t Hoff equation in chemical thermodynamics relates the change in temperature (T) to the change in the equilibrium constant (K) given the enthalpy change (Î”H). ...

$frac {dln K} {dT} = frac{{Delta H_m}^{Theta}} {RT^2}.$

Thus, for exothermic reactions, (ΔH is negative) K decreases with temperature, but for endothermic reactions (ΔH is positive) K increases with temperature. In thermodynamics, the word exothermic describes a process or reaction that releases energy in the form of heat. ... This article is about the physical effect. ...

## Usage

The pH of a solution of weak acid can be expressed in terms of the extent of dissociation. After rearranging the expression defining the dissociation constant, and putting pH = -log10[H+], one obtains For other uses, see PH (disambiguation). ...

$mbox{pH} = mbox{pK}_a - log frac{[HA]}{A^-}$

This is a form of the Henderson-Hasselbalch equation. It can be deduced from this expression that The Henderson-Hasselbalch (frequently misspelled Henderson-Hasselbach) equation in chemistry describes the derivation of pH as a measure of acidity (using pKa, the acid dissociation constant) in biological and chemical systems. ...

• when the acid is 1% dissociated, that is, when $frac{[HA]}{A^-} = 100$, pH = pKa –- 2
• when the acid is 50% dissociated, that is, when $frac{[HA]}{A^-} = 1$, pH = pKa
• when the acid is 99% dissociated, that is, when $frac{[HA]}{A^-} = 0.01$, pH = pKa+ 2

It follows that the range of pH within which there is partial dissociation of the acid is about pKa $pm$ 2.This is shown graphically at the right.

A weak acid may be defined as an acid with pKa greater than about -2. An acid with pKa = -2 would be 99% dissociated at pH 0, that is, in a 1M HCl solution. Any acid with a pKa less than about -2 is said to be a strong acid. Strong acids are said to be fully dissociated. There is no precise pKa value that distinguishes between strong and weak acids because strong acids, such as sulfuric acid, are associated in very concentrated solution. R-phrases S-phrases , , , Flash point Non-flammable Related Compounds Related strong acids Selenic acid Hydrochloric acid Nitric acid Related compounds Hydrogen sulfide Sulfurous acid Peroxymonosulfuric acid Sulfur trioxide Oleum Supplementary data page Structure and properties n, Îµr, etc. ...

On the pKa scale of acid strength, a large value indicates a very weak acid, and a small value indicates a not so weak one.

The pH of a solution of a weak acid can be easily calculated when the analytical concentration of the acid is known. See ICE table for details. Analytical concentration is the concentration of a substance dumped into a solution. ... An ICE table or ICE chart is a tabular system of keeping track of changing concentrations in an equilibrium reaction. ...

Some polyprotic acids can be treated as a set of individual acids. This is possible when successive pK values differ by 4 or more. For example with phosphoric acid

H3PO4 $rightleftharpoons$ H2PO4- +H+, pKa1 = 2.15
H2PO4- $rightleftharpoons$ HPO42- +H+, pKa2 = 7.20
HPO42- $rightleftharpoons$ PO43- +H+, pKa3 = 12.37

Both the hydrogenphosphate and dihydrogenphosphate ions can be treated as acids in their own right. On the other hand, the two pKs for malonic acid are 2.51 and 5.05, so there are pH values at which both malonic acid and the hydrogenmalonate ion co-exist. More elaborate calculations are needed to calculate the composition of solutions of malonic acid.

## Factors that determine the relative strengths of acids

Being an equilibrium constant, the acid dissociation constant Ka is determined by the standard free energy difference ΔGo between the reactants and products, specifically, between the protonated (HA) and deprotonated (A) forms of the substance. In chemistry, the equilibrium constant is a quantity characterizing a chemical equilibrium in a chemical reaction. ...

Pauling's second rule[1] states that the value of the first pK for acids of the formula XOm(OH) n is approximately independent of n and X and is approximately 8 for m=0, 2 for m=1, -3 for m=2 and <-10 for m=3. This correlates with the oxidation state of the central atom, X: the higher the oxidation state the stronger the oxyacid. For example, pKa for HClO is 7.2, for HClO2 is 2.0, for HClO3 is -1 and HClO4 is a strong acid.

With organic acids inductive effects and mesomeric effects affect the pKs. The effects are summarised in the Hammett equation and subsequent extensions. The Inductive effect in chemistry is an experimentally observable effect of the transmission of charge through a chain of atoms in a molecule by electrostatic induction (IUPAC definition). ... To meet Wikipedias quality standards, this article or section may require cleanup. ... The Hammett equation in organic chemistry describes a free-energy relationship relating reaction rates and equilibrium constants for many reactions involving benzene derivatives with ortho- and para-substituents to each other with just two parameters: a substituent constant and a reaction constant [1]. This equation was developed and published by...

Structural effects can also be important. The difference between fumaric acid and maleic acid is a classic example. Fumaric acid is (E)-1,4-but-2-enedioic acid, a trans isomer, whereas maleic acid is the corresponding cis isomer, i.e. (Z)-1,4-but-2-enedioic acid (see cis-trans isomerism). Fumaric acid has pKas of approximately 3.5 and 4.5. By contrast, maleic acid has pKas of approximately 1.5 and 6.5. The reason for this large difference is that when one proton is removed from the cis- isomer (maleic acid) a strong intramolecular hydrogen bond is formed with the nearby remaining carboxyl group. This favors the formation of the maleate H+, and it opposes the removal of the second proton from that species. In the trans isomer, the two carboxyl groups are always far apart, so hydrogen bonding is not observed. Fumaric acid (IUPAC systematic name: 2-butenedioic acid), also called allomaleic acid, boletic acid or lichenic acid, is a colorless crystalline flammable carboxylic acid based on butene and molecular formula C4H4O4. ... Maleic acid or (Z)-Butenedioic acid or cis-butenedioic acid or malenic acid or maleinic acid or toxilic acid is an organic compound which is a dicarboxylic acid (molecule with two carboxyl groups). ... In chemistry, isomers are molecules with the same chemical formula and often with the same kinds of chemical bonds between atoms, but in which the atoms are arranged differently (analogous to a chemical anagram). ... Cis-2-butene Trans-2-butene In chemistry, geometric isomerism or cis-trans isomerism is a form of stereoisomerism and describes the orientation of functional groups at the ends of a bond around which no rotation is possible. ... Intramolecular describes a process or characteristic limited within the structure of a single, or each molecule; a property or phenomenon limited to the extent of a single, or each molecule. ... An example of a quadruple hydrogen bond between a self-assembled dimer complex reported by Meijer and coworkers. ...

## Importance of pKa values

The pKa value(s) of a compound influence many characteristics of the compound such as its reactivity, and spectral properties (colour). In biochemistry the pKa values of proteins and amino acid side chains are of major importance for the activity of enzymes and the stability of proteins. This property is of general importance in chemistry because ionization of a compound alters its physical behavior and macro properties such as solubility and lipophilicity. For example ionization of any compound will increase the solubility in water, but decrease the lipophilicity. This can be exploited in drug development to increase the concentration of a compound in the blood by adjusting the pKa of an ionizable group. This must be done with caution, however, since an ionized compound will pass less easily through cell membranes. Biochemistry (from Greek: , bios, life and Egyptian kÄ“me, earth[1]) is the study of the chemical processes in living organisms. ... A representation of the 3D structure of myoglobin showing coloured alpha helices. ... This article is about the class of chemicals. ... Ribbon diagram of the enzyme TIM, surrounded by the space-filling model of the protein. ... Solubility is a chemical property referring to the ability for a given substance, the solute, to dissolve in a solvent. ... A partition coefficient is a measure of differential solubility of a compound in two solvents. ...

Further information: Protein pKa calculations

In computational biology, protein pKa calculations are used to estimate the pKa values of amino acids as they exist within proteins. ...

## Acidity in nonaqueous solutions

Three properties of a solvent strongly affect acids and bases.

1. A Protic solvent can form hydrogen bonds and will promote ionisation.
2. A solvent with a high donor number is a strong Lewis base.
3. A solvent with a high dielectric constant (Relative permittivity) will promote ionisation.
Donor number dielectric constant
Acetonitrile 14.1 37.5
Dimethyl sulfoxide 29.8 45
Ethanol 31.5 24.3
Pyridine 33.1 12.3
Water 18 81.7

For a given acid, pKa values will vary depending on solvent. The degree of dissociation of any acid increases with the increasing basicity of the solvent. On the other hand, dissociation is relatively less for solvents of low dielectric constant. An acidic solvent will also suppress dissociation of an acid. For example, hydrogen chloride is a weak acid, i.e. poorly ionised, when dissolved in the acidic solvent acetic acid. In chemistry a protic solvent is a solvent that carries hydrogen bond between an oxygen as in a hydroxyl group or a nitrogen as in an amine group. ... In chemistry a donor number or DN is a qualitative measure of lewis basicity. ... A Lewis base is any molecule or ion that can form a new covalent bond by donating a pair of electrons. ... The relative dielectric constant of a material under given conditions is a measure of the extent to which it concentrates electrostatic lines of flux. ... Acetonitrile is an organic molecule, often used as a solvent, with the chemical formula of CH3CN. Also known as methyl cyanide, it is the simplest of the organic nitriles. ... Dimethyl sulfoxide (DMSO) is the chemical compound with the formula (CH3)2SO. This colorless liquid is an important polar aprotic solvent that dissolves both polar and nonpolar compounds and is miscible in a wide range of organic solvents as well as water. ... Grain alcohol redirects here. ... Pyridine is a chemical compound with the formula C5H5N. It is a liquid with a distinctively putrid odour. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... The common (Arrhenius) definition of a base is a chemical compound that either donates hydroxide ions or absorbs hydrogen ions when dissolved in water. ...

Acidity scales have been developed for solvents aside from water, notably for dimethyl sulfoxide (abbrev. 'DMSO') and acetonitrile.[3] It can be seen in the table above that DMSO is more basic than water, but its dielectric constant is less. DMSO is widely used as an alternative to water in evaluating acids and bases. Dimethyl sulfoxide (DMSO) is the chemical compound with the formula (CH3)2SO. This colorless liquid is an important polar aprotic solvent that dissolves both polar and nonpolar compounds and is miscible in a wide range of organic solvents as well as water. ... Acetonitrile is an organic molecule, often used as a solvent, with the chemical formula of CH3CN. Also known as methyl cyanide, it is the simplest of the organic nitriles. ...

In solvents of low dielectric constant, ions tend to associate, which complicates the interpretation of pKas. In particular, in aprotic solvents the process of homoconjugation occurs when the conjugate base forms a hydrogen bond with the parent acid as in the following equilibrium An example of a quadruple hydrogen bond between a self-assembled dimer complex reported by Meijer and coworkers. ...

HA + A- $rightleftharpoons$ HA2-

Typically HA2- would have the structure A---H---A. This process does not occur in water because H2O molecules are strong hydrogen bond donors and acceptors.

In acetonitrile solution, para-toluenesulfonic acid has a homoconjugation constant pKf, of -2.9.[4] This indicates that the toluenesulfonate anion has a strong tendency to form a hydrogen bond with the parent acid. Homoconjugation has the effect of enhancing the acidity of acids, lowering their effective pKas, by stabilizing the conjugate base. Due to homoconjugation, the proton-donating power of toluenesulfonic acid in acetonitrile solution is enhanced by a factor of nearly 800. Tosyl Group with a generic R Group attached (where R can be anything, typically a carbon). ...

## pKa of some common substances

Measurements are at 25ºC in water for those with a pKa at or above -1.76:

 - 25.00: Fluoroantimonic acid - 15.00: Magic acid - 10.00: Fluorosulfuric acid - 10.00: Perchloric acid - 10.00: Hydroiodic acid - 9.00: Hydrobromic acid - 8.00: Hydrochloric acid - 3.00, 1.99: Sulfuric acid - 2.00: Nitric acid - 1.76: Hydronium ion 3.15: Hydrofluoric acid 3.60: Carbonic acid 3.75: Formic acid 4.04: Ascorbic acid (Vitamin C) 4.19: Succinic acid 4.20: Benzoic acid 4.63: Aniline* 4.76: Acetic acid 4.76: Dihydrogencitrate ion (Citrate) 5.21: Pyridine* 6.40: Monohydrogencitrate ion Citrate 6.99: Ethylenediamine* 7.00: Hydrogen sulfide, Imidazole* (as an acid) 7.50: Hypochlorous acid 9.25: Ammonia* 9.33: Benzylamine* 9.81: Trimethylamine* 9.99: Phenol 10.08: Ethylenediamine* 10.66: Methylamine* 10.73: Dimethylamine* 10.81: Ethylamine* 11.01: Triethylamine* 11.09: Diethylamine* 11.65: Hydrogen peroxide 12.50: Guanidine* 12.67: Monohydrogenphosphate ion (Phosphate) 14.58: Imidazole (as a base) 15.76: Water - 19.00 (pKb) Sodium amide 37.00: Lithium diisopropylamide (LDA) 45.00: Propane 50.00: Ethane

• Atkins, Peter, and Loretta Jones. Chemical Principles: The Quest for Insight. 3rd ed. New York: W. H. Freeman and Company, 2005
• Housecroft, Catherine and Sharpe, Alan, Inorganic Chemistry, Prentice Hall, 2nd. edition, 2004. (Non-aqueous solvents)

The Hammett equation in organic chemistry describes a free-energy relationship relating reaction rates and equilibrium constants for many reactions involving benzene derivatives with ortho- and para-substituents to each other with just two parameters: a substituent constant and a reaction constant [1]. This equation was developed and published by... QSAR (Quantitative Structure-Activity Relationship, sometimes the A stands also for Affinity=reactivity) is the quantitative correlation of the biological (ecological, toxicological or pharmacological) activity to the structure of chemical compounds, which allows the prediction of the so-called drug efficacy of a structurally related compound. ... In chemistry and biochemistry, a dissociation constant or an ionization constant is a specific type of equilibrium constant used for reversible reactions or processes. ... The Henderson-Hasselbalch (frequently misspelled Henderson-Hasselbach) equation in chemistry describes the derivation of pH as a measure of acidity (using pKa, the acid dissociation constant) in biological and chemical systems. ...

## References

1. ^ a b Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.  p. 50
2. ^ Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.  Chapter 22
3. ^ March, J. “Advanced Organic Chemistry” 4th Ed. J. Wiley and Sons, 1992: New York. ISBN 0-471-60180-2.
4. ^ Coetzee, J. F. and Padmanabhan, G. R., "Proton Acceptor Power and Homoconjugation of Mono- and Diamines", J. Amer. Chem. Soc.,1965, 87, 5005-5010.

Results from FactBites:

 A - Glossary of chemical terms (2092 words) The strength of an acid depends on the extent to to which it dissociates, and is measured by its dissociation constant. In the Lowry-Brønsted theory of acids and bases (1923), the definition was extended to one in which an acid is a proton donor (a Brønsted acid), and a base is a proton acceptor (a Brønsted base). Amino acids are compounds containing both a carboxylic acid group (-COOH) and an amino group (-NH The most important are the a-amino acids, in which the -NH group in attached to the C atom adjacent to the -COOH group.
 Essential Biochemistry (551 words) The degree of dissociation of a weak acid in water is described by the acid dissociation constant, K For the dissociation of acetic acid (the acid in vinegar) in water, CH the equilibrium (or acid dissociation) constant expression for the reaction is given by
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