Stability constants, formation constants, binding constants, association constants and dissociation constants are all types of equilibrium constant. See also Determination of equilibrium constants for experimental and computational methods. Apparatus for carrying out acid-base titration. ... The acid dissociation constant (Ka), also known as the acidity constant or the acid-ionization constant, is a specific equilibrium constant for the reaction of an acid with its conjugate base in aqueous solution [1]. // When an acid dissolves in water, it partly dissociates forming hydronium ions and its conjugate... The binding constant is a special case of the equilibrium constant K. The equilibrium state of molecular binding, i. ... Apparatus for carrying out acid-base titration. ... 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. ... In biochemistry, equilibrium unfolding is the process of unfolding a protein or RNA molecule by gradually changing its solution conditions, i. ... 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. ... 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). ... It has been suggested that this article or section be merged with chemical equilibrium. ... In thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, and chemical equilibrium. ... Equilibrium constants are determined in order to quantify chemical equilibria. ...

For a general chemical reaction Vapours of hydrogen chloride in a beaker and ammonia in a test tube meet to form a cloud of a new substance, ammonium chloride A chemical reaction is a process that results in the interconversion of chemical substances. ...

αA + βB … σS + τT …

the equilibrium constant can be defined by^[1]

where {A} is the activity of the chemical species A etc. It is conventional to put the activities of the products in the numerator and those of the reactants in the denominator. See Chemical equilibrium for a derivation of this expression. Activity in chemistry is a measure of how different molecules in a non-ideal gas or solution interact with each other. ... Apparatus for carrying out acid-base titration. ...

For equilibria in a gas phase, the activity of a gaseous component is the product of the component's partial pressure (made dimensionless with the utility of standard pressure) and the fugacity coefficient for this component. In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. ... Fugacity is a measure of the tendency of a substance to prefer one phase (liquid, solid, gas) over another. ...

For equilibria in solution activity is the product of concentration and activity coefficient. It is common practice to determine equilibrium constants in a medium of high ionic strength. In those circumstances the quotient of activity coefficients is effectively constant and the equilibrium constant is taken to be a concentration quotient. In chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. ... Activity in chemistry is a measure of how different molecules in a non-ideal gas or solution interact with each other. ... The ionic strength of a solution is a function of the concentration of all ions present in a solution. ...

However, the value of K_c will depend on the ionic strength. All equilibrium constants depend on temperature and pressure (or volume).

A knowledge of equilibrium constants is essential for the understanding of many natural processes such as oxygen transport by haemoglobin in blood and acid-base homeostasis in the human body. 3-dimensional structure of hemoglobin Hemoglobin or haemoglobin is the iron-containing oxygen-transport metalloprotein in the red cells of the blood in mammals and other animals. ... The body is very sensitive to its pH level. ...

Types of equilibrium constants

Association and dissociation constants

In organic chemistry and biochemistry it is customary to use pK_a values for acid dissociation equilibria.

pK_a = − lgK_diss = lg(1 / K_diss)

where K_diss is a stepwise acid dissociation constant (lg stands for log₁₀). For bases the base association constant, pK_b is used. For any given acid or base the two constants are related by pK_a + pK_b = pK_w, so pK_a can always be used in calculations. The acid dissociation constant (Ka), also known as the acidity constant or the acid-ionization constant, is a specific equilibrium constant for the reaction of an acid with its conjugate base in aqueous solution [1]. // When an acid dissolves in water, it partly dissociates forming hydronium ions and its conjugate...

On the other hand stability constants for metal complexes, and binding constants for host-guest complexes are generally expressed as association constants. When considering equilibria such as complex In chemistry, a complex is a structure composed of a central metal atom or ion, generally a cation, surrounded by a number of negatively charged ions or neutral molecules possessing lone pairs. ... In supramolecular chemistry, host-guest chemistry describes complexes that are composed of two or more molecules or ions held together in unique structural relationships by hydrogen bonding or by ion pairing or by Van der Waals force other than those of full covalent bonds. ...

it is customary to use association constants for both ML and HL. Also, in generalised computer programs dealing with equilibrium constants it is general practice to use overall constants rather than stepwise constants and to omit ionic charges from equilibrium expressions. For example, if NTA, nitrilotriacetic acid, HC(CH₂CO₂H)₃ is designated as H₃L and forms complexes ML and MHL with a metal ion M, the following expressions would apply for the dissociation constants. Acronym for Nigerian Television Authority. ... NTA is the chemical compound nitrilotriacetic acid. ...

The overall association constants can be expressed as

Note how the subscripts define the stoichiometry of the equilibrium product.

Stepwise formation constants

The stepwise constant for protonation of ML can be easily derived as follows.

There is no agreed notation for stepwise constants of this kind, though a symbol such as is sometimes found in the literature. It is best always to define a stability constant by reference to an equilibrium expression.

Competition method

A particular use of a stepwise constant is in the determination of stability constant values outside the normal range for a given method. For example, EDTA complexes of many metals are outside the range for the potentiometric method. The stability constants for those complexes were determined by competition with a weaker ligand. EDTA is a widely-used acronym for the chemical compound ethylenediamine tetraacetic acid (and many other names, see table). ...

Micro-constants

When two or more sites in an unsymmetrical molecule may be involved in an equilibrium reaction there are more than one possible equilibrium constants. For example, the molecule L-dopa has two non-equvalent hydroxyl groups which may be deprotonated. Denoting L-Dopa as LH₂, the following diagram shows all the species that may be formed (X=CH₂CH(NH₂)CO₂H) L-DOPA (levodopa, 3,4-dihydroxy-L-phenylalanine). ...

The first protonation constants are

[L¹H] = k₁₁[L][H], [L²H] = k₁₂[L][H]

The concentration of LH^- is the sum of the concentrations of the two micro-species. Therefore, the equilibrium constant for the reaction, the macro-constant, is the sum of the micro-constants.

K₁ = k₁₁ + k₁₂

In the same way,

K₂ = k₂₁ + k₂₂

Lastly, the overall constant is

β₂=K₁K₂=k₁₁k₂₁=k₁₂k₂₂

Thus, althought there are six micro-and macro-constants, only three of them are mutually independent. Moreovever, the isomerization constant, K_i, is equal to the ratio of the microconstants.

K_i=k₁₁/k₁₂

In L-Dopa the isomeriztion constant is 0.9, so the micro-species L¹H and L²H have almost equal concentrations at all pH values.

In general a macro-constant is equal to the sum of all the micro-constants and the occupancy of each site is proportional to the micro-constant. The site of protonation can be very important, for example, for biological activity.

Micro-constants cannot be determined individually by the usual methods, which give macro-constants. Methods which have been used to determine micro-constants include: Equilibrium constants are determined in order to quantify chemical equilibria. ...

• blocking one of the sites, for example by methylation of a hydroxyl group, to determine one of the micro-constants
• using a spectroscopic technique, such as infrared spectroscopy, where the different micro-species give different signals.
• applying mathematical procedures to ¹³C NMR data.^[2]

Infrared spectroscopy (IR Spectroscopy) is the subset of spectroscopy that deals with the IR region of the EM spectrum. ...

pH considerations (Brønsted constants)

pH is defined in terms of the activity of the hydrogen ion The correct title of this article is . ... Activity in chemistry is a measure of how different molecules in a non-ideal gas or solution interact with each other. ...

pH = − lg{H ⁺ }

If, when determining an equilibrium constant, pH is measured by means of a glass electrode, a mixed equilibrium constant, also known as a Brønsted constant, may result.

It all depends on whether the electrode is calibrated by reference to solutions of known activity or known concentration. In the latter case the equilibrium constant would be a concentration quotient. If the electrode is calibrated in terms of known hydrogen ion concentrations it would be better to write p[H] rather than pH, but this suggestion is not generally adopted.

Hydrolysis constants

In aqueous solution the concentration of the hydroxide ion is related to the concentration of the hydrogen ion by Hydrolysis is a chemical reaction or process in which a chemical compound reacts with water. ...

K_W = [H][OH]:[OH] = K_W[H] ^{− 1}

The first step in metal ion hydrolysis ^[3] can be expressed in two different ways

It follows that β * = KK_W. Hydrolysis constants are usually reported in the β * form and this leads to them appearing to have strange values. For example, if lgK=4 and lg K_W=-14, lg β ^* = 4 -14 = -10. In general when the hydrolysis product contains n hydroxide groups lg β ^* = lg K + n lg K_W

Conditional constants

Conditional constants, also known as apparent constants, are concentration quotients which are not true equilibrium constants but can be derived from them.^[4] A very common instance is where pH is fixed at a particular value. For example, in the case of iron(III) interacting with EDTA, a conditional constant could be defined by

This conditional constant will vary with pH. It has a maximum at a certain pH. That is the pH where the ligand sequesters the metal most effectively.

In biochemistry equilibrium constants are often measured at a pH fixed by means of a buffer solution. Such constants are, by definition, conditional and different values may be obtained when using different buffers. Acids and bases: Acid-base reaction theories pH Self-ionization of water Buffer solutions Systematic naming Electrochemistry Acid-base extraction Acids: Strong acids Weak acids Superacids Lewis acids Mineral acids Organic acids Bases: Strong bases Weak bases Superbases Lewis bases Organic bases edit Buffer solutions are solutions which resist change...

Temperature dependence

It can be shown that the equilibrium constant is related to the standard Gibbs energy change of reaction as: Image File history File links Gibbs. ... In thermodynamics, the Gibbs energy or Gibbs energy function is the energy portion of a thermodynamic system available to do work. ...

,

where ΔG° is the standard Gibbs energy change of reaction, R is the gas constant, and T the absolute temperature. The gas constant (also known as the universal or ideal gas constant, usually denoted by symbol R) is a physical constant used in equations of state to relate various groups of state functions to one another. ... Absolute zero is the lowest temperature that can be obtained in any macroscopic system. ...

This relationship is also written as:

A direct consequence of this important relation is the Van't Hoff equation, which relates the change in temperature to the change in the equilibrium constant given the enthalpy change. The Vant Hoff equation in chemical thermodynamics relates the change in temperature to the change in the equilibrium constant given the enthalpy change. ...

Data sources

[IUPAC SC-Database] A comprehensive database of published data on equilibrium constants of metal complexes and ligands

[NIST Standard Reference Database 46] Critically Selected Stability Constants of Metal Complexes

[Inorganic and organic acids and bases] pKa data in water and DMSO Dimethyl sulfoxide The United States DoDs Defense Modeling and Simulation Office This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ...

References

1. ^ F.J,C. Rossotti and H. Rossotti, The Determination of Stability Constants, McGraw-Hill, 1961.
2. ^ D.N. Hague and A.D. Moreton, J. Chem. Soc. Perkin Trans.2, 265-270, 1994; M. Borkovec and G.J.M. Koper, Anal. Chem,,, 72, 3272-3279, 2000.
3. ^ C.F. Baes and R.E. Mesmer, The Hydrolysis of Cations, Wiley, 1976
4. ^ G. Schwarzenbach and H. Flaschka, Complexometric titrations, Methuen, 1969