Ionization dissociation constant

Equilibrium constant in terms of activities ionization (dissociation) constant of an acid. 

Here Kj is an ionization constant and is then a dissociation constant [280], In this case one must then take account of the ion pair in determining the overall ionization/dissociation constant. 

The strength of an acid is measured by the value of its dissociation constant, strong acids, e.g. HCl, HNO3. being substantially fully ionized in solution and weak acids predominately unionized. 

The strength of a weak acid is measured by its acid dissociation constant, which IS the equilibrium constant for its ionization m aqueous solution... 

The C—H bonds of hydrocarbons show little tendency to ionize and alkanes alkenes and alkynes are all very weak acids The acid dissociation constant for methane for exam pie IS too small to be measured directly but is estimated to be about 10 ° (pK 60)... 

The phenoxyalkanoic herbicides are acidic in nature and thus subject to some degree of ionization. The extent to which the herbicide ionizes is controlled by the acid dissociation constant (fQ of the herbicide in question and the soil solution pH (238). The leaching potential is significantly influenced by these reactions. 

H PO is therefore a stronger acid than orthophosphoric acid. The second dissociation constant, pR 2> Th third hydrogen does not ionize in... 

A more concentrated solution of HOBr can be prepared by filtration of one of the above solutions and distillation in vacuum. Or the mercuric oxide reaction can be carried out in Freon 11 without water, yielding a solution of bromine monoxide which is filtered and hydrolyzed. Hypobromous acid is slightly ionized its dissociation constant at 25°C is 2 x 10 . ... 

The numerical values of the terms a and p are defined by specifying the ionization of benzoic acids as the standard reaction to which the reaction constant p = 1 is assigned. The substituent constant, a, can then be determined for a series of substituent groups by measurement of the acid dissociation constant of the substituted benzoic acids. The a values so defined are used in the correlation of other reaction series, and the p values of the reactions are thus determined. The relationship between Eqs. (4.12) and (4.14) is evident when the Hammett equation is expressed in terms of fiee energy. For the standard reaction, o%K/Kq = ap. Thus,... 

Ionization constant A , dissociation constant K, and overall dissociation constant A rx are defined ... 

The ionization eonstant should be a function of the intrinsic heterolytic ability (e.g., intrinsic acidity if the solute is an acid HX) and the ionizing power of the solvents, whereas the dissoeiation constant should be primarily determined by the dissociating power of the solvent. Therefore, Ad is expeeted to be under the eontrol of e, the dieleetrie eonstant. As a consequenee, ion pairs are not deteetable in high-e solvents like water, which is why the terms ionization constant and dissociation constant are often used interchangeably. In low-e solvents, however, dissociation constants are very small and ion pairs (and higher aggregates) become important species. For example, in ethylene chloride (e = 10.23), the dissociation constants of substituted phenyltrimethylammonium perchlorate salts are of the order 10 . Overall dissociation constants, expressed as pArx = — log Arx, for some substanees in aeetie acid (e = 6.19) are perchloric acid, 4.87 sulfuric acid, 7.24 sodium acetate, 6.68 sodium perchlorate, 5.48. Aeid-base equilibria in aeetie acid have been earefully studied beeause of the analytical importance of this solvent in titrimetry. 

Consider the ionization of some weak acid, HA, occurring with an acid dissociation constant, Ai,. Then,... 

An expression for the ionization of H2CO3 under such conditions (that is, in the presence of dissolved CO2) can be obtained from Kh, the equilibrium constant for the hydration of CO2, and from the first acid dissociation constant for H2CO3 ... 

Potentiometric titration curves The procedure involves the addition of a salt of a weak acid to the resin and the determination of the pH of the equilibrated solution. Table 9 shows the pK values of the OH groups and dissociation constants of the studied resin. The first ionization occurs at a pH slightly higher than that of sul-... 

Dicarboxylic acids have two dissociation constants, one for the initial dissociation i into a monoanion and one for the second dissociation into a dianion, i-or oxalic acid, H02C—COoH, the first ionization constant has p/Cal = 1.2 and the second ionization constant has pKa2 = 4.2. Why is the second carboxyl group so much less acidic than the first ... 

The base properties of the amine are represented by its basicity constant (basicity dissociation constant, Kh), which identifies the amount of the amine (in moles) that is ionized (i.e., available to raise pH) in liquid water-condensate at any given temperature and pressure. The dissociation reaction for a primary amine is shown in equation 1, and the value of the dissociation constant is shown in equation 2. 

Ka is also widely called the acid ionization constant or acid dissociation constant. 

Statistical Effects. In a symmetrical diprotic acid, the first dissociation constant is twice as large as expected since there are two equivalent ionizable... 

If the agent is an acid or a base its degree of ionization will depend on the pH. If its acid dissociation constant,is known, the degree of ionization at any pH may be calculated or determined by reference to published tables. 

It is worth mentioning that an attempt was made by Tsao and Willmarth to determine the acid dissociation constant of HO2. The reaction between hydrogen peroxide and peroxydisulphate was used for the generation of the HO2 radical. However, these experiments, like others where the HO2 radical is studied under steady-state conditions, could yield only a value of acidity constant multiplied by a coefficient consisting of a ratio of kinetic parameters. Unfortunately, in this case there are no independent data for the kinetic coefficient, and the value of cannot be evaluated. Considering the kinetic analogue of the titration curve it can be stated only that ionization of HO2 becomes important in the pH range from 4.5-6.5. The value of acidity constant of HO2 obtained by Czapski and Dorfman is (3.5 + 2.0)x 10 mole.l. . ... 

From these equations it is possible to predict the effective lipophilicity (log D) of an acidic or basic compound at any pH value. The data required in order to use the relationship in this way are the intrinsic lipophilicity (log P), the dissociation constant (pKa) and the pH of the aqueous phase. The overaU effect of these relahonships is the effechve hpophilicity of a compound, at physiological pH, is approximately the log P value minus one unit of hpophilicity, for every unit of pH the pKa value is below (for acids) and above (for bases) pH 7.4. Obviously for compounds with mul-hfunchonal ionizable groups the relahonship between log P and log D, as weU as log D as a function of pH become more complex [65, 68, 70]. For diprotic molecules there are already 12 different possible shapes of log D-pH plots. 

Mitchell, R. C Salter, C. J., Tam, K. Y. Multiwavelength spectrophotometric determination of add dissociation constants. Part 111. Resolution of multi-protic ionization systems. J. 

FIG. 4 Thermodynamic equilibria for the interfacial distribution of a solute X which can be ionized n times, and X being its most acidic (or deprotonated) and its most basic (or protonated) forms, respectively. X and are the dissociation constants in the aqueous and organic phase, respectively, and P is the partition coefficient of the various species between the two phases. 

---