Base-dissociation constant defined

Again, by definition, the negative logarithm to the base 10 of Ka, termed pKa, is usually reported. pKa is a fundamental chemical property and the subject of countless physical chemistry textbooks its theory will not be defined further here. In defining pKa we should also define pKb (the base dissociation constant) ... 

The equilibrium constant (Kb) for this reaction is called the base-dissociation constant for the base A-. Because this constant spans a wide range of values, it is often given in logarithmic form. The negative logarithm (base 10) of Kb is defined as pA b-... 

The acidity constant is a measure of the strength of the acid. Base dissociation constants (K),) are similarly defined. The expression ... 

We start by using Eq. (3.9) to define the different chemical species that are assumed to be present and then we will proceed to substitute secondary equilibrium expressions governed by acid/base dissociation constants or... 

Consider a neutral base B of such strength that it can be protonated in dilute aqueous solution in the acidic range, say pH 1-2. In the conventional manner the acid dissociation constant /ibh + is defined. 

The values of Ka and Kb for different acids and bases vary through many powers of ten. It is often convenient to use the dissociation constant exponent pK defined by... 

The pKa or dissociation constant, is a measure of the strength of an acid or a base. The dissociation constant of an organic acid or base is defined by the following equilibria ... 

Hammett acidity function chem An expression for the acidity of a medium, defined as ho = Kbh+ BH / B, where Kbh+ is the dissociation constant of the acid form of the Indicator, and BH+ and B are the concentrations of the protonated base and the unprotonated base respectively. ham at a sid ad e. faijk shan ) hand sugar refractometer analy chem Portable device to read refractive Indices of sugar solutions. Also known as protelnometer. hand shCig ar. re.frak tam-3d-3r1... 

It is possible to compare the strengths of weak acids by the values of their acid dissociation constants Ka. Figure 3.1 shows the titration curves for acids (HA or BH+) of different Ka values. The ordinate shows poH, which is defined by paH = -loga(SI I)). paH corresponds to the pH in aqueous solutions (see Section 3.2). The poH of non-aqueous solutions can be measured with a glass pH electrode or some other pH sensors (see Sections 3.2.1 and 6.2). For the mixture of a weak acid A and its conjugate base B, poH can be expressed by the Henderson-Hassel-balch equation ... 

A weak acid is one that is not completely dissociated. That is, Reaction 9-3 does not go to completion. For a base, B, the base hydrolysis constant, Kb, is defined by the reaction... 

If a solute is an acid or base, its charge changes as the pH is changed. Usually, a neutral species is more soluble in an organic solvent and a charged species is more soluble in aqueous solution. Consider a basic amine whose neutral form, B, has partition coefficient A" between aqueous phase 1 and organic phase 2. Suppose that the conjugate acid, BH+, is soluble only in aqueous phase I. Let s denote its acid dissociation constant as Kir The distribution coefficient, D, is defined as... 

As discussed in Section 3.10.3, in the gas phase the basicity of simple amines follows the order NMe3 > NHMe2 > NH2Me > NH3 because of the electron donating effect of the methyl (Me) groups. In solution, however, we can define a basicity constant as the equilibrium constant for the reaction shown in Equation 3.4. Note it is important to specify temperature, solvent (usually water) and solution ionic strength, 1 Basicity constants are related to the acid dissociation constants (/Q of the base s conjugate acid via the dissociation constant of water, K = 10 14 at 25 °C. Thus Kbx K = Kw. 

Because they span such a wide range, acid-dissociation constants are often expressed on a logarithmic scale. The pKa of an acid is defined just like the pH of a solution as the negative logarithm (base 10) of Ka. 

EXCHANGE MASTER SPECIES defines the interrelation between the name of an exchanger and its master species. Based on this, EXCHANGE SPECIES describes a half-reaction and requires a selectivity coefficient for each exchanger species. In contrast to stability constants or dissociation constants, these selectivity coefficients are dependent on the respective solid phase with the specific features of their inner and outer surfaces (see also chapter 1.1.4.2). Therefore, within thermodynamically data collections they are only to be seen as placeholders that have to be changed according to site specific exchange constants. 

For certain purposes it is useful to define the dissociation constant of the solvent itself as an acid or base by analogy with the conventional method of writing the dissociation constant of any acid or base, the activity of the solvent molecule taking part in the equilibrium is assumed to be unity. In the equilibrium... 

The pK value of an acid (pK ) or base (pK,) is defined as the negative logarithm to base ten of the equilibrium dissociation constant K, whereby the more largely positive the pK value the more weak, or less pronounced, the dissociation of the acid (R" H ) or base (R OH ). 

The Roberts-Moreland inductive constant o is based on the dissociation constants K of 4-substituted bicyclo[2,2,2]octane-l-carboxylic acids in 50 % ethanol by volume at 25°C defined as ... 

Enzyme inhibition data are often presented as IC50, the concentration of the inhibitor to cause 50 percent inhibition at one chosen substrate concentration Kt, the inhibition constant (dissociation constant from the inhibitor-enzyme complex) determined by enzyme kinetic analysis (e.g., Dixon plot) and /Cin lcl, the time-dependent inhibition constant for mechanism-based inhibitors. IC50 values can be estimated from the study described earlier. A positive inhibition, defined as dose-dependent inhibition, with the inhibited activity lower than 50 percent of that of the negative control, will require further experimentation to define Ki for a better evaluation of in vivo inhibitory potential. Further, a study to determine Klwul may be performed to evaluate if the inhibitor acts via covalent binding to the active site of the enzyme, leading to time-dependent irreversible inhibition. 

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