Acid dissociation constant indicators

First (AT ) and second (/fi) stepwise acid dissociation constants (indicated by I and 2 in the K column, respectively) are for the following general reactions and dissociation constant expressions for the acid H A ... 

The plT at which an acid-base indicator changes color is determined by its acid dissociation constant. For an indicator that is a monoprotic weak acid, ITIn, the following dissociation reaction occurs... 

If a methyl group replaces a hydrogen atom on the carbon of the C==N bond across which addition of water occurs, a considerable reduction in the extent of water addition is observed. Conversely, the existence of such a blocking effect can be used as a provisional indication of the site at which addition of water occurs, while the spectrum and acid dissociation constant of the methyl derivative provide a useful indication of the corresponding properties of the anhydrous parent substance. Examples of the effect of such a methyl group on equilibria are given in Table IV. 

SPECTROPHOTOMETRIC DETERMINATION OF THE pAT VALUE OF AN INDICATOR (THE ACID DISSOCIATION CONSTANT OF METHYL RED)... 

Discussion. The dissociation of an acid-base indicator is well suited to spectrophotometric study the procedure involved will be illustrated by the determination of the acid dissociation constant of methyl red (MR). The acidic (HMR) and basic (MR-) forms of methyl red are shown below. 

Spectrophotometric determination of the pK value of an indicator (the acid dissociation constant of methyl red) 718... 

Here, A(BH+) denotes the acid dissociation constant of a basic coloured indicator B (for the theory, see Section 1.4.7). 

Conventional absorptiometric and fluorimetric pH indicators show a shift of band positions in absorption and emission spectra between the protonated and deprotonated forms. This feature allows the spectroscopic measurement of the acid dissociation constant in the ground state, Ka, and also the evaluation of the dissociation constant in the excited state, Ka (Eq. (5.5)), from the Forster cycle under the assumption of equivalent entropies of reaction in the two states.<109 112)... 

The acid has four replacable H+ ions. Its dissociation constants indicate that two H+ ions are strongly acidic while the other two protons are weakly acidic. The first dissociation constant especially is very large ... 

As indicated earlier (Section 3.1.1) the sorption of organic compounds onto dissolved matter can significantly increase the solubility of the compound. This can in turn affect the fate of these chemicals in the environment. We can use physicochemical parameters such as distribution coefficients (log D), aqueous acid dissociation constants (pAia), and octanol-water partition coefficients (p/to )-These attributes are also linked to the acidity and alkalinity of the environment as well as lipohilicity of the compound. The mathematical relationships between these attributes are outlined below to explore how each of these impacts the fate of PPCPs in the environment. 

To measure the amount of dissociation occurring when a weak acid is in aqueous solution, chemists use a constant called the acid dissociation constant (KJ. is a specicil variety of the equilibrium constant. As we explain in Chapter 14, the equilibrium constant of a chemical reaction is the concentration of products over the concentration of reactants, and it indicates the balance between products and reactants in a reaction. 

The acid dissociation constant is simply the equilibrium constant of a reaction in which an acid is mixed with water cmd from which the water concentration has been removed. The water concentration is removed because the concentration of water is a constant in dilute solutions, and a better indicator of acidity is the concentration of the dissociated products divided by the concentration of the acid reactant. The general form of the acid dissociation constant is therefore... 

Whether a reversible reaction favours reactants or products at equilibrium is indicated by the equilibrium constant of the reaction (A eq). Remember that square brackets are used to indicate concentration in moles/litre = molarity (M). The degree to which an acid (HA) dissociates is described by its acid dissociation constant (K ). The acid dissociation constant is obtained by multiplying the equilibrium constant (K q) by the concentration of the solvent in which the reaction... 

The larger the acid dissociation constant, the stronger is the acid. Hydrochloric acid has an acid dissociation constant of 10, whereas acetic acid has an acid dissociation constant of only 1.74 x 10 . For convenience, the strength of an acid is generally indicated hy its pA a value rather than its A a value The of hydrochloric acid, strong acid, is —7, and the pA a of acetic acid, much weaker acid, is 4.76. 

The acid dissociation constant, pKD, was determined for 4-methylimino-pentane-2-one as a function of mole fraction dioxane and the data conform (within 0.03) to the expression, pKD = 10.78 + 14.19 N2, in the range N2=0.08 to 0.173. The half-neutralized ligand was stable in 50 volume % dioxane for at least an hour. Formation constants could not be determined for the ligand again, hydrolysis was indicated. 

The acid dissociation constant Ka is independent of ionic strength, but the acid dissociation constant Kc depends on the ionic strength, as indicated by equation 1.2-7. The equilibrium constant expression in equation 1.2-7 will be used in the rest of the book, but the subscript c will be omitted. This will make it possible for us to deal with concentrations of species, rather than activities. 

A carboxylic acid may dissociate in water to give a proton and a carboxylate ion. The equilibrium constant Ka for this reaction is called the acid-dissociation constant. The pKa of an acid is the negative logarithm of Ka, and we commonly use pKa as an indication of the relative acidities of different acids (Table 20-3). 

Acidity constant — Kan (sometimes also called acid dissociation constant , or acid-ionization constant ) is an equilibrium constant indicating the extent of the dissociation of the nth hydrogen ion from a charged or uncharged acid [i] ... 

The concentration of water is not included explicitly in this expression (it is incorporated into the value of Kg) because in dilute aqueous solution, the concentration of water is a constant, almost equal to the concentration of water when pure (55.6 M). The constant Kg is called the acid dissociation constant. The subscript a indicates that the constant is for the ionization of an acid. 

This table can be helpful in estimating the pATs of other weak acids from their structures. In using this table it is important to remember that -log [H ] is used in the expression for the acid dissociation constant in terms of pH. To obtain pKs based on -log y (H ) [H ], add 0,0.08, 0.11,0.12, and 0.14 at ionic strengths of 0, 0.05, 0.10,0.15, and 0.25 M, respectively, at 298.15 K as indicated by Table 1.3. PaddedForm rounds the output to two figures to the right of the decimal point. There is a list of full names of reactants in the Appendix of this book. The reactants bpg, nmn, pep, and prpp are bisphosphoglycerate, nicotinamidemononucleotide, phosphoenolpyruvate, and 5-phosphoribosyl-alpha-pyrophosphate, respectively. 

In this particular case, Ke is an acid dissociation constant and would be indicated as... 

Physico-chemical properties constitute the most important class of experimental measurements, also playing a fundamental role as - molecular descriptors both for their availability as well as their interpretability. Examples of physico-chemical measurable quantities are refractive indices, molar refractivities, parachors, densities, solubilities, partition coefficients, dipole moments, chemical shifts, retention times, spectroscopic signals, rate constants, equilibrium constants, vapor pressures, boiling and melting points, acid dissociation constants, etc. [Lyman et al, 1982 Reid et al, 1988 Horvath, 1992 Baum, 1998]. 

This expression shows that an indicator with an acid dissociation constant of 1 X 10 - (pA a = 5) typically shows a complete color change when the pH of the solution in which it is dissolved changes from 4 to 6 (Figure 14-2). With a little more algebra, we can derive a similar relationship for a basic-type indicator. 

Determine the transition ranges for Eriochrome Black T in titrations of Mg-+ and Ca at pH 10.0, given that (a) the second acid dissociation constant for the indicator is... 

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