Acid-Ionization Constants at

The acid ionization constants of dibenzepine HCl and opipramol HCl were determined potentiometrically in aqueous solutions at 25 °C. The values pKi = 8.275 and pKi = 3.451 respectively, were obtained for the acid ionization constants at zero ionic strengfli. The value pKi = 8 was obtained for opipramol at Bie ionic strength I = 0.01. The effect of ionic strength on Bie ionization constant, as adjusted with sodium chloride, was determined."... 

In this reaction, the protons from the weak acid are transferred to the water molecules. The acid-ionization constant (AT) of this reaction is shown below ... 

Table 12.5 lists acid ionization constants at 25°C for some common weak acids, and additional values can be found in Appendix F. 

For the purpose of systematizing kinetic and equilibrium data, for literally hundreds of reactions, it is desirable to have a single reference series for all. Hammett adopted as the standard the acid ionization constants for substituted benzoic acids in aqueous solution at 25 °C. This choice was fortunate because the compounds are stable and for the most part readily available. Also, their pA"a s can easily and precisely be measured for nearly every substituent. Thus, one constructs a plot according to either of the following equations, in which Eq. (10-4) constitutes a further example ... 

The freezing point depression of a solvent is proportional to the concentration of solute particles and may be used to measure the extent of ionization once the new particles have been identified qualitatively as ions. The method has the obvious disadvantage of not allowing measurements over a range of temperatures in a single solvent. It is almost certainly not worth while to compute an enthalpy of ionization from ionization constants at two different temperatures in two different solvents. Usable solvents are limited not only by the requirement that the melting point be at a convenient temperature but also by the requirement that the solvent be capable of producing ions yet not be sufficiently nucleophilic to react irreversibly with them once they are formed. For this reason most cryoscopic work has been done in sulfuric acid or methanesulfonic acid.170... 

Table 10-3 gives acid ionization constants for several familiar acids at 25 C. The values for the strong acids are not well defined. Examine the Ions column and see how every acid yields a hydrogen ion and a complementary anion in solution. 

A solution as initially prepared contains 0.050 mol L of phosphate ion (PO ) at 25°C. Given the three acid ionization constants from Table 15.2, calculate the equilibrium concentrations of PO4, HPO4, H2PO4, H3PO4, and OH . 

M oxalic acid (H2C2O4) at 25°C, using the two acid ionization constants for oxalic acid from Table 15.2 and the solubility product = 2.6 X 10 for CaC204. 

The reaction of OH with HCN in aqueous solution at 25°C has a forward rate constant kf of 3.7 X 10 L mol s. Using this information and the measured acid ionization constant of HCN (see Table 15.2), calculate the rate constant in the first-order rate law rate = r[CN ] for the transfer of hydrogen ions to CN from surrounding water molecules ... 

PKa a logarithm of acid ionization constant (Ka) pKg Equivalent to -logioKe. where by convention Kg is the equilibrium dissociation constant of an antagonist. Numerically, identical to pAz for simple equilibrium competition at a single site, The term apparent pKe may be used when there is no evidence whether or not there is competition, but there is assumed to be (e.g. when calculating pKe from a single concentration of antagonist). PKC protein kinase C. 

The acid ionization constant (pKa) of the aqua complex ion determines whether or not the ion would form complexes with a protein. This depends greatly on the pH of the medium. Since the ionization constant of low charge is 12.6, the ion would form a stable complex only with negatively charged protein in alkaline media. It cannot bind to cationic proteins because it does not share electrons to form a covalent bond. This consideration explains why the activity of Ca2+-activated protease is optimum in the alkaline pH range. Thus a decrease in its activity at acidic pH values may partly be due to a change in the electronic state of Ca2+ (Asghar and Bhatti, 1987 Barbut and Mittal, 1985). 

Sometimes we are given the value of for a compound and asked to calculate the compound s molar solubility. For example, the of silver bromide (AgBr) is 7.7 X 10. We can calculate its molar solubility by the same procedure as outlined on p. 608 for acid ionization constants. First we identify the species present at equilibrium. Here we have Ag+ and Br ions. Let i be the molar solubihty (in mol/L) of... 

The ionization constants at various temperatures and thermodynamic functions for sulphamic acid and several other aminosulphonic acids have been determined in formamide from electromotive force measurements40. The initial thermoelectric power (TEP) for hydrogen electrode thermocells has been determined for aqueous solutions of sulphamic acid at 29 °C over the concentration range 0.001 to 0.01 molal41. 

Acids ionize and produce hydrogen ions in aqueous solution. The strength of an acid depends on how completely it ionizes. Strong acids undergo essentially complete ionization, whereas weak acids are only partly ionized at equilibrium. Like other reversible reactions, the ionization of a weak acid can be represented by an equilibrium constant expression. The value of this expression, called the acid ionization constant, K, is a measure of the strength of a weak acid. 

Calculate the acid ionization constant, K, for each acid, using the equilibrium concentrations of acid and H in wells Al-Dl and A3-D3. (Hint Remember that the concentration of the conjugate base, A , equals the concentration of H+.) Record your results. If the concentration of acid at equilibrium is zero in any well, draw a dash in the column for that cell. 

Spinaceamines are fairly strong bases with two basic centres. The highest protonation constants correspond to the N-5 atom of SpA (8.90) and the N-5 atom of SpN (8.66), while the smaller constants characterize the basicity of the imidazole ring in the monocation of SpA (4.89) and SpN (4.96). The acid ionization constant of SpN (pKg) is 1.65 and is comparable with the pK of histidine (1.96) (73JCS(D)323). SpNs 517, 519, 563 and 564 substituted at an N-atom easily take up methyl halides to afford monoquaternary salts 559-562 at the N-5 atom. More stringent conditions are required to obtain diquaternary salts 565 (82MI5). 

With the concentrations at equilibrium we can substitute the concentrations in the acid-ionization constant expression. 

Acid ionization constant of the substituted benzoic acid in water at 25°. 

Thus for substituted acetic acids in water at 25°, the maximum error to be expected is 0.015 a units. As S is probably about 0.1 and u — vch2x will generally be less than 0.30, the error in values obtained from acetic acid ionization constants should be negligible for most substituents. 

Acid ionization constants are shown for some common acids at 25°C. Larger values of indicate stronger acids. 

Calculating the hydronium ion concentration in weak acid solutions isn t as straightforward as it is in strong solutions, because not all of the weak acid that dissolves initially has ionized. In order to calculate the hydronium ion concentration, you must use the equilibrium constant expression for the weak acid. Chapter 8 covers the K, expression that represents the equilibrium system. For weak acid solutions, you use a modified equilibrium constant expression called the K, — the acid ionization constant Take a look at the generalized ionization of some weak acid HA ... 

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