Ionization constants, determination

Ultraviolet (UV) spectra of some selected furo[3,2-3]- and furo[2,3 ]pyrroles were published and their apparent ionization constants determined by spectrometric titration (Table 5), and correlated with their structures <2001CCC1615>. 

The tautomeric ratios characterizing the whole complex scheme of uracil cannot be evaluated. Only the tautomeric equilibrium constant Kt32,28 for uracil was calculated by Katritzky and Waring.97 The ionization constants determined by two different methods (see Table XVII) give the values of 104° and 103 3 for X,32,28 of 1-methyluracil. 

Ionization Constants Determined from Variation of Reaction Velocity and Michaelis Constant with pH... 

B) Ionization Constants. Determine the pH of 0.1 solutions of acetic, chloroacetic, and trichloroacetic acids. Convert the pH to hydrogen-ion concentration as moles per liter then calculate the ionization constants for acetic and monochloroacetic acid. 

The carboxyl ionization [pK]) is low and easily identified. However, the ammonium and thiol groups have similar pK values (compare methylamine with methyl mercaptan, Table 2.1) and so an uncertainty exists as to which group ionizes first. Ki, K2, and K3 represent macroscopic ionization constants determined experimentally from a titration curve. K2 and are the composite of four microscopic ionization constants. Once the proton is lost from the carboxyl group, one of two ionization pathways may be followed ... 

Each acid and base has its own ionization constant. Some acids (or bases) have two or three hydrogen (or hydroj l) ions per atom, each of which has its own constant. The ionization constant determines the pH for a given strength of acid or base. Consider the example of an acid... 

Although the dehydration of (4) to (7) is rapid, the initial ultraviolet absorption could be obtained accurately by extrapolation and the ionization constant determined. This is because the pKa value (7.77) is in a pH region where hydrogen ion catalysis is small. In contrast the pK value for the equilibrium is low and in a region where the process (8) (3) is strongly catalyzed by hydrogen ions—... 

It can be seen from Table 2 that the intrinsic values of the pK s are close to the model compound value that we use for Cys(8.3), and that interactions with surrounding titratable residues are responsible for the final apparent values of the ionization constants. It can also be seen that the best agreement with the experimental value is obtained for the YPT structure suplemented with the 27 N-terminal amino acids, although both the original YPT structure and the one with the crystal water molecule give values close to the experimentally determined one. Minimization, however, makes the agreement worse, probably because it w s done without the presence of any solvent molecules, which are important for the residues on the surface of the protein. For the YTS structure, which refers to the protein crystallized with an SO4 ion, the results with and without the ion included in the calculations, arc far from the experimental value. This may indicate that con-... 

A double end point, acid—base titration can be used to determine both sodium hydrosulfide and sodium sulfide content. Standardized hydrochloric acid is the titrant thymolphthalein and bromophenol blue are the indicators. Other bases having ionization constants in the ranges of the indicators used interfere with the analysis. Sodium thiosulfate and sodium thiocarbonate interfere quantitatively with the accuracy of the results. Detailed procedures to analyze sodium sulfide, sodium hydro sulfide, and sodium tetrasulfide are available (1). 

The use of UV spectroscopy as an identification method is continuously decreasing in relative importance compared to the use of NMR or mass spectrometry. However, due to the general validity of Beer s law, it continues to be an appropriate method for quantitative studies such as the measurement of ionization constants (Section 4.04.2.1.3(iv) and (v)) and the determination of tautomeric equilibrium constants (Section 4.04.4.1.5). 

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. 

It is a simple matter to determine an ionization constant and also to predict its magnitude. When these values do not agree, and if ringopening has been carefully excluded, the likelihood of covalent hydration must be considered. Equilibria encountered during the determination of the ionization constant of a hydrating heteroaromatic base are shown in the following diagram. Similar equilibria exist for... 

It is presumptuous to report that a substance is not hydrated simply because there are no drifts in the readings obtained during potentio-metric measurements or because the experimentally determined p a value is not very different from the predicted value. A small amount of hydration may cause only a small difference in the ionization constant and hence other tests should be applied. A number of heterocyclic compounds which have seemingly normal pvalues may well be partially hydrated. 

Most of the chemistry of PA is determined by its acidic nature. It is a strong acid whose ionization constant of 1.6 x 10"1 (Ref 31) makes it comparable in acid strength to pyrophosphoric acid and trichloroacetic acid. PA readily forms salts with bases and esters with alcohols. The salts are known as Picrates. Many of them are expl and will be described in a separate article in this Vol. The esters are phenol ethers, eg, Trinitro-anisolc (see Vol t, A450-L)... 

Values were selected from the 47 entries given in Ref. 19. The rate constants were determined in acetone-water at 25.0 °C. and the ionization constants in water at 25.0 °C. 

Hydroxyl radicals. The acid ionization constant of the short-lived HO transient is difficult to determine by conventional methods but an estimate can be made because HO, but not its conjugate base, O -, oxidizes ferrocyanide ions HO + Fe(CN) — OH- + Fe(CN)g . Use the following kinetic data26 for the apparent second-order rate constant as a function of pH to estimate Ka for the acid dissociation equilibrium HO + H20 =... 

Several 1 -phosphates of deoxyfluoro sugars were prepared, and their acid-catalyzed hydrolysis was studied. 2-Deoxy-2-fluoro- (580), 3-deoxy-3-fluoro- (582), 4-deoxy-4-fluoro- (583), and 6-deoxy-6-fluoro-a-D-gluco-pyranosyl phosphates (584) were prepared by treatment of the corresponding per-( -acetylated )9-D-glucopyranoses with phosphoric acid [the p anomer (581) of 580 was prepared by a different method]. The first and second ionization constants (pA a, and pA a2) of these compounds were determined potentiometrically, as well as by the F-n.m.r. chemical shifts at a series of pH values, and then the rate constants of hydrolysis for neutral (B) and monoanion (C) were decided. The first-order rate-constants (k) for 580-584 and a-D-glucopyranosyl phosphate (in Af HCIO4,25 °) were 0.068, 0.175, 0.480, 0.270, 1.12, and 4.10 (all as x lOVs), respectively. The rate... 

All equilibrium constants in the present discussion are based on the concentration (not activity) scale. This is a perfectly acceptable thermodynamic scale, provided the ionic strength of the solvent medium is kept fked at a reference level (therefore, sufficiently higher than the concentration of the species assayed). This is known as the constant ionic medium thermodynamic state. Most modern results are determined at 25 °C in a 0.15 M KCl solution. If the ionic strength is changed, the ionization constant may be affected. For example, at 25 °C and 0.0 M ionic strength, the pXj of acetic acid is 4.76, but at ionic strength 0.15 M, the value is 4.55 [24]. 

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