Hydrolysis constant, observed

Chemical/Physical. The estimated hydrolysis half-life of acetonitrile at 25 °C and pH 7 is >150,000 yr (Ellington et al., 1988). No measurable hydrolysis was observed at 85 °C at pH values 3.26 and 6.99. At 66.0 °C (pH 10.42) and 85.5 °C (pH 10.13), the hydrolysis half-lives based on first-order rate constants were 32.2 and 5.5 d, respectively (Ellington et al., 1987). The presence of hydroxide or hydronium ions facilitates hydrolysis transforming acetonitrile to the intermediate acetamide which undergoes hydrolysis forming acetic acid and ammonia (Kollig, 1993). Acetic acid and ammonia formed react quickly forming ammonium acetate. 

Table V summarizes rate constants, and activation volumes for water exchange on [M(0H)(H20)5] together with those of [M(H20)6] . Accuracy of exchange rate constants /jqh and its activation parameters AH and AS relies on the knowledge of AHj , and ASa° (39,52). The hydrolysis of aqueous M " " ions is complicated by oligomerization and ranges of hydrolysis constants have been reported for example for Al (91,92) and Ga (93-95). As a general trend a strong increase in the lability of the coordinated water molecules is observed for the hydrolyzed species when compared to the hexa-aqua ions. Even at very low pH, where the mole fraction of the hydrolyzed species is extremely small, the water exchange observed by NMR on the bulk water can be dominated by the fast exchange on [M(0H)(H20)5] and not by the much slower exchange on...

In most cases such solubility diagrams are calculated from solubility constants which have been evaluated for acidic or basic solutions. The results obtained by extrapolating such data to other regions should be regarded with much reserve. Figure 4 shows that for iron (III) hydroxide the observed solubility is considerably higher than that expected from solubility and hydrolysis constants. At log [H+] = —8.1, the calculated value is log[Fe(OH)2 ] = —10.85, whereas the observed value is log [FetotnI] = —7, which is approximately the iron concentration in sea... 

As in the case of the base-catalyzed hydrolysis, the observed rate constant for the reaction of ester 9c in the presence of amine 5a was found to decrease with increasing concentration of CTAB. Using equation (28), the second order rate constants for the aminolysis and those for the alkaline hydrolysis were calculated as a function of CTAB concentration and were found to be decreased by factors of 30-6 and 13-3, respectively, by micellar CTAB. These results can be interpreted by an explanation analogous to that for the alkaline hydrolysis in the absence of amines. However, the magnitude of the inhibition for the aminolysis (30-6) as compared to that for the hydroxide ion-catalyzed hydrolysis (13-3) is not readily explicable. It is conceivable, however, that deep penetration of ester into the micelle could result in an environment for the ester group in which the amino group is either not suitably oriented for nucleophilic attack or is excluded to a greater extent than water and hydroxide ion. 

Figure 11.22. Observed hydrolysis constants (log k ) versus pH for some esters. (Data from Mabey and Mill, 1978 adapted from Brezonik, 1994.)...

Cations that form strong inner-sphere aqueous complexes are often strongly and specifically adsorbed when the same ligand is involved in solution and at the sorbent surface. Conversely, species forming weak aquocomplexes or ion pairs tend to be weakly adsorbed. These observations have formed the basis for most estimates of intrinsic adsorption constants (cf. Dzombak and Morel 1990 Smith and Jenne 1991). The extensive literature of solution complexation constants (cf. Smith and Martell 1976 Baes and Mesmer 1976, 1981) has facilitated such estimation methods. As an example, log K - values for cation adsorption by specific metal oxyhydroxides plotted against the first hydrolysis constants of the same cations are often strongly correlated. Such plots, called linear free... 

Motekaitis and Martell (1984) also proposed the mechanistic formation of Al(H icit) (flg) instead of AlOHcit (aqr), arguing that the formation of the former was complete at pH values below the first AP+(aqr) hydrolysis constant (log Kio-i = —5.30). In support of the Motekaitis and Martell model, Greenaway (1986) observed several distinct Al NMR resonances in the pH range 2 to 10, which differed from the findings of Karlik et al. (1983). The findings of... 

In the first step, the hydrolysis constant for Zr, (OH) formation was obtained by re-interpretation of a series of data (solvent extraction data of [75TRI/SCH] and heat capacity data of [76VAS/LYT]). The data of [75TRI/SCH] were re-evaluated (see Appendix A) in terms of Zr, (OH) formation. The authors of [75TRI/SCH] assumed ZrOH to be the dominant monomeric hydrolysis species in 4 m HCIO4. In contrast, the first hydrolysis constant, y ,, (see Section V.3.1.1.1) clearly indicates that Zr is the dominant species in the experimental solutions. The observed increase in polymer concentration, proportional to the third power of the monomeric Zr concentration, therefore, indicates trimer formation not with the composition Zr, (OH) (as invoked by [75TRI/SCH]) but of Zr, (OH) according to the reaction ... 

PROBABLE FATE photolysis . C-Cl bond photolysis is possible, could be important, may photolyze on the soil surface, when released to the atmosphere, it will react with photo-chemically produced hydroxyl radicals with an estimated half-life of 1.23 hr, adsorption onto atmospheric particles will increase this half-life oxidation , probably not important, photooxida-tion by u.v, light in aqueous medium 90-95°C, 25% CO2 formation 5.0 hr, 50% 9.5 hr, 75% 31.0 hr, oxidation rate constant 9.7x10 at pH 7, half-life 71.4 days hydrolysis , hydrolysis of sulfite group may be rapid, probably important above pH 7, hydrolyzed rapidly by alkalies, when released to water, hydrolytic half-life 37.5 and 187.3 days for pH 7 and 5.5 respectively, in the presence of ferric hydroxide, a higher rate of hydrolysis was observed at pH 7 and 20°C, in a solution of ferric oxide, hydrolysis half-life was 9.4 days volatilization could be important sorption sorption is an important process biological processes not important... 

Though the values for ai,m, etc. in equation (1) can of course be expressed in terms of the hydrolysis constants for the separate bonds, the treatment becomes extremely complicated as a different constant will be required for each polypeptide in which the particular bond occurs. In other words, the constant will vary as the reaction proceeds. Thus Kuhn et al. (1932) found that a three line formula was required to express the rate of hydrolysis of tetraglycine. Thus it does not seem that any rigid mathematical approach can be given at present, and we shall have to be content with a few generalizations derived largely from experimental observations. 

The authors determined mononuclear hydrolysis constants logn, j (Th(OH) ) and logjo (Th(OH)2 ) in water and in aqueous ethanol mixtures containing 19.8 and 40.3 mass% ethanol at / = 0.05 M and 25°C. Similar measurements were made for the trivalent lanthanide, yttrium, and scandium ions, which do not concern this review. Usherenko and Skorik [1972USH/SKO] observed a linear relationship between logic A°i (Th(OH) ) and the reciprocal of the dielectric constant of the solvent. 

In order to determine a precise value of the first mononuclear hydrolysis constant, additional potentiometric measurements were performed at each temperature in the pH range 1.8-3.1. Besides a minor contribution from Th (OH)g", the mononuclear complex ThOH was the only hydroxide complex observed in this pH range. The following equilibrium constants were calculated ... 

Belyakov(14) has shown that the pK value decreases to 6.5 for high polymers. Schindler and Kamber(15) have reported a pK value of 6.8 0.2 for surface silanol groups of silica gel. Maatman, et. al.,(16) and Schindler, et. al.,(17) have shown that multivalent metal ions associate with a silica gel surface in a manner that indicates a linear correlation between the ligand properties of the surface silanol groups and metal ion hydrolysis. For Cu, Fe ", Cd " and Pb , Schindler observed that the log of the stability constant of surface complex on silica gel was roughly 60% of the log of the metal ions hydrolysis constant... 

Kale and Nand (1982) investigated the oxidation of /-sorbose in perchlorate medium. The rate is first-order in both Ce(IV) and substrate indicating that the reaction proceeds without formation of a precursor complex. From the observed acid concentration dependence, the authors determine a value for the first hydrolysis constant of Ce(IV) of 0.66 M. The authors conclude that the rate-determining step is... 

The authors reject a simple mechanism which relates the acid dependent process to Ce(IV) hydrolysis in favor of a more complex mechanism involving multiple transition states. This mechanism requires high-order Ce(IV)-I complexes, for which there is no corroborating evidence (even in reasonable analogue systems). If the first hydrolysis constant recommended by Baes and Mesmer (1976) are taken, the simpler explanation of acid concentration dependence due to Ce(IV) hydrolysis becomes reasonable, and probably is the more plausible explanation for the observed results. The specifics of this proposed alternate mechanism are discussed below. 

Figure 9 also reveals that the decomposition rate constants observed at 0% RH were greater than those at 79% RH in samples from CPG 75 to CPG 350. In the cases of the CPG 1000 and CPG 3000 mixtures, the decomposition at 79% RH was faster than that at 0% RH. Although hydrolysis in the solid state was generally accelerated by humidification, the opposite situation was observed in the mixtures with small-pore CPGs. The decomposition rate constant in the CPG 75 mixture at 0% RH and 50 C was determined as 4.90 x 10 h , indicating a half-life of 14.1 h for aspirin decomposition. 

Observed Hydrolysis Constants of Some Sugar Phosphates [Compiled by L. F. Leloir 161)... 

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