Metal ions, hydrolysis constants

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... 

The species present in the very alkaline lower ratio silicate solutions do not >ossess the capability of competing with OH as a ligand or of adsorbing these nucleating hydroxide species thus they do not enhance nucleation and their behavior is essentially predictable from metal ion hydrolysis constants. 

The increased acidity of the larger polymers most likely leads to this reduction in metal ion activity through easier development of active bonding sites in siUcate polymers. Thus, it could be expected that interaction constants between metal ions and polymer sdanol sites vary as a function of time and the sihcate polymer size. The interaction of cations with a siUcate anion leads to a reduction in pH. This produces larger siUcate anions, which in turn increases the complexation of metal ions. Therefore, the metal ion distribution in an amorphous metal sihcate particle is expected to be nonhomogeneous. It is not known whether this occurs, but it is clear that metal ions and siUcates react in a complex process that is comparable to metal ion hydrolysis. The products of the reactions of soluble siUcates with metal salts in concentrated solutions at ambient temperature are considered to be complex mixtures of metal ions and/or metal hydroxides, coagulated coUoidal size siUca species, and siUca gels. 

K j is the effective formation constant at a fixed pH and fixed concentration of auxiliary complexing agent. Box 12-2 describes the influence of metal ion hydrolysis on the effective formation constant. 

Box 12-2 Metal Ion Hydrolysis Decreases the Effective Formation Constant for EDTA Complexes... 

The interplay of extensive metal-ion hydrolysis, hgand deprotonation, parallel reaction paths, arising from combinations of these deprotonated and hydrolized species, together with metal-ligand redox reactions, which follow complex formation and which are in turn inhibited by the product, make the Fe(III)/ascorbic acid system a complicated one to study quantitatively. This challenge has been dealt with by Jordan. The initial reaction can be attributed to formation of a blue Fe(III)-ascorbate complex and a number of acid-dependent parallel paths are proposed. The rate constant for Eq. (18) (5.5 X 10 s ) is the highest observed for substitution at the hexaaquo... 

K2 is called the hydrolysis constant for sodium ethanoate. Hydrolysis occurs when salts involving weak acids or bases are dissolved in water. It is often also found with metal ions in solution. The ion [M(H20) ] dissociates to the hydroxy species [M(H20) , (OH)]f 1. ... 

Perhaps the most extensively studied catalytic reaction in acpreous solutions is the metal-ion catalysed hydrolysis of carboxylate esters, phosphate esters , phosphate diesters, amides and nittiles". Inspired by hydrolytic metalloenzymes, a multitude of different metal-ion complexes have been prepared and analysed with respect to their hydrolytic activity. Unfortunately, the exact mechanism by which these complexes operate is not completely clarified. The most important role of the catalyst is coordination of a hydroxide ion that is acting as a nucleophile. The extent of activation of tire substrate througji coordination to the Lewis-acidic metal centre is still unclear and probably varies from one substrate to another. For monodentate substrates this interaction is not very efficient. Only a few quantitative studies have been published. Chan et al. reported an equilibrium constant for coordination of the amide carbonyl group of... 

The rates of hydrolysis of the ester group in compounds A and B have been compared. The effect of an added metal ion (Np+) on the rate of hydrolysis has been studied, and the observed rate constants for attack by OH are tabulated. Suggest the most favorable transition-state stmcture for the addition step of the hydrolysis reaction for each substrate under each set of conditions. Discuss the relationship between the stmctures of these transition states and the relative rates of attack by hydroxide ion. 

Most biological environments contain substantial amounts of divalent and monovalent metal ions, including Mg, Ca, Na, K, and so on. What effect do metal ions have on the equilibrium constant for ATP hydrolysis and the... 

Through all these calculations of the effect of pH and metal ions on the ATP hydrolysis equilibrium, we have assumed standard conditions with respect to concentrations of all species except for protons. The levels of ATP, ADP, and other high-energy metabolites never even begin to approach the standard state of 1 M. In most cells, the concentrations of these species are more typically 1 to 5 mM or even less. Earlier, we described the effect of concentration on equilibrium constants and free energies in the form of Equation (3.12). For the present case, we can rewrite this as... 

It should be noted that whereas a completely soluble hydroxide (e.g. NaOH) will give a solution of high pH in which the pH will increase with concentration of the hydroxide, the pH of a solution of a sparingly soluble hydroxide will depend upon the equilibrium constant for hydrolysis and the activity of metal ions. 

The extent of hydrolysis of (MY)(n 4)+ depends upon the characteristics of the metal ion, and is largely controlled by the solubility product of the metallic hydroxide and, of course, the stability constant of the complex. Thus iron(III) is precipitated as hydroxide (Ksal = 1 x 10 36) in basic solution, but nickel(II), for which the relevant solubility product is 6.5 x 10 l8, remains complexed. Clearly the use of excess EDTA will tend to reduce the effect of hydrolysis in basic solutions. It follows that for each metal ion there exists an optimum pH which will give rise to a maximum value for the apparent stability constant. 

The behavior of metal ions in reversed micelles may be more interesting, since the reversed micelle provides less solvated metal ions in its core (Sunamoto and Hamada, 1978). Through kinetic studies on the hydrolysis of the p-nitrophenyl ester of norleucine in reversed micelles of Aerosol OT and CC14 which solubilize aqueous cupric nitrate, Sunamoto et al. (1978) observed the formation of naked copper(II) ion this easily formed a complex with the substrate ester (formation constant kc = 108—109). The complexed substrate was rapidly hydrolyzed by free water molecules acting as effective nucleophiles. 

The dependences of pH and C-potential on the adsorbed amount of M(H20)2+ at the total metal ion concentrations of 3 x10-3 mol dm-3 are shown in Figures 7 and 8, respectively. The amount adsorbed for each M2+ increases with the pH, and the inflection points are shifted toward the lower pH region in the order of Co2+, Zn2+, Pb2+, Cu2+, which corresponds to the order of the hydrolysis constant of metal ions. To explain the M2+-adsorption/desorption, Hachiya et al. (16,17) modified the treatment of the computer simulation developed by Davis et al. (4). In this model, M2+ binds coordina-tively to amphoteric surface hydroxyl groups. The equilibrium constants are expressed as... 

There is no reason to believe that the conjugate base mechanism does not apply with the other metal ions studied. Complexes of Cr(III) undergo base hydrolysis, but generally rate constants are lower, often 10 —10 less than for the Co(III) analog, Table 4.10. The lower reactivity appears due to both lower acidity (A"i) and lower lability of the amido species (kf) in (4.49) (provided k i can be assumed to be relatively constant). The very unreactive Rh(III) complexes are as a result of the very low reactivity of the amido species. The complexes of Ru(III) most resemble those of Co(III) but, as with Rh(III), base hydrolyses invariably takes place with complete retention of configuration. ... 

Metal-ion catalysis has been extensively reviewed (Martell, 1968 Bender, 1971). It appears that metal ions will not affect ester hydrolysis reactions unless there is a second co-ordination site in the molecule in addition to the carbonyl group. Hence, hydrolysis of the usual types of esters is not catadysed by metal ions, but hydrolysis of amino-acid esters is subject to catalysis, presumably by polarization of the carbonyl group (KroU, 1952). Cobalt (II), copper (II), and manganese (II) ions promote hydrolysis of glycine ethyl ester at pH 7-3-7-9 and 25°, conditions under which it is otherwise quite stable (Kroll, 1952). The rate constants have maximum values when the ratio of metal ion to ester concentration is unity. Consequently, the most active species is a 1 1 complex. The rate constant increases with the ability of the metal ion to complex with 2unines. The scheme of equation (30) was postulated. The rate of hydrolysis of glycine ethyl... 

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