Hydrolysis constant, first

Table 3.6. The first hydrolysis constants of selected elements in soil chemistry3...

Assuming a correlation between surface complexation and aqueous hydrolysis exists, the trend in strengths of surfaces complexes for An in different oxidation states onto a given mineral would be in the order An4+ > AnC>2+ > An3+ > AnOj. Several authors have provided evidence for linear relations between the first hydrolysis constant of metals and the intrinsic constant associated to the formation of surface species of metals as S-OMamorphous silica (Schindler Stumm 1987), hydrous ferric oxides (Dzombak Morel 1990), aluminum (hydr-)oxides and kaolinite (Del Nero et al. 1997, 1999a). 

Spectrophotometric experiments, however, do not provide straightforward evidence for the existence of small amounts of americium hydroxypolymers. In order to compare the literature data with one another and with our data, they are transformed to thermodynamic data at I = 0 according to Baes and Mesmer (1), using the salt effect constants assessed by them. These data are given in parentheses in Table I. The first hydrolysis constants given in the literature differ from one another considerably. For lanthanide ions, log Bi values are reported from 3.7 to 6.3 (1), which are distinctly lower than that of americium shown in Table I. 

The first hydrolysis constant for plutonium(III), that is, the equilibrium constant for the reaction... 

Plutonium (IV) is the most readily hydrolyzed of the four oxidation states, but only the first hydrolysis constant is known with any confidence. For the reaction... 

Strong acid organic ion exchange resins. Dugger et al. has shown that AF values for ion-exchange reaction of 20 metal ion/H systems relate linearly to the logarithm of the first hydrolysis constant of these metal ions (Fig. 18) [205]. In alkaline media, the selectivity sequence is reversed in the order of Cs" < Rb+ < K" " < Na+ < Li for the hydrous oxides of Zr and Sn. 

Figure 15 Correlation between metal-humate stability constants (log knu-Me) and the first hydrolysis constant (log koH-Me) of the corresponding metal (Dupre et al, 1999). Squares correspond to humic-metal association constants determined by ultrafiltration of organic-rich waters. knu-Me corresponds to the conditional constant of the reaction Me"t" - - Hu — ...

Figure 6.4. Hydrolysis of metal ions, (a) Predominant pH range for the occurrence of aquo, hydroxo, hydroxo-oxo, and 0x0 complexes for various oxidation states. The scheme attempts to show a useful generalization, but many elements cannot be properly placed in this simplified diagram because other factors, such as radius and those related to electron distribution, have to be considered in inteipreting the acidity of metal ions, (b) The linear dependence of the log,o of the first hydrolysis constant Ki = MOH H / M on the ratio of the charge to the M-O distance (z/d) for four groups of cations (25°C). (Note change of abcissa zero for different groups.) (From Baes and Mesmer, 1976.) (c) Hydrolysis constants of some important metal ions.

Figure 6.7. Correlation of the solubility product K q with the first hydrolysis constant AT, for M, and M cations. The lines have slopes of -1, -2, -3,8ind...

Hydrolysis of hydrated small, highly charged cations may occur beyond the first step. In many cases these reactions are quite complex. They may involve two or more cations reacting with each other to form large polymeric species. For most common hydrated cations, consideration of the first hydrolysis constant is adequate for our calculations. 

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... 

Figure 10,26 Correlation plot for some metal cations, of their first hydrolysis constants ( /fii) versus intrinsic surface complex constants i Ku) for their adsorption by Si02(am) assuming the constant capacitance model. The equation of the solid line is log = 0.09 -( 0.62 log A. Hydrolysis and adsorption reactions are written A,i -t- H2O = +...

Figure 10.27 Correlation plot for some soft and borderline hard-soft acid cations, of their first hydrolysis constants ( Tn) versus intrinsic surface complex constants for their ad-...

The effects of pH. The effects of pH on adsorption of metal ions by oxides are remarkably consistent as the pH is increased, reaction begins at a characteristic pH and increases rapidly so that over about 2 pH units reaction is nearly complete (Fig.l.). The sequence of reaction is similar to the sequence of their first hydrolysis constants (Fig. 1.). 

Mercury differs from the other metals in that it reacts at much lower pH (Fig. 2.). However, this is indeed consistent in that its first hydrolysis constant is also much lower (negative log 3.4). There are also large effects of chloride concentration on mercury reaction (Fig. 2.). This occurs because the strong affinity between mercury and chlorine means that the concentration of the species present in solution change with chloride concentration. Thus, this result shows that reaction does depend on the species in solution. 

Inoue et al. [303] found that the correlation between log (hydrous Nb20s) at pH 3 for four trivalent and three divalent metal cations and logarithm of stability constant of l-I acetate complex (r=0.96) was better than with the first hydrolysis constant (r = 0.91). An equally good correlation between log Ap (hydrous Zr02) at pH 4 for three trivalent and four divalent metal cations and logarithm of the stability constant of 1-1 acetate complex (r = 0.98) and of the first hydrolysis constant (r = 0.99) was found [407]. In another publication Inoue et al, [304] arranged the cations into three groups on basis of the valence and electronic structure, and the... 

Examples of correlations between stability constants of surface complexes (calculated using different adsorption models, cf. Chapter 5) on the one hand, and the first hydrolysis constant and other constants characterizing the stability of solution complexes on the other are more munerous than the studies of correlations involving directly measured quantities. It should be emphasized that there is no generally accepted model of adsorption of ions from solution, and stability constants of surface complexes are defined differently in particular models, thus, the numerical values of these constants depend on the choice of the model. Moreover, some publications reporting the correlations fail to define precisely the model. 

A linear correlation between the binding constant of metal cations to silica and their first hydrolysis constant was reported by Schindler et al. [357]. Similar results have been obtained for other materials. These correlations can be conveniently expressed as ... 

This initial pH drop was also reported by other authors (Ma et al., 1993 Zhang et al., 1998). They demonstrated that dissolution of HA, a relatively slow process [Eq. (15.1)], causes solution pH to increase. On the contrary, the precipitation of pyromorphite is instantaneous and causes the pH to decrease sharply [Eq. (15.2)]. Thus, the initial pH drop in the system of Mavropoulos et al. (2002) occurred mainly because, initially, the precipitation of pyromorphite (instantaneous) is more rapid than the dissolution of HA (slow and pH dependent), resulting in a decrease in solution pH. In their case, dissolution of HA is the rate-limiting step. As P is exhausted in solution, the precipitation of pyromorphite stops, and hence there is a gradual increase in solution pH. Only at later stages of Pb immobilization, as the system pH approaches the PZCha and the first hydrolysis constant of Pb, could surface complexation be contributing to Pb immobilization. 

Li (1981) has proposed that the distribution coefficients reflect adsorption-desorption reactions at the surface of mineral grains. To emphasize this point, Li plotted a slightly different distribution coefficient (log Cop/Q , where Qp and Cs are the concentrations in oceanic pelagic clay sediments and seawater respectively vs the first hydrolysis constants of the metals or the dissociation constant of the oxyanion acids. The argument is that those elements that hydrolyze the strongest will adsorb the... 

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