Stability hydrolysis species

Such equilibria and their stability constants are summarized in Table 9.1. The total concentration of dissolved iron (Fe-r) at any pH is given by the sum of the concentrations of the free metal iron and all the soluble hydrolysis species, i. e. 

The scope of the living cationic polymerizations and synthetic applications of these functionalized monomers will be treated in the next chapter on polymer synthesis (see Chapter 5, Section III.B). One should note that the feasibility of living processes for these polar monomers further attests to the formation of controlled and stabilized growing species. Conventional nonliving polymerizations, esters, ethers, and other nucleophiles are known to function as chain transfer agents and sometimes as terminators. In addition, the absence of other acid-catalyzed side reactions of the polar substituents, often sensitive to hydrolysis, acidolysis, etc., demonstrates that these polymerization systems are free from free protons that could arise either from incomplete initiation (via addition of protonic acids to monomer) or from chain transfer reactions (/3-proton elimination from the growing end). 

A plot of p([Cu2 ]/[Cu7ot]) 3S a function of pH for three separate titrations fall on a single curve despite up to fivefold differences in measured dissolved copper concentration at a given pH (Figure 2). This behavior of the ratio [Cu2+]/[Cujot] is indicative of the formation of mononuclear hydrolysis species and excludes the possibility that the observed reduction in free cupric ion may have been caused by precipitation of Cu(0H)2 (solid) or the formation of polynuclear complexes. Analysis of data for p[Cu2+], pECujoj] and pH in the pH range 7.7 to 10.8 indicated the presence of two hydrolysis species (CuOH and Cu(0H)2) whose stability constants are given in Table I. Our value of the stability constant for the monohydroxo complex (106.48) falls... 

The stability of the hydrolysis species may also be expressed in terms of the hydroxo corriplex formation (cf. Table 6.2, where in case of hydrolysis L = OH" and HL = H2O). 

The results of simultaneous weighted least-squares regression of the data and some of the unfitted but derived quantities are shown in Tables la, lb, and 2. Table la displays elemental arsenic, its simple oxides, and the reactions for arsenic oxidizing to arsenic trioxides. Table lb introduces the hydrolysis species for As(III) and As(V) in solution, the hydrolysis reactions, and the solubility reactions for the simple oxides. Single species are shown at the top of each table with the reactions underneath. The following discussion describes some of the mineral occurrences for these substances, describes their relative stabilities from field observations, and considers the implications of the evaluated thermodynamic data in terms of these occurrences. 

Clearfield [64CLE] reviewed the structural aspects of zirconium chemistry. One of the important findings of this review was the substantial amount of evidence that was presented for the absence of the zirconyl (ZrO ) ionic structure in aqueous solution and in solids. Even in monoclinic zirconium oxide, Zr-O-Zr bonds are present rather than the Zr=0 double bond. Many compounds in both aqueous solution and the solid state contain hydroxo-zirconium bridges (see Section V.2.2). Clearfield presented evidence of the moiety [Zr4(0H)g(H20)i6] occurring in both aqueous solution and the solid state and argued that measurements by Zielen and Connick [56Z1E/CON] to determine the stability of polymeric zirconium hydrolysis species confirmed the existence of this species in aqueous solution. 

A recent review on the aqueous complexes of zirconium has been published by Nagra/Paul Scherrer Institute (PSI) as part of an update to their thermochemical database [2002HUM/BER]. For the hydrolysis species, these authors included in the Nagra/PSI update the stability constants proposed by [76BAE/MES] for ZrOH, Zr(OH)4 and Zr(OH)j. As indicated above, the stability constant derived for Zr(OH)4 may be questionable due to the fact it was obtained from solutions that may contain a... 

Figure V-11 Stability fields of monomer and polymer hydrolysis species of Zr in 1 m NaC104. Stability field boundaries between two solution species are defined here by equal Zr-concentrations bound to the two species. It is important to note that these boundaries are, in most cases, not equal to the boundaries at which both species have the same concentration.

Variations of the metal ion concentration (at a fixed hydrochloric acid concentration (0.2 M)) were used to indicate the formation of polynuclear zirconium hydrolysis species. There was insufficient data to allow an estimation of stability constants. 

Table A-2 Stability constants for zirconium hydrolysis species.

Table A-9 Stability constants for zirconium hydrolysis species. Uncertainties are given as 95% confidence intervals.

The interpretation of the titration data given by the authors starts from the premise that Zr(OH)2 is the main hydrolysis species. Moreover, as in [99VEY], the implicit assumption is made that all Zr is bound to carbonate complexes, i.e. a very high formation constant is assumed a priori. Therefore, the stability constant ( "4 = 8.0 x 10 °) derived by [80MAL/CHU] for the formation reaction Zr(C03)j" + CO " = Zr(C03)4, had to be rejected. We re-interpreted the potentiometric data on the base of full equilibrium calculations and using the Zr hydroxo, chloride and sulphate stability constants selected in this review (see below). 

Stability fields of monomer and polymer hydrolysis species of Zr... 

The results in basic solutions indicate the occurrence of the hydrolysis species Ni(OH)3 owing to the increase of the solubility at higher NaOH concentrations, see Figure A-31. Due to the scatter of solubility data, the stability constants for Ni(OH)2(aq) were not determined. For [NaOH]jnj,jai < 10 " moFkg, the predominant species is NiOH. The stability constants of the complex Ni(OH)3 are listed in Table A-20 for the temperature range 423 - 573 K, neglecting the formation of Ni(OH)2(aq). 

Note that the stability constants of monomeric species (and their stepwise constants) are often referred to without the preceding 1 (i.e. as rather than Biq)- The formation of the stepwise hydrolysis species M(OH) l can also be described via the reaction of the metal with the hydroxide ion, as shown by reaction (2.11) ... 

As indicated earlier, the stability constant of a hydrolysis species is dependent on the activity of water as shown in Eq. (2.67). The activity of water is related to the osmotic coefficient through Eq. (2.70). Thus, for an electrolyte N Xi, of molahty m, the activity of water can be described by... 

Figure 3.2 shows that the inflection occurs at a hydrogen ion concentration of about 0.4 mmol 1 . This corresponds to a pH of about 3.4 which is very close to the actual determined stability constant for the first hydrolysis species ofthorium(IV), ThOH , as shown by Ekberg et al. (2000) (the data shown in Figures 3.1 and 3.2 originate from this latter study). 

The equation for the average ligand number, as shown by Eq. (3.8), has been derived where only monomeric hydrolysis species form. As can be seen, the equation is independent of the metal ion concentration, and, as such, plots of average ligand number data against pH should be independent of the metal ion concentration. This behaviour is illustrated in Figure 3.3. The data shown in the figure are for potentiometric measurements to determine the stability constants of the hydrolysis species of thallium(III). This metal ion only forms monomeric species. 

Equation (3.8) can, however, be easily extended to include polymeric species. The same technique can then be utilised to derive any hydrolysis species stability constant from the average ligand number formulation. Where polymeric species form, the average ligand number is dependent upon the metal concentration utilised in each experiment as is shown in Figure 3.4. These data have been obtained for the hydrolysis of cerium(IV). When polymeric hydrolysis species are to be determined, it is necessary to perform a series of experiments at different total metal concentrations. In this case, it is necessary to assume the stoichiometry of the species likely to form, and typically, a computer program is then used to investigate which of the proposed species, and their associated... 

For this methodology, dedicated adsorbents have been developed with selectivity with respect to different elements, pH dependence and so on. Thus, like was the case for liquid- liquid extraction, solid adsorbents can be used for selective purification or separation to achieve high-purity materials. In this section, however, the focus is on the use of solid adsorbents for the determination of the stability constants of hydrolysis species. 

There are considerable limitations in the use of the solubility technique for the determination of stability constants of hydrolysis species. The most important is identification of the actual solid phase that is dissolved (precipitated) as well as those of the species formed in the aqueous solution. When performing undersaturation experiments, the starting sohd can be fairly well characterised, but during the dissolution experiment it is not uncommon that the phase actually changes, and it is essential that the solid phase is ascertained after the experiment to assess whether any changes occurred to the sohd phase. The speciation in the aqueous phase can be considerably more tricky to obtain, more specificaUy the... 

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