Hydrolysis of Aqueous Cations

A water molecule is strongly polarized by coordination to a metal cation, toward which its electrons are attracted  

a typical aqueous metal ion M +(aq) can act as a Br0nsted (i.e., proton-donating) acid, of which (aq) is the conjugate base.  

From Eq. 13.24, it is seen that pAa corresponds to the pH value at which hydrolysis of M- +(aq) is just half complete—assuming no other reactions intervene (but see later). Typical values of pAa are about 14 for z — 1, 9 3 for 2 = 2, and 3 2 for z = 3, at least for the lighter elements. The pAa values for the lanthanide(III) ions, for example, are on the order of 7 to 9 because of their larger radii  

However, simplistic generalizations based on 2 and r are only partly successful in understanding metal ion hydrolysis. For instance, it is not obvious why Ka. for Fe +(aq) (r = 64.5 pm) is about 100-fold greater than for Cr (aq) (r = 61.5 pm). 

Like stability constants and other thermodynamic properties of metal ions in solution, hydrolysis constants are affected by ionic strength and temperature, and these should be specified when quoting precise values. For the ballpark figures cited here, 25 °C and high dilution are assumed. 

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Structure Modification. Several types of stmctural defects or variants can occur which figure in adsorption and catalysis (/) surface defects due to termination of the crystal surface and hydrolysis of surface cations (2) stmctural defects due to imperfect stacking of the secondary units, which may result in blocked channels (J) ionic species, eg, OH , AIO 2, Na", SiO , may be left stranded in the stmcture during synthesis (4) the cation form, acting as the salt of a weak acid, hydrolyzes in aqueous suspension to produce free hydroxide and cations in solution and (5) hydroxyl groups in place of metal cations may be introduced by ammonium ion exchange, followed by thermal deammoniation. 

The most common oxidation state of niobium is +5, although many anhydrous compounds have been made with lower oxidation states, notably +4 and +3, and Nb can be reduced in aqueous solution to Nb by zinc. The aqueous chemistry primarily involves halo- and organic acid anionic complexes. Virtually no cationic chemistry exists because of the irreversible hydrolysis of the cation in dilute solutions. Metal—metal bonding is common. Extensive polymeric anions form. Niobium resembles tantalum and titanium in its chemistry, and separation from these elements is difficult. In the soHd state, niobium has the same atomic radius as tantalum and essentially the same ionic radius as well, ie, Nb Ta = 68 pm. This is the same size as Ti ... 

Another phenomenon that is closely associated with acid-base equilibria is the so-called hydrolysis of metal cations in aqueous solution, which is probably better considered as the protolysis of hydrated cations, e.g. ... 

Electrodeposition of metal oxide thin films in aqueous solutions is made possible by promoting hydrolysis of metal cations with electrochemically... 

The reaction mechanism is still obscure, and the authors just suppose that the acidity necessary for getting the condensation of the OPUF nearby the anode, could arise from the acidic hydrolysis of Fe3+ cations originated by dissolution of the stainless steel anode. The applicative aspects of this system are noteworthy and further knowledge is necessary. Tidswell and Train110 ni>112) studied in great detail the homo- and copolymerization of vinyl acetate in aqueous emulsions. The possible initiation mechanisms are compared and the hypothesis of the formation of a vinyl acetate-hydrogen radical acting as true chain initiator is discussed. 

This book on the chemistry of hydrolysis of Inorganic cations contains a substantial amount of equilibrium data pertinent to hydrolysis reactions. For each of the elements which produces a cation or cations in aqueous solution, the available equilibrium data for the hydrolysis reactlon(s) at or about 298 K has been critically assessed In order to obtain "best" values for equil Ibriuffl constants and quotients applicable to a given medium. When available, and AS data for the hydrolysis reactions are also presented. The data, with references and comments. Is arranged under the element of Interest. 

The measured equilibrium constants for this stepwise de-protonation scheme for Mo and W have been collected from the literature [111] see also Aqueous Chemistry of the Transactinides . They show that Mo is more hydrolyzed than W, and that the de-protonation sequence for Mo and W at pH = 1 reaches the neutral species M02(0H)2(H20)2. Assuming the de-protonation processes for Sg to be similar to those of Mo and W as in Eqs. 14—17, Pershina and Kratz [111] predict that the hydrolysis of the cationic species to the neutral species decreases in the order Mo > W > Sg. This is in agreement with the experimental data on hydrolysis of Mo and W and with the result for Sg [110]. For Sg, the de-protonation sequence ends earlier with a cationic species such as Sg0(0H)3(H20)2, which adsorbs on a cation-exchange resin. 

Contrarily to the observed behavior of the copper chloride, the diffusion of Fe2(S04)3 is affected by the presence of lactose in aqueous solutions for [lactose]/[Fe2(SO )3] = 2, Pactose]/[Fe2(S04)3] = 3, and [lactose]/ [Fe2(SO )3] = 4 (Fig. 1.4). This behavior can be explained considering the main interactions that can occur (a) interactions between lactose and iron cation (b) interactions between lactose and sulphate anion (c) interactions between lactose and water (d) hydrolysis of iron cation. The transport of an appreciable fraction of ferric sulfate may occur as a result of the initial Fe2(SO )3 gradient, but also by a further hydrogen ion flux, resulting of the hydrolysis of Fe(III). "... 

Similarly, the cyanide, acetate and carbonate are unstable in aqueous solution. Hydrolysis of the halides and other salts such as the nitrate and sulfate is incomplete but aqueous solutions are acidic due to the ability of the hydrated cation [AI(H20)6] to act as proton donor giving [A1(H20)5(0H)]-+, (AI(H20)4(0H)2]+, etc. If the pH is gradually increased this deprotonation of the mononuclear species is accompanied by aggregation via OH bridges to give species such as... 

Recently, Deligoz and Yilmaz [51] prepared three polymeric calix[4]arenes, which were synthesized by reacting chloromethylated polystyrene with 25,26,27-tribenzoyloxy-28-hydroxy calix[4]arene (2a, 3a) and po-lyacryloyl chloride with 25,26,27,28-tetraacetoxy ca-lix[4]arene (4a). After alkaline hydrolysis of the polymers, they were utilized for selective extraction of transition metal cations from aqueous phase to organic phase. 

Sulfate radical anion may be converted to the hydroxyl radical in aqueous solution. Evidence for this pathway under polymerization conditions is the formation of a proportion of hydroxy end groups in some polymerizations. However, the hydrolysis of sulfate radical anion at neutral pi I is slow (k— 107 M"1 s 1) compared with the rale of reaction with most monomers (Ar=l08-109 M 1 s 1, Table 3.7)440 under typical reaction conditions. Thus, hydrolysis should only be competitive with addition when the monomer concentration is very low. The formation of hydroxy end groups in polymerizations initiated by sulfate radical anion can also be accounted for by the hydration of an intermediate radical cation or by the hydrolysis of an initially formed sulfate adduct either during the polymerization or subsequently. 

In a classic study in 1940, Crossley and coworkers demonstrated that the rates of nucleophilic substitution of the diazonio group of the arenediazonium ion in acidic aqueous solution were independent of the nucleophile concentration, and that these rates were identical with the rate of hydrolysis. Since that time it has therefore been accepted without question that these reactions proceed by a DN + AN mechanism, i.e., that they consist of a rate-determining irreversible dissociation of the diazonium ion into an aryl cation and nitrogen followed by rapid reactions of the cation with water or other nucleophiles present in solution (Scheme 8-6). 

Henry M, Jolivet JP, Livage J (1991) Aqueous Chemistry of Metal Cations Hydrolysis, Condensation and Complexation. 77 153-206 Hider RC (1984) Siderophores Mediated Absorption of Iron. 57 25-88 Hill HAO, Rdder A, Williams RJP (1970) The Chemical Nature and Reactivity of Cytochrome P-450. 8 123-151... 

The running of parallel reactions of hydrolysis, ammonolysis and depolymerization of apple pectin in aqueous solution of ammonia (IM) at 25 C were investigated. It was examined the effects of monovalent cations (Na, K", NH4 ) and divalent cations (Ca, Mg ) when they were added as chloride salts. It was found that the relative rates of the above mentioned reactions, depend on the nature and concentration of the added salts as well. The chlorides of sodium, potassium and calcium accelerate hydrolysis and depolymerization, while magnesium chloride delays these reactions. Ammonolysis was increased in cases of ammonium chloride addition. 

Oae found that for both base- and acid-catalyzed hydrolysis of phenyl benzenesul-fonate, there was no incorporation of 0 from solvent into the sulfonate ester after partial hydrolysis. This was interpreted as ruling out a stepwise mechanism, but in fact it could be stepwise with slow pseudorotation. In fact this nonexchange can be explained by Westheimer s rules for pseudorotation, assuming the same rules apply to pentacoordinate sulfur. For the acid-catalyzed reaction, the likely intermediate would be 8 for which pseudorotation would be disfavored because it would put a carbon at an apical position. Further protonation to the cationic intermediate is unlikely even in lOM HCl (the medium for Oae s experiments) because of the high acidity of this species a Branch and Calvin calculation (See Appendix), supplemented by allowance for the effect of the phenyl groups (taken as the difference in between sulfuric acid and benzenesulfonic acid ), leads to a pA, of -7 for the first pisTa of this cation about -2 for the second p/sTa. and about 3 for the third Thus, protonation by aqueous HCl to give the neutral intermediate is likely but further protonation to give cation 9 would be very unlikely. 

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