Hydrolysis phenomena

The calculation of the affinity scale, in terms of differences in free-energy content between the various ionic forms of these materials, implies that one is dealing with equilibrium systems and that the reaction is both reversible and stoichiometric, i.e., hydrolysis phenomena are absent in the zeolite. Within certain limits, these conditions are generally met however, it is apparent that some discrepancies between experimental data have sometimes been attributed (1) to a failure in the fulfillment of one or more of these basic prerequisites. 

Tn the past 20 years several physical inorganic chemistry laboratories have re-examined extensively the nature of the simple inorganic solutes in water. Substantial contributions to man s knowledge of the hydrolytic behavior of metal cations and metallate anions in water solvent have resulted. Various new techniques have been applied with varying degrees of efficacy. The details of the behavior, within specified conditions, of a number of solute ions are now matters of fact. In many more cases and over wider variations of conditions, all of the important variables which affect hydrolysis phenomena have not yet been identified. 

In the electroless deposition, the hydrolysis phenomena, as has already been published before,3,5,6 plays a very significant role in the reduction of metallic ions from aqueous solutions. 

When dealing with aqueous solutions, an important question arises why hydrolysis phenomena play such an important role in the electroless deposition of metals It is generally believed that the metallic surfaces upon immersion in an aqueous solution can attract the OH- ions. If this were true, then at the metallic surface a pH rise should be observed due to the adsorption of OH- ions by the metal itself. 

Compared to the case of the dissolution of the halides, the increased complexity of the hydrolysis phenomena and the appearance of small amounts of Th(OH)4(am) precipitate in the case of the dissolution of the oxychloride and the oxybiomide do not permit us to treat the data on the three oxyhalides with sufficient accuracy for any values to be selected. 

It becomes clear that a broad pore size range can be covered by the different polyoxycations, though the pore size distribution of PlLCs is not uniform as is the case for zeolites. One must keep in mind that careful control of the hydrolysis conditions is necessary in order to obtain the narrowest pore size distribution. Variations in the pillar composition (size, charge, etc.) due to hydrolysis phenomena result in variations of both the sht width and the interpillar distance. 

A calcination step at 573-773 K in air converts the Al- and Zr-polyoxycation precursors into rigid alumina and zirconia oxide pillars. This heating process is necessary to obtain a stable PILC with a permanent microporosity, not subject to swelling or hydrolysis phenomena. During calcination, dehydration and dehydroxylation reactions of the charged pillar precursors occur to give neutral oxide particles. The equilibrium in electrical balance is maintained by the release of protons during the conversion at elevated temperature ... 

The absorption of humidity by the joints is one of the main causes of ageing, physically to begin with, often involving the elimination of the plasticizers and other additives or impurities, and possibly chemically as well, with hydrolysis phenomena. This suggests that controlling the reduction in adhesive properties, an active area of research and development across many industries could be achieved through a regulated absorption of humidity. 

Modes of passive degradation are not different from what we can observe in other aqueous media. We may, for example, observe a dissolution at grain boundaries associated with the release of particles and hydrolysis phenomena, sometimes limited to the siuface of the material. In the case of multiphase ceramics, phenomena connected to the transfer of material between phases of different solubility may appear. The release of particles in tissues or biological fluids may sometimes produce inflammatory reactions which we will analyze later. Certain plasma sprayed coatings of calcium phosphate can be associated with small proportions of calcium oxide, whose hydration is accompanied by a large increase in molar volume, leading to the formation of cracks and to a detachment from the metallic substrate. 

One often needs to calculate the pH of a salt solution that comes from a weak acid and a strong base, such as sodium acetate. For this salt, Kb = 10 u/(2 x 10-5) = 5 x 10-10, where 2 x 10-5 is the Ka of acetic add. Because sodium acetate solutions are alkaline because of the hydrolysis phenomenon,... 

PU and silicone rubber are biocompatible materials which are commonly used in a variety of medical applications [53], e.g., as a raw material for central venous catheters and tracheotomy tubes. Although these materials are biocompatible, the side effects which occur during clinical use include inflammation, infection and biofilm formation and growth. This in turn initiates the degradation of the material, e.g., previous studies have proven that the degradation of PU catheters is caused by either oxidation or hydrolysis of the material [54]. The degradation of silicone rubber is a hydrolysis phenomenon [55], which could be catalysed by an acidic environment. 

The hydrolysis phenomenon may play a considerable role during the formation of complexes for several reasons. The first one could have been studied in the first section of this chapter. Indeed, the hydrolysis phenomenon induces a competition between the ligand L and the hydroxo ions L for the metallic ion. The second reason results from the fact that hydroxo complexes may precipitate as insoluble hydroxides or oxides. Hence, in this case, a phenomenon of precipitation also occurs, and we can speak of a complexation-precipitation interaction. Of course, all these processes depend on the pH value since the hydroxo ligand and the hydroxo complexes intervene in the complexation phenomenon. 

In order to facilitate the discussion, it is interesting first to know the concentrations of the hydroxo complexes resulting from the hydrolysis phenomenon, no other ligand being present at that time. With the mercuric ion Hg +, we calculate (see Exercise 1 ahead) that in an aqueous solution containing 10 mol/L of mercuric nitrate and whose acidity is such that pH = 3.00, the concentrations are [Hg +] = 5.9 X 10 mol/L, [Hg(OH)+] = 1.2 x 10 mol/L, and [Hg(OH)2] = 2.9 x 10 mol/L. This is an interesting result. It shows that in these conditions, only half of the mercuric ions remain as such, although Hg(N03)2 is totally ionized. 

In this example, the fraction of mercury(II) escaping from complexation to chloride ions due to the hydrolysis phenomenon is negligible. This is due to the great value of the overall stability constant of the [HgCb] complex on the one hand and, on the other, to the fact that the concentration [OH ] intervening in expressions (25.5) and (25.6) (see Exercise 2) is weak. In this example, the hydrolysis of the metallic ion (Hg " ") does not preclude the formation of the expected complexes. 

The system seems to be difficult to solve. However, some judicious simplifications make its resolution considerably easier. Hence, the hydrolysis phenomenon, which induces an acidification of the medium, incites neglecting [OH ] in the proton condition relation. From another standpoint, the stability constant K2 of [Hg(OH)2] also incites neglecting [Hg(OH)+] in the same expression, which therefore becomes... 

The kinetics of vinyl acetate emulsion polymeriza tion in the presence of alkyl phenyl ethoxylate surfactants of various chain lengths indicate that part of the emulsion polymerization occurs in the aqueous phase and part in the particles (115). A study of the emulsion polymerization of vinyl acetate in the presence of sodium lauryl sulfate reveals that a water-soluble poly(vinyl acetate)—sodium dodecyl sulfate polyelectrolyte complex forms, and that latex stabihty, polymer hydrolysis, and molecular weight are controlled by this phenomenon (116). 

Either acid or base catalysis may be employed. Alkaline catalysts such as caustic soda or sodium methoxide give more rapid alcoholysis. With alkaline catalysts, increasing catalyst concentration, usually less than 1% in the case of sodium methoxide, will result in decreasing residual acetate content and this phenomenon is used as a method of controlling the degree of alcoholysis. Variations in reaction time provide only a secondary means of controlling the reaction. At 60°C the reaction may takes less than an hour but at 20°C complete hydrolysis may take up to 8 hours. 

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. ... 

Poly(L-malate) decomposes spontaneously to L-ma-late by ester hydrolysis [2,4,5]. Hydrolytic degradation of the polymer sodium salt at pH 7.0 and 37°C results in a random cleavage of the polymer, the molecular mass decreasing by 50% after a period of 10 h [2]. The rate of hydrolysis is accelerated in acidic and alkaline solutions. This was first noted by changes in the activity of the polymer to inhibit DNA polymerase a of P. polycephalum [4]. The explanation of this phenomenon was that the degradation was slowest between pH 5-9 (Fig. 2) as would be expected if it were acid/base-catalyzed. In choosing a buffer, one should be aware of specific buffer catalysis. We found that the polymer was more stable in phosphate buffer than in Tris/HCl-buffer. 

Whereas the cleavage of /S-poly(L-malate) at neutral pH is at random [2], alkaline hydrolysis reveals characteristic patterns of the cleavage products, which is due to nonrandom chain scission (Fig. 3). The phenomenon is explained by an autocatalytic ester hydrolysis. Assuming that one (or both) of the polymer ends bends... 

The importance of occluded cells cannot be overemphasised, and Brown considers that pitting, crevice corrosion, intergranular attack, filiform corrosion and hydrogen cracking are characterised by local acidification due to hydrolysis of metal ions, and that this phenomenon is of major significance in the overall mechanism. 

The phenomenon of salt hydrolysis may be regarded as a simple application of the general Bronsted-Lowry equation... 

Since the catalytical behaviour of titanium derivatives depends on their degree of condensation, these catalysts are used under conditions where the water concentration is very low, for instance in transesterifications or in the last steps of esterifications. However, the amounts of water required to hydrolyse these compounds are so low (less than 0.5 ppm for Ti(OBu)4) that hydrolysis is probably the determining phenomenon in most studies. It seems that before Fradet and Marshal230 the contribution of this side effect has not been taken into consideration. 

A reaction with a rate constant that conforms to Eq. (10-21)—particularly to the feature that the catalysts are H+ and OH-, and not weak acids and bases—is said to show specific acid-base catalysis. This phenomenon is illustrated by the kinetic data for the hydrolysis of methyl o-carboxyphenyl acetate16 (the methyl ester of aspirin— compare with Section 6.6) ... 

General acid/base catalysis is less significant in natural fresh waters, although probably of some importance in special situations. This phenomenon can be described fairly well via the Bronsted law (relating rate constants to pKa and/or pKb of general acids and bases). Maximum rates of general acid/base catalysis can be deduced from a compound s specific acid/base hydrolysis behavior, and actual rates can be determined from relatively simple laboratory experiments (34). 

Gums are hydrophilic materials that easily undergo chemical hydrolysis with depolymerization, oxidation and microbiological attack. Leaching is a well-known phenomenon, and the inversion between arabinose and xylose content has been reported [164,165],... 

The unequal basicities of the three nitranilines can be illustrated by the following experiment. It is a general property of the salts of weak bases—as well as of weak acids—that in aqueous solution they are stable only if an excess of acid (or alkali) is present. When such solutions are diluted with water hydrolysis occurs as a result of the operation of the law of mass action. In the present case this phenomenon shows itself in the appearance of the yellow colour characteristic of the bases and finally, since the nitranilines are sparingly soluble in water, in their precipitation in crystalline form. The weaker the base the smaller is the amount of water which must be added in order to make the hydrolysis perceptible. 

Hydrolysis of metal ion complexes in homogeneous solutions of increasing pH is an important phenomenon ... 

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