Precipitation kinetics aging

Kinetic aging of iron hydroxides precipitated from true solution is depicted in Fig. 53b, which illustrates the dependence of the second moment... 

Aging temperature was also an important parameter with catalysts prepared at 70 °C being more active than those prepared at 50 °C. It was concluded that where pH exerted its effect through modification of the chemical composition, temperature mainly affects the precipitation kinetics. In general, the presence of basic nitrate in the precipitate was detrimental, whereas malachite was beneficial [32]. 

From these somewhat discordant investigations, it would seem that both the detailed chemist and kinetics involved are very dependent on both precipitation and ageing conditions. At the temperatures used in most of the studies (60-80 C) the time for malachite to crystallise from the initial amorphous precipitate is less than 9 minutes [ 17 ]. Most of the samples examined were consequently well "aged" only Pollard and Shen sampling early in a precipitation of rapid addition have detected amorphous phases which transform or react to produce crystalline copper/zinc minerals. 

The time-temperature behaviors of Alloys C and C-276 are compared in Figure 12.14, which shows much slower precipitation kinetics in AUoy C-276. Therefore, the evolution of AUoy C-276 from AUoy C enabled the use of this alloy system in the as-welded condition. However, because only carbon and sihcon were controlled in C-276, there remained the problem of intermetallic p-phase precipitation, which occurred at longer times of aging. Alloy C-4 was developed with lower iron, cobalt, and tungsten levels to prevent precipitation of /4-/i-phases. 

Altman and King made the first detailed study of this system, using Cr(ril) solutions containing only the monomeric species and Cr(VI) solutions which had been allowed to age. The isotopic method and lead chromate precipitation separation were used to obtain kinetic data at a temperature of 94.8 °C. Over the range of concentrations, Cr(VI) 2.3x10 to 8.4x10 A/, Cr(III) 1.8x10 to... 

The prepared solution will slowly deposit Se as a black precipitate over a period of weeks. Also, the solution is considerably less stable than thiourea. If kept out of excessive contact with air, it will be usable for about a month if high reproducibility of the deposition kinetics is not important. However, it is important to be aware of the slow decomposition and loss of reactivity of this reactant. If a freshly made NaiSeSOs solution is used to deposit CdSe, the reaction will proceed much faster than if an aged solution is used this fact should be taken into account in preparing the overall deposition solution. 

The reverse of the coagulation process is peptization. It is well known from analytical chemistry that fresh precipitates are easier to disperse than old ones, which indicates qualitatively that an aggregate of colloidal particles is not in equillibrium and that irreversible, time-dependent processes occur in coagulation. As pointed out Frens and Overbeek (1971) the interpretation of peptization phenomena with aggregated systems is not possible unless the data are obtained in experiments with a shorter time scale than the aging time of the aggregate. They demonstrated that it was possible to follow the kinetics of peptization by suddenly diluting the sol... 

The establishment of hydrolysis equilibria is usually very fast, as long as the hydrolysis species are simple. Polynuclear complexes are often formed rather slowly. Many of these polynuclear hydroxo complexes are kinetic intermediates in the slow transition from free metal ions to solid precipitates and are tlius thermodynamically unstable. Some metal-ion solutions age, that is, tliey change their composition over periods of weeks because of slow structural transformations of the isopoly ions. Such nonequilibrium conditions can frequently be recognized if the properties of metal-ion solutions (electrode potentials, spectra, conductivity, light scattering, coagulation effects, sedimentation rates, etc.) depend on the history of the solution preparation. 

In an earlier study, Larsen and Ravnholt [55] showed that the dissolution kinetics of differently sourced CaF2 and the calcium-fluoride-like material that is actually formed in saliva (as opposed to in water) varied markedly, depending on both the different solid preparations and on the fluid (water or saliva). Saliva inhibits CaF2 s precipitation and dissolution. This is illustrated in figure 3, which shows results of a salivary clearance study taken from a patent of Clarkson et al. [56], Aqueous solutions of calcium chloride and NaF were mixed at various times relative to the application of the resultant CaF2 -precipitating mixture to a group of individuals as a form of mouth rinse. Saliva samples were collected at least 1 h after the 1-min mouth rinse and were analysed for F after the addition of TISAB buffer, as described elsewhere [42], The effect of the age of the precipitated material was dramatic salivary F concentrations... 

Ferrihydrite obtained by hydrolysis of Fe2(S04)3 is very slowly transformed into goethite at pH 7 [20], After one year only 7% of the initially formed ferrihydrite was transformed, while for ferrihydrite obtained by hydrolysis of FeCls or Fe(N03)3 the degree of transformation into mixture of goethite and hematite was 30% at the same pH. On the other hand at pH 11 the transformation into goethite was almost complete after one year and the difference in the transformation kinetics between ferrihydrite obtained by hydrolysis of Fe2(S04>3, FeCU and Fe(N03)3 was less significant. At pH 8 10 substantial amount of hematite is present after a one year aging. These results show that in adsorption experiments with fresh precipitates we can deal with two different adsorbents in the beginning and in the end the experiment. 

At first, the effect of ageing time on precipitation and growth of particles has been studied. The characterisation of particles aged for various duration shows that one week aged boehmite particles have reached their thermodynamic stability (table 1), so that their different morphologies do not result from any kinetic effect. Hereafter, we discuss only boehmite materials obtained for one week ageing. 

The effect of ageing of Th02(am, hyd) precipitates may be interpreted in terms of increasing particle size via dissolution-precipitation equilibria, so called Ostwald ripening. This effect is kinetically favoured at higher aqueous Th concentrations, i.e., at low pH. 

The laboratory studies were designed to examine in detail the composition of aluminum species in dilute solutions having values of r between 0.5 and 4.0, after various periods of aging. The behavior of OH boimd to aluminum was examined by kinetic methods, precipitated solids were identified by the electron microscope and x-ray diffractometer, and equilibrium solubilities were calculated for stable and metastable aluminum hydroxides. 

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