Rates of precipitation

The hydrolysis of urea is strongly temperature-dependent, with the rate being negligible at room temperature. The rate of hydrolysis, and thus the rate of precipitate formation, can be controlled by adjusting the solution s temperature. Precipitates of BaCr04, for example, have been produced in this manner. 

Precipitatioa (2,13—17) techniques employ a combination of nucleation and growth iaduced by adding a chemical precipitant, or by changing the temperature and/or pressure of the solution. Chemical homogeneity is controlled by controlling the rate of precipitation. FFeterogeneous precipitation iavolves the precipitation of a soHd of different composition from the solution, and the composition of the precipitate may change as precipitation continues. Coprecipitation iavolves the simultaneous precipitation of similar size cations ia a salt as a soHd solutioa. 

The rate of precipitation is the rate at which molecules return to the surface and fit into the crystal lattice. To do this, the molecules in solution must first strike the crystal surface. Again, the more surface, the more frequently will dissolved molecules encounter a piece of crystal. The rate of precipitation is proportional to A. 

In addition, the rate that molecules strike the surface depends upon how many molecules there are per unit volume of solution. As the concentration rises, more and more molecules strike the surface per unit time. The rate of precipitation is proportional to the iodine concentration,... 

We have, then, three factors that determine the rate of precipitation ... 

Polymerization Rate and Radical Occlusion. In modeling the rate of precipitation polymerization, the reaction can occur at three different loci in the diluent, at the surface of the particles, or in the interior of the particles. 

The speed of precipitation of silver required depends on the surface to be silvered. If the rate of precipitation is too great, the silvering agent may be spent before it can be removed from the surface, and a bloom will appear because the silver is in contact with the spent solution. This is a discolouration of the silver. Thus if a rapid silvering agent is used for Dewar flasks, it may be difficult to avoid spoiling the silvering. 

The dependency of rate of precipitation of barite from aqueous solution on time at room temperature studied by Nielsen (1958) suggests that the precipitation of barite from solutions of high levels of supersaturation (i.e., more than 30 as saturation index (S.I.) which is defined as the ratio, where A sp is the solubility... 

There are various parameters and assumptions defining radionuclide behavior that are frequently part of model descriptions that require constraints. While these must generally be determined for each particular site, laboratory experiments must also be conducted to further define the range of possibilities and the operation of particular mechanisms. These include the reversibility of adsorption, the relative rates of radionuclide leaching, the rates of irreversible incorporation of sorbed nuclides, and the rates of precipitation when concentrations are above Th or U mineral solubility limits. A key issue is whether the recoil rates of radionuclides can be clearly related to the release rates of Rn the models are most useful for providing precise values for parameters such as retardation factors, and many values rely on a reliable value for the recoil fluxes, and this is always obtained from Rn groundwater activities. These values are only as well constrained as this assumption, which therefore must be bolstered by clearer evidence. 

The severe interaction of the zinc bromide fluid, 19.2 ppg (2.32 g/cc), was unexpected. Severe plugging of the core occurred, caused by precipitation of zinc hydroxide, as the injected solution mixed with and was neutralized by formation brine. Tests in which the zinc bromide fluid was simply titrated with distilled water also produced a precipitate, 0.0036 g/cc. Titration in the presence of the common reservoir clay, montmorillonite, increased both the rate of precipitation, and total quantity to 0.03 g/cc. 

The proposed mechanism of effect of surfactant and ultrasound is reported in Fig. 7.5. The long chain surfactant molecules attach to surface of nanoparticles due to physical adsorption. Only thin layer is adsorbed onto the CaC03 nanoparticles. Due to presence of ultrasound and use of surfactant will control the nucleation. Surfactant keeps the particles away from each other by preventing flocculation due to change in surface tension of reaction mass. The concentration of additives was changed from 0.2 to 1.0 g/L. Addition of 0.2 g/L tripolyphosphate shows the increase in the rate of precipitation which is determined from the Ca(OH)2 consumption. Polyacrylic acid shows the least rate of precipitation (0.115 mol/1), which... 

I assume that the rate of precipitation of calcium carbonate depends on the degree of saturation of the waters with respect to calcium carbonate. This degree of saturation is expressed by the quantity... 

The dissolution rate, according to the theory, does not depend on the mineral s saturation state. The precipitation rate, on the other hand, varies strongly with saturation, exceeding the dissolution rate only when the mineral is supersaturated. At the point of equilibrium, the dissolution rate matches the rate of precipitation so that the net rate of reaction is zero. There is, therefore, a strong conceptual link between the kinetic and thermodynamic interpretations equilibrium is the state in which the forward and reverse rates of a reaction balance. 

Post-precipitation involves the deposition of a sparingly soluble impurity of similar properties to the precipitate on the surface of that precipitate after it has been formed. It is particularly a problem where similar materials are being separated on the basis of their different rates of precipitation, e.g. calcium and magnesium oxalates or zinc and mercury sulphides. Copreci-... 

Let us now And out how the system works. Assume that it starts at a large reduced flow-rate (point A) and reduce the input slowly. Up to the point C, any deviation from the equilibrium curve will die out rapidly. At C, concentration fluctuations become unstable and the system evolves quite rapidly towards D (p is fixed) where it finds a stable steady-state. The system has become unstable because reducing the flow-rate enhances crystallization which through the kinetic factor enhances the rate of precipitation and thereby depletes the residual liquid. The system quenches. Upon reducing the flow-rate further, the stable evolution continues towards point E. 

The rate of precipitation depends on the rate of change in concentrations... 

There is evidence that the value of X for type I systems depends on the degree of supersaturation during the precipitation process with a somewhat better separation being reached at lower rates of precipitation, and hence, at lower degrees of supersaturation (29). This may mean that the events at the interface of solid phase and liguid phase are not completely described by Eguation (12), e.g. adsorption might also be involved. 

Figure 4. The partitioning of Baz + into calcite at high rates of precipitation. Reduction of precipitation times from several days to hours yields larger values of kcBa, as compared with Figure 1. Note change of horizontal scale.

The time behaviors for the various types of processes are shown in Figure 3. The three upper curves correspond to a kinetic control while the three lower ones result from thermodynamic control. It should be noted that at intermediate rates of precipitation the two types of mechanisms may act at the same time in terms of the difference between the ion product and K ... 

Solubility products can be used to predict the stability of a mineral by comparing the observed ion product, [A (aq)][B (aq)], to the mineral s K. K the ion product is greater than K, the solution is supersaturated with respect to that mineral. In this case, precipitation should proceed spontaneously until the ion concentrations are decreased to levels that lower the ion product to the value dictated by the K. Conversely, if the ion product is less than the K, the solution is undersaturated with respect to that mineral. Dissolution should proceed spontaneously until the ion concentrations are increased to levels that raise the ion product to a value equal to the K. At equilibrium, where the ion product has a value equal to that of the K, the rate of mineral dissolution is equal to the rate of precipitation, so the ion concentrations remain constant over time. 

Precipitation is generally much slower than adsorption. Table 3.14 compares rates of precipitation with rates of adsorption and other surface phenomena in soil systems. Rates vary greatly between precipitating compounds. They are also often subject to inhibition and catalysis by other solutes and solid phases present. 

However, rates of precipitation in soil systems may be quite different from those in solutions because precipitation is catalysed by adsorption of the reacting solutes onto soil surfaces the nature of the solid phases formed may be different and sorption may also alter the effects of inhibitors. There are very few data in the literature on these effects actually measured in soils. Figure 3.15 shows data of Huang (1990) for calcite precipitation in three soils incubated with urea. Precipitation was induced as the pH increased during urea hydrolysis ... 

The values of k were 0.18, 1.98 and 1.16 x 10 (moldm s basis) for Soils A, B and C in Figure 3.15, respectively. These values are more than four times k for the solution system. The values of the inhibition coefficients-a = —1686, = 6.13, c = 3854—were smaller than in the solution system. As a result the concentrations of P and DOC required to halve the rate of precipitation were 10 times those in the solution system. Also the interaction between [Pl] and [Cl] was negligible in the solution system but important in the soils. Figure 3.16 shows plots of Equation (3.54) for different valnes of [Pl] and [Cl] and w = 0.75 gdm. For the values used, which are realistic for submerged-soil solutions, the combined inhibitory effect of P and DOC was snch that an order of magnitude greater degree of supersaturation [(Ca +)(C03 )/A sp] is necessary to produce the same rate of precipitation as in the absence of inhibitors. 

An instrument was proposed for measurement of Li concentration in human serum, based on the turbidimetric or nephelometric determination of the maximum rate of precipitation of a Li complex, which is related to the Li concentration in the serum. The precipitating reagent was prepared by dissolving KIO4 and FeCls in a KOH solution. Measurements were made every 10 s for 5 min at 550 nm. No interference was recorded for the presence of triglycerides, cholesterol, Na or... 

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