Equilibrium precipitation processes

Equations (141) and (142) describe the equilibrium between the hydrolysis of complex fluoride acids (shift to the right) and the fluorination of hydroxides (shift to the left). Near complete precipitation of hydroxides can be achieved by applying an excessive amount of ammonia. Typically, precipitation is performed by adding ammonia solution up to pH = 8-9. However, the precipitate that separates from the mother solution can be contaminated with as much as 20% wt. fluorine [490]. Analysis of niobium hydroxides obtained under different precipitation conditions showed that the most important parameter affecting the fluorine content of the resultant hydroxide is the amount of ammonia added [490]. Sheka et al. [491] found that increasing the pH to 9.6 toward the end of the precipitation process leads to a significant reduction in fluorine content of the niobium hydroxide. 

These opposing tendencies may defeat the purpose of the fractional precipitation process. The fractional precipitation of crystalline polymers such as nitrocellulose, cellulose acetate, high-melting polyamides, and polyvinylidene chloride consequently is notoriously inefficient, unless conditions are so chosen as to avoid the separation of the polymer in semicrystalline form. Intermediate fractions removed in the course of fractional precipitation may even exceed in molecular weight those removed earlier. Separation by fractional extraction should be more appropriate for crystalline polymers inasmuch as both equilibrium solubility and rate of solution favor dissolution of the components of lowest molecular weight remaining in the sample. 

Let us consider the dissolution-precipitation process in seawater in the following example. The normal concentrations of calcium and of carbonate in the near-surface oceanic waters are about [Ca2+] = 0.01 and [C032-] 2 x lO"4 M. The CaC03 in solution is metastable and roughly 2U0% saturated (1). Should precipitation occur due to an abundance of nuclei, TC032-] will drop to 10-4 M but [Ca2+] will change by no more than 2%. Therefore, the ionic strength of the ionic medium seawater will remain essentially constant at 0.7 M. The major ion composition will also remain constant. We shall see later what the implications are for equilibrium constants. 

Although crystals can be grown from the liquid phase—either a solution or a melt—and also from the vapour phase, a degree of supersaturation, which depends on the characteristics of the system, is essential in all cases for crystal formation or growth to take place. Some solutes are readily deposited from a cooled solution whereas others crystallise only after removal of solvent. The addition of a substance to a system in order to alter equilibrium conditions is often used in precipitation processes where supersaturation is sometimes achieved by chemical reaction between two or more substances and one of the reaction products is precipitated. 

If the ion product, [Cd ][S ], exceeds the solubility product, K p, of CdS (ca. 10 Table 1.1), then CdS can form as a solid phase, although a larger ionic product may be required if supersaturation occurs. If the ion product does not exceed Ksp, no solid phase will form, except possibly transiently due to local fluctuations in the solution, and the small solid nucleii will redissolve before growing to a stable size. For that reason, the precipitation process is shown as an equilibrium rather than as a one-way reaction. 

The main driving force of a salting-out precipitation process is the supersaturation generated by the different solubilities of the solute in the solvent and antisolvent. The one-shot mode generally means that the necessary amount of the antisolvent is added in one portion to the initial solution (either saturated or undersaturated) as it is schematically shown in Fig. 2. In our cases the aqueous solution was added to the antisolvent. As a result of the antisolvent addition the original solubility of the substance will change until the new equilibrium solubility, ceq has been reached. To characterize the supersaturation driving force in a precipitation system the so called initial supersaturation,. S, is used ... 

Order-disorder, or rod-to-coil , transitions in dilute solution have been reported for polydiacetylenes (2, 5-11), polysilylenes (12-15), and alkyl-substituted polythiophenes (16). The interpretation of the experimental observations has been the subject of considerable controversy with respect to whether the observations represent a single-polymer-molecule phenomenon or a many-chain aggregation or precipitation process (3-16). Our own experimental evidence (12, 13) and that of others (5-8, 10, 16) weigh heavily in favor of the single-chain interpretation. In our theoretical interpretation, we will assume that the order-disorder transitions observed in dilute pol-ysilylene solutions represent equilibrium, single-chain phenomena. 

The lack of understanding of the dolomite precipitation process is reflected in the discrepancy of solubility products reported by different investigators. Published figures range from 10 to 10 . As mentioned before, solubility equilibrium can be reached (under atmospheric conditions) only from undersaturation. The time of approaching equilibrium is unknown. Thus it is very difficult to ascertain equilibrium in laboratory experiments. 

The sol-gel process was first used for the preparation of silicates used in phase equilibrium studies. Often the sol-gel process makes use of a concentrated hydrous sol, a colloidal dispersion of (hydrated) oxide particles produced by controlled precipitation. It is also a precipitation process that makes use of immobilization of ions in a gel or glassy structure. 

The evidence from field studies is somewhat contrary to the predictions based on equilibrium chemistry. Abiological precipitation of CaC03 seems to be very limited, restricted to geographically and geochemically unusual conditions. The reasons why carbonate minerals are reluctant to precipitate from surface seawater are still poorly understood, but probably include inhibiting effects of other dissolved ions and compounds. Even where abiological precipitation is suspected—for example, the famous ooid shoals and whitings of the Bahamas (Box 6.5)—it is often difficult to discount the effects of microbial involvement in the precipitation process. 

Notice that the equilibrium constant is an exhemely small number compared to the hydroxide Ksp. This shows that for the same copper ion concentration, an immeasurable amount of sulhde ions is needed to achieve saturahon. In other words, it will take exhemely low sulhde concentration to precipitate more of the copper than just a pH change. As we have discussed previously, the sulhde ion concenhahon is a function of the soluhon pH. That is, the sulhde ion concentra-hon will be considerably lower at low pH (where metal ions are more stable), and become higher as the pH is raised. So like the hydroxide precipitation process, the pH can be an important variable. 

Ostwald ripening is often important in proeesses in which crystallization is rapid and crystal sizes are small. This is especially true in a precipitation process and will be diseussed in detail in Chapter 6 of this volume. It is important to remember that the effect of Ostwald ripening is to alter the crystal size distribution with time in a suspension of erystals that is an apparent equilibrium with its saturated solution. Ostwald ripening is an important phenomena if you are concerned with obtaining fine particles or are concerned about changes in the crystal size distribution of your product prior to drying. 

It is interesting to see that all precipitation processes with SCFs rely on some kind of special thermodynamic behavior of mixtures at high pressure. Also, it is essential to understand phase equilibrium phenomena to appreciate any process development in this field. 

In these processes, the most frequently used supercritical solvent is supercritical carbon dioxide (SC-CO2). The reason for this are the favorable properties of SC-CO2, including no toxicity, no flammability, low critical temperature that allows to carry out the precipitation with mild operating conditions, and phase equilibrium properties relatively favorable for many precipitation processes. Supercritical precipitation processes with CO2 have been used to process a large variety of materials, including natural substances, pharmaceuticals, polymers, explosives, and inorganic substances. - ... 

See solution to problem 8-3 for analogous problem. The difference curve will show that ApKgp < 2 is a reasonable lower limit. It should be kept in mind that precipitation processes do not achieve equilibrium as rapidly as acid-base reactions so that the actual titration curve will differ from this calculated one. Coprecipitation can also introduce changes in the actual titration curves. 

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