Aqueous solution-precipitate system

In addition, at very low water contents, ampicillin accumulation curves do not exhibit a clear-cut maximum, inherent in the enzymatic acyl transfer reactions in aqueous medium (including quite concentrated heterogeneous aqueous solution-precipitate systems), because of the secondary hydrolysis of the target product by penicillin acylase (Figure 12.6) [84]. 

Youshko MI, van Langen LM, de Vroom E et al. (2000) Penicillin acylase-catalyzed synthesis of ampicilUn in aqueous solution-precipitate system. High substrate concentration and supersaturation effect. J Mol Catal B Enzym 10 509—515... 

Surfactants are employed in nanoparticle suspensions. Chen et al. (2002) evaluated the pre paration of amorphous nanoparticle suspensions containing cyclosporine A using the evaporative precipitation into aqueous solution (ERAS) system. The effect of particle size was studied varying the drug surfactant ratios, type of surfactants, temperature, drug load, and solvent. Acceptable particle sizes suitable for both oral and parenteral administration were also studied. Additional articles in the nanoparticle delivery of poorly water-soluble drugs include Kipp (2004), Perkins et al. (2000), Young et al. (2000), and Tyner et al. (2004). 

Example 3.11 Polymerization of Acrylonitrile with a Redox System in Aqueous Solution (Precipitation Polymerization)... 

The hydrolysis (which in the case of Fe produces acidic solutions) is virtually absent, and in aqueous solution the addition of C03 does not result in the evolution of CO2 but simply in the precipitation of white FeC03. The moist precipitate oxidizes rapidly on exposure to air but in the presence of excess CO2 the slightly soluble Fe(HC03)2 is formed. It is the presence of this in natural underground water systems, leading to the production of FeC03 on exposure to air, followed by oxidation to iron(III) oxide, which leads to the characteristic brown deposits found in many streams. 

The diversity of chemical reactions is immense. To make sense of this vast expanse of chemistry, we need a system for grouping chemical reactions into categories. The reactions within each category should share some characteristics or follow a common theme. One relatively simple category is precipitation reactions, in which cations and anions in aqueous solution combine to form neutral insoluble solids. 

Another way of analyzing these combinations is by type of substance. Parts a and c involve mixing of two aqueous solution, parts b and d involve adding a metal to an aqueous system, and part e is the interaction of a metal with O2 gas. When solutions mix, we look first for acid-base reactions (part c), then for formation of a precipitate (part b). When a metal contacts an aqueous system, the most likely reaction, if any, is oxidation of the metal. Any time molecular oxygen is present, we can expect oxidation to be one possibility. 

The process of precipitation of a metal from an aqueous solution of its salt by another metal is the well-known cementation process, so named because the precipitated metal is usually cemented on the metal introduced into the system. The process prediction stems from consideration of electrode potentials of metals. The metal positioned with greater (oxidation) potential in the electromotive series will pass into solution and remove a metal positioned with a less positive potential. The larger the spread of the positions of the two metals in the series, the greater is the possibility or feasibility of cementing out one by the other. 

Siroc [Silicate rock] Also known as the one-shot system. A chemical grouting system for hardening ground formations. Aqueous solutions of sodium silicate and formamide are mixed and injected into the ground the formamide slowly reacts with the silicate, precipitating hydrated silica, which binds the soil particles together. Invented in 1961 by the Diamond Alkali Company. See also Joosten. 

The use of the enzyme system then known as invertin, which was extracted from beer yeast with water and precipitated from the aqueous solution, was available to Fischer when he began his classical studies of the enzymic hydrolysis of glucosides reported in 1894. The stage was also set by another enzyme known as emulsin, which Fischer purchased from E. Merck, Darmstadt, and which was known to hydrolyze several natural aromatic glucosides such as salicin, coniferin, arbutin, and the synthetic phenyl glucoside. These aryl glucosides were already known to not be cleaved by invertin. 

The geochemical fate of most reactive substances (trace metals, pollutants) is controlled by the reaction of solutes with solid surfaces. Simple chemical models for the residence time of reactive elements in oceans, lakes, sediment, and soil systems are based on the partitioning of chemical species between the aqueous solution and the particle surface. The rates of processes involved in precipitation (heterogeneous nucleation, crystal growth) and dissolution of mineral phases, of importance in the weathering of rocks, in the formation of soils, and sediment diagenesis, are critically dependent on surface species and their structural identity. 

---