Evaporation sequence

A position sensitive detector (PSD) is employed, of which there are several types used effectively around the world. One type is essentially a square array of multianodes, as shown in Figure 1.6. By measuring the time-of-flight and the coordinates of the ions upon the PSD, it is possible to map out a two-dimensional elemental distribution. The elemental maps are extended to the z-direction by ionizing atoms from the surface of the specimens. The z position is inferred from the position of the ion in the evaporation sequence, so that the atom distribution can be reconstructed in a three-dimensional real space. 

Figure 10.7 Evaporation sequence for seawater, showing the minerals formed (right column) and the brine density (g enr3) at which each mineral precipitates as 1000 L evaporates to dryness. Also shown are the weights of the major precipitated phases (left column) and which sum to the salinity of typical seawater, namely 35%o (parts per thousand, or g kg-1) (after Valyashkov, 1972).

Alternatively addition of cold n-hexane or u-hexanc-diethyl ether (1 1) gives complete precipitation of the catalyst as a light green solid. The product can then be separated by a filtration/evaporation sequence. 

Figure 5 illustrates a typical distillation train in a styrene plant. Benzene and toluene by-products are recovered in the overhead of the benzene—toluene column. The bottoms from the benzene—toluene column are distilled in the ethylbenzene recycle column, where the separation of ethylbenzene and styrene is effected. The ethylbenzene, containing up to 3% styrene, is taken overhead and recycled to the dehydrogenation section. The bottoms, which contain styrene, by-products heavier than styrene, polymers, inhibitor, and up to 1000 ppm ethylbenzene, are pumped to the styrene finishing column. The overhead product from this column is purified styrene. The bottoms are further processed in a residue-finishing system to recover additional styrene from the residue, which consists of heavy by-products, polymers, and inhibitor. The residue is used as fuel. The residue-finishing system can be a flash evaporator or a small distillation column. This distillation sequence is used in the Fina-Badger process and the Dow process. 

A detailed procedure for the use of MCPBA recently appeared in Reagents for Organic Synthesis by Fieser and Fieser. The commercially available MCPBA (Aldrich) is 85% pure the contaminant, m-chlorobenzoic acid, can be removed by washing with a phosphate buffer of pH 7.5. The epoxidation is usually performed as follows a solution of 3 -acetoxy-5a-androst-16-ene (2.06 g, 6.53 mmoles) in 25 ml of chloroform (or methylene dichloride) is chilled to 0° in a flask fitted with a condenser and drierite tube and treated with a solution of commercial MCPBA (1.74 g, 20% excess) in 25 ml chloroform precooled to the same temperature. The mixture is stirred and allowed to warm to room temperature. After 23 hr (or until TLC shows reaction is complete) the solution is diluted with 100 ml chloroform and washed in sequence with 100 ml of 10% sodium sulfite or sodium iodide followed by sodium thiosulfate, 200 ml of 1 M sodium bicarbonate and 200 ml water. The chloroform extract is dried (MgS04) and evaporated in vacuo to a volume of ca. 10 ml. Addition of methanol (10 ml) followed by cooling of the mixture to —10° yields 0.8 gof 16a,17a-epoxide mp 109.5-110°. Additional product can be obtained by concentration of the mother liquor (total yield 80-90%). 

Crystallization-based separation of multi-component mixtures has widespread application. The technique consists of sequences of heating, cooling, evaporation, dilution, diluent addition and solid-liquid separation. Berry and Ng (1996, 1997), Cisternas and Rudd (1993), Dye and Ng (1995), Ng (1991) and Oyander etal. (1997) proposed various schemes based on the phase diagram. Cisternas (1999) presented an alternate network flow model for synthesizing crystallization-based separations for multi-component systems. The construction... 

MesSiNs 19 (0.34 rtiL, 2.,4 mmol) and of Et2NCOCl (0.32 mL, 2.4 mmol) are added, in this sequence, under argon, to a solution of 3-aminopyrazine-l-oxide 934 (1 mmol) in abs. MeCN (8 mL) and the reaction mixture is heated under reflux for 18 h with exclusion of humidity. After evaporation in vacuo the residue is chromatographed with hexane-ethyl acetate (10 1 to 3 1) on a column of 20 g silica gel to give almost quantitative yield of microcrystalline 2-amino-3-azidopyrazine 935, m.p. 225 °C (dec.) [55] (Scheme 7.56). 

According to Hardie and Eugster s (1970) model and its later variants (see discussions in Eugster and Jones, 1979 Drever, 1988, pp. 232-250 and Jankowski and Jacobson, 1989), a natural water, as it evaporates, encounters a series of chemical divides that controls the sequence of minerals that precipitate. The reaction pathway specific to the evaporation of a water of any initial composition can be traced in detail using a reaction model like the one applied in this section to Sierra spring water. 

Harvie, C.E., J. H. Weare, L. A. Hardie and H.P. Eugster, 1980, Evaporation of seawater, calculated mineral sequences. Science 208,498-500. 

Nuclear bombardment reactions in which the product is radioactive constitute the basis of radioactivation analysis (p. 456). Although in principle any bombardment-decay sequence may be used the analyst is largely concerned with thermal neutron activation. Equation (10.13) relates the induced activity to the amount of the parent nucleide (analyte). However, practical difficulties arise because of flux inhomogeneities. It is common therefore to irradiate a standard with very similar characteristics alongside the sample, e.g. for a silicate rock sample a standard solution would be evaporated on to a similar amount of pure silica. On the assumption that identical specific activities for the analyte are then induced in the sample and standard, the amount w2 of analyte is readily calculated from... 

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