Solvent absorption, negligible

Another consequence of the strong absorption properties of water is the spectral impact of the displacement of water by dissolved solutes. Generally, in absorption spectroscopy, the solvent is selected not to absorb over the wavelength range of interest. When the absorption properties of the solvent are negligible, any displacement of solvent molecules from the optical path by the dissolution of solute molecules has a negligible effect on the measured spectrum. For near-infrared spectra of aqueous solutions, however, the absorption spectrum depends heavily on the degree of water displacement by solutes in the sample. 

The balance equation can be most easily formulated when the loadings are introduced as the measure of concentration. Initially, the carrier gas is regarded as insoluble in the solvent, and it is assumed that the vapor pressure of the solvent is negligible. The loading of the gas stream (G in moles per unit time) or of the absorption liquid (F in moles per unit time) with A is given by Equations (2.3.3-3) and (2.3.3-4), respectively ... 

The type of solvent used in the analysis may influence the absorption. In the case of a nonpolar substance in a nonpolar solvent, the influence of the solvent is negligible but in the case of polar molecules in polar solvents, there are fairly strong solute-solvent interactions, which usually lead to a decrease in resolution of the spectrum. 

It is implied that reflection losses and solvent absorption are compensated for and that scattering is negligible. 

The former exhibits absorption tjrpical of an isolated keto group, whereas the latter shows a high intensity -band associated with the conjugated system HO—C=C—C=0. The proportions of the two forms under various conditions are readily determined from the ultraviolet spectra. The ultraviolet spectra in various solvents are shown in Fig. A, 7, 2. Since the absorption of the keto form is negligible, the percentage of enol present is 100(em/e ), where e is the observed extinction at 245 mp. and that of the pure enol. It was shown that in alcoholic solution is 1900 and the percentage of enol is 12. Thus e is ca. 16000, and use of this value permits the approximate evaluation of the enol content in different solvents. The results are collected in Table XII. 

It should be noted that the highest possible absorption rates will occur under conditions in which the hquid-phase resistance is negligible and the equilibrium back pressure of the gas over the solvent is zero. Such situations would exist, for instance, for NH3 absorption into an acid solution, for SO9 absorption into an alkali solution, for vaporization of water into air, and for H9S absorption from a dilute-gas stream into a strong alkali solution, provided there is a large excess of reagent in solution to consume all the dissolved gas. This is known as the gas-phase mass-transfer limited condition, wrien both the hquid-phase resistance and the back pressure of the gas equal zero. Even when the reaction is sufficiently reversible to allow a small back pres-... 

In a gas absorption process, the solute gas A diffuses into a solvent liquid with which it reacts. The mass transfer is one of steady state unidirectional molecular diffusion and the concentration of A is always sufficiently small for bulk flow to be negligible. Under these conditions the reaction is first order with respect to the solute A. 

Several approaches may be used in modeling absorption with heat effects, depending on the job at hand (1) treat the process as isothermal by assuming a particular temperature, then add a safety factor (2) employ the classical adiabatic method, which assumes that the heat of solution manifests itself only as sensible heat in the liquid phase and that the solvent vaporization is negligible (3) use semitheoretical shortcut methods derived from rigorous calculations and (4) employ rigorous methods available from a process simulator. 

In the case of compound 32, no change in the absorption spectrum compared with that of 33 could be observed, neither in cyclohexane nor in benzonitrile. This indicates that intramolecular CT interaction is negligible in the ground-state. Nevertheless, fluorescence and transient absorption spectroscopy show that processes after excitation strongly depends on solvent polarity. In cyclohexane the Si state... 

One of the main degrees of freedom when designing absorption processes is the choice of solvent. The main criteria are that (1) the component which is to be removed absorbs well, and fast, into the solvent and (2) the absorption of the other components present in the vapour is negligible. The choice, therefore, depends on the thermodynamic properties of the components in the feed and will not be considered here. 

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