Dilute solutions absorption

Table IX. Dilute Solution Absorption Values for Polyethylene, Poly (vinyl Chloride), and Nitrile Polymer...

DILUTE SOLUTION ABSORPTION SPECTRA. Figure 1 shows the absorption spectra of a dilute solution of PMPS in 2MTHF at room temperature and at 77°K. At room temperature the spectrum is characterized by a structureless band with a maximum absorbance, at 337nm with an extinctloi coefficient, , ... 

Figure 1. Dilute solution absorption spectra of poly(methyl-phenylsilylene) in 2-methyltetrahydrofuran at room ten5)erature and 77 K.

Salame (35 ) has considered the scalping of flavor constituents into polymers by dilute solution absorption. He found sorption in hydrophilic polymers to be dependent on surrounding water activity (Table III). In the most hydrophilic polymers such as nylon and EVOH, there was an increase both in non-polar organics and polar organics in these two polymers in the presence of water. The increase in non-polar organics could be attributed to increased free volume of the polymer in the sorbed polymer state, while the increase in polar organics could be attributed to additional interactions available for the polymer molecule. 

Other than optically dilute solutions, absorption within these optical path lengths can distort excitation and emission spectra (so called inner filter effects). Emission cells of shorter pathlength for standard 1 cm cell holders must either be made with the solution held in the middle of the cell, e.g., 1 mm square wells in a 1 cm block, or held in an adapter which does this. Again many instrument manufacturers offer a variety of emission cells and cell holders, allowing measurement down to volumes of a few pL. 

Figure 21 Potential energy diagram of the ground and the first excited electronic states of [Ag(CN)32 (eclipsed configuration) as plotted from extended Huckel calculations. The excimer [Ag(CN)32 corresponds to the potential minimum of the excited state. The optical transitions shown are (a) excimer emission, (b) solid state excitation and (c) dilute solution absorption. (Reproduced with permission from Omary MA and Patterson HH (1998) Luminescent homoatomic exciplexes in dicyanoargentate 0) ions doped in alkali halide crystals 1. Exciplex tuning by site-selective excitation. Journal of the American Chemical Society 120 7606-7706.

When the ascending solvent-front has reached a convenient height, the strip is removed, the position of the solvent-front marked, and the paper strip dried. The positions of the various solutes, if they are coloured compounds, now appear as clear separate spots. Frequently however, the solutes are colourless, and the position of their spots must be determined by indirect methods, such as their fluorescence in ultraviolet light, or their absorption in such light (when the spots appear almost black), or by spraying the paper with a dilute solution of a reagent which will give a coloured insoluble derivative with the solutes. 

Band 3, 3-93y. (2548 cm. ). This absorption is characteristic of carboxylic acids and is due to the 0—H stretching absorption in the reson-ance-stabUised dimer. (Carboxylic acids generally exist as dimers in the solid state and in all but very dilute solutions.)... 

It must be emphasised that the above Tables must be used with caution. The presence of a specific group cannot always be established with certainty from the presence of the absorption band, particularly in the deformation vibration region on the other hand, the absence of the appropriate absorption band indicates that the grouping is not present. The physical state in which the substance is examined may have an appreciable influence the Tables apply generally to dilute solutions in organic solvents (see Table I). 

The thiazolium ring, as most heterocycloammoniums, is a Lewis acid conferring to the carbon atom in the 2-position the carbocationic property of adding the free pair of a base either organic or mineral that may be the molecule of solvent as ROH (Scheme 11). For many nuclei of suitable acidity, these equilibria can be observed in dilute solution by means of absorption spectra when species A and C possess different characteristics (24). For example, benzothiazolium and benzoxazolium in methanol and ethanol give at 10 mole liter 8 and 54% of the alkoxy derivatives for the former and 29 and 90% for the latter respectively. 

Description of Method. Quinine is an alkaloid used in treating malaria (it also is found in tonic water). It is a strongly fluorescent compound in dilute solutions of H2SO4 (f = 0.55). The excitation spectrum of quinine shows two absorption bands at 250 nm and 350 nm, and the emission spectrum shows a single emission band at 450 nm. Quinine is rapidly excreted from the body in urine and is easily determined by fluorescence following its extraction from the urine sample. 

If the exponent in Eq. (6.10) is small-which means dilute solutions in practice, since most absorption experiments are done where a is large—then the exponential can be expanded, e = 1 + x + , with only the leading terms retained to give... 

The drying mechanisms of desiccants may be classified as foUows Class 1 chemical reaction, which forms either a new compound or a hydrate Class 2 physical absorption with constant relative humidity or vapor pressure (solid + water + saturated solution) Class 3 physical absorption with variable relative humidity or vapor pressure (soHd or liquid + water + diluted solution) and Class 4 physical adsorption. 

The simplest possible case occurs when (1) both the operating and the equilibrium hues are straight (i.e., there are dilute solutions), (2) Henry s law is valid y /x = yifXj = m), and (3) absorption heat effects are negligible. Under these conditions, the integral term in Eq. (14-20) may be computed by Colburn s equation [Trans. Am. Jn.st. Chem. Eng., 35,211 (1939)] ... 

With a reactive solvent, the mass-transfer coefficient may be enhanced by a factor E so that, for instance. Kg is replaced by EKg. Like specific rates of ordinary chemical reactions, such enhancements must be found experimentally. There are no generalized correlations. Some calculations have been made for idealized situations, such as complete reaction in the liquid film. Tables 23-6 and 23-7 show a few spot data. On that basis, a tower for absorption of SO9 with NaOH is smaller than that with pure water by a factor of roughly 0.317/7.0 = 0.045. Table 23-8 lists the main factors that are needed for mathematical representation of KgO in a typical case of the absorption of CO9 by aqueous mouethauolamiue. Figure 23-27 shows some of the complex behaviors of equilibria and mass-transfer coefficients for the absorption of CO9 in solutions of potassium carbonate. Other than Henry s law, p = HC, which holds for some fairly dilute solutions, there is no general form of equilibrium relation. A typically complex equation is that for CO9 in contact with sodium carbonate solutions (Harte, Baker, and Purcell, Ind. Eng. Chem., 25, 528 [1933]), which is... 

The interpretation of these remarkable properties has excited considerable interest whilst there is still some uncertainty as to detail, it is now generally agreed that in dilute solution the alkali metals ionize to give a cation M+ and a quasi-free electron which is distributed over a cavity in the solvent of radius 300-340 pm formed by displacement of 2-3 NH3 molecules. This species has a broad absorption band extending into the infrared with a maximum at 1500nm and it is the short wavelength tail of this band which gives rise to the deep-blue colour of the solutions. The cavity model also interprets the fact that dissolution occurs with considerable expansion of volume so that the solutions have densities that are appreciably lower than that of liquid ammonia itself. The variation of properties with concentration can best be explained in terms of three equilibria between five solute species M, M2, M+, M and e ... 

The PL spectrum and onset of the absorption spectrum of poly(2,5-dioctyloxy-para-phenylene vinylene) (DOO-PPV) are shown in Figure 7-8b. The PL spectrum exhibits several phonon replica at 1.8, 1.98, and 2.15 eV. The PL spectrum is not corrected for the system spectral response or self-absorption. These corrections would affect the relative intensities of the peaks, but not their positions. The highest energy peak is taken as the zero-phonon (0-0) transition and the two lower peaks correspond to one- and two-phonon transitions (1-0 and 2-0, respectively). The 2-0 transition is significantly broader than the 0-0 transition. This could be explained by the existence of several unresolved phonon modes which couple to electronic transitions. In this section we concentrate on films and dilute solutions of DOO-PPV, though similar measurements have been carried out on MEH-PPV [23]. Fresh DOO-PPV thin films were cast from chloroform solutions of 5% molar concentration onto quartz substrates the films were kept under constant vacuum. 

Figure 16-36 shows the absorption spectra of thin films of four differently substituted five-ring OPVs. in contrast to the solution spectra, which show structureless low-energy absorption bands, the absorption bands of the films are structured. In the solid slate, the molecules are spatially constrained, whereas in solution different conformers exist, resulting in a distribution of accessible levels. As a consequence, some details appear in the absorption spectrum of the films which can be attributed to vibronic coupling, while, in dilute solution, the spectrum is a broad featureless band. For oct-OPV5 and Ooci-OPV5 films, the absorption maxima are red-shifted over approximately 0.1 eV relative to the solution (see Fig. 16-12). The low-energy absorption band of a thin film of Ooct-OPV5-CN" displays an appreciably larger...

The substituted five-ring OPVs have been processed into poly crystal line thin films by vacuum deposition onto a substrate from the vapor phase. Optical absorption and photolumincscence of the films are significantly different from dilute solution spectra, which indicates that intermolecular interactions play an important role in the solid-state spectra. The molecular orientation and crystal domain size can be increased by thermal annealing of the films. This control of the microstruc-ture is essential for the use of such films in photonic devices. 

Suppose one wishes nevertheless to use x-ray emission spectrography for the determination of iron in these materials. In principle, this can be done if the absorption effect is eliminated or kept constant—-if, for example, the material is placed in dilute solution in a relatively transparent solvent (7.8). 

Absorption values, dilute solution Acid on applesauce stored in cans,... 

If a dilute solution is mixed with pure solvent, without performance of external work, there is no evolution or absorption of heat, so that AU = 0. 

In the development of the theory of freezing-points for very dilute solutions (Chap. XI.) it was assumed that both the change of total volume and the heat absorption on further dilution are zero. With solutions of moderate concentration (say up to 5N), neither assumption is true, but we know that the change of volume is always very small, and possibly negligible, whilst the heat absorption is not usually of such small magnitude. It... 

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