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Nonaqueous problem

Acryhc and methacryhc nonaqueous dispersions (NADs) are primarily utilized by the coatings industry to avoid certain difficulties associated with aqueous dispersion (emulsion) polymers. Water as a suspension medium has numerous practical advantages, but also some inherent difficulties a high heat of evaporation, a low boiling point, and an evaporation rate that depends on the prevailing humidity. Nonaqueous dispersions alleviate these problems, but introduce others such as flammabihty, increased cost, odor, and toxicity. [Pg.268]

Nonaqueous phase Hquids (NAPLs) present special problems for soil and ground water cleanup. Contaminant transport through ground water depends in part on the water solubiHty of the compound. Because NAPLs cling to subsurface particles and are slow to dissolve in ground water, they hinder cleanups and prolong cleanup times. Dense nonaqueous phase Hquids (DNAPLs) migrate downward in the aquifer and can coUect in pools or pockets of the substmcture. Examples of DNAPLs are the common solvents tetrachloroethylene (PCE) and trichloroethylene (TCE) which were used extensively at many faciHties before the extent of subsurface contamination problems was realized. [Pg.169]

Although the ethyleneamines ate water soluble, soHd amine hydrates may form at certain concentrations that may plug processing equipment, vent lines, and safety devices. Hydrate formation usually can be avoided by insulating and heat tracing equipment to maintain a temperature of at least 50°C. Water cleanup of ethyleneamine equipment can result in hydrate formation even in areas where routine processing is nonaqueous. Use of warm water can reduce the extent of the problem. [Pg.46]

A more difficult criterion to meet with flow markers is that the polymer samples not contain interferents that coelute with or very near the flow marker and either affect its retention time or the ability of the analyst to reproducibly identify the retention time of the peak. Water is a ubiquitous problem in nonaqueous GPC and, when using a refractive index detector, it can cause a variable magnitude, negative area peak that may coelute with certain choices of totally permeated flow markers. This variable area negative peak may alter the apparent position of the flow marker when the flow rate has actually been invariant, thereby causing the user to falsely adjust data to compensate for the flow error. Similar problems can occur with the elution of positive peaks that are not exactly identical in elution to the totally permeated flow marker. Species that often contribute to these problems are residual monomer, reactants, surfactants, by-products, or buffers from the synthesis of the polymer. [Pg.549]

The use of totally permeated flow markers in aqueous GPC offers similar advantages along with many of the same shortcomings that one finds in nonaqueous GPC. One problem commonly found in aqueous GPC is that salt peaks due to the on-column ion exchange of counter ions of a polyelectrolyte with dissimilar ions in the GPC mobile phase will occur at or near the total permeation volume of the column. These salt peaks will often obscure the flow marker used in the analysis. Short of preconditioning the sample to exchange... [Pg.549]

The problem with the Arrhenius definitions is that they are specific to one particular solvent, water. When chemists studied nonaqueous solvents, such as liquid ammonia, they found that a number of substances showed the same pattern of acid-base behavior, but plainly the Arrhenius definitions could not be used. A major advance in our understanding of what it means to be an acid or a base came in 1923, when two chemists working independently, Thomas Lowry in England and Johannes Bronsted in Denmark, came up with the same idea. Their insight was to realize that the key process responsible for the properties of acids and bases was the transfer of a proton (a hydrogen ion) from one substance to another. The Bronsted-Lowry definition of acids and bases is as follows ... [Pg.97]

The single most important consideration in nonaqueous GPC is sample solubility. Although adsorption is not an infrequent problem, finding a solvent for a polymer is usually the hard step in analysis. The most common solvents for nonaqueous GPC are toluene, tetrahydrofuran, chloroform, and DMF. A number of potentially useful solvents are toxic, corrosive, or expensive,... [Pg.332]

Lin and coworkers disclosed that, at room temperature, nonenzymatic chemical addition was still observed in a water-organic solvent biphasic reaction system, though the volume of aqueous phases was relative small. Lin developed a method of preparing an active enzyme meal that contained essential water to retain its power for catalysis and found a new catalytic reaction system by application of the prepared meal in a nonaqueous monophasic organic medium (Figure 5.7). There was no problem over a wide range of temperature (from 0-30 °C) when the reactions were carried out under micro-aqueous conditions [50]. [Pg.111]

Last but not least of the liquid calorimetric media are aqueous solutions used in the hydrolysis of simple and complex fluorides. Stepwise replacement of F by OH occurs, and mixed products are not unusual. Thus the BFj ion hydrolyzes to species BF (OH)l and one has to ensure that the same product composition is formed in the auxiliary heat experiments (99). The problem is accentuated when polynuclear species form, as the equilibration can be slow. The inconsistencies in the heats of alkaline hydrolysis of MoF6 and WFe found by various authors and of the enthalpy of SbF5—derived by assuming SbF5 and Sb205 dissolved in 10 M HF produced the same species in solution—illustrate the difficulties. It is as well to confirm enthalpies of higher valent fluorides obtained by hydrolysis by alternative nonaqueous methods, especially since uncertainty in the Afl (Fderived enthalpy. The advantage of hydrolysis methods, apart from the simplicity of technique, is that the heats are small and one can tolerate... [Pg.23]

Before considering different theoretical approaches to determining the free energies and other thermodynamic properties of ionic solvation, it is important to be aware of a problem on the experimental level. There are several methods available for obtaining these quantities for electrolyte solutions, both aqueous and nonaqueous some of these have been described by Conway and Bockris162 and by Padova.163 For example, enthalpies of solvation can be found via thermodynamic cycles, free energies from solubilities or galvanic cell potentials. However the results... [Pg.59]

The techniques and apparatus which have been developed to measure electrolytic conductivities in nonaqueous solutions have been adapted from aqueous conductivity measurements with some modifications. Direct current measurements suffer the limitation of requiring reversible electrodes - a serious limitation in nonaqueous solvents. Although this problem can be circumvented U in some instances, virtually all precision conductance data have been taken using the alternating current method. General descriptions of this method are given in several sources. 2>3)... [Pg.5]

Several problems arise in the preparation of solutions in nonaqueous solvents. The large thermal coefficient of expansion of many solvents necessitates the use of weight methods to establish concentrations, with subsequent calculation of molarities from weight concentrations. Also, solutions must be prepared and maintained under strictly anhydrous conditions during the course of the experiment. Further, since the preparation of quantities of highly pure solvent is difficult, the use of minimum amounts is desirable. Finally, salts sometimes dissolve very slowly in certain solvents, which makes efficient stirring to hasten dissolution important. [Pg.7]

Variations of resistance with frequency can also be caused by electrode polarization. A conductance cell can be represented in a simplified way as resistance and capacitance in series, the latter being the double layer capacitance at the electrodes. Only if this capacitance is sufficiently large will the measured resistance be independent of frequency. To accomplish this, electrodes are often covered with platinum black 2>. This is generally unsuitable in nonaqueous solvent studies because of possible catalysis of chemical reactions and because of adsorption problems encountered with dilute solutions required for useful data. The equivalent circuit for a conductance cell is also complicated by impedances due to faradaic processes and the geometric capacity of the cell 2>3( . [Pg.9]

Amokrane and Badiali proposed a semiempirical approach to the determination of the solvent contribution C, to the capacitance of the double layer in aqueous and nonaqueous " solutions. They used the relation C = Cf - C m, where Q is the experimentally determined capacity of the inner layer and Cm is the contribution of the metal. The plots ofC, vs. (Tm were presented for various solvents and correlated with their properties.However, the problem of the supporting electrolyte was entirely neglected in the quoted papers. It was shown recently that the height and position of the maximum on the C, vs. Gm plots depend on the type of the supporting electrolyte. Experimental differential capacity data obtained on the Hg electrode in methanol and ethanol containing various electrolytes with nonadsorbing anions (F , PFg, ClOi) indicate that the type as well as concentration of the electrolyte influences the position and the height of the maximum on the C, vs. plots (Fig. 13). [Pg.58]

All the kinetic features expected for a D mechanism and rate law (4.9) i. e. marked effects of L and Lj on the rate constants, are shown in the comprehensive studies in nonaqueous solution of substitution in low-spin Fe(II) complexes of the type FeN4XY where N4 are planar porphyrins, phthalocyanins and macrocycles and X and Y are neutral ligands, CO, R3P, pyridines etc. Small discrimination factors (Ar, /kj) suggest that the five-coordinated intermediate in these systems is very reactive.There have been problems in the confirmation of curvature in the plots of A o,j/[L ] for classical reactions of a number of aquapent-ammine complexes. ... [Pg.210]

The aqueous batteries use water based electrolytes (e.g., KOH electrolyte for NiCd and NiMH and H2-SO4 electrolyte for lead acid), which are less resistive then nonaqueous electrolytes. Polyolefin materials are generally suitable for use in the manufacture of separators for these batteries, but they are not inherently wettable by aqueous electrolytes. Such electrolytes are therefore unable to penetrate the pores of a separator formed from such a material, so that ion migration through the pores in solution will not occur without modification. This problem is sometimes overcome by treating the polyolefin material with a surfactant, which allows an aqueous electrolyte to wet the material. However, such surfactant can be removed from the surfaces of the polyolefin material when electrolyte is lost from the device, for example during charging and discharging cycles, and it is not subsequently replaced on the material when the electrolyte is replenished. [Pg.206]


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See also in sourсe #XX -- [ Pg.307 , Pg.308 ]




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Nonaqueous

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