Choosing the solvent

Paraffinic HC have an unfavorable Kp which, however, allows crude desalters to be considered as excellent phenol extractors. This function has been used to remove phenols from spent caustic recycled to the desalter inlet, but has been almost totally phased out. Tliis practice is dangerous for two reasons  

The advantage of LCO is its own low phenol content which is lower than 200 ppm. This is possible because phenols are preferentially carried over to fractionation by the main 

Its surface tension is about 23 dyn cm and has the advantage of being constant between 30 and 50 C. 

However, recent improvements in FCC efficiency have modified the specific gravity and aromatics content of LCO which may as a result become less favorable for extraction. 

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The polarity index is a measure of the polarity of the solvent, which is often the most important factor in the solvent choice for the particular application. In extraction processes, the tenet that like dissolves like (and conversely, opposites do not attract ) is the primary consideration in choosing the solvent for extraction, partitioning, and/or analytical conditions. For example, hexane often provides a selective extraction for nonpolar analytes, and toluene may provide more selectivity for aromatic analytes. 

Solubilities of the most frequently used hydrides and complex hydrides in most often used solvents are listed in Table 3. In choosing the solvent it is necessary to consider not only the solubility of the reactants but also the boiling points in case the reduction requires heating. 

Certain solvent combinations (a water solution of sodium hydroxide and chloroform) lead to emulsions when shaken together. Emulsification cannot always be anticipated, so choose the solvent wisely, or wait along time after shaking for the emulsion to dissipate. 

For example, an ink may contain a mixture of cyclohexanone with an evaporation rate of 0.2 and butoxyethanol (Butyl Cellosolve) with an evaporation rate of 0.07. Additional factors to take into account when choosing the solvents are the interaction of the solvent with the substrate, e.g., swelling of polymeric substrates enhances adhesion. Surface tension will influence print quality and dot gain with high surface tension solvents leading to smaller drops. 

Unfortunately, donor numbers have been defined in the non-SI unit kcal mol b Marcus has presented a scale of dimensionless, normalized donor numbers DN, which are defined according to DN = DNl i%.% kcal mol ) [200]. The non-donor solvent 1,2-dichloroethane [DN = DN = 0.0) and the strong donor solvent hexamethyl-phosphoric triamide (EiMPT DN = 38.8 kcal mol DN = 1.0) have been used to define the scale. Although solvents with higher donicity than EiMPT are known [cf. Table 2-3), it is expedient to choose the solvent with the highest directly [i.e. calori-metrically) determined DN value so far as the second reference solvent [200] f The DN values are included in Table 2-3. 

The correct choice of a solvent system is of paramount importance for a successful CPC separation. Selection of a two-phase (biphasic) solvent system for CPC is similar to choosing the solvents for other chromatographic methods such as for a normal column or HPLC (50). Important criteria are the polarity of the sample components and their solubility, charge state, and their ability to form complexes, etc. The most critical points in selecting solvent systems for CPC are two-fold one is solubility of the sample and the second is the difference in partition coefficients of the molecular species that are to be separated. 

In choosing the solvent the chemist is guided by the dictum like dissolves like. Even the nonchemist knows that oil and water do not mix and that sugar and salt dissolve in water but not in oil. Hydrocarbon solvents such as hexane will dissolve hydrocarbons and other nonpolar compounds, and hydroxylic solvents such as water and ethanol will dissolve polar compounds. Often it is difficult to decide, simply by looking at the structure of a molecule, just how polar or nonpolar it is and therefore which solvent would be best. Therefore, the solvent is often chosen by experimentation. 

Choosing the solvent. Like dissolves like. Some common solvents are water, methanol, ethanol, ligroin, and toluene. When you use a solvent pair, dissolve the solute in the better solvent and add the poorer solvent to the hot solution until saturation occurs. Some common solvent pairs are ethanol-water, diethyl ether-ligroin, and tolu-ene-ligroin. 

Regulatory issues—Environmental regulations should be considered when choosing the solvent. 

Diffusion of molecules in a solvent will depend on the nature of the molecules and on the nature of the solvent. Two important factors are the size of the molecules and the viscosity of the solvent. The analyst has no control on the size of the molecules, but should always choose the solvent that has the lower viscosity. For example when facing the choice between methanol (viscosity =... 

Incidentally, we note that for neutral polymer solutions, the form function can also be determined in a simpler way, by choosing the solvent composition so as to get zero average contrast (C, b" + C2 bj = 0). 

Dissolve a sample of the mixture in hot solvent Choose the solvent such that B and C are soluble at any temperature reached in the crystallization, while compound A is not... 

Note To use Table 7.18, place the solute in row and choose the solvent from the column groups that lowers the activity coefficient. Sign (+) means that the column group component tends to increase the activity coefficient, sign (-) means decrease, while sign (0) means no effect. 

Repeat Example 1.1 for a solvent for which E = 0.90. Display your results with a plot like Fig. 1.15. Does countercurrent flow still have a marked advantage over crosscurrent flow It is desirable to choose the solvent and solvent rate so that E > 1 Explain. 

This section has been devoted to discussing the factors to be taken into account in choosing the solvent and supporting electrolyte. The dual role of the supporting electrolyte is established and in particular its role in ensuring diffusion control as the dominant mechanism for mass transport of analyte to the electrode illustrated. The marginal effect of temperature has been explained. 

Ta types of anions with chemical potentials /ii- Using Hansen s reference system and choosing the solvents a and P as reference substances, one can write down Hansen s rendition of Gibbs adsorption equation ... 

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