Polarity index measurement

This polarity index measures the intermolecular attraction between a solute and a solvent, whereas the Hildebrand solubility parameter is defined for pure solvent. For example, ether is not very polar and has a Hildebrand value of 7.4—about the same as hexane, which has a value of 7.3. However, ether can accept protons in the form of hydrogen bonds to its nonbonding electron pairs, and consequently its polarity index is 2.8 compared to 0.1 for hexane. 

Optical properties also provide useful stmcture information about the fiber. The orientation of the molecular chains of a fiber can be estimated from differences in the refractive indexes measured with the optical microscope, using light polarized in the parallel and perpendicular directions relative to the fiber axis (46,47). The difference of the principal refractive indexes is called the birefringence, which is illustrated with typical fiber examples as foUows. Birefringence is used to monitor the orientation of nylon filament in melt spinning (48). 

By monitoring the insulation condition of the windings during maintenance, at least once a year, which can be carried out by measuring (a) the polarization index (Section 9.5.3) and (b) the dielectric loss factor, tan S (Section 9.6) and making up the insulation as in Section 9.5.2, when the condition of the insulation is acceptable and only its level is less than permissible. 

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. 

Another measure of polarity, the polarity index, P, is calculated from solubility data. This quantity again increases with increasing polarity. Fig. 3.If shows the values of both of these quantities for a range of solvents, in order of increasing P. The order of polarity is... 

Quantitative determination of solvent polarity is difficult, and quantitative methods rely on physical properties such as dielectric constant, dipole moment and refractive index. It is not possible to determine the solvent polarity by measuring an individual solvent property, due to the complexity of solute-solvent interactions, and for this reason empirical scales of solvent polarity based on chemical... 

A general account of dipole-moment measurements and the use of these in conformational analysis has been given.3 At the University of East Anglia, for a study of the conformational equilibria of piperidines,119 121 electronic polarizations were estimated from tabulated bond polarizations122 (neglecting contributions of atomic polarization to the total polarization) or determined from refractive-index measurements. Most measurements were carried out in benzene or cyclohexane. 

Table 11.3 Ultraviolet Cutoff Wavelength Ac (at which the Solvent in a Cell of Path Length L = 1 cm has an Absorbance of 1 unit). Dielectric Constant e, Scalar Refractive Index nD (Measured at 589 nm, the Na D-line), Dipole Moment ft (Debyes30), and Reichardt s31 Solvent Polarity Index ET...

Figure 5. (a) Profile of the hydrophobic barrier in a phosphatidylcholine liposome. The circles are measured points based upon the polarity index [54] the solid line is the dielectric constant as determined with fluorescent probes [55]. (b) Calculated relative free energy for diffusion of CH across a dimyristoylphosphatidylcholine bilayer (adapted from [57h]). In both diagrams, distances are measured from the center of the bilayer. 

The polarity index or the solubility parameter may be used as a measure of solvent strength, which would be a measure of polarity in those cases. For reversed phase HPLC, solvent strength parameters have been proposed for the four most common solvents used, i.e. water (Si = 0), methanol (Si = 2.6), acetonitrile (Si = 3.2) and THF (Si = 4.5). Using these values water makes no contribution to the eluting power of the mobile phase and the solvent strength is measured by the volume fraction of organic modifier. 

For the purpose of estimating the solubility of a solute it is necessary to have some measure of the polarity of a solute or a solvent. Based on Eqs. (1) and (2), a useful polarity index should be a measure of a material s intermolecular forces, Cn and C22-Table 1 contains a list of solvents that are typically used in liquid pharmaceutical formulations and three measures of solvent polarity. Each measure of solvent polarity, or polarity index, is based upon a different measure of a material s property. For example, dielectric constant is a measure of the electrical insulating properties of a solvent, solubility parameter is determined from the molar energy of vaporization, and... 

Rohrschneider and McReynold extended the RI system to predict a PI for various stationary phases measured at a column temperature of 120°C with a 20% (w/w) loading, to minimise retention contributions from the diatomite support [11,12]. A set of five reference compounds were selected to reflect a range of polar characteristics and functional groups benzene, X butanol, Y 2-pentanone, Z nitromethane, [/ and pyridine, S. Squalane, 2, 6, 10, 15, 19, 23-hexamethyltetracosane (C30H62), is used as the reference stationary phase as it is a readily available completely non-polar, non-volatile liquid, bp = 176 at 0.05 mm. The values of X, Y, Z, U and S represent the relative affinities of the reference compounds for the stationary phase, calculated as the differences, ARI, between the RI of the reference on a chosen stationary phase compared to the RI on squalane. The polarity index, PI, is the mean of the RI values (Table 5.2). 

The polarity index, P, is a numerical measure of the relative polarity of various solvents as determined from their solubility in some specific solvents. The polarity index for a mixture can then be readily determined from the polarity indices of the pure components and their respective volume fractions (< A, < b), thus... 

Effect of Solvent Strength on Retention Factors. Solvents that interact strongly with. solutes are often termed strong" solvents. Strong solvents are often, but not always, polar solvents. Solvent strength depends on the nature of the analyte and stationary phase. Several indexes have been developed fur quantitatively describing the polarity of solvents. The most useful of these for partition chromatography is the polarity index P which was developed by Snyder." This parameter is based on. solubility measurements for the substance in question in three solvents dioxane (a low ... 

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