Polarity indices

Choosing a Mobile Phase Several indices have been developed to assist in selecting a mobile phase, the most useful of which is the polarity index. Table 12.3 provides values for the polarity index, P, of several commonly used mobile phases, in which larger values of P correspond to more polar solvents. Mobile phases of intermediate polarity can be fashioned by mixing together two or more of the mobile phases in Table 12.3. For example, a binary mobile phase made by combining solvents A and B has a polarity index, of... 

A reverse-phase HPLC separation is carried out using a mobile-phase mixture of 60% v/v water and 40% v/v methanol. What is the mobile phase s polarity index ... 

From Table 12.3 we find that the polarity index is 10.2 for water and 5.1 for methanol. Using equation 12.30, the polarity index for a 60 40 water-methanol mixture is... 

A useful guide when using the polarity index is that a change in its value of 2 units corresponds to an approximate tenfold change in a solute s capacity factor. Thus, if k is 22 for the reverse-phase separation of a solute when using a mobile phase of water (P = 10.2), then switching to a 60 40 water-methanol mobile phase (P = 8.2) will decrease k to approximately 2.2. Note that the capacity factor decreases because we are switching from a more polar to a less polar mobile phase in a reverse-phase separation. 

Changing the mobile phase s polarity index, by changing the relative amounts of two solvents, provides a means of changing a solute s capacity factor. Such... 

Motors rated 7500 kW and less must be considered suitable for dielectric tests or operation only when the polarization index or the value of the insulation resistance (at 40°C) is at least the minimum recommended values. 

Motors rated above 7500 kW must have both the polarization index and the insulation resistance above the minimum recommended values. 

The recommended minimum value of the polarization index for motors having class B or F insulation must be 2 when determined according to lEC 60034-18-1. 

This is a resin-rich system. The stator is wound with pre-formed coils on similar lines as noted above. After thorough testing on each coil for the polarization index and dissipation factor (tan S), as discussed in Sections 9.5 and 9.6, and the impulse voltage withstand test, as discussed in Section 11.4.9, the individual coils are completely cured and toughened before inserting them into the slots. The rest of the process is as noted earlier. 

These test results are then compared with similar test data obtained from the manufacturer on similar windings carried out during manufacture. If the manufacturer s original test results are available, the results obtained at site can be quickly compared and the condition of the insulation assessed easily and accurately. If the test facility to obtain test results at 1 minute and 10 minutes is not available, the results may also be obtained for 15 seconds and 60 seconds and a graph plotted as shown in Figure 9.5(a) to determine the polarization index. 

Check for winding insulation by polarization index (Section 9.5) and dissipation factor, tan S (Section 9.6)... 

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. 

A d.c. insulation resistance test or polarization index reveals only the surface condition of the insulation and does not allow a realistic assessment of internal condition. Loss tangent values are true reflections of the insulation condition to detect moisture content, voids, cracks or general deterioration. The tan 5 versus test voltage curve may be drawn and compared with the original curve provided by the manufacturer, and inferences drawn regarding the condition of the insulation. The different starting tan lvalues will reveal the condition of the insulation in terms of amount of contamination, as noted in Table 10.4 (See lEE, Vol. 127, May 1980). 

Solvent strength determines the value, but not the selectivity. The mobile phase can be established by using the polarity index P proposed by Snyder. The highest values of P represent the strongest solute adsorbed in conventional TLC but represent the weakest for the separation in reversed phases. Sometimes aqueous polar mixtures cannot totally wet the chemically bonded layer. For this reason, checking... 

The model has the advantage that it requires only a simple table eontaining the polarity index P and selectivity group for a number of solvents (Table 4.2). The model is based on Snyder s elassifieation of solvents [41,42] aeeording to their eharaeteristies to internet as proton aeeeptors (xj, proton donors (x, or dipoles (xj. 

Solvent Polarity index Solubility in water (%, w/w) Density (gmL-i) Viscosity (mN s Expansion volume Boiling point (°C)... 

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. 

However, not withstanding the above objections, further discussion of the Snyder solvent triangle classification method is justified by its common use in many solvent optimization schemes in liquid chromatography. The polarity index, P, is given by the sum of the logarithms of the polar distribution constants for ethanol, dioxane and nltromethane and the selectivity parameters, X, as the ratio of the polar distribution constant for solute i to... 

The solvent triangle classification method of Snyder Is the most cosDBon approach to solvent characterization used by chromatographers (510,517). The solvent polarity index, P, and solvent selectivity factors, X), which characterize the relative importemce of orientation and proton donor/acceptor interactions to the total polarity, were based on Rohrscbneider s compilation of experimental gas-liquid distribution constants for a number of test solutes in 75 common, volatile solvents. Snyder chose the solutes nitromethane, ethanol and dloxane as probes for a solvent s capacity for orientation, proton acceptor and proton donor capacity, respectively. The influence of solute molecular size, solute/solvent dispersion interactions, and solute/solvent induction interactions as a result of solvent polarizability were subtracted from the experimental distribution constants first multiplying the experimental distribution constant by the solvent molar volume and thm referencing this quantity to the value calculated for a hypothetical n-alkane with a molar volume identical to the test solute. Each value was then corrected empirically to give a value of zero for the polar distribution constant of the test solutes for saturated hydrocarbon solvents. These residual, values were supposed to arise from inductive and... 

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... 

Fig. 3.1f. Solubility parameter and polarity index for a range of solvents...

Grathwohl (1990) found a relationship between sorption capacity and the the atomic H/O ratio of NOM. Similarly, there is a good relationship between log Koc and the polarity index (PI) of SOM, defined as the (0+N)/C ratio (DePaolis and Kukkonen 1997 Rutherford et al. 1992 Xing 1997 Xing et al. 1994a). The effect of SOM polarity on sorption of organic compounds is consistent with the well-known theory of solvent polarity on solute solubility. In studying the influence of SOM composition... 

Fig. 6. Relationship between the log Koc for phenanthrene sorption and the polarity index of humic acids and humin, sequentially extracted from a soil. F-l, F-4, F-7, and F-9 are the first, fourth, seventh, and nineth extracted humic acids, respectively. 0.005, 0.05, and 0.5 ig mL 1 are selected liquid-phase equilbrium concentration of phenathrene (Kang and Xing 2005).

Fig. 7. Relationship between polarity index, (N+0)/C, and the Koc of a-Naphthol sorption to peat, black shale, and humic acids (HA) from different horizons of a soil. Modified from Chen et al. (1996).

Adsorbent Organic carbon content, % Polarity Index, (N+0)/C Aromatic carbon, % Koc of naphthalene (mL g-1)... 

Fig. 8. Relationship between polarity index ((O + N)/C) and aromaticity of organic sorbents from the literature. The aromatic carbon was calculated as the product of aromatic carbon content (108-165 ppm) from NMR distribution and percentage of carbon contents from elemental analysis.

Fig. 10. Relationship between polarity index ((O + N)/C) and sorption capacity of HOCs from the literature.

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