Empirical Scales

Wind chill index An empirical scale that correlates well with the sensation of bare dry skin due to the chilling effect of the outdoor air temperature and wind speed. 

An empirical temperature scale is based on some arbitrary physical property (such as density, electrical resistance, magnetic susceptibility, etc.) that changes in a way that is continuous and single valued. The ITS-90 temperature scale described in Appendix 2 is an empirical scale that is designed to closely approximate the absolute (ideal gas) temperature scale. 

The dielectric constant and refractive index parameters and different functions of them that describe the reactive field of solvent [45] are insufficient to characterize the solute-solvent interactions. For this reason, some empirical scales of solvent polarity based on either kinetic or spectroscopic measurements have been introduced [46,47]. The solvatochromic classification of solvents is based on spectroscopic measurements. The solvatochromic parameters refer to the properties of a molecule when its nearest neighbors are identical with itself, and they are average values for a number of select solutes and somewhat independent of solute identity. 

Particularly useful is the physical classification of surfactants based on the hydrophile-lipophile balance (HLB) system [67,68] established by Griffin [69,70]. More than 50 years ago he introduced an empirical scale of HLB values for a variety of nonionic surfactants. Griffin s original concept defined HLB as the percentage (by weight) of the hydrophile divided by 5 to yield more manageable values ... 

For a flow boiling crisis, an empirical scaling factor was suggested by Stevens and Kirby (1964). It is a graphical correlation developed from uniform-flux round-tube data. They suggested an empirical function ... 

Figure 5.30 Empirical scaling factor. (From Steven and Kirby, 1964. Reprinted with permission of UK AEA Technology, Didcot, Oxfordshire, UK.)... 

Mohs scale of hardness An empirical scale that grades the hardness of minerals from 1 (softest, talc) to 10 (hardest, diamond). mohair See angora. 

When the experimental values of I and A are known, one can determine through these expressions the values of /a and tj. Since for atoms and molecules, the trends shown by these values of /a and tj are, in general, in line with those provided by several empirical scales constructed intuitively by chemists, the identification of these global DFT descriptors with their associated chemical concepts is strengthened. In other words, the quantity (I+A)/2 shows, in general, the same behavior as that of the electronegativity concept, while the quantity (I—A) shows, also in general, the same behavior as that of the chemical hardness concept. 

This analysis therefore provides some theoretical justification for the rather empirical scaling rules which have been used to simulate the effect of orthogonalisation in earlier work (5,24). 

Empirical scales of solvent polarity based on solvatochromic shifts... 

It should be emphasized again that there are many parameters underlying the concept of polarity, and therefore the validity of empirical scales of polarity based on a single parameter is questionable. 

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... 

The variations in the absorption energies of various dyes have been used to characterise the polarity of various media and create empirical scales. For this purpose, the most widely used dye is the highly, negatively solvatochromic betaine (1.98), known as Reichardt s dye, whose transition energy f .j,(30) in kcal mol, measured in a particular solvent, characterises the polarity of that solvent. 

Mohs was first to call attention to the significance of the hardness parameter for identification of minerals, and he provided a sufficiently precise and generally accessible tool for this purpose. The 10-degree, empirical scale of hardness he devised, Figs. 1.1 and 4.1.1 (10—diamond, 9—corun-... 

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