Solid reflectance

In terms of their optical properties, all solids fall into one of two classes. Either they are transparent to light (here we are restricting the term "light" to visible radiation) or they are opaque. In the latter case, all of the radiation may be reflected. However, most solids reflect some wavelengths and absorb others. This is the condition that we call "color". If all visible wavelengths are absorbed, the solid is said to be "black" while reflectance of all visible wavelengths results in a "white" solid. We intend to show how "color" is measured but first must define the nature of "light". 

Fig. 10.3 Colour contrast of human erythrocytes on different solid reflective substrates Al -polycrystalline aluminum Cu - polycrystalline copper Mo - monocrystalline molybdenum Ni - polycrystalline nickel Pt - chemically polished platinum Si(m) - monocrystalline silicon Si(p) - polycrystalline silicon Ti - chemically polished titanium W- monocrystalline tungsten...

The red perchlorate complex is diamagnetic and the solid reflectance spectrum shows a band at 18.7 kK typical of square planar nickel(II). >" The isothiocya-nato complex is six-coordinate and high-spin (pgj-f = 3.21 and exhibits... 

How is the ionic character of an ionic solid reflected in the band picture ... 

Al room temperature. 10 M in MeCN. ° IR frequencies in cm UV-vis frequencies in 10 cm (solid), reflectance spectra (MeCN), solution spectra with s in parentheses,... 

Clearly, the treatment of a solid involving of the order of 10 3 electrons is even a more complicated matter than that of an isolated molecule or complex in spite of the simplifications introduced by symmetry, and the use of effective potentials, and thus of a band theoretical approach, is probably not adequate in the discussion of wave function sensitive parameters such as spin distributions. But many important properties of solids reflect the electronic energy levels, rather than the finer details of the electronic distributions, and in spite of the fact that band calculations are rarely carried through to self consistency, band structures and energies of simple compounds may be determined sufficiently well to provide a good comparison with experimental data. The main effort has been directed to metals, where the valence electrons are weakly bound, and to simple compounds of high symmetry with the sodium chloride or diamond-like structure. In the latter case this effort also reflects the importance of these compounds in solid state physics and electronics and the elucidation of the band structure was essential for an understanding of many of the important properties of these materials. 

The basic thesis of Zisman s work was that the contact angle of a liquid on a solid reflects in some way the chemical constitution of the solid surface. He took the point of view Langomuir that the range of forces acting at the surface of a non-polar solid are short of the order of a few nanometers. Therefore, the contact angle must reflect the properties and in some fashion the actual composition of the surface of the solid. In this context, contact angle measurements are highly sensitive to the outer most chemical characteristics of solid surfaces. 

The exit geometry at the top of the riser influences solids reflection and holdup in the upper part of the reactor, affecting solids flow patterns, overall solids holdup, and hence mixing. 

A The well-defined angles and smooth faces of crystaUine solids reflect the underlying order of the atoms composing them. 

If the ratio of cations to anions in a solid reflects the chemical formula, the system is said to be stoichiometric. For example, if the ratio of Na to Cl in NaCl is 1 1, the system is stoichiometric. Nonstoichiometry arises when a cation or anion with a different valency is substituted into an ionic crystal. Anion or cation vacancies must then result in order to maintain charge neutrality. For example, if a Ca were substituted for Na in NaCl, a cation vacancy would be needed to accommodate the extra charge. If was substituted for Cl , an anion vacancy would have to occur. Iron can exist with a valence of -1-2 or - -3. If a trivalent Fe was substituted for Fe in FeO, one Fe vacancy would have to be formed for each pair of Fe + incorporated. Remember, Mother Nature is pretty serious about charge neutrality. 

Some solids interact with solvents. In this case, the absorption spectrum of the solution will be different from that of the solid. For these solids and for insoluble solids, reflectance spectrophotometry would be a suitable technique for characterising the solids. More expensive spectrophotometers are provided with an integrating sphere, which allows for diffuse reflectance measurements. Less satisfactory results are obtained by preparing a nujol mull of a small amount of the solid. Impregnate a rectangular strip of filter paper with the mull and press it against the wall of a spectrophotometer cell. 

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