Optical and UV spectrophotometry

Although this example, at face value, looks to be a case of the use of the absorption of UV/visible radiation to determine the concentration of a single ionic species (the Cu2+ ion) in solution, and, therefore, the province of the previous chapter, it is, in fact, the quantification of a molecular absorption band. In a sulfate solution, the copper ion actually exists, not as a bare ion, but as the pentaquo species, in which the central copper ion is surrounded by five water molecules and a sulfate ion in an octahedral structure (Fig. 4.1). The color of the transition metal ions arises directly from the interaction between the outer d orbital electrons of the transition metal and the electric field created by the presence of these co-ordinating molecules (called ligands). Without the aquation 

Of course, not all dissolved ions produce colored solutions, and therefore not all ions in solution can be quantified by colorimetry. Noncolored solutions can sometimes, however, be converted to colored solutions by introducing chromophore species which complex with (i.e., attach themselves to) the target ion to produce a colored solution, which may then be measured by UV/visible colorimetry. An important archaeological example of this is the determination of phosphorus in solution (which is colorless) by com-plexation with a molybdenum compound, which gives a blue solution (see below). The term colorimetry applies strictly only to analytical techniques which use the visible region of the spectrum, whereas spectrophotometry may be applied over a wider range of the electromagnetic spectrum. 

The angular dispersion of a prism is the rate of change of 6, the refraction angle, with A, the wavelength of the radiation, i.e., dd/dX. Now  

For a given r, a number of combinations of n and k can satisfy this equation (e.g., 1 x 800 nm, 2 x 400 nm, 3 x 266.7 nm, etc.). The value of n is the order of the diffraction. It is usual to design echellette blazings such that nearly all the power is concentrated in the first order. 

The ability of a diffraction grating to separate different adjacent wavelengths is known as its dispersion. The angular dispersion of a grating (dr/dA.) is given by differentiation of the above equation at constant / and inversion  

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