Như thảo luận tại Chương 5, các tính chất hấp thụ nguyên tử và phân tử là khá khác nhau. Hấp thụ ánh sáng của cá nhân, nguyên tử nonbonded khác của các phân tử đó. Do đó, các kỹ thuật và thiết kế nhạc cụ khác nhau đáng kể và Must Be Bàn luận riêng. Sự hấp thu của ánh sáng bởi các phân tử hoặc các ion gây ra hai loại năng lượng trao đổi: điện tử (thay đổi về năng lượng của các electron của phân tử) và rung động (thay đổi trong khoảng cách internuclear của. | 6 Molecular Spectrophotometry As discussed in Chapter 5 the absorption properties of atoms and molecules are quite different. Light absorption by individual nonbonded atoms differs from that of molecules. Consequently the techniques and instrument designs differ significantly and must be discussed separately. Absorption of light by molecules or ions causes two types of energy changes electronic change in the energy of the electrons of a molecule and vibrational change in the internuclear distance of two or more atoms in the molecule . Electronic transition requires more energy and thus occurs in the visible-ultraviolet spectral region. Vibrational changes in a molecule result from the absorption of low-energy infrared radiation. MOLECULAR ABSORPTION AND COLOR The color of a molecule in solution depends on the wavelengths of light it absorbs. Thus when a sample solution of a molecule or ion is exposed to white light certain wavelengths are absorbed and the remaining wavelengths are transmitted to the eye. The color perceived by the eye is determined only by the wavelengths transmitted. The substance exhibits the color that is complementary to wavelengths absorbed. In simple terms the color seen is the complementary color of the color absorbed. Table shows the general relationship between wavelengths of visible light absorbed and the color observed. For example if the color of the test solution is yellow the selected wavelength should be 450 nm and if the test solution is dark blue the absorbance measured is at the 580-nm wavelength. MOLECULAR ABSORPTION SPECTROPHOTOMETRY The instrument used for measurement of absorbance is known generally as a spectrophotometer. In a spectrophotometer radiant energy of a very narrow wavelength range is selected from a source and passed through the sample solution which is contained in a glass or quartz cell called a cuvette. The chemicals in the sample absorb some of the radiant energy and the rest passes on through. .
