The result is a number of closely spaced absorption bands that merge together to form a single broad absorption band. Sodium, for example, has a single valence electron in its 3 s atomic orbital. The wavelengths in wavenumbers corresponding to several transitions are shown. The valence shell energy level diagram in Figure The reasons for this splitting are unimportant in the context of our treatment of atomic absorption.
Absorption of a photon is accompanied by the excitation of an electron from a lower-energy atomic orbital to an orbital of higher energy. Not all possible transitions between atomic orbitals are allowed. The atomic absorption spectrum for Na is shown in Figure The most obvious feature of this spectrum is that it consists of a small number of discrete absorption lines corresponding to transitions between the ground state the 3 s atomic orbital and the 3 p and 4 p atomic orbitals.
Note that the scale on the x -axis includes a break. Another feature of the atomic absorption spectrum in Figure Natural line widths for atomic absorption, which are governed by the uncertainty principle, are approximately 10 —5 nm. Other contributions to broadening increase this line width to approximately 10 —3 nm. As light passes through a sample, its power decreases as some of it is absorbed.
This attenuation of radiation is described quantitatively by two separate, but related terms: transmittance and absorbance. All methods of detecting photons—including the human eye and modern photoelectric transducers—measure the transmittance of electromagnetic radiation. Equation We will show that this is true in Section What is its absorbance?
A percent transmittance of Click here to review your answer to this exercise. As we saw in Figure For this reason, atomic absorption requires a line source instead of a continuum source. When monochromatic electromagnetic radiation passes through an infinitesimally thin layer of sample of thickness dx , it experiences a decrease in its power of dP Figure Integrating the left side of equation The absorptivity and molar absorptivity are proportional to the probability that the analyte absorbs a photon of a given energy.
Nassau, K. The Physics and Chemistry of Color. New York: John Wiley and Sons, Periodicals Walker, J. Absorption spectrum —The record of wavelengths or frequencies of electromagnetic radiation absorbed by a substance; the absorption spectrum of each pure substance is unique. Band spectrum —A spectrum in which the distribution of values of the measured property occurs in distinct groups.
Continuous spectrum —A spectrum in which there are no breaks in the distribution of values associated with the measured property. Electromagnetic spectrum —The continuous distribution of all electromagnetic radiation with wavelengths ranging from approximately 10 15 to 10 6 meters which includes: gamma rays, x rays, ultraviolet, visible light, infrared, microwaves, and radio waves. Emission spectrum —The record of wavelengths or frequencies of electromagnetic radiation emitted by a substance which has previously absorbed energy, typically from a spark or a flame.
Line spectrum —A spectrum, usually associated with isolated atomic absorbers or emitters, in which only a few discrete values of the measured property occur.
Visible spectrophotometers, in practice, use a prism to narrow down a certain range of wavelength to filter out other wavelengths so that the particular beam of light is passed through a solution sample. Figure 1 illustrates the basic structure of spectrophotometers. It consists of a light source, a collimator, a monochromator, a wavelength selector, a cuvette for sample solution, a photoelectric detector, and a digital display or a meter.
Detailed mechanism is described below. Figure 2 shows a sample spectrophotometer Model: Spectronic 20D. A spectrophotometer, in general, consists of two devices; a spectrometer and a photometer. A spectrometer is a device that produces, typically disperses and measures light. A photometer indicates the photoelectric detector that measures the intensity of light. You need a spectrometer to produce a variety of wavelengths because different compounds absorb best at different wavelengths.
For example, p-nitrophenol acid form has the maximum absorbance at approximately nm and p-nitrophenolate basic form absorb best at nm, as shown in Figure 3. Looking at the graph that measures absorbance and wavelength, an isosbestic point can also be observed.
An isosbestic point is the wavelength in which the absorbance of two or more species are the same. The appearance of an isosbestic point in a reaction demonstrates that an intermediate is NOT required to form a product from a reactant. Figure 4 shows an example of an isosbestic point.
Referring back to Figure 1 and Figure 5 , the amount of photons that goes through the cuvette and into the detector is dependent on the length of the cuvette and the concentration of the sample. Once you know the intensity of light after it passes through the cuvette, you can relate it to transmittance T.
Transmittance is the fraction of light that passes through the sample. This can be calculated using the equation:. Where I t is the light intensity after the beam of light passes through the cuvette and I o is the light intensity before the beam of light passes through the cuvette. Transmittance is related to absorption by the expression:. Where absorbance stands for the amount of photons that is absorbed. With the amount of absorbance known from the above equation, you can determine the unknown concentration of the sample by using Beer-Lambert Law.
Figure 5 illustrates transmittance of light through a sample. Beer-Lambert Law also known as Beer's Law states that there is a linear relationship between the absorbance and the concentration of a sample. For this reason, Beer's Law can only be applied when there is a linear relationship. Beer's Law is written as:.
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