Quantitative Analysis by Spectrophotometric Methods

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Abstract

In this experiment, the absorbance of KMnO4 was measured by spectrophotometric method to determine the molar concentration and the molar extinction coefficient of KMnO4. In part 1, in order to determine the maximum absorbance wavelength of KMnO4, we measured the absorbance of the sample solution which contains KMnO4 at the wavelengths between 330nm and 660nm, and plotted the λ and A points; the λmax was 530nm. In part 2, the effect of concentration on the absorbance was examined. We prepared five differently concentrated (but, same path length) solutions, and measured the absorbance of them at the λmax(530nm) discovered in part 1; According to the results, higher concentrated solution had higher absorbance value. The extinction coefficient(ε) could be calculated from the results determined in part 2 and Beer’s Law; ε = 1.7 x 103. In part 3, the absorbance of the KMnO4 solution of unknown concentration was measured, and using Beer’s law and dilution equation, the initial concentration of the unknown was determined; The concentration of the solution (unknown # : 15) was calculated to be 3.3 x 10-3M.

Introduction
Our eyes are sensitive to light which lies in a very small region of the electromagnetic spectrum labeled "visible light". This "visible light" corresponds to a wavelength range of 400 - 700 nanometers (nm) and a color range of violet through red. The human eye is not capable of "seeing" radiation with wavelengths outside the visible spectrum. The visible colors from shortest to longest wavelength are: violet, blue, green, yellow, orange, and red. Ultraviolet radiation has a shorter wavelength than the visible violet light. Infrared radiation has a longer wavelength than visible red light. The white light is a mixture of the colors of the visible spectrum. Black is a total absence of light. Figure 5.1 The electromagnetic spectrum.

Although visible light acts as a wave in some respects, it also displays properties characteristic of particles. The particle-like properties of visible light are exhibited through small, energy-bearing entities known as photons. The energy of a photon is: E photon = hc / λ (1) where h = Planck's constant, 6.626 x 10-34 J/s, c = speed of light, 3.00 x 108 m/s, and λ = wavelength of light. Light is energy, and when energy is absorbed by a chemical it results in a change in energy levels of the chemical. Molecules normally exist in discrete energy levels. Vibrational energy levels exist because molecular bonds vibrate at specific frequencies. Electronic energy levels exist because electrons in molecules can be excited to discrete, higher energy orbitals. The energy (E) of light depends on its wavelength. Longer wavelengths (infrared) have less energy than shorter wavelengths (ultraviolet). A molecule will absorb energy (light) when the energy (or wavelength) exactly matches the energy difference between the two energy states of the molecule. In absorption, light — sunlight which is white light — strikes an object and part of the light may be absorbed by the object. The light we see coming from that object is the light which was not absorbed by the object. We see the "not-absorbed" light as the color of the object. If no light is absorbed, the object appears to be colorless. A spectrophotometer is employed to measure the amount of light that a sample absorbs. The instrument operates by passing a beam of light through a sample and measuring the intensity of light reaching a detector. The beam of light consists of a stream of photons. When a photon encounters a molecule, there is a chance the molecule will absorb the photon. This absorption reduces the number of photons in the beam of light, thereby reducing the intensity of the light beam. The ratio of transmitted light intensity(I) to the incident light intensity(I0) is the transmittance, T: T = I / I0...
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