ANALYSIS OF ASPIRIN SPECTROPHOTOMETRYThe Purpose Of
ANALYSIS OF ASPIRIN: SPECTROPHOTOMETRY
The purpose of this experiment is to evaluate the percent aspirin on a commercial aspirin tablet using an instrumental method, spectrophotometry.
In a spectrophotometer, light from a strong lamp passes through a monochromator, which breaks the light into its component colors using a grating, then uses mirrors and slits to direct a light beam of the desired wavelength through the sample compartment, where you place a tube (cuvette) of solution. A detector on the other side of the sample compartment measures the amount of light that passes through the solution and compares that to the amount of light that passes through a control solution.
The result is displayed as either transmittance or absorbance. Transmittance is the ratio of light passing through the sample (I) to light passing through the control (I0). Absorbance is the negative logarithm of transmittance: The absorbance of a solution depends on three things: the nature of the solution, the distance the light travels through the solution (called the path length), and the concentration of the solution. This is summarized by the Beer-Lambert Law, A = abc
• A is the measured absorbance
• a is a constant (the absorptivity); it depends on and the nature of the solution
• b is the path length
• c is the solution concentration
If the type of material and the cuvette diameter are always the same, ab is constant and the measured absorbance depends directly on the concentration of the solution. The absorbance is most sensitive to concentration if measured at a wavelength of high absorbance for that material.
To use the spectrophotometer, turn it on and let it warm up for 5 minutes, so the lamp and detector outputs are stable. Select the desired wavelength, and use one knob (usually on the left) to set the detector to read A = ∞ or T = 0% when no light passes through the sample compartment (this is called the dark current). Put a cuvette of control solution in
The purpose of this experiment is to evaluate the percent aspirin on a commercial aspirin tablet using an instrumental method, spectrophotometry.
In a spectrophotometer, light from a strong lamp passes through a monochromator, which breaks the light into its component colors using a grating, then uses mirrors and slits to direct a light beam of the desired wavelength through the sample compartment, where you place a tube (cuvette) of solution. A detector on the other side of the sample compartment measures the amount of light that passes through the solution and compares that to the amount of light that passes through a control solution.
The result is displayed as either transmittance or absorbance. Transmittance is the ratio of light passing through the sample (I) to light passing through the control (I0). Absorbance is the negative logarithm of transmittance: The absorbance of a solution depends on three things: the nature of the solution, the distance the light travels through the solution (called the path length), and the concentration of the solution. This is summarized by the Beer-Lambert Law, A = abc
• A is the measured absorbance
• a is a constant (the absorptivity); it depends on and the nature of the solution
• b is the path length
• c is the solution concentration
If the type of material and the cuvette diameter are always the same, ab is constant and the measured absorbance depends directly on the concentration of the solution. The absorbance is most sensitive to concentration if measured at a wavelength of high absorbance for that material.
To use the spectrophotometer, turn it on and let it warm up for 5 minutes, so the lamp and detector outputs are stable. Select the desired wavelength, and use one knob (usually on the left) to set the detector to read A = ∞ or T = 0% when no light passes through the sample compartment (this is called the dark current). Put a cuvette of control solution in