How to Calculate Molar Absorptivity
Understand the Beer-Lambert law for absorbance, A = ɛlc., Rearrange the Beer-Lambert equation to solve for molar absorptivity., Obtain values for the variables in the equation using spectrophotometry., Plug in the values for the variables and solve...
Step-by-Step Guide
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Step 1: Understand the Beer-Lambert law for absorbance
The standard equation for absorbance is A = ɛlc, where A is the amount of light absorbed by the sample for a given wavelength, ɛ is the molar absorptivity, l is the distance that the light travels through the solution, and c is the concentration of the absorbing species per unit volume.Absorbance can also be calculated using the ratio between the intensity of a reference sample and the unknown sample.
It is given by the equation A = log10(Io/I).Intensity is obtained using a spectrophotometer.
The absorbance of a solution will change based on the wavelength that is passed through the solution.
Some wavelengths will be absorbed more than others depending upon the makeup of the solution.
Remember to state which wavelength is being used for your calculation. -
Step 2: A = ɛlc.
Using algebra we can divide absorbance by the length and the concentration to get molar absorptivity on one side of the equation: ɛ = A/lc.
We can now use this basic equation to calculate molar absorptivity for a given wavelength.
Absorbance between readings can vary due to the concentration of solution and the shape of the container used to measure intensity.
Molar absorptivity compensates for these variations., A spectrophotometer is a piece of equipment that passes a specific wavelength of light through a substance and detects the amount of light that comes out.
Some of the light will be absorbed by the solution and the remaining light that passes through can be used to calculate the absorbance of that solution.
Prepare a solution of known concentration, c, for analysis.
Units for concentration are molar or moles/liter.To find l, measure the length of the cuvette.
Units for path length are centimeters.
Using a spectrophotometer, obtain a measurement for absorbance, A, at a given wavelength.
The unit for wavelength is meters, but most wavelengths are so small, they are actually measured in nanometers (nm).Absorbance has no units. , Using the values you obtained for A, c, and l, plug them into the equation ɛ = A/lc.
Multiply l by c and then divide A by the product to solve for molar absorptivity.
For example:
Using a cuvette with a length of 1 cm, you measured the absorbance of a solution with a concentration of
0.05 mol/L.
The absorbance at a wavelength of 280 nm was
1.5.
What is the molar absorptivity of this solution? ɛ280 = A/lc =
1.5/(1 x
0.05) = 30 L mol-1 cm-1 -
Step 3: Rearrange the Beer-Lambert equation to solve for molar absorptivity.
-
Step 4: Obtain values for the variables in the equation using spectrophotometry.
-
Step 5: Plug in the values for the variables and solve the equation for molar absorptivity.
Detailed Guide
The standard equation for absorbance is A = ɛlc, where A is the amount of light absorbed by the sample for a given wavelength, ɛ is the molar absorptivity, l is the distance that the light travels through the solution, and c is the concentration of the absorbing species per unit volume.Absorbance can also be calculated using the ratio between the intensity of a reference sample and the unknown sample.
It is given by the equation A = log10(Io/I).Intensity is obtained using a spectrophotometer.
The absorbance of a solution will change based on the wavelength that is passed through the solution.
Some wavelengths will be absorbed more than others depending upon the makeup of the solution.
Remember to state which wavelength is being used for your calculation.
Using algebra we can divide absorbance by the length and the concentration to get molar absorptivity on one side of the equation: ɛ = A/lc.
We can now use this basic equation to calculate molar absorptivity for a given wavelength.
Absorbance between readings can vary due to the concentration of solution and the shape of the container used to measure intensity.
Molar absorptivity compensates for these variations., A spectrophotometer is a piece of equipment that passes a specific wavelength of light through a substance and detects the amount of light that comes out.
Some of the light will be absorbed by the solution and the remaining light that passes through can be used to calculate the absorbance of that solution.
Prepare a solution of known concentration, c, for analysis.
Units for concentration are molar or moles/liter.To find l, measure the length of the cuvette.
Units for path length are centimeters.
Using a spectrophotometer, obtain a measurement for absorbance, A, at a given wavelength.
The unit for wavelength is meters, but most wavelengths are so small, they are actually measured in nanometers (nm).Absorbance has no units. , Using the values you obtained for A, c, and l, plug them into the equation ɛ = A/lc.
Multiply l by c and then divide A by the product to solve for molar absorptivity.
For example:
Using a cuvette with a length of 1 cm, you measured the absorbance of a solution with a concentration of
0.05 mol/L.
The absorbance at a wavelength of 280 nm was
1.5.
What is the molar absorptivity of this solution? ɛ280 = A/lc =
1.5/(1 x
0.05) = 30 L mol-1 cm-1
About the Author
Alice Ellis
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