Q&A for How to Calculate Molar Absorptivity

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What is Beer Lambert's law?

Mohsin257

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In simple words it states that light absorbed by the sample is directly proportional to the path length (l or x) and concentration.

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Is the molar absorptivity constant, or does it change as the length of the cuvette changes?

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It is constant. Units of molar absorptivity constant is in M^-1 cm^-1, which is essentially how much is absorbed per unit length. As the length of cuvette increases, more is absorbed as a whole, but the constant is independent of length of cuvette!

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How do I know the path length?

Mohsin257

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It is known by sample compartment. Path-length is the area of the cell/sample compartment. It is mostly 1cm and depends on that compartment may be .5cm etc.

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How do I calculate the concentration of the unknown?

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Concentration is molarity. So you use Mol's/solute, which is the M=Mol's/ml.

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How do I calculate molar concentrations?

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Molarity is expressed in moles/liters. So find the number of moles using dimensional analysis, and then divide that by a volume measured in liters.

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How do I find the absorbance of radiation?

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Use the formula A = ebc, where A is absorbance, e is molar extinction coefficient, b is path length (cm), and c is concentration (mol/L).

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Shouldn't the slope be multiplied by path length and not divided in the 4th method? The absorptivity will be more at 1 cm path length than at 0.5 cm, won't it?

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The formula is A = ɛlc. To solve for ɛ you use the formula A/lc, but the slope or gradient is A/c. So solving for ɛ would be the gradient multiplied by 1/l or the gradient divided by l.

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How do I calculate the absorbance?

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In this procedure the absorbance is measured on a spectrophotometer. If, for some reason, you know all values except absorbance you can solve for it as A=e*l*c.

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How do I calculate the concentration of an extract if I don't know the molar absorbity?

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In this procedure, you're preparing solutions of known concentration for analysis. You can't solve for the molar absorptivity if you don't know the concentration.

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How do I calculate the concentration from absorbance?

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If you know the value of all variables except concentration, you can rearrange the equation as c = A/le.

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How do I calculate the molar absorptive if the concentration value is in PPM? How do I get the slop in moles per liter?

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You must convert your units from ppm to mol/L. To do this, look up the molar mass (g/mol) of the compound of interest. Also, 1 ppm can be equated to 1 mg/L (or 0.001 g/L). For example, nitrate (NO3) is 62 g/mol. If you have a 62,000 ppm NO3 solution, it is equal to 1 mol/L NO3. (62,000 ppm = 62,000 mg/L * 1g/1000mg * 1mol/62g = 1 mol/L) The slope will not be in mol/L. It will be in your units for molar absorptivity (L mol^-1 cm ^-1).

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What is the maximum wavelength of absorption for each dye?

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You would have to run a scan of the wavelength spectrum rather than at a single wavelength to determine this.

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