How to Calculate Molar Extinction Coefficient Without Concentration

Molar Extinction Coefficient Formula:

\[ \varepsilon = \frac{\text{Slope of A vs l plot}}{\text{Concentration}} \]

Molar Extinction Coefficient (ε):

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is Molar Extinction Coefficient?

The molar extinction coefficient (ε) is a measure of how strongly a chemical species absorbs light at a particular wavelength. It is a fundamental parameter in spectrophotometry and is used in the Beer-Lambert law to relate absorbance to concentration.

2. How to Calculate Without Direct Concentration

When concentration is unknown, the molar extinction coefficient can be determined by measuring absorbance at varying path lengths:

\[ \varepsilon = \frac{\text{Slope of A vs l plot}}{c} \]

Where:

\( \varepsilon \) — Molar extinction coefficient (L/mol·cm)
Slope — Slope from absorbance vs path length plot
\( c \) — Concentration (mol/L)
\( A \) — Absorbance (unitless)
\( l \) — Path length (cm)

Explanation: By plotting absorbance against path length at constant concentration, the slope equals ε × c, allowing calculation of ε when c is known.

3. Beer-Lambert Law Application

Details: The Beer-Lambert law states that absorbance (A) is proportional to the concentration (c) and path length (l): A = εcl. By varying path length while keeping concentration constant, we can determine the extinction coefficient.

4. Using the Calculator

Tips: Enter the slope obtained from your absorbance vs path length plot and the known concentration. Ensure all values are positive and concentration is greater than zero.

5. Frequently Asked Questions (FAQ)

Q1: Why calculate extinction coefficient without direct concentration measurement?
A: This method is useful when concentration is known from preparation but cannot be directly measured, or when verifying concentration-independent properties.

Q2: What are typical values for molar extinction coefficients?
A: Values range from 0 to over 100,000 L/mol·cm, depending on the compound and wavelength. Strong chromophores have higher extinction coefficients.

Q3: What factors affect extinction coefficient accuracy?
A: Temperature, solvent, pH, wavelength accuracy, and instrument calibration can all affect the measured extinction coefficient.

Q4: Can this method be used for mixtures?
A: For mixtures, the method becomes more complex due to overlapping absorbances. Separate measurements or spectral deconvolution may be needed.

Q5: How many data points are needed for reliable slope determination?
A: At least 5-7 different path lengths are recommended for a reliable linear regression and accurate slope calculation.