1. What is Molar Absorption Coefficient?

The molar absorption coefficient (ε) is a measure of how strongly a chemical species absorbs light at a particular wavelength. It is a fundamental parameter in spectroscopy and is expressed in L/mol·cm.

2. How to Calculate Without Concentration

When concentration is unknown, you can determine ε by measuring absorbance at varying path lengths:

Where:

• \( \varepsilon \) — Molar absorption coefficient (L/mol·cm)
• \( \Delta A \) — Change in absorbance
• \( \Delta l \) — Change in path length (cm)
• \( c \) — Concentration (mol/L) - held constant

Explanation: By plotting absorbance vs. path length at constant concentration, the slope gives ε·c. Since c is constant, the slope is proportional to ε.

3. Slope Method Explanation

Details: The slope of the absorbance vs. path length plot equals ε·c. If you know either ε or c, you can determine the other. This method is particularly useful when concentration cannot be easily measured.

4. Using the Calculator

Tips: Enter absorbance values, path length in cm, and the slope from your A vs. l plot. Ensure all measurements are at the same wavelength and concentration.

5. Frequently Asked Questions (FAQ)

Q1: Why calculate ε without concentration?
A: When dealing with unknown samples, unstable compounds, or when concentration measurement is difficult or impossible.

Q2: What are typical ε values?
A: ε ranges from 0 to over 100,000 L/mol·cm. Values above 10,000 are considered strong absorbers.

Q3: What instruments are needed?
A: Spectrophotometer with variable path length cuvettes or a series of cuvettes with different path lengths.

Q4: Are there limitations to this method?
A: Requires constant concentration, linear response, and may be affected by scattering or other optical effects.

Q5: How accurate is this method?
A: Accuracy depends on the linearity of your A vs. l plot and precision of path length measurements.