Molar absorptivity is a key concept in IB Chemistry Topic 11 (Measurement & Data Processing). It appears in Beer–Lambert law calculations and helps determine how strongly a substance absorbs light at a particular wavelength. Understanding molar absorptivity makes spectrophotometry easier and allows you to interpret calibration curves, compare substances, and perform accurate quantitative analysis.
What Is Molar Absorptivity?
Molar absorptivity (ε) is a constant that measures how strongly a substance absorbs light at a particular wavelength.
It appears in the Beer–Lambert law:
A = ε c l
Where:
- A = absorbance
- ε = molar absorptivity (L mol⁻¹ cm⁻¹)
- c = concentration of the solution
- l = path length (usually 1 cm)
In simple terms:
A higher ε value means the substance absorbs light more strongly.
Units of Molar Absorptivity
The units of ε are:
L mol⁻¹ cm⁻¹
These units show that absorbance depends on:
- Amount of substance per liter (concentration)
- Distance the light travels through the sample (path length)
Absorbance itself is unitless.
What Molar Absorptivity Tells You
1. How strongly a substance absorbs light
A high ε value means:
- The substance absorbs intensely at that wavelength
- Even small concentrations give high absorbance
A low ε value means:
- Weak absorption
- Higher concentrations are needed to get measurable absorbance
2. Which wavelength is best
Each substance has a wavelength where absorption is highest—called λmax.
At λmax:
- ε is at its maximum
- Sensitivity is highest
- Measurements are most accurate
This is why spectrophotometry always uses λmax.
3. How easy a substance is to analyze
Substances with very high ε values give strong signals and are ideal for quantitative analysis.
Substances with very low ε may require:
- A different wavelength
- A different ligand (complexing agent)
- A more sensitive instrument
Why Molar Absorptivity Matters in the Beer–Lambert Law
Beer–Lambert law links absorbance, concentration, and path length.
But ε determines how absorbance responds to concentration.
Two solutions at the same concentration can show completely different absorbance values if their ε values differ.
Example:
- A dye with ε = 30,000 absorbs strongly.
- A compound with ε = 50 absorbs weakly.
This is why some solutions appear very dark even in low concentrations, while others barely absorb.
Factors That Affect Molar Absorptivity
Although ε is considered a constant, it depends on:
1. Wavelength of light
ε varies with wavelength—hence the importance of λmax.
2. Chemical environment
pH changes can:
- Alter electronic structure
- Change charge
- Shift λmax
- Modify molar absorptivity
3. Solvent polarity
Different solvents change how molecules interact with light.
4. Temperature
A small effect, but can slightly alter absorption.
Using Molar Absorptivity in Calculations
Finding concentration
Using A = ε c l:
c = A / (ε l)
If l = 1 cm (standard cuvette), then:
c = A / ε
This calculation appears frequently in IB Paper 2.
Finding absorbance at a given concentration
A = ε c l
Useful for predicting calibration curve values.
Comparing substances
A compound with ε = 25,000 is much easier to detect than one with ε = 100.
Real-World Applications of Molar Absorptivity
It is essential in:
- Water quality testing
- Environmental monitoring
- Food dye analysis
- Medical assays (e.g., hemoglobin)
- Pharmaceutical concentration measurements
- Chemical kinetics (monitoring absorbance changes)
Spectrophotometry depends entirely on ε for accuracy.
Common IB Misunderstandings
“ε is the same at all wavelengths.”
Incorrect—ε varies dramatically across the spectrum.
“High absorbance means high concentration.”
Only true if ε and l are constant.
“Absorbance has units.”
Absorbance is dimensionless; ε carries the units.
“Molar absorptivity depends on path length.”
It does not—A varies with l, not ε.
FAQs
Why is molar absorptivity important?
It determines how much light a molecule absorbs and how sensitive the spectrophotometric method will be.
Does every compound have a molar absorptivity?
Yes—any compound that absorbs UV or visible light has ε values at specific wavelengths.
Why does ε change with pH?
Because protonation or deprotonation alters electronic structure, shifting absorption.
Conclusion
Molar absorptivity (ε) measures how strongly a substance absorbs light at a specific wavelength. As a key part of the Beer–Lambert law, it determines absorbance for a given concentration and path length. Understanding ε is essential for spectrophotometry, calibration curves, and quantitative analysis in IB Chemistry.
