Absorbance

Absorbance (A) is a dimensionless quantity that measures the ability of a medium (usually a solution) to absorb light at a given wavelength. It is directly proportional to the concentration of the absorbing species (Beer-Lambert law). This concept is covered in 11th and 12th grade curriculum.

Discover FizziQ

How to measure it in class

With the FizziQ app, the colorimeter enables simplified colorimetric titrations.

Steps:

Prepare solutions of known concentrations (color scale)
Open FizziQ and select the “Colorimeter” instrument
Place each solution in front of a white light source
Measure the intensity of transmitted R, G, B components
Build a calibration curve (intensity vs concentration)
Determine the concentration of an unknown solution

Scientific activities on this topic

Colorimetric titration: https://www.fizziq.org/en/activities/beer-lambert-law/
Calibration curve with FizziQ
Kinetic monitoring of a colored reaction

Learn more

Definition of absorbance:

A = log10(I0/I) = -log10(T)

where I0 is the incident intensity, I is the transmitted intensity, and T is the transmittance.

Beer-Lambert Law:

Absorbance is proportional to concentration and the path length through the solution:

A = epsilon x l x c

Validity conditions:

Dilute solutions (A < 2)
Monochromatic light
Homogeneous solution
No chemical reaction induced by light

Wavelength selection:

Measurements are taken at the wavelength where absorbance is maximum (lambda_max) for better sensitivity.

Formula

Definition: A = log10(I0/I) = -log10(T)

Beer-Lambert Law: A = epsilon x l x c

where:

A: absorbance (dimensionless)
I0: incident light intensity
I: transmitted light intensity
T = I/I0: transmittance
epsilon: molar absorption coefficient (L.mol-1.cm-1)
l: optical path length (cm)
c: concentration (mol/L)

Application examples

Iron titration in water by colorimetry
Measurement of chlorophyll concentration in leaves
Quality control of colored foods
Monitoring of kinetic reactions (discoloration)

FAQ

Q: Can absorbance be measured with FizziQ? A: Not directly, but transmitted intensity can be measured. By comparing with a reference (pure water), absorbance can be calculated or a calibration curve can be used.

Q: Why is logarithm used? A: To linearize the relationship with concentration. Transmitted intensity decreases exponentially with c, but A is proportional to c.

Q: What is the difference between absorbance and absorption? A: Absorption is the physical phenomenon. Absorbance is the measurable, dimensionless quantity.

Q: Why measure at maximum absorption? A: Sensitivity is maximum at lambda_max. A small variation in concentration produces a large variation in absorbance.

Beer-Lambert Law - Colorimetry - Transmittance - Spectrophotometry - Concentration - Titration