Beer's Law
Definition and meaning of Beer's Law in chemistry.
Beer's Law states that the absorbance of light by a solution is directly proportional to the concentration of the solute and the path length of light through the solution. This relationship is expressed as A = εbc, where absorbance depends on molar absorptivity, path length, and concentration.
In more detail
When light passes through a colored solution, some of it is absorbed by dissolved molecules. The more concentrated the solution or the longer the light path, the more light is absorbed. Beer's Law quantifies this relationship, making it essential for quantitative analysis in spectrophotometry. By measuring absorbance and knowing the molar absorptivity and path length, chemists can determine the unknown concentration of a solution.
Key facts
| Mathematical Formula | A = εbc |
|---|---|
| Field | Analytical Chemistry |
| Also Known As | Beer-Lambert law |
| Primary Application | Calculating concentration from absorbance measurements in spectrophotometry |
A solution containing potassium permanganate (KMnO4) has a molar absorptivity of 2,300 L/(mol·cm) at 525 nanometers. If the absorbance is measured as 0.46 using a 1-centimeter cuvette, the concentration is calculated as: c = A/(ε·b) = 0.46/(2,300 × 1) = 0.0002 M or 0.2 mM.
Frequently asked questions
Why is it called Beer's Law if Beer-Lambert is the full name?
The law was developed through contributions from Pierre Bouguer, Johann Heinrich Lambert, and August Beer. Different fields emphasize different scientists; Beer-Lambert law is most common in analytical chemistry, while Beer's Law is frequently used in English-speaking countries.
What are the limitations of Beer's Law?
Beer's Law assumes ideal behavior and works best at moderate concentrations (typically below 0.01 M). At very high concentrations, chemical interactions and instrumental effects can cause deviations from linearity.