Beer's Law: Absorbance & Concentration

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  • 0:00 Intro to Light…
  • 1:11 Background for Beer's Law
  • 2:43 The Equation
  • 4:24 Lesson Summary
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Lesson Transcript
Instructor: Amy Lange

Amy has taught university-level earth science courses and has a PhD in Geology.

Beer's Law relates the absorbency of a substance to its chemical concentration. In this lesson, we'll review light transmittance and absorbance and how Beer's Law is used to determine chemical concentrations.

Intro to Light Transmittance & Absorbance

If you've ever made hot tea in a clear glass, you've observed that the tea grows darker the longer you leave the tea bag in the water. As the tea gets darker, less light is able to travel through the glass. In this example, the strength of the tea, or its concentration, is directly related to how much light travels through the glass. When incident light hits a substance, part of the light is transmitted through the substance and part is absorbed by the substance. Incident light is another term for incoming light. The incident light shining on an object is either reflected, transmitted, or absorbed. Transmittance is the fraction of light that passes through a substance. Absorbance is the measure of light that is absorbed by the substance. The fraction of light that is reflected is normally very small, so we will ignore this fraction for simplicity. So if 80% of light is transmitted through a liquid, 20% of the light is absorbed by that liquid.

Background for Beer's Law

Beer's law is an equation that allows scientists to relate the decrease of light intensity to the concentration of a substance. Before we learn the equation, we first must understand the concepts of cell path length and analyte concentration, which are both used in Beer's law.

Cell path length is the two-dimensional distance that light travels through in the sample. Imagine we have a light source facing a light detector. If we place our glass of tea in front of the light source, some of the light is absorbed by the tea so less light reaches the light detector. The length across the tea in the glass is called the cell path length. If we place another glass of tea in front of the light source, even less light reaches the light detector. In this case, you have doubled the cell path length, which decreases the transmittance. This effect explains why tea will look lighter in a glass than a larger pitcher, because the pitcher is larger and the cell path length is larger.

The other concept we must understand is analyte concentration. The analyte is the substance that we are interested in measuring in Beer's law and what is absorbing the light. Concentration in chemistry is the amount of solute in the total volume. If we are adding food coloring to a glass of water, the color will get darker the more food coloring you add. In this case, we are increasing the solute, which is increasing the concentration of food coloring in water.

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