Name ______

Group Members______

POGIL: Chromatography

Why?

Chromatography is a method for separating the components of a mixture, and among its many applications, it is useful in the field of forensic toxicology. This POGIL will help us to understand the chemistry behind chromatography.

Resources:

Zumdahl and Zumdahl (Chemistry, Cengage Learning, 2010) pp. 443-448.

Prerequisites:

Intermolecular forces, polarity

Objective:

Relate the separation process to the features of the chromatogram.

Explain how the chemical interactions between a solute and both the stationary phase and the mobile phase impact retention.

New Concepts:

Cohesion, adhesion, capillary action

Model:

Chromatography is a method for separating the components of a mixture. The term chromatography is a general name that is applied to a series of methods that employ a system with two phases (states) of matter; a mobile phase and a stationary phase. The stationary phase is a solid, and the mobile phase is either a liquid or a gas. The separation occurs because the components of the mixture have different affinities for the two phases and therefore move through the system at different rates.

One simple type of chromatography, paper chromatography, is employs a strip of porous paper, such as filter paper, for the stationary phase. A drop of the mixture to be separated is placed on the paper, which is then dipped into a liquid (the mobile phase) that travels up the paper as though it were a wick. In our lab later, we will use water as our mobile phase.

Chromatography works by something called capillary action. The attraction of the water to the paper (adhesion force) is larger than the attraction of the water to itself (cohesion force), hence the water moves up the paper. The mixture to be separated will be attracted to the paper, to itself, and to the water differently, and thus a different component will move a different distance depending upon the strength of attraction to each of these objects.

To measure how far each component travels, we calculate the retention factor (Rf value) of the sample. The Rf value is the ratio between how far the component travels and the distance the solvent travels from a common starting point (the origin). If one of the sample components moves 2.5 cm up the paper and the solvent moves 5.0 cm, then the Rf value is 0.5. In the image below, the light shading represents the solvent and the dark spot is the chemical sample.

When measuring the distance the sample traveled, you should measure from the origin (where the middle of the spot originally was) and then to the center of the spot in its new location.

To calculate the Rf value, we use the equation:

Key Questions:

  1. In paper chromatography, what is the stationary phase and what is the mobile phase?
  1. Why must Rf values always be less than one?
  1. Why do different compounds travel different distances on the piece of paper?
  1. How is an Rf value useful?
  1. To use Rf values to identify components of mixtures, what variables must remain constant to make a good comparison?
  1. What are the units for Rf values?

Exercises:

  1. What is the Rf value of a component of a mixture if the distance traveled is 1.27 cm and the distance traveled by the solvent is 7.2 cm?
  1. What is the distance traveled by the mixture component if the distance traveled by the solvent is 4.5 cm and the Rf factor is 0.67?