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ETHANOL AGAR DIFFUSION

Learning Objectives:

  1. Analyze the benefits of Raman and IR spectroscopies

  2. Differentiate similar compounds with vibration-based spectroscopy

 

Background:

There are two common demonstrations used to show diffusion. One of them involves equimolar counter-diffusion of HCl and NH3 in a glass tube. In this experiment, one cotton ball is doused in HCl solution and another in NH3 solution. These cotton balls are then stuck into opposite ends of a long glass tube. Gas-phase HCl and NH3 diffuse towards each other until they come into contact, at which point they form the visible compound NH4Cl. This lab primarily demonstrates the effect of mass on diffusion; because HCl has approximately double the molecular mass of NH3, the compound will form closer to the HCl side of the tube.

The second lab entails analyzing the diffusion of a solution by measuring the distance it travels as a function of time. Although this method accounts for distance and concentration, it fails to incorporate concentration. Some labs adjust for this by using colorimeters, thus allowing concentration to be measured and plotted as a multi variable function of distance and time. 

The Ethanol Agar diffusion lab integrates elements of both the aforementioned labs by giving the mobile phase a distinctive color and a finite, pre-measured distance to traverse.  More specifically, all three variables will be measured in order to approximate the diffusion constant of ethanol in a water-based gel using Raman spectroscopy.