BIEN 435 Laboratory

Measurement of the Diffusion Coefficient of a Dye in Water

Introduction: Diffusion coefficients are difficult to obtain directly. An alternative to direct measurement is to obtain measurements of the distribution of a diffusing species and apply a known mathematical analysis to back out the desired quantity.

Overview: You will need some way to measure concentration of a substance, which will be an optical technique where the attenuation of the light passed through a substance depends on the concentration of the species. You will then look at diffusion in a test tube, which will represent one-dimensional diffusion. A critical part of this experiment is the calibration of the optical device. Two different dyes will be used, and you will test the results to determine whether the diffusion coefficients of the two dyes are different.

Null Hypothesis: The calculated diffusion coefficient of the dye is independent of the time at which the measurements are taken..

Calibration: The optical device sends laser light through the sample. The sample attenuates the laser light by an amount that depends on both the path length of the light and the concentration of the dye. A simple relation for this is Beer’s law:

where I is the intenstity at the detector, I₀ is the intensity of the incident light, α is an attenuation coefficient that depends on concentration, and l is the path length of light through the medium.

The figure to the right shows the setup for the test tube. The detector is a photoresistor. Its resistance changes according to the amount of light striking it. You need to determine the relationship between its resistance (output) and concentration (input)

1. Fill the test tube with water.

2. To determine the volume of water, weigh the test tube, then weigh the test tube when filled with water. The difference is the weight of the water. The volume can be obtained by dividing by density.

3. With just the water in the test tube, take a reading of the resistance on the multimeter.

4. Add 1 drop of the dye to the water and mix thoroughly. Take another reading of the voltage.

5. Continue this process, adding the dye a drop at a time until you have reached 5 drops.

6. From this data set determine the calibration of concentration as a function of resistance. (Is it linear?)

Experimental Procedure: Carefully place as drop of dye on the cap of the test tube. Obtain measurements of voltage at 5 distances from the cap, 0.5 cm, 1.0 cm, 1.5 cm, 2 cm and 2.5 cm. Take these readings after 2.5 minutes and after 5 minutes. Repeat these measurements so that you have 5 total sets of data.

Data analysis:

Eq. 2

Examine the equation for 1-dimensional diffusion as a function of time and space Eq. 2. You have all the data to compare this to theory, except the Diffusion coefficient. Use a least squares method to determine the Diffusion coefficient.

If you take the logarithm of this equation, it becomes:

Assuming that all data were taken at the same time, you can plot as a function of and perform a linear least squares fit. The slope of this will be , and the y-intercept will be . From the slope and knowledge of the time you can determine D. That is, if

, then .

Next you will want to know if this method gives you different values for D at the two different times. Calculate the values of D for all 5 sets of data and for both times. Now perform a Student’s T test to test the null hypothesis.