Experiment 11 - Kinetics of Food Dyes >> Step 1: Making a kinetic trace

Experiment 11 - Kinetics of Food Dyes

Step 1. Follow the disappearance of the dye by measuring the absorbance of the dye during the reaction

We are now ready to move to the first step of our experiment: mixing the dye with bleach and observing how the absorbance of the dye decreases with time: we will be creating a kinetic trace.

In the laboratory and in the simulation below, you will be working with a 10.00 mL solution, which you will make up of the dye and the bleach. You will be using a 10 mL serological pipet to deliver the dye and a 1 mL syringe to deliver the bleach to a vial, which you will place into the spectrophotometer (the MicroLab). You will have to ensure that the solution is mixed well throughout the experiment by means of a magnetic stirrer. You are given a dye, Yellow 6, whose concentration is 3.40 x 10^-5M and two choices of bleach solution: 0.090M and 0.180M.

From the movie you saw at the beginning of this tutorial it is clear that the concentrations of dye and bleach in solution determine the length of time it takes for the colour of dye to disappear. The spectrophotometer takes a reading every second, so your reaction must be long enough for you to be able to observe a clear kinetic trace. We recommend to make your reaction last at least 20 seconds. Try out the simulation first to obtain a long enough reaction and then leave its kinetic trace on the screen and move on to the next step of the tutorial.

The sequence of steps in the simulation is as follows:
1. Use the serological pipet to deliver between 9.00 mL and 9.90 mL of dye into the vial
2. Move the vial into the spectrophotometer
3. Drop the magnetic bar into the vial
4. Use the 1 mL syringe to deliver between 1.00 mL and 0.10 mL of bleach into the vial (to make up the volume to 10.00 mL)
5. Immediately cover the vial with the black lid to start the measurement.
6. When you are satisfied with your kinetic trace, print it out. Make sure to include your name and Lab ID, as well as a descriptive title, which will identify your experiment (i.e. 9.80 mL dye with 0.20mL 0.090M bleach). If you don't have a printer, save your graph on your device and show it to your TA in the lab.
7. Record all data on the feedback form. You will be handing it in to your TA.

Note: Simulation may take a few minutes to completely load, depending on your internet connection speed, please be patient.

The following tutor will lead you through the calculations required to determine the initial concentrations of dye and bleach in solution you have just prepared in your simulation. Choose the volumes and concentrations you have actually used to create your kinetic trace.

Tabulate this data on the feedback form

Step 2. Getting information from a kinetic trace

Beer's law tells us that absorbance is directly proportional to concentration of the colour species, and so the above behaviour of data is the same for absorbance as it is for the concentration. We can collect our data using the kinetic traces for absorbance vs time and use them to determine reaction orders, k' and k.

Use the tabs in the upper left hand corner in the simulation above to change the manner in which the data is plotted. The choices are absorbance, ln(absorbance), or 1/absorbance versus time. Remember that the type of plot that leads to a straight line gives you the information you need to determine the order of the reaction with respect to dye.

Now that you have determined the order of reaction with respect to dye, predict what will happen to the rate of your reaction if the concentration of the dye is doubled?