Kinetics Studies of the Bleaching of Food Dyes >> Step 1: Making a kinetic trace
Kinetics Studies of the Bleaching 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 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. 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 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.
Step 2a. 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 observed decay of absorbance versus time has the same shape as the decay of concentration versus time. We can collect our data using the kinetic traces for absorbance vs time and use them to determine reaction orders.
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?