An article recently published in the Journal of Chemical Education by Ruth E. Nalliah¹ describes a great experiment for studying chemical kinetics that is extremely simple to set up and carry out. The experiment involves monitoring the reaction between blue food dye and hydrogen peroxide in the presence of base:

Blue dye #1 + H₂O₂    à  colorless products                 Equation 1

This article piqued my interest so I decided to try it out for myself. When doing so, I made a few modifications. I found that it was quite simple to monitor the kinetics of this reaction using an iPhone outfitted with the RGB analyzer app called “Color Name”. You can see the overall process of how I carried out the experiment as well as collected and analyzed the data in the video below.

The goal of the experiment is to determine the order of the reaction with respect to the blue dye:

Rate = k[dye]^x[H₂O₂]^y                        Equation 2

Where “Rate” is the rate of the reaction, k is the rate constant for the reaction, x is the order of the reaction with respect to the dye, and y is the order of the reaction with respect to H₂O₂. The reaction is run with a very large excess of H₂O₂, therefore the concentration of H₂O₂ essentially remains constant throughout the reaction. As a result, we can treat k[H₂O₂]^y as a constant, and Equation 2 may be written as:

Rate = k’[dye]^x                                   Equation 3

Where k’ =  k[H₂O₂]^y. If the reaction is first order in the dye, the following equation can be used to describe the concentration of the dye at various time intervals:

ln[dye]_t = ln[dye]₀ - k't                         Equation 4

Where [dye]_t is the concentration of the dye at the beginning of the experiment and [dye]₀ is the concentration of the dye at some time, t, during the experiment. Equation 4 takes on the form of a line if we let y = ln[dye]_t , b = ln[dye]₀, m = k’ and x = t. Because of this, if we plot the natural log of the dye concentration vs. time, a straight line will result if the reaction is first order in the dye. Furthermore, the slope of the resulting line is equal to the opposite of k’ (Figure 1).

Figure 1: First order plot of ln[dye] vs. time. The straight line indicates the reaction is first order with respect to the dye concentration. The slope of the line yields the rate constant for the reaction (k’ = 0.47 s^-1)

On the other hand, if the reaction is second order in the dye, then the equation below should be used to describe the concentration of the dye at various time intervals:

1/[dye]_t = 1/[dye]₀ +k't                   Equation 5

In this case, we get a line if we let y = 1/[dye]_t, b = 1/[dye]₀, k’ = m and x = t. Thus, if the reaction is second order in dye concentration, a plot of 1/[dye] vs. time results in a straight line with a slope = k' (Figure 2). Thus collecting [dye] vs. time data and constructing the accompanying first and second order plots can provide insight into whether the reaction described in Equation 1 is first or second order with respect to the dye. 

Figure 2: Second order plot of 1/[dye] vs. time. The straight line indicates the reaction is second order with respect to the dye concentration. The slope of the line yields k' in Equation 3 (k’ = 1.38 M^-1 s^-1)

Because the dye is colored blue, it can be monitored by measuring the amount of red light that passes through the dye in the reaction mixture (recall that blue dye absorbs red light quite well).² Thus, the RGB app on the smartphone can be used to measure the red light that passes through the solution. Recall that the absorbance, A, of a dye is directly proportional to its concentration:

 A a [dye]                     Equation 6

Because of this direct relationship we can use absorbance as a measure of dye concentration. Note that in the context of this experiment, the RGB app on an iPhone is used to monitor the amount of red light that passes through the blue dye. The absorbance at each time interval is calculated as:

A = -log(R/R₀)                                    Equation 7

Where R is the amount of red light transmitted through the solution and R₀ is the amount of red light that passes through the solution when it is colorless (which is essentially the “blank”).

I had the opportunity to try this experiment out with students and it worked very well. If you use this experiment, let me know how it works for you. Also let me know if you monitored the reaction using the method described in the article or by using the RGB app. I’m curious to hear about any other variations you might try!

References: 

1. Ruth Nalliah, Reaction of FD&C Blue 1 with Sodium Percarbonate: Multiple Kinetics Methods Using an Inexpensive Light Meter, Journal of Chemical Education, 2019, 96, 7, 1453 -1457.  (accessed 11/26/19)
2. Kuntzleman, Tom, Use Your Smartphone as an "Absorption Spectrophotometer", ChemEd X, 3/30/16. (accessed 11/26/19)