Methylene Blue Kinetics Lab 4p6t1c

Methylene Blue Kinetics Lab Materials 

0.03 M Ascorbic Acid

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0.10 M HCl

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1.0 x 10-4 M Methylene Blue

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Ocean Optics spectrometer and cuvettes

Methods After the Ocean Optics spectrometer was set up, a spectrum of methylene blue was taken to determine the wavelength at which the methylene blue has the strongest absorption. This was done by placing one drop of methylene blue into a cuvette full of distilled water. The parameters that were used to aquire the kinetics data were the following: integration time = 140 ms, scans to average = 25, boxcar = 5, wavelength = 663 nm. Two data aquistions were performed with solutions of the following composition: Run # 1 2

0.10 M HCL 1.5 mL 2.25 mL

0.03 M Ascorbic Acid 1.0 mL 1.5 mL

1.0 x 10-4 M Methylene Blue 0.10 mL 0.15 mL

The cuvettes were filled with the proper amounts of 0.1 M HCl and 0.03 M ascorbic acid and then placed into the spectrometer. Immediately before data aquistion, the methylene blue was added to the cuvette. This prevented the reaction from starting prior to data acquisition. The instrument took absorption measurements of the samples every 10 seconds for 500 seconds. Discussion To determine the order of the reaction that was observed in this lab, three different functions of the absorbance verses time data were plotted. A linear regression was then performed for each function of absorbance versus time. The linear regression plot that is the most linear is said to correspond to the correct order of the reaction. In both cases observed in this experiment, the function of absorbance versus time that produced the most linear regression was the 1/absorbance plot. The corresponding order for the 1/absorbance plot is second. This seems to fluctuate from a literature value for the order of the reduction of methylene blue by ascorbic acid, which is one. A possible explanation for this error is the fact that the results were altered. An offset was subtracted from the data in order to ensure that the final absorbance readings were zero. Error could have easily occurred in that step.

Not only do the linear regressions determine the order of the reaction, they also provide the value of the pseudo rate constant for the reaction, k’. K’ in this situation is called a pseudo rate constant since it is not the actual rate constant of the reaction. The actual rate constant of the equation, however, is encorporated into the value of k’ since k’= k [H2A]y [HCl]z. The way in which the pseudo rate constant is determined from the linear regression is that the slope of the linear regression is the value of k’. In this case, k’ = 0.1361 for the 1 molar sample of methylene blue and k’ = 0.193 for the 1.5 molar sample of methylene blue. Even though in this experiment we changed the concentration of methylene blue, the pseudo rate constant should have remained the same in both of the reactions. Since they weren’t the same, the average of the two pseudo rate constants is k’ = .1645. Now that both the reaction order and the pseudo reaction constant are known, the rate law for the methylene blue plus vitamin C reaction can be written as: Rate = .1654 [MB+] 2