Bleaching of Allura Red
Katie Berry TA: Jason Fahrion April 10, 2013 A Kinetic Study of the Bleaching of Allura Red Introduction: In this laboratory experiment, we studied the kinetics of the reaction between Allura Red (C18H14N2O8S2) and bleach. C18H14N2O8S2 (aq) + NaOCl (aq) → products Allura Red is a food dye, which is an organic molecule with alternating single and double bonds. The color of the dye comes from the delocalization of electrons in the πbonding system (Exton, 149). Bleach is an aqueous solution that contains sodium hypochlorite (NaOCl). The hypochlorite of bleach breaks the double bonds of the food dye through oxidation, forming molecules that do not absorb visible light and thus bleaching the product. During this process, hypochlorite is reduced to chloride and hydroxide ions. OCl (aq) + H2O (l) + 2e → Cl (aq) + 2OH (aq) Our goal in this experiment was to observe the reaction and concentration changes between Allura Red and bleach and determine the rate law for the reaction. The rate of reaction can be found by the equation rate = ∆concentration / ∆time = k[Allura Red]x[NaOCl]y The x and y values indicate the order with respect to each reactant, and k represents the rate constant for the reaction. These values had to be determined experimentally. In order to determine the rate law of the reaction, we had to use spectrophotometry in order to monitor the change in absorption of the various reactions. Since Allura Red was the only colored species in the reaction, we were able to monitor the rate by observing the decrease in color of the solution with time. From the absorption values, we were able to calculate the concentrations from Beer’s law: A=ϵBC (1). Five different trials were performed with various concentrations of Allura Red, bleach, and water. The equation: M1V1 = M2V2 (2) was used in order to calculate the initial concentrations of Allura Red and NaOCl for these trials. Graphs of concentration of Allura Red ([AR]), ln[AR], and 1/[AR], as a function of time