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1 | P a g e Experiment 3 Analysis of Dichromate and Permanganate Mixture Laboratory Report KEZIAH MAY VALENCIA KIMBERLY ADENIX (Group 3) Chemistry 126 Laboratory –Instrumental Methods Laboratory Section 1 January 28, 2016 JAY MARTIZANO Laboratory Instructor
2 | P a g e ABSTRACT A spectrophotometric way of analyzing a mixture of two compounds was done in this experiment. UV-Vis Spectrophotometer was used to obtain the absorption maximas of potassium permanganate and potassium dichromate and was found to be at 526 nm and 437.5 nm wavelengths, respectively. The lambda maximums were used in measuring the absorbances of the standard solutions of each compound which were then used in constructing their respective calibration curves. Also, the absorptivities of the four calibration curves as well as the absorptivity of the sample were the determination. From the measured absorbance of the mixture at each wavelength, a system of two equations in two unknowns was solved algebraically to determine the molar concentrations of the two ions, MnO4-and Cr2O72-, and they are found to be 3.067 M with 25% error, and 0.345M with 27.95% error, respectively. INTRODUCTION Simultaneous spectrophotometric determination of two solutes in a solution is feasible when the two solutes are unreactive. However, there is an overlap in the spectra and generation of calibration curve of either one of the species is not possible. In this experiment, the concentrations of dichromate and permanganate ions were determined using UV VIS spectrophotometer. Dichromate was quantified at λ = 440 nm whilepermanganate was measured at λ = 545 nm. These wavelengths correspond to the wavelengths where the absorbance was not changing much over a range of wavelengths. Since the absorbances were additive, the amount of the ions in solution can be computed using these equations:
3 | P a g e Aλ1= λ1A1+λ1A2(1) Aλ2= λ2A1+ λ2A2(2) where A1 and A2are the measured absorbances at the two wavelengths λ1and λ2respectively. The subscripts 1 and 2 refer to the two different substances, λ1and λ2refers to the different wavelengths. The wavelengths were selected to coincide with the absorption maxima of the two species and the absorption spectra of the two solutes should not overlap so that substance 1 absorbs strongly at wavelength λ1and weakly at wavelength λ2, and substance 2 absorbs strongly at λ2and weakly at λ1. Now, A = εcl, where ε is the molar absorption coefficient at any particular wavelength, c is the concentration (mol L-1) and l is the thickness or length of the absorbing solution (cm). Aλ1= λ1ε1c1+ λ1ε2c2 (3) Aλ2= λ2ε1c1+ λ2ε2c2 (4) The values of molar absorption coefficients ε1and ε2 can be deduced from measurements of the absorbances of pure solutions of substances 1 and 2. By measuring absorbance of the mixture at wavelengths λ1& λ2, the concentrations of the two components can be calculated.