To each flask add 10 ml conc h 2 so 4 carefully using

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volume in each flask to about 50 mL. To each flask add 10 mL conc. H2SO4 (CAREFULLY, using a graduated cylinder) and 0.5 g solid KIO4 (potassium periodate or metaperiodate, depending on the manufacturer). Heat each to boiling for about 10 min, cool, transfer quantitatively to 250 mL volumetric flasks, and dilute to the mark with distilled water. Determine the absorbance of each solution at 440 nm and 545 nm, using 0.5 M H2SO4 as a blank solution. Permanganate solutions containing periodate are stable. Note: The absorbance at 440 nm will be less than 0.1 and, hence, the spectrophotometric error (precision) will be large. But this is acceptable, in fact, desirable, because the correction for manganese absorption at this wavelength is small; that is, a relatively large error in determination a small correction results in only a small error. i.Chromium: You need three 250 mL volumetric flasks, and one burette. Using burette, add aliquots of 10.00, 15.00, and 25.00 mL of the standard K2Cr2O7 solution to 250 mL volumetric flasks, add about 100 mL distilled water and 10 mL conc. H2SO4, mix thoroughly, and dilute to the mark with distilled water. Determine the absorbance of each solution at 440 nm and 545 nm, using 0.5 M H2SO4 as the blank solution. The absorbance in this case will also be small (< 0.1) at 545 nm. j.Determination of k values: Using a spreadsheet, plot absorbance versus concentration in units of mg/L for each solution at each wavelength, and plot the least-squares straight line through each set of data points. The lines should intercept at zero absorbance and zero concentration; under “Chart Options”, you can instruct Excel to plot the intercept at zero. In this case we are independent on the y-intercept, and the slopes of these lines are the coefficients (k = A/C) to be used in determining the concentrations of chromium and manganese in the unknown. These slopes relate absorbance and concentration for the instrumental parameters used. Therefore, one should use the same instrument, cuvettes, cuvette position, and volumes of solutions for all determination in this experiment. k.Analysis of unknown: You need four 250 mL volumetric flasks and three 250 mL Erlenmeyer flasks. Bring one 250 mL volumetric flask to TA to obtain 25.00 mL of a mixture of Mn2+ and Cr2O72- unknown. Dilute to the mark with distilled water. Transfer with volumetric pipette three 50.00 mL aliquots into 250 mL Erlenmeyer flasks. To each flask add 5 mL conc. H2SO4 (BE CAREFUL!), then mix well. Add about 1 or 2 mL of 0.1 M AgNO3 solution and 1.0 g solid potassium peroxidisulfate (K2S2O8)***. Dissolve peroxydisulfate and heat the solution to boiling and boil gently for about 5 min. Cool the solution and then add 0.5 g KIO4. Again heat to boiling for 5 min.
62 *** BE CAREFUL! PEROXYDISULFATE IS A STRONG OXIDIZING AGENT THAT CAN REACT VIOLENTLY WITH REDUCING AGENTS! USE ONLY AS DIRECTED. DO NOT SPILL! a.Cool each solution to room temperature, quantitatively transfer to 250 mL volumetric flasks, and dilute to the mark with distilled water. The solutions at this point (or before solution) are stable and can be saved until another laboratory period if necessary. l. From the unknown absorbance at the two wavelengths, calculate the parts per million of Cr and Mn in your unknown using Beer’s law for the mixture and the determined constants. The calculated results will have the same units as used in determining the constants. Keep in mind the dilutions made. Reports the results for each portion analyzed.

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