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Determining the Concentration of KMnO, Procedure 1) You will prepare five (5) KMnO solutions for use as given in Table. Using one of these standards you will need to determine 2max and then analyze all samples to construct a calibration graph. The graph will be used to determine the concentration of an unknown KMnO4 sample. Once these solutions are prepared and mixed well, you are ready to use the spectrophotometer. To correctly use a Colorimeter cuvette, remember: • All cuvettes should be wiped clean and dry on the outside with a tissue. Handle cuvettes only by the top edge of the ribbed sides. • All solutions should be free of bubbles. • Always position the cuvette with its reference mark facing toward the white reference mark at the right of the cuvette slot on the Colorimeter 2. Determine Imax a. Fill a cuvette with the standard solution (sample 5), the most concentrated solution). b. Holding the cuvette by the upper edges, place it in the cuvette slot of the Colorimeter. c. You should see on the screen an A (absorbance) vs. A (wavelength) graph with the visible spectrum showing. If not click on the icon in the toolbar that shows the visible spectrum and select from the popup "Absorbance vs wavelength". Click collect. d. Find the wavelength of maximum absorbance. Record the wavelength. 3. Measure Absorbance of the standard solutions. Begin with the least most dilute solution and end with the most concentrated. You do not need to wash the cuvette in between solutions. a. From the wavelengths at the right, select the Amax you determined. Click OK. b. Now measure the absorbance of each of your solutions. Put a cuvette, full of solution in the holder, click COLLECT, then KEEP. Enter the concentration of the solution. Repeat for all samples. Click STOP when data collection is complete.

c. Using a clean cuvette measure the Absorbance of the Unknown. Measure the absorbance of your unknown, just as you measure the absorbance of your standards. 4. Place a cuvette with unknown in the holder. The Absorbance of the Unknown will display and record the unknown concentration. DATA SAMPLE Absorbance Concentration of KMnO. (ppb) 2 4 هایم اس امام ا 1 2 3 4 5 6 COON 8 10 Unknown Calculations 1. Plot of Absorbance vs concentration for Concentration by excel. 2. Calculate the concentration in ppb of unknown solution using the equation from your calibration curve. 3. The equation relating absorbance and concentration is 4 - ul. Our variables are 1 dependent, and c, independent. Assume our path length, 1, is 1.00 cm. A plot of A vs. c will give a line with slope equal to . . 4. Determine a percent error for each value. Ask your instructor for the actual concentration of your unknown for a comparison. (Concentration of unknown solution is 5 ppb)

Questions a. A sample absorb 80% of the light passed through it at a certain wavelength. What is the absorbance of the sample at this wavelength? b. What would the absorbance be at the same wavelength if the sample were diluted by half? Assume the same size sample cell was used. 7 C. A 0.070 M solution has an absorbance of A -0.689. If a 1.00 cm path length was used.calculate the molar absorptivity constant (€) for this solution. Be sure to include the units! d. Use the equation for the best fit line for the plot of Absorbance vs concentration below to calculate the concentration of the standard that corresponds to an absorbance value of 0.25.