1 Experiment 7 Synthesis and Analysis of those same old Moldy Green Crystals from Two Weeks Ago Part 3: Spectrophotometric Determination of Iron Content CH 204 Fall 2006 Dr. Brian Anderson Unknown Summary Sheets If you got a low grade for your acid concentration in Experiment 4, you can recalculate and resubmit. Make sure you include: Ø Your original unknown summary sheet Ø Your new unknown summary sheet all filled out Ø A copy of the redone calculations OR the signature of your TA on the new sheet. Experiment 7 in a nutshell Convert our green crystals into an orange solution, Fe(phen) 3 2+ , then use a spectrophotometer to measure how much light this solution absorbs at 510 nm. Make some solutions with known concentrations of Fe(phen) 3 2+ compare the absorbance of our sample to these standards to determine how much Fe(phen) 3 2+ is in our solution.2 The Plan for Today 1) Part 1 (free up Fe 2+ from sample)... until the hot solution has to cool 2) Part 2 (make up standard solution)... until the solution has to sit for 20 minutes 3) Finish Part 1, start Part 4 (make up sample solution)... until the solution has to sit for 20 minutes 4) Do Part 3 (measure standards) 5) Finish Part 4 (measure sample) Lab Procedure, Part 1 1. Weigh out 0.15 g of green crystals and dissolve in deionized H 2 O. Transfer the dissolved sample to a 25 mL volumetric flask. 2. Add 8 mL of 6 M H 2 SO 4 , and fill to the line with deionized water using a disposable pipette. Your sample is now dissolved in 25 mL of solution. Part 1, continued... 3-4. Pipet 5 mL into a 30 mL beaker, add about 10 mL deionized water, heat and stir. 5. Add KMnO 4 dropwise until the solution turns pink. 6-8. Transfer the solution to a clean 25 mL volumetric flask. Let the sample cool and go on to Part 2.3 Part 2 – make up the standard iron solution 1. Get 10 mL of the iron solution from the hood, and pipette 5 mL into a 25 mL volumetric flask. That’s a 1 to 5 dilution of the original concentration. 2. Add 1 mL of hydroxylamine, NH 2 OH 2 mL sodium acetate, and 8 mL 1,10 phenanthroline 3. Fill the volumetric flask up to the line with deionized water using a dropper pipette, then mix it, cap it off and let it sit for 20 minutes for the reaction to occur. Finish Part 1 9. The sample has cooled off in a 25 ml volumetric flask, and needs to be filled to the mark. The sample has now been diluted 1 to 5 from the original concentration. On to Part 4 1. Pipette 5 mL of your sample from part 1.9 into a 25 mL volumetric flask. The sample has now been diluted 1 to 5 TWICE, or 1 to 25. Add 1 mL of hydroxylamine, NH 2 OH 2 mL sodium acetate, and 8 mL 1,10 phenanthroline Swirl and mix, and allow it to sit for 20 minutes to let the reaction proceed.4 After 20 minutes is up... ...FILL TO THE MARK WITH PHENANTHROLINE!!! In Part 2 (making the standard) you used water. In Part 4 (working with your sample) use phenanthroline to fill up the 25 ml volumetric flask. Part 3 – Make Individual Standards 1. Get five test tubes and label them 1, 2, 3, 4, 5. Write directly on the glass with your marker. Using a graduated pipette, add that many milliliters of the orange solution that you prepared in Part 2 to each test tube. Using the graduated pipette again, fill each test tube to 5 mL total by adding 4, 3, 2, 1, and 0 mL of deionized water to test tubes 1-5 respectively. A whole lotta dilutin’ goin’ on! When we mix up the standards in the test tubes, each one is diluted by a different factor: 1 was diluted 1 to 5 2 was diluted 2 to 5 3 was diluted 3 to 5 4 was diluted 4 to 5 5 was not diluted in this step.5 Correcting for dilutions To find the actual concentration of each of the standards, we have to multiply by the dilution factor for each one: Concentration on the bottle × 1/5 × test tube dilution factor 1: Conc. × 1/5 × 1/5 2: Conc. × 1/5 × 2/5 3: Conc. × 1/5 × 3/5 4: Conc. × 1/5 × 4/5 5: Conc. × 1/5 × 1 Next Step: Spectrophotometry! Spectrophotometry! Spectrophotometers are the most widely used analytical instruments in the world, except for the analytical balance, and they’re about as easy to use as an analytical balance. “But what does a spectrophotometer look like?” you are wondering, “And how does it work?” I’m glad you asked! Looks like this6 Works like this Anything that is colored has color because it absorbs some wavelength (or wavelengths) of visible light. absorbance units Using the spectrophotometer Place a cuvette full of deionized water into the instrument. This is your blank. Press the button that says 0 ABS. Remove the blank and put in a cuvette containing your first standard. The display will automatically read out the absorbance. Record this value. Lather, Rinse, Repeat Repeat this procedure for each of your standards and your sample. You will have to dilute your sample until the measured absorbance is between 0.1 and 1.0. Insert the blank before each measurement to make sure the blank reads 0 absorbance units, then insert the next sample.7 How not to screw up this part 1) Rinse the cuvette twice with the sample you are about to measure before you put it in the instrument 2) Wipe the outside of the cuvette clean using Kim-Wipes. No fingerprints, no wetness on the outside. 3) No bubbles in the solution. 4) Fill the cuvettes at least 3/4 of the way up. But what do these absorbance values tell us? Beer’s Law Beer’s Law says that absorbance depends on three factors: molar absorptivity, concentration, and path length. A = εcl Sometimes written as A = εbc or A = abc Beer’s Law plots When we plot Absorbance versus Concentration, the slope of the line is equal to εl. In our case l=1, so the slope of the line is equal to the molar absorptivity for Fe(phen) 3 2+ .8 After you have your data Enter the absorbance and concentration values into Excel. Plot Absorbance (y-axis) versus concentration (x-axis). Include 0,0 as a data point – that is your blank. You should get a straight line, and the slope of the line is your molar absorptivity, ε, in units of M -1 cm -1 . Get a least squares fit so you have an equation for the line. Don’t forget about those dilutions When you determine the concentration of your sample from the graph, remember that you have to back-calculate through all the dilutions you made in order to figure out the original concentration you started with. Looking ahead • The final three labs (Thermochemistry, Kinetics,