Embedded Files
PROCEDURAL VIDEOS TO WATCH:
PROCEDURAL VIDEOS TO WATCH:
CHEMICALS PROVIDED (FOUND ON SHELF UNLESS OTHERWISE NOTED):
CHEMICALS PROVIDED (FOUND ON SHELF UNLESS OTHERWISE NOTED):
REQUIRED VOLUMETRIC GLASSWARE:
REQUIRED VOLUMETRIC GLASSWARE:
500-mL volumetric flasks (2)
100-mL volumetric flasks (8)
50-mL volumetric flask (2)
SPECIAL EQUIPMENT (FOUND ON SHELF UNLESS OTHERWISE NOTED):
SPECIAL EQUIPMENT (FOUND ON SHELF UNLESS OTHERWISE NOTED):
100-1000 µL adjustable micropipetter (on the bench next to the sink)
PROCEDURE:
PROCEDURE:
A. Preparation of Initial Solutions
Prepare 1.0 L of 0.01 M HCl by diluting an appropriate volume of the provided HCl stock to 1.0 L with ultrapure water. (The exact concentration and volume of the final solution aren’t critical, so you can prepare the solution using a graduated cylinder and a 1-L plastic bottle instead of a pipet and a volumetric flask.)
Pour approximately 20 mL of Mello Yello into a 125-mL Erlenmeyer flask, then sonicate the sample for ~5 minutes to remove dissolved carbon dioxide. (VIDEO)
Using a micropipetter (with a new tip) and a 50-mL volumetric flask, dilute exactly 1 mL of the soft drink to 50 mL with 0.01 M HCl.
Using a micropipetter (with a new tip) and another 50-mL volumetric flask, dilute exactly 2 mL of the caffeine/benzoic acid unknown solution to 50 mL with 0.01 M HCl.
Sample Prep Summary
B. Preparation of Standard Solutions
Prepare a stock solution of benzoic acid by accurately weighing ~0.12 g benzoic acid (record the mass to four decimal places) and quantitatively transferring it to 500-mL volumetric flask. Fill the flask about half-full with ultrapure water. The solid is difficult to get the solid to dissolve, so you'll want to use the sonicator to help dissolve the acid (see picture below). Once the solid is dissolved, fill the flask to the mark with ultrapure water. (VIDEO)
2. Prepare a stock solution of caffeine by dissolving ~0.22 g caffeine (accurately weighed and transferred) with ultrapure water in a second 500-mL volumetric flask. (You'll probably want to use the sonicator again to help dissolve the caffeine.) Once the solid is dissolved, fill the flask to the mark with ultrapure water. (VIDEO)
3. Make four standard benzoic acid solutions by using a micropipetter (with a new tip) to deliver 1, 2, 3, and 4 mL of benzoic acid stock solution into four clean 100-mL volumetric flasks. Dilute each flask to the mark with 0.01 M HCl (not water). Make sure the flasks are well-labeled.
4. Make four standard caffeine solutions by using the micropipetter (with another new tip) to deliver 1, 2, 3, and 4 mL of caffeine stock solution into four clean 100-mL volumetric flasks. Dilute each flask to the mark with 0.01 M HCl (not water). Make sure the flasks are well-labeled.
Standard Prep Summary
C. Setting up the Instrument Software
Note: When using the fiber optic dip probe, it is important that it first be properly aligned. This will have already been done by either myself or a TA at the start of class.
Turn on the Cary 60 by pressing the button in the lower right hand corner of the instrument. The button light will flash until the instrument is ready to use, and then become steady. (Note that if the button is lit when you come to the instrument, it's already on.)
You may need to log onto the computer. The username is cary60 and there is no password.
Open the software by first double clicking on the Cary WinUV icon on the desktop.
5. In the directory that appears, double click the Scan shortcut.
6. Click the Setup... button to open up the experimental parameters.
7. You'll now click through the various tabs to set up the parameters. Beginning with the Cary tab, set all values to be the same as in the image below. Note that the wavelength range (X mode) is set from 400 to 220 nm, the Beam Mode is Dual Beam, and the Scan Controls setting is Medium.
8. Click the Baseline tab and select Baseline Correction.
9. Skip to the Reports tab and make the window look like the image below. Be sure to enter your name and sample information.
10. Click the Auto Store tab and tell the software to store data either at the start (as shown below) or at the end of a run. Then click OK to save all the parameters.
11. If an Information window appears with the heading of Baseline Information, it simply means that you'll have to measure a baseline. Click OK to continue.
D. Absorbance Measurements
Bring all ten solutions (diluted soft drink, diluted unknown, four caffeine standards, and four benzoic acid standards) to the instrument. Also pour the remainder of your 0.01 M HCl into a small beaker and bring that as well to use as your blank solution.
If necessary, remove the protective plastic cover from the dip probe. NOTE: When using the dip probe, always make certain that there are no bubbles in the sensor area when you take a measurement. To eliminate any bubbles present, swirl/shake the solution or remove the dip probe from the solution and reinsert it until the sensor is clear. (VIDEO)
Click the Zero button (circled below). When the Zero window appears, place the dip probe in your beaker of 0.01 M HCl and click OK.
4. Now click the Baseline button (circled below). When the Scan window appears, again place the dip probe is in your beaker of 0.01 M HCl and click OK. Make sure to hold the solution in place until the scan is complete.
5. You'll now record the full absorbance spectrum of all ten solutions. The probe should fit into the 100-mL volumetric flasks, but you'll want to pour the solutions from the 50-mL volumetric flasks into clean, dry beakers or Erlenmeyer flasks. NOTE: Make sure to gently wipe the dip probe with a Kim-wipe each time you change solutions. (VIDEO)
6. Press the Start button to begin taking measurements. You'll be prompted for a place to save your data. I suggest creating your own directory. Make sure also to change the file type to Batch (*.BSW) as in the image below.
7. Place the dip probe in your first sample. Enter a Sample Name (see below) and click OK.
8. Continue through each of your ten samples, changing the sample name accordingly for each. IMPORTANT: Except for your blank, each spectrum you acquire should have a baseline at zero absorbance. If this isn't the case you probably have an air bubble in the sensor area (see below) and you'll need to re-run that solution. Simply pull the probe out of the solution, put it back in, and run the spectrum again. The figure below shows a spectrum acquired with a bubble.
9. When you have acquired your last spectrum, let me look at your data. If it is ok, then click Finish.
10. Print a report by clicking the printer icon, selecting CutePDF Writer as the printer, then saving it on a flash drive so that you can upload it to LabArchives.
11. Go to the File menu and select Save Data As... Choose Spreadsheet Ascii(*.CSV) as the Type of File, then save the data to a flash drive so you can use it for data analysis and for uploading to LabArchives.
12. Close the software and shut off the instrument by pressing the power button. Replace the protective plastic cover on the dip probe.
E. Waste Disposal / Clean Up
Caffeine and benzoic acid stock solutions can be poured down the drain.
All remaining acidic solutions should be collected in a single container (I recommend an Erlenmeyer flask). Put the container on a stir plate and drop a magnetic stir bar in the solution. Add ~1-2 mL of universal indicator and begin stirring, then neutralize the solution with sodium carbonate (found in the hood) until the color is green or yellow. Pour the solution down the drain to dispose.
Pour any remaining sample solutions down the drain. Rinse any used sample containers and place them in the labeled bin in the prep hood.
Place any used pipets in the "Pipets to Be Washed" bin.
Rinse all volumetric flasks 5-6 times with tap water and then 2-3 times with RO water, and return to the shelf. (Use acetone to wash off any markings.) Similarly rinse all glassware from your drawer and return it to your drawer.
Use the sponge in your drawer to clean your bench area.
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