Lab took the entire period today with the first students leaving 45 min early.
Student Checkout: laptop, 25mL volumetric flask
TA Pick-up: Ocean Optics Spectrometers, cords, cuvettes
The spectrometers should already be set up on the back benches. Just double check that the chalk is the correct height and angle so that the light is reflected up to you. The students can do Part A at any time during the period. They turn the wavelength dial on the instrument and record the corresponding colors. Make sure they do not go below 425nm because that is UV range and can damage their eyes.
Assign dyes to each group to ensure that each dye is used at least once (this is important for next week). Dyes are in the plastic carboys. Remind students to record the name and concentration of their dye.
Follow Set-Up Appendix G ("Collecting a Complete Spectrum"). Note the importance of keeping cuvettes clean and orienting them consistently. The cuvettes only need to be filled about 3/4 full.
To copy & paste their data into Excel, have them highlight the entire column by clicking on the "wavelength" or "absorbance" tab at the top of the column, instead of trying to click & drag. They only need the data in the visible region (360-750nm), so they can scale the x-axis on their Excel graph accordingly (right-click on the x-axis, select "format axis" and change the fixed min and max values).
Make sure they HIGHTLIGHT THE GRAPH BEFORE PRINTING or they will have about 70 pages of numbers. They should connect to the network before printing.
Have them think about the volumes themselves rather than suggesting numbers. 5, 10, 15 and 20 mL of stock solution work well, except for Yellow #5 where the lowest dilution might be too pale to get a good absorbance.
Make sure they use their burets (get rid of bubbles in the tip, record volumes to the nearest 0.01 mL) and that they dispense the stock solution DIRECTLY into their volumetric flasks. I have seen groups dispensing from the buret to a beaker and then pouring it into their volumetric flask.
They can collect their data in one of two ways:
1) Collect a full spectrum as they did in Part B, then scroll through their data to find the absorbance at their maximum wavelength (as found in Part B). They can record each of these absorbance values in their table then make their own graph in Excel. (I think this is easier for most students)
2) Follow Appendix G "Collecting Absorbance vs. Concentration Data". They'll just have to insert their own concentration values, and a graph will be generated for them.
When typing their concentrations into Excel, several students tried "4.5 x 10^-5". Excel doesn't recognize this as a number and gives them an axis with points 1,2,3,4 rather than their data points. They have to type "4.5E-5" or "=4.5*10^-5" for Excel to recognize the numbers.
Give them the hint that only one of their two graphs in Part C will be linear - have them graph both before answering Q12 and Q13. They can use the R2 value to determine which is more linear.
A lot of students have trouble determining the units and understanding Beer's Law - remind them that slope is "rise over run", so the units of slope will be the units of "rise" divided by the units of "run".
Remind them that they have to bring their own unknown food dye for next week. Things that work well are skittles, M&Ms (only leave them in water for a few seconds), jello, mtn dew, jolly ranchers, gatorade and gummy worms. Things that don't work well are brown sodas, vitamin water, teas, and cranberry or orange juice (color is not from a dye).
Also, you might want to remind your students that they need to be dressing appropriately for lab now that the weather is warming up.