Download 33 PRELAB amino acids with counter NEW

Identification of an Unknown Amino Acid by Titration PRELAB
On this procedure, answer the questions, box the equipment (draw if you don’t know), and highlight
all chemical (give name/formula)
Procedure :
1. Lower the Drop Counter plate onto a ring stand and clamp it into
position (see Figure 1).
2. Connect the Drop Counter to LabQuest DIG 1
3. Obtain the plastic 60 mL reagent reservoir. Close both valves by
turning the handles to a horizontal position.
4. Follow the steps below to set up the reagent reservoir for the titration.
a. Rinse the reagent reservoir with a few mL of the NaOH
solution and pour the rinse NaOH into an empty 250 mL
beaker.
b. Use a utility clamp to attach the reservoir to the ring
Figure 1. Drop Counter
stand. (see Figure 1).
Apparatus
c. Fill the reagent reservoir with ~ 60 mL of the NaOH solution.
Note: Record the precise concentration of the NaOH solution
in your data table.
d. Place the 250 mL beaker, which contains the rinse NaOH, beneath the tip of the reservoir.
e. Drain a small amount of the NaOH solution into the 250 mL beaker so that it fills the reservoir’s tip.
To do this, turn both valve handles to the vertical position for a moment, then turn them both back
to horizontal.
f. Discard the drained NaOH solution in the 250 mL beaker as directed.
5. Calibrate the drops that will be delivered from the reagent reservoir.
a. On the top row of the LabQuest toolbar, open the SENSORS menu and choose Calibrate ! DIG 1:
Drop Counter.
b. Place a 10 mL graduated cylinder directly below the slot on the Drop Counter, lining it up with the
tip of the reagent reservoir.
c. Open the bottom valve on the reagent reservoir (vertical). Keep the top valve closed (horizontal).
d. Click the Calibrate Now button.
e. Slowly open the top valve of the reagent reservoir so that drops are released at a slow rate (~1
drop every two seconds). You should see the drops being counted on the screen.
f. When the volume of the NaOH solution in the graduated cylinder is between 9 and 10 mL, close
the bottom valve of the reagent reservoir. Note: Do NOT adjust the top valve for the remainder of
the experiment.
g. Enter the precise volume of NaOH (read to the nearest 0.1 mL) in the edit box.
h. Click OK
i. Discard the NaOH solution in the graduated cylinder as directed, and set the graduated cylinder
aside.
6. Assemble the apparatus. (See Figure 1.)
a. Place the magnetic stirrer under the Drop Counter.
b. Connect pH Sensor to CH 1 at the top of the LabQuest.
c. Remove the storage bottle from the electrode by first unscrewing the lid, then removing the bottle
and lid. Thoroughly rinse the lower section of the probe, especially the region of the bulb, using
distilled or deionized water.
d. Insert the pH Sensor through the large hole in the Drop Counter.
e. Attach the Microstirrer to the bottom of the pH Sensor. Rotate the paddle wheel of the Microstirrer,
and make sure that it does not touch the bulb of the pH Sensor.
f. Adjust the positions of the Drop Counter and reagent reservoir so they are both lined up with the
center of the magnetic stirrer.
g. Ensure reagent reservoir has a sufficient amount of NaOH solution.
h. Add 10 mL of unknown acid solution to a 150 mL beaker.
^^^ Draw the structure of amino acid in acidic solution.
i.
Add 50 mL distilled water into beaker.
^^^ Does the amount of water make difference to the titration? Why, why not?
j.
Lift up the pH Sensor, and slide the 150 mL beaker containing the acid solution onto the magnetic
stirrer. Lower the pH Sensor into the beaker.
k. Adjust the position of the Drop Counter so that the Microstirrer on the pH Sensor is just touching
the bottom of the beaker.
l. Adjust the reagent reservoir so its tip is just above the Drop Counter slot.
7. Turn on the magnetic stirrer so that the Microstirrer is stirring at a fast rate.
8. You are now ready to begin collecting data. Press
. No data will be collected until the first drop goes
through the Drop Counter slot. Fully open the bottom valve. The top valve should still be adjusted so drops
are released at a rate of about 1 drop every 1 second. When the first drop passes through the Drop
Counter slot, check the data table to see that the first data pair was recorded.
a. Stop the titration if the pH reaches 13.0 or if 50 mL NaOH is added. Turn the bottom valve of the
reagent reservoir to a closed (horizontal) position, then press
.
^^^ Draw the reaction between amino acid (in HCl!!) with NaOH. The reaction is NOT between HCl
and NaOH.
9. Repeat steps (6g) to (8) if more than one run is required.
10. To save data:
a. Insert a FAT32 formatted USB drive or SD card into the top of the LabQuest.
b. On any screen use the stylus to tap on the word file at the top of the screen.
c. Tap EXPORT to save your file as a .txt file.
d. Tap the picture of the USB or SD card to enter that device.
e. Tap on word Untitled at the top of the screen and use the backspace key on the onscreen
keyboard to change the file name.
f. This text file can later be exported into Excel for data analysis.
*** Make sure you have a way to distribute the data to all of your group members OR
give the data to Jackie and it will be linked to the class website.***
11. To finish using the LabQuest:
a. To turn off the machine hold down the power button until the screen goes black.
b. If you get an unsaved data message tap discard.
c. Return the stylus to its holder and remove the pH probe and any save devices.
d. When you are finished making measurements, rinse the tip of the electrode with distilled water.
Slide the cap onto the electrode body, then screw the cap onto the storage bottle.
e. Note: When the level of storage solution left in the bottle gets low, you can replenish it with pH
storage solution. It should NEVER be stored in distilled water!
Prelab Questions : Answer these directly here.
(PL1) Sketch a theoretical titration curve for HCl vs. NaOH.
(PL2) Sketch a theoretical titration curve for H3PO4.
(PL3) Discuss the titration curves for (PL1) and (PL2). What can you say about strong/weak acid? What can
you say about mono/tri protic acid?
(PL4) On the titration curve given in the INTRODUCTION of this lab, write H-H equation that goes with Point
A, Point B, or Point C? You do not need to solve for anything. Just show what values goes where in the H-H
equation.
(PL5) Under what condition is pKa = pH? Does this happen at Point A, B,C,D and/or E? Prove it using H-H
equation.
(PL6) During which points of the titration should you be most accurate? Explain.
(PL7) All protein is made up with 20 amino acids. All 20 amino acids share common structure with one varying
section. Draw the structure of one amino acid. Name it. On the drawing put a circle around the common part.
(PL8) Cut and paste a (or two) table that has 20 amino acids. It should include at least the names, structures,
pKa’s. Site the source. ATTACH THIS TO THESE PAPERS AND TURN IT IN! YOU WILL ALSO NEED
THIS INFORMATION TO IDENTIFY YOUR UNKNOWN.
(PL9) Draw a data table you will be filling in during the lab. Include all the measurements that will be made in
this lab and qualitative observation if needed.
Objective : WRITE YOUR OWN.