Titrations of Acetic Acid, Formic Acid, Unknowns X and Y            Nov 17, 2015

Generally we do acid-base titrations with NaOH solution in the burette, and the acid in a 250 ml Erlenmeyer flask or 400 ml beaker.  You will use beakers so that you can use a pH probe and a magnetic stirrer.

            Be sure to coat the inside of your burette with the standardized 0.10xx M NaOH solution and rinse before filling the burette.  You'll need to clear bubbles from the burette tip; sometimes they're hiding where you can't see them.  Once you’ve filled the burette with NaOH solution and cleared bubbles from the tip, you should not have to do this again; just refill the burette after each titration and don’t let the burette get emptied.  Don’t bother trying to start at exactly the 0.00 ml mark on the burette.  You should never use a titration that consumes more than 50 ml of titrant.  Before you start, think about how much titrant each titration will consume.

Spread the acid bottles selected by your instructor across an unused bench top.  Get a 25 ml pipette and a 5 ml pipette for each acid bottle.  Coat the inside of the pipette with acid  solution and rinse before you use it.  DON’T MIX THE PIPETTES!  If you have forgotten how to use a pipette bulb, practice on water first before you pipette acid solution in to the beaker.  In the past you may have used a 250 ml Erlenmeyer flask for titrations, but since you will be using a pH probe today, you should use a beaker and a magnetic stirrer.

Do a quick and dirty titration of 5 ml of each acid so that you have an idea where the end point will be.  That way you can do the big titrations faster, and you’ll know were the buffer region is.  You can always add more DI water to the titration if the volume is too small for the pH probe.  Make a rough estimate of how much water you add.  Remember to add 2 drops of phenolphthalein to each titration before you start.

Set up the Vernier LabPro to record data during the big titrations.  (Instructions for the LabPro are below.)  Plan your titration!  You want to record more points as you pass through the buffering region and again during the endpoint region--but you don't want to waste time recording lots of unneeded points.  While passing through the buffering region (where pH = pKa) and also while passing through the endpoint, add just 0.20 ml of titrant at a time and read the pH after each addition of titrant.  You want readings close together in these regions so that you can make an accurate graphical estimate of the pKa and the end point volume..

    Compare the shape of the titration curve to the curves in Harris’s textbook. Your instructor may tell you to repeat a titration if your curve doesn’t make sense.

Write your results on the board.  Compare the results among groups; if one group gets values for acetic acid that are rather different from everyone else’s, then that group should check their calculations and repeat the titration.  Write on the board results for formic acid, X, and Y; check to make sure all groups get consistent results. On each acid bottle the concentration is given in grams of solute/liter of solution. (Don't mistake this for solution density!) Using this information and the titration data, compute the molecular weight of acetic acid, formic acid, X, and Y.  X and Y are both monoprotic acids listed in Appendix G of Harris’s textbook. Use the molecular weight and the pKa to determine identity of the unknown acids.

Lemon juice, fruits juices, and sodas:

Again, spread out the bottles of juice, and give each bottle its own set of pipettes.  Never pipette directly from the original bottle!  Pour out some of the juice into a beaker and then fill you pipette from the beaker.  It's a good idea to do a quick and dirty titration before investing time in a big titration.  As you work with the juices and sodas, you should share information with the other groups about how much to titrate--that way you can get down faster.  Compute the concentration of acid in each solution as g/100 ml (% acidity, as it is called) as if the acid is always acetic acid.

Setting up LabPro:

            Obtain a LabPro and supply it with power using the AC Adapter.  Using the USB cable, connect one end to the LabPro and the other to one of the USB ports on the left side of a laptop.  Connect the pH probe to the one of the channels in the side of the LabPro and then open up Logger Pro (located on desktop).  Once this program opens it should display a table on the left-hand side and a graph on the right.  Open up the Experiment menu and select Data Collection.  Under the Collection tab, select Events with Entry from the pull-down menu.  This will allow you to label the column as volume of base added, or something to that effect.  There is also a spot for the units, mL, or whatever is appropriate.  A modified table on the left should have two columns, one for the volume and one for the pH. 

            You will need to calibrate the pH probe using pH 4 and pH 7 buffers.  If the pH probe is not calibrated, your value of the pKa will be wrong.

            To adjust the scale on the graphs, right-click on the graph and select Autoscale from the menu.  You can also select a value on the axis and type over it with the value you want.  Get the axes right so that the titratio curve will be big and easy to read on the computer screen.

To start recording readings: Click the Collect button at the top right of the screen in Logger Pro.  At this point the Keep button should also become active.  The current readings for the pH probe should be shown right above the table and graphs.  Once the readings have had a chance to stabilize, click the Keep button and enter “0” in the blank.  Now add two or three mL of titrant at a time to the beaker, allow the reading to stabilize, and click Keep once again to enter the amount of titrant added.  Around the endpoint, add titrant 0.2 mL at a time.  Be sure to take seven or eight data points beyond the endpoint so that you see the shape of the entire curve.  Once your last data point has been collected, click Stop to end data collection.  You MUST click Stop before you can print and save the data to a drive.

You may then print out the table and graphs separately by opening up the File menu and selecting Print Data Table and Print Graph, respectively.  A menu will come up in which you can add a footer if you would like. 

To save your data, open up the File menu and select Export as Text.  This will allow you to save the data as a text file to open in Excel.  Then, open up Excel and select the Open command.  Change the file type in the pull-down menu to Text Files and find your file and open it.  A window will appear which helps to import the data into Excel.  Make sure the Delimited option is selected, then click Next twice, and then Finish.  The data should appear exactly as it was in your table in Logger Pro.  Alternatively, you can highlight the data in Logger Pro, select Copy, switch to Excel, and finally select Paste.  However, do not forget to save your data from Logger Pro.