Acid-Base Titrations Using ph Measurements Prelab

Transcription

1 1. What is the purpose of this experiment? Acid-Base Titrations Using ph Measurements Prelab. The following data were collected in the titration of 1. ml of.1 M weak acid, HA, with.1 M NaOH solution. Tabulate and plot three graphs: titration curve, first derivative and second derivative, and find the equivalence point. ml.1 M NaOH ph An example of the procedure is given below v (ml NaOH) ph v ' (ml) f ' (ΔpH/v) v'' (ml) f '' (Δ(ΔpH) /v ) v ' is the average of two consecutive volumes: =.7 and f ' is calculated by taking the difference in ph and dividing by the difference in volume of NaOH = =

2 ph ΔpH/ΔmL v '' is the average of consecutive average volumes : = 1. and f '' is calculated by taking the difference in f ' and dividing it by the difference in v'..4.. = = A spreadsheet to do this calculation for all your tabulated data looks like: A B C D E F 1 Volume ph v' (ml) f' ( ph/ v) v'' (ml) f'' ( ph / v) A B =(A+A3)/ =(B3-B)/(A3-A) =(C+C3)/ =(D3-D)/(C3-C) 3 A3 B3 =(A3+A4)/ =(B4-B3)/(A4-A3) =(C3+C4)/ =(D4-D3)/(C4-C3) 4 A4 B4 =(A4+A)/ =(B-B4)/(A-A4) =(C4+C)/ =(D-D4)/(C-C4) A B =(A+A6)/ =(B6-B)/(A6-A) =(C+C6)/ =(D6-D)/(C6-C) 6 A6 B6 =(A6+A7)/ =(B7-B6)/(A7-A6) 7 A7 B7 Note: The last cell in C and D and the last two cells of E and F will contain no data. Your graphs, which will be used as a template for your lab data, should look like: Titration Curve of.1 M HA with.1 M NaOH First Derivative of Titration Curve volume NaOH (ml) Second Derivative of Titration Curve average volume (ml) 1 Δ ph/δml average average volume (ml)

3 Acid-Base Titrations Using ph Measurements Introduction When you titrate a weak acid with a strong base, such as NaOH, the reaction will go essentially to completion, giving an acidic solution until just before the equivalence point OH + HCH3O CH3O + HO (1) At the equivalence point, the ph will not be 7. This is because the conjugate base, acetate ion, is the major species in solution. In this experiment, you will titrating acetic acid. A ph electrode will be used to determine the equivalence point. A plot of ph vs. ml NaOH added will show an inflection point at the equivalence point. The size of the inflection point depends on the Ka of the acid. If the acid is too weak, this change is so small that it may not be seen. In order to clearly find the equivalence point, you will plot the first and second derivatives of the titration curve. This will make the equivalence point easier to locate. Procedure Titration of Acetic Acid 1. Rinse a buret with a few milliliters of the provided standardized sodium hydroxide solution, fill and remove the air bubble from the tip. Make sure you record the molarity of this sodium hydroxide solution since you will need it to calculate the molarity of the acetic acid solution. Record the initial volume to.1 ml.. Obtain approximately 6 ml an unknown acetic acid solution and record its letter. Pipet. ml of the solution into a -ml beaker. Add ml of distilled water (graduated cylinder) and two drops of phenolphthalein to help visualize the equivalence point. 3. Place a stir bar in the beaker and set it a little off to one side on a stir plate. Suspend a ph electrode in the solution with an electrode clamp and position it to the same side as the offset of the beaker (so the stir bar does not hit it). Adjust the stir rate to about 3 rpm (no splashing). Plug in the electrode to a lab interface port on the right side of the netbook (white Labquest Mini interface). Position the buret tip about 1 ½ inches from the solution in the beaker, over the stir bar s vortex. Keep the ph electrode in the solution throughout the following steps. The first titration you will perform will be a trial titration so you can get the approximate volume of sodium hydroxide that is required to reach the equivalence point. 3

4 4. Click Experiment, Data Collection. On the menu, on the Collection tab, change Mode: from Time Based to Events With Entry (on pull down menu). On the new menu, make Column Name: Volume, Short Name: Volume, Units: ml. Click Done. This sets ph data to be collected as a function of volume of standard NaOH added.. Make a table in your notebook of buret reading, total volume added and ph. 6. Click. When the ph stabilizes (should not take long), click. A dialog box will appear asking you to enter the volume. Enter, then click OK. 7. Add about 1 ml of NaOH solution from the buret. Record the final volume to.1 ml. 8. When the ph has stabilized, click. Enter the total volume added (final reading minus initial reading), then click OK. Both partners should record all data in their notebooks. 9. Repeat steps 7 and 8. Enter the total volume added up to that point each time. You will see a sharp increase in the ph of the solution at the equivalence point. Go past the equivalence point in the same way until the ph reaches about 11. Click. Click to better visualize the ph curve. Click Analyze, Interpolate. Find the approximate equivalence point. The titration will be repeated, but with more data recorded near the equivalence point. Clear the data (do not save): On the file menu, click Data, Clear All Data. 1. Fill the buret and record the initial volume of the buret to.1 ml. 11. Pipet. ml of unknown solution into a clean -ml beaker. Add ml of distilled water (graduated cylinder). Set up the beaker, ph electrode and buret as in step Make a table in your notebook of buret reading, total volume added and ph. 13. Click. When the ph stabilizes (should not take long), click. Enter in the dialog box, then click OK. 14. Add about 1 ml of NaOH solution from the buret. Record the final volume to.1 ml. 1. When the ph has stabilized, click. Enter the volume added (final reading minus initial reading), then click OK. Record the data in your notebook. 16. Repeat 14 and 1. When you are within about ml of the equivalence point (determined by interpolation of the first titration), add the NaOH in increments of about. ml for the next 3 ml, then in increments of about. ml or less for the next ml. Continue for ml on the other side of the equivalence point in a mirror image (about., then. ml increments). Record all volumes to.1 ml accuracy. Click On the menu bar, click File, then Export As. Choose CSV (Excel, InspireData, etc). Save the data. 4

5 Data Treatment Using the same spreadsheet from the prelab, tabulate just the second titration data. Calculate the first and second derivatives. Plot ph vs. ml of NaOH solution added, the first derivative plot and the second derivative plot. The equivalence point is found at the volume corresponding to the x-intercept of the second derivative curve. This point should be coincident with the inflection point (the point at which the curve changes direction) of the original data plot and the peak of the first derivative plot. Read the equivalence point volume, to the appropriate number of significant figures, off the graph. Expand the x axis (and attach the plot) to read it accurately. Calculate the molarity of the acetic acid solution using the equivalence point determined from the graph. The volume of the original solution was. ml. Do not include the ml of water added from the graduated cylinder; you are titrating moles. Conclusion Give the molarity of the acetic acid solution. Be sure to give the unknown letter. Also address: What is the advantage to using a ph electrode and meter to find the equivalence point versus a visual indicator?