Acid-Base Titrations Using ph Measurements

Transcription

1 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 + HC 2 H 3 O 2 C 2 H 3 O 2 + H 2 O (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 the titration, 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 K a of the acid. If the acid is too weak, this change becomes so small that it is not possible to identify an inflection point. In part I of this experiment, acetic acid (a monoprotic acid) will give only one inflection point on the plot. In the second part, you will titrate a phosphoric acid (H 3 PO 4 ), a weak triprotic acid. The dissociation of each proton (hydrogen ions) occurs in steps. The first proton dissociates more readily than the second proton, which dissociates more readily than the third. When you plot your data (ph vs ml), you will see that the first equivalence point will be very clear, the second equivalence point will be harder to notice with the third probably not being evident. In order to clearly find the equivalence points for both experiments, you will plot the first and second derivatives of the titration curve. This will make the equivalence points easier to locate. Procedure Part I 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. 2. Obtain an acetic acid solution with an unknown concentration. Record the unknown letter. 3. Pipet ml of the unknown solution into a 250-mL beaker. Add 25 ml of distilled water (graduated cylinder) and a teflon-coated stir bar. Place the beaker on the stir plate. 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. Add a few drops of phenolphthalein indicator to give help visualize the equivalence point. 26

2 4. Place the ph electrode in the acetic acid solution and record the ph before any sodium hydroxide is added. Keep the electrode in the solution for the duration of the titration. 5. Record the initial volume of the buret to an accuracy of 0.01 ml. Start the stirrer. 6. Add approximately 1 ml of NaOH solution. Record the final volume to 0.01 ml. 7. Record the ph after the reading has stabilized. 8. Repeat steps 6 and 7 until you observe the equivalence point (you will notice a sharp increase in the ph of the solution). Go past the equivalence point in the same way until the ph reaches about 11 to 12. Now that you know approximately what volume of sodium hydroxide you need to reach the equivalence point, repeat the titration, but with more data recorded in the vicinity of the equivalence point. Sketching a rough plot in your notebook will help you find that point. 9. Pipet ml of the unknown solution into a 250-mL beaker and add 25 ml of distilled water (graduated cylinder). Do not add phenolphthalein indicator. 10. Record the initial volume of the buret to 0.01 ml and the ph. 12. Add approximately 1 ml of NaOH solution. Record the final volume to 0.01 ml. 13. Mix the solutions thoroughly and record the ph. 14. Repeat 12 and 13. When you are within about 5 ml of the equivalence point (determined in the first titration), add the NaOH in increments of about 0.5 ml for the next 3 ml and then in increments of about 0.2 ml or less for the next 2 ml. Continue for 5 ml on the other side of the equivalence point in a mirror image (about 0.2, then 0.5 ml increments). Record all volumes to 0.01 ml accuracy. Part II Titration of Phosphoric Acid As in Part I, obtain a phosphoric acid solution of unknown concentration. Record the unknown designation. Add 3 drops of bromocresol green before you begin. This indicator will undergo a color change when you have reached the first equivalence point. Proceed as before with a trial titration. Record the volume, color and ph as you titrate. You will be able to clearly determine when you have reached the first equivalence point by a color change. You will not clearly see the second, but it will be approximately two times the first. You will not see the third equivalence point at all. Proceed as before (Part I) with a good titration. Do not add any indicator. Obtain good data points in the vicinity of the first and the second equivalence points. You can take longer intervals after the first and within about 5 ml of the second (which should be twice the first point). Record the volume and ph as you titrate. 27

3 Question 1. The following data were collected in the titration of 10.0 ml of 0.10 M weak acid, HA, (K a = 1.0 x 10 5 ) with 0.10 M NaOH solution. Tabulate and plot three graphs: the titration curve, the first derivative and the second derivative (see page 29). ml 0.10 M NaOH ph A example of the procedure is given below v (ml NaOH) ph v ' (ml) f ' ( ph/v) v'' (ml) f '' ( ( ph) /v ) v ' is the average of two consecutive volumes: = and f ' is calculated by taking the difference in ph and dividing by the difference in volume of NaOH = =

4 v '' is the average of consecutive average volumes : = and f '' is calculated by taking the difference in f ' and dividing it by the difference in v' = = 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 2 / 2 v) 2 A2 B2 =(A2+A3)/2 =(B3-B2)/(A3-A2) =(C2+C3)/2 =(D3-D2)/(C3-C2) 3 A3 B3 =(A3+A4)/2 =(B4-B3)/(A4-A3) =(C3+C4)/2 =(D4-D3)/(C4-C3) 4 A4 B4 =(A4+A5)/2 =(B5-B4)/(A5-A4) =(C4+C5)/2 =(D5-D4)/(C5-C4) 5 A5 B5 =(A5+A6)/2 =(B6-B5)/(A6-A5) =(C5+C6)/2 =(D6-D5)/(C6-C5) 6 A6 B6 =(A6+A7)/2 =(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. Data Treatment and Discussion Using the same spreadsheet from the Questions, tabulate the good titration data of the acetic acid and the phosphoric acid unknowns. Calculate the first and second derivatives. Show one sample calculation of the first and second derivative in your notebook. For Part I, make a plot of ph vs ml of NaOH solution added, a first derivative plot and a second derivative plot. The plots are column B versus column A, column D versus column C, and column F versus column E. 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 4 significant figures, off the graph. Expand the x axis to read it accurately. For Part II, find the volume of sodium hydroxide at the first and the second equivalence points by plotting the first and second derivative of your good data as in #2 above. Calculate the molarity of the original acetic acid solution using the equivalence point off the graph. The volume of the original solution was ml. Do not include the 25 ml of water added from the graduated cylinder. You are still titrating moles. 29

5 Calculate the molarity of the original phosphoric acid solution using both equivalence points and give the average. In order to calculate the molarity of the H 3 PO 4 solution, use the first equivalence point in a 1:1 reaction and use the difference between the second equivalence point and the first also in a 1:1 reaction. Take the average of the two molarities. Conclusion Give the unknown number and the molarity of the acetic acid solution and the unknown number and the molarity of the phosphoric acid solution. Also address: In part II, are the first equivalence point and the difference from the second equivalence point identical? If not, suggest a reason. What is the advantage to using a ph electrode and meter to find the equivalence point versus a visual indicator? 30