CHAPTER 8 Acid Base Titration Curves Objectives The objectives of this experiment are to: Understand the titration curves for the following solutions: a strong acid: hydrochloric acid, HCl. a weak acid: acetic acid, CH 3 COOH. an acidic commercial cleanser. a basic commercial cleanser. Use the titration curves to calculate the percent of the active ingredients in the commercial cleansers. Determine the K a of a weak acid. Background A plot of the ph of a solution against the volume of titrant added is called a titration curve. The ph can be measured directly with a ph meter while titrant is added through a buret. For the acids used in this experiment the titrant will always be a diluted solution of the strong base NaOH. From the form of the titration curve, it can be determined whether the solution consists of a strong or weak acid. Furthermore, if it is a weak acid, the equilibrium constant for its dissociation can be calculated. Strong Acids For a strong acid, HA, the equilibrium constant K a for the process Eq. 1 HA 1 H 2 O H 3 O 1 1 A is so large that HA is completely dissociated into H 3 O 1 and A at most concentrations. Hence, as titrant is added, the H 3 O 1 concentration simply equals the initial acid concentration minus the amount of H 3 O 1 that has been neutralized by the titrant. This is true until the titration has reduced the concentration of HA to about 10 6 M. At this point the dissociation of water according to the equation Eq. 2 H 2 O 1 H 2 O H 3 O 1 1 OH (K w 5 [H 3 O 1 ][OH ] 5 10 14 ) begins to govern the H 3 O 1 concentration. At the exact equivalence point (where the moles of base added equal the initial moles of acid present), the H 3 O 1 concentration comes entirely from the dissociation of water; it is, therefore, equal to 10 7 and the ph is seven. 67
Chemistry 143 Experimental Chemistry Weak Acids For a weak acid the equilibrium constant for the reaction in Equation 1 is small (between 10 2 and 10 7 ), so the concentration of [H 3 O 1 ] and the ph are governed by K a. Eq. 3 K a 5 [H 3 O 1 ][A ] / [HA] The titration curve that you obtain should be considerably different from that obtained when the same concentration of strong acid is titrated. Obviously the [H 3 O 1 ] concentration will be lower (and the ph therefore higher) throughout the titration because not all of the acid is dissociated. You should also find that the shape of the curve is somewhat different. The reason for this will become clear if the following three points along the titration curve are considered in some detail. a. Zero titrant. Since the dominant source of [H 3 O 1 ] is the acid dissociation described by Equation 1, from the stoichiometry of the equation [H 3 O 1 ] 5 [A ]. Therefore Equation 3 reduces to Eq. 4 K a [HA] 5 [H 3 O 1 ] 2 If we further assume that [HA] is given by the nominal molarity of HA in the solution, then the [H 3 O 1 ] concentration and hence the ph can be calculated if K a is known. K a is most conveniently obtained from the following point on the titration curve. b. Halfway to the equivalence point. At this point half of the acid has been titrated; hence, [A ] 5 [HA] Therefore, Equation 3 reduces to the simple expression Eq. 5 K a 5 [H 3 O 1 ] or pk a 5 ph This is a rather remarkable point on titration curves, since the ph is determined by the value of pk a ( log K a ) and is independent of the initial acid concentration and any subsequent dilution. To determine the pk a of a weak acid, we need to locate this half equivalent point accurately, and this can be done by first finding the following point on the titration curve. c. The equivalence point. When the monoprotic acid has been completely titrated by addition of an equal number of moles of base, we have reached the equivalence point. On the titration curve this will appear as the point of maximum slope. If the data are obtained by adding titrant in equal portions, the ph at the equivalence point probably will be hard to locate on the titration curve, because there will be a sudden jump between two points. We therefore attempt to add small amounts of titrant in the vicinity of the equivalence point and thus accurately determine the point of inflection. Once this has been determined, the halfway point can be calculated and pk a can be determined as discussed in part (b). 68
Acid Base Titration Curves Chapter 8 Let us now consider the calculation of the exact ph at the equivalence point. The solution contains an amount of A equal to the original acid present, but in a volume which is larger by an amount depending on how much titrant was added. The concentration of A is then easily calculated by considering the dilution factor. To calculate the ph at this point, we consider the following equilibrium in which A is said to be hydrolyzed: Eq. 6 A 1 H 2 O HA 1 OH It is not difficult to show that the equilibrium constant, K b (sometimes called K h ), for the reaction in Equation 6 is related to K a and K w by the equation Eq. 7 K b 5 [HA][OH ] / [A ] 5 K w / K a (K w 5 10 14 ) From the stoichiometry of Equation 6, [HA] 5 [OH ] and Equation 7 can be written as Eq. 8 K b 5 [OH ] 2 / [A ] When K a is obtained from part (b), K b can be calculated with Equation 7. Then [OH ] and poh can be calculated using Equation 8. The ph at the equivalence point, then, is 14 poh. Acid Base Titration of Commercial Cleansers Potentiometric titration can be applied to the determination of acidic and basic compounds in household cleansers. The cleansers generally contain one or two acids or bases and their concentrations can be determined from the one or two equivalence points on the titration curves. The acidic household cleansers usually contain hydrochloric acid, phosphoric acid, sodium bisulfate, or hydroxyacetic acid, which remove alkaline scale deposits and stains. The basic cleansers usually contain ammonium hydroxide, sodium hydroxide, sodium hypochlorite, or sodium carbonate. The weaker bases cut grease while the stronger bases and oxidizing agents dissolve animal matter such as hair, grease, and foodstuffs. Lysol is an acidic cleanser, containing hydrochloric acid as an active ingredient. The percentage content of HCl can be calculated from the titration data. Liquid-Plumr is a two component basic cleanser containing sodium hydroxide and sodium hypochlorite (NaOCl). Sodium hypochlorite is an oxidizing agent as well as a weak base and, together with sodium hydroxide, is used as a drain cleaner. The titration of a base with a strong acid is just the reverse of the titration of an acid with a strong base. It is easier to understand what happens during the titration of a mixture of strong and weak bases if you consider the following two points on the titration curve. 69
Chemistry 143 Experimental Chemistry 70 (a) Zero titrant. The strong base will be completely dissociated, giving an equivalent OH concentration. The weak base will be only partially dissociated and will make a much smaller contribution to the total [OH ] present. The presence of OH from the strong base will, in fact, suppress the dissociation of the weak base, since the equilibrium B 1 H 2 O BH 1 1 OH will be shifted to the left. Thus, the first equivalence point gives the volume of the titrant reacting with the strong base. (b) When the strong base has been titrated. At this point we simply have a solution of the weak base that has been diluted. The difference between the first and the second equivalence points gives the volume of strong acid required to react with the weak base. Safety Precautions Safety goggles must be worn in the lab at all times. Any skin contacted by chemicals should be washed immediately. Be extra careful with 2.5 M NaOH, toilet bowl cleaner, and Liquid- Plumr. Materials Equipment MicroLAB Buret, 25 ml volumetric pipette, pipette bulb (check out from stockroom) ph electrode, plastic pop bottle, ring stand (provided in lab) Beakers, graduated cylinder, buret clamp (from your locker) Supplies Disposable plastic gloves Liquid-Plumr, Lysol (or Vanish) toilet bowl cleaner Reagents 2.5 M NaOH, 0.1 M HCl, 0.1 M NaOH ph 7 buffer solution Phenolphthalein indicator Experimental Procedures Calibrate your ph electrode with ph 4.0, 7.0, and 10.0 buffer, placing ph on the y-axis, Digital Display and Spreadsheet. Add Keyboard as a sensor, label it volume (ml) and place it on the x-axis, Digital Display, and spreadsheet. Obtain a plastic pop bottle and rinse it thoroughly with deionized water. Use a clean 50 ml graduated cylinder to measure 15 to 18 ml of 2.5 M NaOH solution. The exact amount is not critical, but it should be in this range. Pour this solution into the plastic pop bottle and use the graduated cylinder to add 400 ml of deionized water to the NaOH solution. Again, your measurements do not have to be exact, but the volume
Acid Base Titration Curves Chapter 8 of water added should be close to 400 ml. Mix the diluted NaOH solution well. It will be your titrant for all of the acidic solutions. You will standardize it (determine its concentration) when you titrate the HCl solution. Titration of the Acids Rinse your buret with deionized water; then rinse it 3 times with 5 ml portions of the NaOH from the pop bottle. Fill the buret with the NaOH solution to a little above the zero mark and clamp it to a ring stand. Put a beaker under the buret and open the stopcock until the titrant level is below the zero mark. Check to see that there are no air bubbles in the buret tip. Using a volumetric pipette, measure 25.00 ml of 0.100 HCl into a clean 250 ml beaker. Add about 100 ml of deionized water and 2 3 drops phenolphthalein. Be sure to record the concentration of HCl, since it will be used to calculate the concentration of your NaOH. Rinse the ph electrode with deionized water, put it in the beaker, and use it to gently stir the diluted HCl solution for 20 or 30 seconds. Read the level of titrant in the buret and record it in your lab notes. Then start your program, and enter the initial buret reading with your computer keyboard. Titrate the sample with the NaOH solution, beginning with increments of about 1 ml. Each time you add some NaOH, stir the solution with the ph electrode, make sure that the reading has stabilized within at least ± 0.1 ph units, take a buret reading, and enter the value. When the ph curve begins to rise more steeply, reduce the volume of your additions of titrant. As you approach the equivalence point (indicated by a more persistent pink color from the phenolphthalein indicator), reduce the increments to two or three drops between buret readings. When you get to the indicator end point (the pink color doesn t go away when you stir), you should be adding a drop at a time. Record the buret reading at the indicator end point in your notes; then continue the titration, one drop at a time, for at least an additional 5 drops. Gradually increase the volume increments (up to a maximum of 1.0 ml) until you have added approximately 10 ml of NaOH beyond the end point. For a good titration curve, you will want more than 30 data points. Stop the program and save you data to your USB flash drive. Do a second titration of another 25 ml HCl sample. This time you will have a better idea of when to decrease your volume increments and when to expect the indicator end point. Then use the same procedure to titrate a 25 ml sample of 0.1 M acetic acid solution. Record the buret readings at the beginning and at the indicator end point for each titration in your lab notes, and save your titration curve data to disk files. Titration of Toilet Bowl Cleaner Weigh a clean 150 ml beaker and bring it back to your work area. Add about 0.5 g of Lysol to the beaker. Caution: handle the toilet bowl cleaner with care; it is strongly acidic. Reweigh the beaker, add about 60 ml of deionized water, and titrate with NaOH, as you did with the other acidic samples. 71
Chemistry 143 Experimental Chemistry Titration of Liquid-Plumr Rinse the 150 ml beaker thoroughly with deionized water and weigh it. Return to your work area and put about 2 g of Liquid-Plumr into the beaker. Caution: handle Liquid-Plumr with care; it is strongly basic. Reweigh the beaker and add 60 ml of deionized water. Rinse your buret with deionized water; then rinse and fill the buret with 0.1 M HCl solution. Titrate the diluted Liquid-Plumr with HCl. This time the ph will drop as you add titrant. Use smaller volume increments when the titration curve gets steep, as you did with the acids. Continue the titration to a ph near 2. Your data should show two inflections. Interpreting the ph Titration Curve A ph titration curve can be analyzed at the equivalence point of the ph versus volume curve. It is the steepest region of the titration curve. To better find this region, you can perform a derivative on the ph vs. volume (ml) data, revealing the volume at which this maximum slope or where the equivalence point occurs. Your lab instructor will assist you in performing this function on your data when you get to this point in the lab period. Data Analysis Using your titration curves and derivative plots, determine the equivalence points, ph, and volume of titrant added for all the titrations. For the acid samples, compare the equivalence point volumes you obtained from the graphs to the indicator end points you recorded in your notes. Which values are more precise, the values obtained from the indicator or the values obtained from the graphs? If the indicator changed color at a different ph than the ph of the equivalence point, which method would give the more accurate volume measurement for equivalence, the indicator or the graphs. Did performing a derivative on the data assist you in solving for the equivalence points? Were you careful in collecting enough points throughout the titration? Calculate the exact molarity of the NaOH solution you prepared in the pop bottle, using the HCl titration data. Don t forget to subtract the initial buret reading to determine the volume of titrant used. To calculate the NaOH concentration, you can use the average from your two HCl titrations, if they agree, or you can use the titration that gave the more distinct equivalence point. You can also choose to use the indicator end point(s) or the equivalence point(s) from one or both graphs. Justify your choices in your lab report. Determine the half equivalence point for the titration of acetic acid, and use it to calculate its K a value. Compare your experimental K a to the accepted value for acetic acid, 1.8 3 10 5. For the toilet bowl cleaner, determine the equivalence point and calculate the percentage content of HCl. Use the exact concentration of the NaOH solution calculated above. For the Liquid-Plumr, determine the ph and the volume of the HCl added at both the first and second equivalence points. Calculate the percentage content of both the NaOH and NaOCl in Liquid-Plumr. 72
Acid Base Titration Curves Chapter 8 Lab Report Your laboratory report should include the graphs of your titration data, your discussion of the questions posed above, and your data and calculations on the summary sheets that follow this experiment. Pre-Lab Questions Answer these questions before you come to lab. 1. a. Calculate the ph of a solution in which the concentration of the hydronium ion is 2.0 3 10 4 M. b. If the ph at the half equivalence point for a titration of a weak acid with a strong base is 4.6, what is the value of K a for the weak acid. 2. When a 0.5725 g sample of Lysol toilet bowl cleaner was titrated with 0.100 M NaOH, an end point was obtained at 15.00 ml. Calculate the percent (by weight) of hydrochloric acid in the Lysol sample. 3. When a 3.529 g sample of Liquid-Plumr was titrated with 0.100 M HCl, two end points were obtained. The first end point occurred at 15.00 ml, the second at 42.00 ml. Calculate the percentages of both NaOH and NaOCl in Liquid-Plumr. 73
Chemistry 143 Experimental Chemistry 74
Acid Base Titration Curves Chapter 8 CHAPTER 8 Name ID No. Worksheet Instructor Partner s Name (if applicable) Course/Section Date (of Lab Meeting) Data Summary Titration of HCl First Titration Second Titration ph at equivalence point Buret reading at equivalence point from graph Initial buret reading Volume of NaOH added at equivalence point Buret reading at indicator end point Volume of NaOH added at indicator end point Concentration of HCl sample Calculate moles of HCl in each sample. Calculate the molarity of the NaOH titrant solution that you prepared. Use either an average or best titration volume for this calculation (see Data Analysis ). Explain your choice in your lab report. 75
Chemistry 143 Experimental Chemistry Titration of Acetic Acid Buret reading at equivalence point from graph Initial buret reading Volume of NaOH added at equivalence point Volume of NaOH added at half equivalence ph at half equivalence point Calculate your experimental value of K a for acetic acid. Titration of Toilet Bowl Cleaner Mass of beaker with cleaner Mass of empty beaker Mass of cleaner ph at equivalence point Buret reading at equivalence point Initial buret reading Volume of NaOH added Molar mass of HCl Calculate moles of HCl in the sample of toilet bowl cleaner and percentage of HCl in the cleaner. 76
Acid Base Titration Curves Chapter 8 Titration of Liquid-Plumr Mass of beaker with Liquid-Plumr Mass of empty beaker Mass of Liquid-Plumr ph at first equivalence point ph at second equivalence point Molar mass of NaOCl Buret reading at second equivalence point Buret reading at first equivalence point Volume of HCl to titrate NaOCl Initial buret reading Volume of HCl to titrate NaOH Molar mass of NaOH Calculate moles of NaOCl in the Liquid-Plumr sample and percentage of NaOCl in Liquid-Plumr. Calculate moles of NaOH in the Liquid-Plumr sample and percentage of NaOH in Liquid-Plumr. 77