Experiment 7 Potentiometric Titration Valdosta State University Purpose To accurately determine the molar mass and pKa of an unknown weak acid. Valdosta State University Background • In this experiment, the pH of a solution will be measured directly as the sodium hydroxide is added. • The pH will be measured using a glass combination electrode. • This device measures pH (or [H3O+]) as voltage. Valdosta State University Background pH Combination Electrode • combination refers to the ion sensing cell and the reference cell. Valdosta State University Background pH Combination Electrode • combination refers to the ion sensing cell and the reference cell. Valdosta State University Background Consider the following graph: 13 12 11 10 pH 9 8 7 6 5 4 3 0 5 10 15 20 Volum e NaOH (m L) Valdosta State University 25 30 35 Background Consider the following graph: 13 12 11 10 pH 9 8 7 6 5 4 3 0 5 10 15 20 25 30 35 Volum e NaOH (m L) In this region H+ dominates, the small change in pH is the result of relatively small changes in H+ concentration. Valdosta State University Background Consider the following graph: 13 12 11 10 pH 9 8 7 6 5 4 3 0 5 10 15 20 25 30 35 Volum e NaOH (m L) In this region, relatively small changes in H+ concentration cause large changes in pH, The midpoint of the vertical region is the equivalence point. Valdosta State University Background Consider the following graph: 13 12 11 10 pH 9 8 7 6 5 4 3 0 5 10 15 20 25 30 35 Volum e NaOH (m L) In this region OH- dominates, the small change in pH is the result of relatively small changes in OH- concentration. Valdosta State University Background HA(aq) + NaOH(aq) NaA(aq) + H2O(l) • This experiment will use monoprotic acids. • Therefore, at the equivalence point, moles HA = moles NaOH • Using the definition of moles, the molar mass can be determined: Valdosta State University Background HA(aq) + NaOH(aq) NaA(aq) + H2O(l) • This experiment will use monoprotic acids. • Therefore, at the equivalence point, moles HA = moles NaOH • Using the definition of moles, the molar mass can be determined: sample weight moles molar mass Valdosta State University Background HA(aq) + NaOH(aq) NaA(aq) + H2O(l) • pKa can be determined by using the Henderson-Hasselbach equation. Valdosta State University Background HA(aq) + NaOH(aq) NaA(aq) + H2O(l) • pKa can be determined by using the Henderson-Hasselbach equation. conjugatebase pH pK a log weak acid Valdosta State University Background HA(aq) + NaOH(aq) NaA(aq) + H2O(l) • pKa can be determined by using the Henderson-Hasselbach equation. • If the pH is measured at the point where [conjugate base] = [weak acid] conjugatebase pH pK a log weak acid Valdosta State University Background HA(aq) + NaOH(aq) NaA(aq) + H2O(l) • pKa can be determined by using the Henderson-Hasselbach equation. conjugatebase pH pK a log weak acid • If the pH is measured at the point where [conjugate base] = [weak acid] pH pK a log 1 pH pK a Valdosta State University Background HA(aq) + NaOH(aq) NaA(aq) + H2O(l) • This assumption is valid at the half-titration point. • If the equivalence point is 20.00mL, the half-titration point is at 10.00mL. • The pH is then obtained directly from the graph. Valdosta State University Background 13 12 11 10 pH 9 8 7 6 5 4 3 0 5 10 15 20 Volum e NaOH (m L) Valdosta State University 25 30 35 Background 13 12 11 10 pH 9 8 7 6 5 4 3 0 5 10 15 20 Volum e NaOH (m L) Valdosta State University 25 30 35 Background 13 12 11 10 pH 9 8 7 6 5 4 3 0 5 10 15 20 Volum e NaOH (m L) Valdosta State University 25 30 35 Procedure – Experiment 7 - For this experiment, work in pairs. Technique - Titration Clean The Buret 1. Rinse a buret twice with distilled water. 2. Rinse the buret twice with 10-mL portions of your standardized NaOH solution. 3. Run some of the NaOH solution through the tip of the buret. 4. Drain the buret, then fill it with the standardized NaOH solution. 5. Let some NaOH solution run through the tip of the buret so that all air bubbles are removed. Technique - Titration Clean The Buret Technique - Titration Prepare the Unknown 1. Weigh 0.300 – 0.400g of solid unknown or 0.120 – 0.180g of liquid unknown to the nearest 0.001 g into a 250mL beaker. 2. Add approximately 50 mL of distilled water to the beaker and stir until all the unknown is dissolved. Technique - Titration Set-up the pH electrode – GLX Computer Technique - Titration Set-up the pH meter - Calibration • Get a 400 mL beaker of distilled water to rinse the electrode between readings. Rinse the electrode with distilled water. Technique - Titration Performing The Experiment 1. Put the electrode tip in the unknown acid solution. “Start” button on 2. Fill the buret to 0.00mL and record this initial pH and buret reading. 3. Slowly add NaOH with stirring until either the pH increases by 0.20 units or 2.0 mL of NaOH have been added, whichever occurs first. Stop and record both the pH and the buret reading. 4. Repeat step 3 until the pH reaches 12, or until the titration has gone 15 mL past the equivalence point. Data Mass of unknown: 0.449g mL NaOH pH mL NaOH pH 0.00 4.02 18.80 10.08 2.30 4.36 19.00 10.60 3.00 4.47 19.40 10.98 4.80 4.67 19.92 11.24 6.10 4.81 21.25 11.46 8.48 5.03 23.95 11.69 10.61 5.21 25.10 11.84 12.80 5.42 27.28 11.94 15.08 5.72 28.12 11.94 16.90 6.09 29.80 12.00 17.82 6.47 Data Experiment 7, trial 1 13.00 12.00 11.00 10.00 9.00 pH 8.00 7.00 6.00 5.00 4.00 3.00 2.00 0.00 5.00 10.00 15.00 20.00 Volum e NaOH (m L) 25.00 30.00 35.00 Data Experiment 7, trial 1 13.00 12.00 11.00 10.00 9.00 pH 8.00 7.00 6.00 5.00 4.00 3.00 2.00 0.00 5.00 10.00 15.00 20.00 Volum e NaOH (m L) 25.00 30.00 35.00 Calculation – Molar Mass 18.50 mL NaOH 1 L NaOH 0.116 mol NaOH 0.00215 mol NaOH 1000 mL NaOH 1 L NaOH Calculation – Molar Mass 18.50 mL NaOH 1 L NaOH 0.116 mol NaOH 0.00215 mol NaOH 1000 mL NaOH 1 L NaOH 1 mol unknown 0.00215 mol unknown 0.00215 mol NaOH 1 mol NaOH Calculation – Molar Mass 18.50 mL NaOH 1 L NaOH 0.116 mol NaOH 0.00215 mol NaOH 1000 mL NaOH 1 L NaOH 1 mol unknown 0.00215 mol unknown 0.00215 mol NaOH 1 mol NaOH 0.449 g unknown 209 g / mol unknown 0.00215 mol unknown Calculation - pKa Experiment 7, trial 1 13.00 12.00 11.00 10.00 9.00 pH 8.00 7.00 6.00 5.00 4.00 3.00 2.00 0.00 5.00 10.00 15.00 20.00 Volum e NaOH (m L) 25.00 30.00 35.00 Calculation - pKa Experiment 7, trial 1 13.00 12.00 11.00 10.00 9.00 pH 8.00 7.00 6.00 5.00 4.00 3.00 2.00 0.00 5.00 10.00 15.00 20.00 Volum e NaOH (m L) 25.00 30.00 35.00 IMPORTANT REMINDER Save the unused solution in a location identified by your laboratory instructor. Valdosta State University Safety The NaOH solutions are corrosive. If you get some on your skin, wash with water for at least five minutes. If any NaOH solution gets in the eyes IMMEDIATELY wash with water for at least fifteen minutes. Call for medical assistance! Valdosta State University Waste Disposal All wastes from this experiment can be flushed down the drain with plenty of running water. Valdosta State University