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Experiment 7 Potentiometric Titration

Experiment 7 Potentiometric Titration. Valdosta State University. Purpose. To accurately determine the molar mass and pK a 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.

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Experiment 7 Potentiometric Titration

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  1. Experiment 7Potentiometric Titration Valdosta State University

  2. Purpose To accurately determine the molar mass and pKa of an unknown weak acid. Valdosta State University

  3. 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

  4. Background • pH Combination Electrode • combination refers to the ion sensing cell and the reference cell. Valdosta State University

  5. Background • pH Combination Electrode • combination refers to the ion sensing cell and the reference cell. Valdosta State University

  6. Background Consider the following graph: Valdosta State University

  7. Background Consider the following graph: In this region H+ dominates, the small change in pH is the result of relatively small changes in H+ concentration. Valdosta State University

  8. Background Consider the following graph: 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

  9. Background Consider the following graph: In this region OH- dominates, the small change in pH is the result of relatively small changes in OH- concentration. Valdosta State University

  10. 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

  11. 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

  12. Background HA(aq) + NaOH(aq)  NaA(aq) + H2O(l) • pKa can be determined by using the Henderson-Hasselbach equation. Valdosta State University

  13. Background HA(aq) + NaOH(aq)  NaA(aq) + H2O(l) • pKa can be determined by using the Henderson-Hasselbach equation. Valdosta State University

  14. 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] Valdosta State University

  15. 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] Valdosta State University

  16. 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

  17. Background Valdosta State University

  18. Background Valdosta State University

  19. Background 5.2 9.1mL 18.2mL Valdosta State University

  20. Procedure – Experiment 7 - For this experiment, work in pairs.

  21. Technique - Titration • Clean The Buret • Rinse a buret twice with distilled water. • Rinse the buret twice with 10-mL portions of your standardized NaOH solution. • Run some of the NaOH solution through the tip of the buret. • Drain the buret, then fill it with the standardized NaOH solution. • Let some NaOH solution run through the tip of the buret so that all air bubbles are removed.

  22. Technique - Titration Clean The Buret

  23. Technique - Titration • Prepare the Unknown • 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. • Add approximately 50 mL of distilled water to the beaker and stir until all the unknown is dissolved.

  24. Technique - Titration Set-up the pH electrode – GLX Computer

  25. 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.

  26. Technique - Titration • Performing The Experiment • Put the electrode tip in the unknown acid solution. “Start” button on • Fill the buret to 0.00mL and record this initial pH and buret reading. • 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. • Repeat step 3 until the pH reaches 12, or until the titration has gone 15 mL past the equivalence point.

  27. Data Mass of unknown: 0.449g

  28. Data

  29. Data 18.5 mL

  30. Calculation – Molar Mass

  31. Calculation – Molar Mass

  32. Calculation – Molar Mass

  33. Calculation - pKa 9.25 mL 18.5 mL

  34. Calculation - pKa 5.08 9.25 mL 18.5 mL

  35. IMPORTANT REMINDER Save the unused solution in a location identified by your laboratory instructor. Valdosta State University

  36. 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

  37. Waste Disposal All wastes from this experiment can be flushed down the drain with plenty of running water. Valdosta State University

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