Chemistry Potentiometric Titration of a Chloride-Iodide Mixture

Transcription

1 Chemistry 3200 Silver iodide, AgI, is much less soluble than AgCl. The solubility products of the two salts are 9.8 x and 1.78 x 10 10, respectively. Therefore, if a mixture of I and Cl is titrated with Ag +, AgI will precipitate almost completely before AgCl begins to precipitate. If the concentration of Ag + in such a titration is measured, it will be observed to rise abruptly at the point corresponding to complete precipitation of I and again at the point corresponding to complete precipitation of Cl. The volumes of standard Ag + added at these two points ( breaks in the titration curve ) may be used to compute the moles of I and Cl present in solution. The concentration of Ag + in solution may be determined by measuring the potential difference between a silver wire electrode and a reference electrode both in contact with the solution. The equation relating the concentration of silver, [Ag + ], and the potential difference E is E = E log[ag + ] where E 0 is a constant, depending upon the electrode system used. Since [Ag + ] changes by several orders of magnitude in going from a solution of excess halide to one of excess silver, E rather than [Ag + ] is plotted versus volume of standard silver solution in making the titration curve. Procedure (Students will work in groups for this experiment) IMPORTANT NOTE: Use distilled or ultra-purified water to prepare solutions 1. ~0.7 M AgNO 3 standard solution in an amber vial is provided. Record the exact standard concentration in your lab notebook. Use a clean, dry 10 ml volumetric pipet to deliver ml of the standard to a 250 ml volumetric flask and dilute to the mark with water. This is the titrant. 2. Using a volumetric pipet, pipet ml of the unknown solution into a 250 ml volumetric flask and dilute to the mark. The unknown solution contains a mixture of Cl and I. 3. Use a clean, dry 25 ml pipet to deliver ml of the diluted unknown to a 400 ml beaker. Add an additional ~65 ml of water and ~10 ml of ph 2 bisulfate buffer solution to the beaker. Immerse the electrodes in the solution and start the magnetic stirrer. If the silver bullet electrodes are tarnished, wash them with a mild abrasive without bleach, like Soft Scrub. Also, be sure to remove the plastic protective cap

2 from the Ag/AgCl glass electrode! The two electrodes should be kept close to the same position throughout a titration. 4. Use a burette to add the titrant (dilute AgNO 3 ) in 1 ml increments to the unknown solution. Wait for the potential reading to stabilize, then record the potential (E) from the meter. Plot E versus volume of titrant onto an EXCEL worksheet. This is a rough titration to show you the locations of the two equivalence points and may be done rapidly. The E versus volume plot will be the inverse of that in the text where the glass electrode was used as the reference. 5. Perform three more titrations in which readings are taken every 0.2 ml in the immediate vicinities of the equivalence points and at 2-5 ml intervals elsewhere. No more than about 20 readings should be required. 6. Determine the end point volumes from your graph at the inflection points of the volume versus potential plots. The inflection points may be located precisely by selecting the point where the first derivative of the curve is maximized. Using EXCEL plot ΔE/ΔV. If the measurement and analysis are done properly, the endpoints will be at the peaks in the first derivative curve. 7. Report total moles I and total moles Cl in your sample as determined from the titrations using stoichiometry, total moles KI and total moles KCl from the potentiometric titration. The total moles of KI are obtained from the volume of AgNO 3 required to reach the first equivalence point. The additional volume of AgNO 3 required to reach the second equivalence point gives the moles of KCl present. Because all the KI initially present was converted to AgI by the nitrate added prior to the first equivalence point, the moles of nitrate added between the first and the second equivalence points gives the moles of KCl in the sample. Don t forget to account for sample dilution.

3 Chemistry 3200 Date: Lab Instructor: Section: Unknown Number: Molarity of standard AgNO3: Molarity of AgNO3 used for titrations: ml to 1 st endpoint: ml to 2 nd endpoint: Average: Average: Moles I in 250 ml sample: Moles Cl in 250 ml sample: Molarity of I in original sample: Molarity of Cl in original sample: Attach a copy of each titration and derivative plot to this report.

4 Calculations for [I ] in 250 ml sample and in original unknown: Calculations for [Cl ] in 250 ml sample and in original unknown:

5 Calculation for error analysis (Include a list of the errors and their sources):