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  1. Class, Monday, Oct 25, 2004 ______________________________________ • Announcements • Calcium Determination Lab is due this Friday at class time. • Discussion of time for Exam 2 – schedule shows Wed, Nov 5 (typo, should be Nov 3)

  2. Chapter 16 – Redox Titrations ______________________________________ Review Redox is a combined word from Reduction and Oxidation. OIL RIG Reduction is the gain of electrons as in the example Cu+2 + 2 e = Cu Oxidation is the loss of electrons as in Zn = Zn+2 + 2 e

  3. Chapter 16 – Redox Titrations ______________________________________ Neither of the two processes occur without the other one. That is, both oxidation and reduction occur simultaneously so that the electrons gained by one species is lost from a second one. The representation in the previous slide showing electrons as products in the oxidation reaction and as reactants in the reduction reaction are known as half-cell reactions.

  4. Chapter 16 – Redox Titrations ______________________________________ The net balanced redox equation is the sum of the two half-cell reactions. It may be necessary to multiply one or both half-cells by some coefficient so that the same number of electrons are lost by the substance that is oxidized as are gained by the substance reduced.

  5. Chapter 16 – Redox Titrations ______________________________________ Redox reactions may serve as the basis for analytical determinations. If the net cell potential (Enet) for the reaction is positive, that reaction is spontaneous. The larger the value of E, the greater is the equilibrium constant for that reaction. [ ln K = nFE/RT, K = e nFE/RT ]

  6. Chapter 16 – Redox Titrations ______________________________________ Many of the end point indicators used in redox titrations are substances that are either oxidized or reduced at given potentials as shown in Table 16-1, page 343. Much like acid-base indicators, the electrical potential at the end point oxidizes or reduces the indicator to one of the colored forms to signal the end point.

  7. Chapter 16 – Redox Titrations ______________________________________

  8. Chapter 16 – Redox Titrations ______________________________________ The titration curve also has a shape similar to the acid-base titration. Note that the y-axis is electrical potential

  9. Chapter 16 – Redox Titrations ______________________________________ Equipment for obtaining a titration curve for a redox titration.

  10. Chapter 16 – Iodine Methods ______________________________________ Iodine either as I2, oxidizing agent or I, reducing agent serve as the basis for a large number of redox titrations. The half-cell reactions are: Oxidizing Agent (I2 is reduced) I2 (aq) + 2e  2I Reducing Agent (I is oxidized) 2I  I2 (aq) + 2e

  11. Chapter 16 – Iodine Methods ______________________________________ • If I2 is the titrant, the method is called iodimetry or iodimetric. (sometimes called direct) • Alternately, if the redox reaction has produced I2 the method is called iodometry or iodometric. (sometimes called indirect, or displacement reactions) • Due to the difficulty is maintaining the concentration of I2, (limited solubility in water, appreciable vapor pressure of the I2.) iodometric methods are more commonly used. The amount of I2 produced by the action of the oxidizing analyte on excess iodide is usually titrated with standardized sodium thiosulfate, Na2S2O3.

  12. Chapter 16 – Iodine Methods ______________________________________ • Table 16-2 shows some titrations that are done using a standardized I2 solution as the titrant; many of these are done as a back titration where excess (unreacted I2) is determined.

  13. Chapter 16 – Iodine Methods ______________________________________ • Table 16-3 shows titrations where the oxidizing analyte reacts with excess iodide (I) to produce I2 (actually I3). The amount of I3is determined by titration with a standard solution of thiosulfate.

  14. Chapter 16 – Iodine Methods ______________________________________ • The indicator for both iodimetric and iodometric titrations is a starch solution; in the presence of iodine, it shows a blue or purple color. The starch indicator should not be added when I2 is in large excess because the desorption of I2 from the starch molecule is not reversible.

  15. Chapter 16 – Starch Indicator ______________________________________ The repeating amylose unit in the starch molecule. Starch is a polymer of amylose.

  16. Chapter 16 – Starch Indicator ______________________________________ The starch-iodine complex where the sugar chain forms a helix about I6units.

  17. Thiosulfate, Reducing Titrant ______________________________________ Thiosulfate is a quantitative reducing agent at pH < 9; however, it is not stable is acidic solutions, so the acid is added to the titration mixture, not the titrant.

  18. Thiosulfate, Reducing Titrant ______________________________________ In an acidic solution, thiosulfate undergoes an auto-oxidation reduction (disproportionation) reaction: S2O32 (aq)+ H+ (aq)  HSO3 (aq)+ S (s) Even dissolved CO2 (the anhydride of H2CO3) causes the solution to be sufficiently acidic to allow the above reaction.