Redox Titrations Lecture 2 Nov 15, 2002

1
Redox TitrationsLecture 2Nov 15, 2002
2

• Homework Chapter 15
• Problems 11, 12, 13, 16, 17
• Due Wed, Nov 20

3
Titrations Involving Iodine

• One of the most common redox titrations involve
  either using iodine (I2) as a mild oxidizing
  agent or iodide (I-) as a mild reducing agent.
• Iodine as oxidizing agent
• I2 2 e - 2 I -
• When iodine is used as the titrant the method is
  known as iodimetry.

4
Titrations Involving Iodine

• I2 is not very soluble in water (only about
• 1.3 x 10 -3 mol/L). It solubility is increased
  in the presence of excess iodide by the formation
• of the triiodide (I3-) species,
• I2 I - I3
• So it is really the triiodide species, though it
  will commonly be referred to as iodine that is
  involved in the chemical reactions.

5
Titrations Involving Iodine

• Iodide as reducing agent
• 2 I - I2 2 e -
• When iodine is produced by the addition of an
  oxidizing analyte to an excess of iodide, the
  method is as iodometry.

6
Titrations Involving Iodine

• Thiosulfate (S2O3-2) is commonly used in
  titration reactions involving iodine, both for
  iodimetric and iodometric methods. As shown in
  Tables 15-2 the iodimetric methods generally
  involve an excess of standard I2 (as I3 -)
  followed by back titration with standard
  thiosulfate.

7
Titrations Involving Iodine

• In the iodometric methods, an excess of iodide is
  added to the sample of an oxidizing analyte and a
  stoichiometric amount of iodine (I2 or I3- ) is
  produced. This iodine is titrated with a
  standard solution of thiosulfate. This reaction
  is shown on the following slide.

8
Titrations Involving Iodine
9
Titrations Involving Iodine

• The reaction with iodine needs to occur in a
  solution whose pH lt 9 to prevent side reactions
  which produce iodates (IO3-). Generally acetic
  acid is added to the analyte mixture before
  titration to assure the proper pH. In some
  cases, appropriate pH buffers may also be added.

10
Titrations Involving Iodine

• The titrant solution of thiosulfate cannot be
  prepared directly. It is made to an approximate
  concentration and then standardized with a
  primary standard oxidizing agent by iodometry.
  The thiosulfate solution is unstable if the pH lt
  5, undergoing the following disproportionation
  reaction.
• S2O3-2 2 H lt gt H2SO3 S

11
Titrations Involving Iodine

• This disproportionation reaction is prevented by
  using freshly boiled deionized water as the
  solvent and adding a small amount of NaOH.
  Although the thiosulfate needs to be stored in a
  basic solution, as mentioned earlier, its
  reaction as a reductant titrant needs to occur in
  an acid solution.

12
TABLE 15-2 Iodimetric titrations Titrations
with standard iodine (actually I3?)

• Species analyzed Oxidation reaction
• SO2 SO2 H2O lt gt H2SO3
• H2SO3 H2O lt gt SO24- 4H 2e-
• H2S H2S lt gt S(s) 2H 2e-
• Zn2, Cd2, M2 H2S ? MS(s) 2H
• Hg2, Pb2 MS(s) lt gt M2 S 2e-

13
TABLE 15-2 Iodimetric titrations Titrations
with standard iodine (actually I3?) continued

• Species analyzed Oxidation reaction
• Cysteine, glutathione, 2RSH lt gt RSSR
  2H 2e-
• mercaptoethanol
• Aldehydes H2CO 3OH- lt gt HCO2-
  2H2O 2e-
• Glucose (and other reducing sugar)
• O
• RCH 3OH- lt gt HCO2- 2H2O 2e-
• Ascorbic acid See next slide
• (or vitamin C)

14
Ascorbic acid (or vitamin C)
15
TABLE 15-3 Iodometric titrations Titrations of
iodine (actually I3?) produced by the analyte

• Species analyzed Reaction
• HOCl HOCl H 3I- lt gt Cl - I3-
  H2O
• Br2 Br2 3 I- lt gt 2 Br - I3-
• IO3- 2 IO3- 16 I - 12 H lt gt 6 I3-
  6 H2O
• IO4- 2 IO4- 22 I - 16 H lt gt 8
  I3- 8 H2O
• O2 O2 4 Mn(OH)2 2 H2O lt gt 4
  Mn(OH)3
• 2 Mn(OH)3 6 H 6 I - lt gt 2 Mn2
  2 I3- 6 H2O
• H2O2 H2O2 3 I - 2H lt gt I3- 2 H2O

16
TABLE 15-3 Iodometric titrations Titrations of
iodine (actually I3?) produced by the analyte
continued

• Species analyzed Reaction
• O3 O3 3 I - 2 H lt gt O2 I3-
  H2O
• NO2- 2 HNO2 2 H 3 I - lt gt 2 NO
  I3- 2 H2O
• S2O82- S2O82- 3 I - lt gt 2 SO42-
  I3-
• Cu2 2 Cu2 5 I - lt gt 2 CuI(s)
  I3-
• MnO4- 2 MnO4- 16 H 15 I - lt gt 2
  Mn2 5 I3- 8 H2O
• MnO2 MnO2(s) 4 H 3 I - lt gt Mn2
  I3- 2 H2O