Page 34 - Pharmaceutical Analytical Chemistry II - Pharm D Clinical- 07-PA202
P. 34

Back Titrations

      Determination of Glycerol

      C3H8O3 + 8Ce4+ + 3H2O       3HCOOH + 8Ce3+ + 8H+
                    xss St ceric                3C = +2 x 3 = +6

      3C = -2

      Eq wt glycerol = Mw / 8

      - Xss unused st Ce4+ ≠ st Na2C2O4, Ferroin ind
                      50 oC
      2Ce4+ + C2O42−
                                  2Ce3+ + 2CO2

      Color change: Ferriin (blue) to Ferroin (red)

III-                  Potassium dichromate, K2Cr2O7

      Properties
      - Slightly weaker oxidizing agent than MnO4−

      - Its reactions are slow (Back titn)
      - It does not oxidize Cl−

      - Primary standard, its solution needs not to be standardized (pure

       grade)

      - Titrations are carried out in acidic solution (WHY)
       In basic solution, Cr2O72− is converted to yellow chromate ion
       (CrO42−), whose oxidizing power is nil

      Reduction half-reactions

      Cr2O72− + 14H+ + 6e         2Cr3+ + 7H2O E0 = +1.33 V
      Orange                                 green
      2 Cr = +6 x 2 = +12                    2 x +3 = +6

      Change in oxidation state of Cr: +6 to +3
      For 1 mole of Cr2O72− , the electrons transfer = 6

      Eq wt = Mw / 6

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