Page 35 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

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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