Page 36 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

Ep detection
Diphenylamine sulfonic acid sodium salt
Ep is marked by change in color from colorless to violet
OR
ferroin ind
Ep is marked by change in color from red to pale blue

Applications

Direct Titrations

Determination of reducing species

Determination of Fe2+

6Fe2+ + Cr2O72− + 14H+ 6Fe3+ + 2Cr3+ + 7H2O

- A mixture of sulfuric / phosphoric acid is added in this titration.
H3PO4
It complexs Fe3+ to [Fe(HPO4) ]+, accordingly it will:
(a) Mask its yellow color→ sharp end point

(b)  the E of Fe3+/Fe2+ system →  reducing power of Fe2+

⎯→ feasibility of titration of Fe2+ against Cr2O72-

H2SO4: for acidification

- Ind: diphenylamine sulfonic acid
Color is changed from colorless to violet

Back Titrations

Determination of Glycerol
- Add xss st Cr2O72− (heat/ c. sulfuric acid)
- back tit un-reacted Cr2O72− iodometrically

3 C3H8O3 + 7Cr2O72− + 56 H+ c. H2SO4

14 Cr3+ + 9 CO2 + 40 H2O

3C = -2 9 C= +4 x 9 = +36

9C = -2 x 3 = -6

-6 to +36---------loss of 42 e

Eq wt glycerol = 3 Mw / 42 = Mw / 14

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