Page 48 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

2. Air oxidation
Although thiosulfate is resistant to air oxidation, metal ions and
acidity catalyze it.

(S2O32− → S4O62−)

3. Bacterial growth

Bacteria metabolize S2O32− into SO32−, SO42−, and S
So, preservatives like sod. Benzoate or CHCl3 (few drops per liter)
can be used

Standardization

Use primary st KIO3 solution, iodometric (K2Cr2O7 can also be
used)

IO3− + 5I− + 6H+ 3I2 + 3H2O

Liberated I2 is titrated ≠ S2O32− solution, to be standardized

I2 + 2S2O32− 2I− + S4O62−

IO3− ≡ 3I2 ≡ 6S2O32−

Oxidation Half reaction

2S2O32− S4O62− + 2e (Oxidation with I2)

Eq wt = 2 Mw / 2
Eq wt = Mw / 1

Application

used to determine oxidizing agents by indirect procedure,
iodometric

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