Page 39 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

5- Applications

Direct Titrations
Determination of Na2S2O3

2S2O32− + I2 S4O62− + 2I−
tetrathionate

Determination of As3+ H3AsO4 + 2I− + 2H+
H3AsO3 + I2 + H2O

- Starch ind, added at the beginning of titration
- The solution is neutral, use NaHCO3

Back Titrations Technique
Determination of Aldehydes and aldoses

- Formaldehyde,
- Glucose (aldehydic reducing sugar),
- Sucrose (Non reducing sugar, after acid hydrolysis → equivalent
amount of glucose)

St. I2 + 2OH− IO− (stronger oxidizing agent) + I− + H2O

RCHO + IO− + OH− RCOO− + I− + H2O
aldose

IO− (Xss unreacted) + I− + 2H+ (acidify) I2 + H2O

I2 + 2S2O32− 2I− + S4O62−
Starch is used as ind

6- Sources of error
1- Due to I2

Lack of stability (volatility, air oxidation of iodide)

2- Due to starch
Aqueous starch suspension decomposes within a few days,
because of bacterial action.
Decomposition products, e.g. glucose may be oxidized with iodine,
other products give irreversible reddish color with I2.
To Avoid, add preservative like boric acid or formamide.

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