Page 42 - Pharmaceutical Analytical Chemistry II - Pharm D Clinical- 07-PA202
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Karl Fischer reagent for determination of moisture content
Reagent
Iodine, sulfur dioxde dissolved in pyridine (I2, SO2 exist as complex to
prevent their volatilization) and xss methanol
C5H5N C5H5N.I2 C5H5N.SO2 MeOH
Application
Determination of water in organic liquids and solids which are soluble
in methanol and can be analyzed by direct titn (acids, alcohols, esters,
anhydrides…)
Reaction
Redox reaction
I2 + SO2 + (H2O) 2HI + SO3
C5H5N.I2 + C5H5N.SO2 + C5H5N + H2O 2C5H5N.HI + C5H5N.SO3
Role of MeOH
Pyridine / SO3 complex is capable of consuming water;
C5H5N.SO3 + H2O C5H5NHSO4H
Pyridinium hydrogen sulfate
This reaction is undesirable and can be prevented by xss MeOH
(more reactive than water)
C5H5N.SO3 + CH3OH C5H5N(H)SO4CH3
Pyridinium methyl sulfate
Method Direct titration
Ep detection
Color change from yellow (reaction products) to brown (xss reagent,
pyridine iodine cpx) OR potentiometric
Stoichiometry
1 mole of I2, 1 mole of SO2, for each mole of H2O.
I2 ≡ H2O
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