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Test method for analysis of water content in Methanol by Karl Fischer
Scope
This method covers the determination of the moisture content, at trace levels, in organic solvents, formic acetic and propionic acids i.e. organic materials, and liquid under ambient condition. The range may be varied by suitable adjustment of sample size.
Summary of the Method
The method is based up on that originally proposed by Karl Fischer. Dissolved water is titrated with a solution of iodine and sulphur dioxide in pyridine. The Karl Fischer Reagent in the presence of methanol, according to the reaction:
3C5H5N + S02 + I2 + H20 ------------- 2C5H5NHI + C5H5NS03
C5H5NS03 + CH30H --------------------- C5H5NH(S030CH3)
The end point is determined potentiometrically.
Precautions
a) The sampler must be aware of the known hazards associated with the organic material being tested.
b) Karl Fischer reagent contains 4 toxic compounds, namely iodine, sulphur dioxide, pyridine and methanol. The reagent should be handled with care to avoid unnecessary inhalation or direct contact with the skin.
c) It should be emphasised that the equivalent of the Karl Fischer reagent will decrease with time, and the reagent must, therefore, be re-calibrated frequently.
Apparatus
 | Balance, heavy duty
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| Deionizer
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| Hydrometer
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| Karl Fischer Apparatus |
Reagents
Karl Fischer reagent (single solution pyridine free)
Methanol, Anhydrous
3C5H5N + S02 + I2 + H20 -------- 2C5H5NHI + C5H5NS03
C5H5NS03 + CH30H --------------- C5H5NH (S030CH3)
The end point is determined potentiometrically.
Preparation of Apparatus
Add to the cell, 10 ml of dry methanol. Add Karl Fischer reagent, slowly, to the cell unit there is just an excess of reagent. Dry the cell by inverting making sure that the surface of cell has been completely rinsed with the liquid. Add more Karl Fischer reagent if necessary, to the instrument end-point.
Standardisation of Karl Fischer Reagents
Add 50 microlitre (50 mg) of water and titrate with the Karl Fischer reagent. Repeat the above procedure until consistent results have been obtained, and calculate the equivalent of the Karl Fischer reagent (mg of water/ ml reagent).
Procedure
To the titration mixture already adjusted to the instrument end-point, add a suitable volume of sample (25 ml) by means of graduated syringe of pipette.
Titrate immediately with the Karl Fischer reagent to the instrument end-point.
Calculation
Water content % by weight = (A x E) / 10 W
Where A = Karl Fischer reagent required to titrate the organic sample, in ml
E = Water equivalent of Karl Fischer regent, in mg/ml.
C = Weight of sample, in g = Volume, in ml x sp. gr. of sample. |
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