Introduction: Karl Fisher titration is
the method used to determine amount of the moisture present in the substances. Here we use the reagent called Karl-Fisher reagent and it is composed of iodine sulphur dioxide, anhydrous methanol and pyridine.
Principle: Here the principle involved is, the contents of the reagent react stoichiometrically (from the equation below) with the water. We can determine the end point electrometrically (there will be an increase in the passage of the current with slight excess of reagent added) or by colorimetrically
(change in the color).
The determination of the water takes place in two steps.
3Pyr + SO2 + I2+ H2O a 2Pyr.HI +Pyr-SO3
Pyr-SO3 + CH3OH a Pyr(H)SO4.CH3
Here we can conclude that amount of the water present is equal to the iodine reacted.
Apparatus: It contains an instrument which consists of two electrodes made of platinum and is present in the vessel which consists of the solution whose moisture content is to be determined. The vessel should be closed tightly and consists of one or two burettes. It is also provided with the magnetic stirrer.
Preparation of the reagent: Add 63g of iodine to the 100ml of dehydrated pyridine and dissolve it. Cool the solution keeping it in ice and then allow the sulphur dioxide into the solution and continue passing till there is an increase in 32.3g weight. Then make to the solution to 500ml.
Equivalent factor: 1ml of the reagent is equal to 5mg of water.
Note: The above prepared reagent will deteriorate quickly and hence we have to standardize the reagent before using it.
Standardization of the reagent:
Primary standardization: In the vessel keep 36ml of methyl alcohol and then and supply karl fisher reagent until we get the end point. Then we have to add 150 to 250mg of sodium tartarate to the vessel and titrate it with the reagent until we get the end point.
Here we can find the water equivalence factor F=0.1566W/V.
Here W represent the weight of the sodium tartarate that we added to the solution and V represent the volume of the reagent that took for getting the end point.
Secondary standardization: Take 1000ml of the methyl alcohol and add 2ml of the water. Then take 25ml from the above solution and keep it in the vessel. Then titrate the solution to get the end point. Even we have to do the blank titration, by taking 25ml of the methyl alcohol and titrate it with the reagent until we get the end point. Correct the first value using the blank.
He water content is given by the formula: VF/25 where V is the volume of the karl fisher reagent (value is corrected with the help of the blank titration) and F represents the water equivalence factor.
Determination of the water content in the given sample: Now we can determine the amount of the moisture present in the sample. If it is given, first add the 25ml of the solvent i.e. methy alcohol to the vessel and add karl fisher reagent until we get the end point. This value will not be included in the calculation. Then add the sample under test, make sure that sample contains the water between 10 to 50ml. Then titrate with karl fisher reagent to get the end point.
Here the water present in the sample is given by the formula SF
Where S represents the volume of the reagent and F represents water equivalence facor.
Conclusion: The drugs and other pharmaceuticals must be evaluated for the presence of the water and Karl fisher method is useful for getting the amount of the moisture present.
A text book of Pharmaceutical Analysis by G. Devala Rao, volume-1, 5th edition, Birla publications, page no148 to 150.
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