Title: Determination of Water by KF
1. Objective: To measure the amount of Water content in the sample.
- Principle: The Titrimetric determination of water by the Karl Fischer method depends on the reaction that takes place quantitatively between water and a reagent consisting of sulfur dioxide and iodine in anhydrous pyridine and usually methanol. The reaction is carried out in a suitable solvent such as methanol.
3. Procedure:
- Method I: Titrimetric Method:
- Karl Fischer reagent (KF) reagent:
The reagents and solutions used for preparing the KF reagent should be kept anhydrous and care should be taken throughout the determination to prevent exposure to atmospheric moisture. The reagent should be protected from light and stored in a bottle to which is fitted an automatic burette.
- Primary Standardisation of the reagent:
Place about 36 ml of dehydrated methanol in the titration vessel and add sufficient Karl Fischer reagent to give the characteristics end-point. Add quickly 150 to 350 mg of Sodium Tartrate, C4H4O6Na2, 2H2O, accurately weighed by difference and titrate to the endpoint.
The water equivalence factor F, in mg of water per ml of the reagent is given by the expression 0.1566 x (w/v).
Where, w is the weight in mg of the Disodium Tartrate and v is the volume in ml of the reagent required.
- Secondary Standardisation of the reagent:
The Karl Fisher reagent may alternatively be standardised for each day’s use against a water-methanol solution standardised as follows. Add 2.0 ml of water to 1000.0 ml of dehydrated methanol. Retain a portion of the methanol used for a blank determination. Place 25 ml, accurately measured of the water methanol solution in the titration vessel and titrate with Karl Fisher reagent. Perform a blank titration on 25 ml accurately measured, of the methanol used and make any necessary correction. The water content in mg per ml of the water methanol solution is given by the expression VF/25 in which V is the volume in ml of Karl Fischer reagent required and F is the water equivalent factor of the reagent determined against Disodium Tartrate as directed under Primary standardisation of the reagent.
Water content of water methanol solution: V x F__
25
- Apparatus:
A titration vessel of about 60 ml capacity is fitted with two platinum electrodes, about 0.05 sq. cm in area and about 2.5 cm apart a nitrogen inlet tube, a stopper which accommodates the burette tip and a vent tube protected by a suitable desiccant such as phosphorous pentoxide or silica gel. The substance being examined is introduced through an inlet or side arm which can be closed by a ground stopper. Stirring is done magnetically or by means of a stream of dried nitrogen passed through the solution during the titration. The air in the entire system should be kept dry during the titration.
The end point is determined by amperometry. The circuit consists of a potentiometer of about 2000 ohms. Connected across a 1.5 v battery. The resistance is adjusted so that an initial low current passes through the electrodes. On adding the reagent the needle of the microammeter shows a deflection but returns immediately to its starting position. At the end point of the titration a slight excess of the reagent produces a deflection which persists for not less than half a minute.
The actual procedure to be employed are dependent on the particular instrument used. Refer the manufacture’s literature and/or the analysis literature for the most appropriate technique for a given instrument.
Use Method A unless otherwise directed.
- Method A:
Unless otherwise directed add about 20 ml of dehydrated methanol to the titration vessel and titrate to the electrometric end point with the Karl Fischer reagent. Transfer quickly the prescribed amount of the substance being examined, accurately weighed to the titration vessel. Stir for 1 minute and titrate again to the electrometric end point using the Karl Fischer reagent.
The water content of the sample, in mg is given by the expression S x F, in which S is the volume, in ml of the Karl Fischer reagent used to titrate the sample and F is the water equivalent factor.
Water Content (%w/w) = S x F x 100______
Weight of sample
- Method B:
This method should be followed for samples that react with difficulty or too slowly for convenient direct titration with the Karl Fischer reagent.
Unless otherwise directed add about 10 ml of dehydrated methanol to the titration vessel and titrate to the electrometric end point with the Karl Fischer reagent. Transfer quickly the prescribed amount accurately weighed, of the substance being examined to the titration vessel followed by an accurately measured amount of Karl Fischer reagent sufficient to given an excess of about 1 ml. Allow to stand , protected from light, for 1 minute, stirring well. Titrate the excess of the reagent to the electrometric endpoint with dehydrated methanol to which has been added an accurately known amount of water equivalent to about 0.25% w/v.
Calculate the content of water from the expression S x F, where S is the volume in ml, of the KF reagent used to titrate the sample and F is equivalence factor.
Unless otherwise directed, express the result as a percentage w/w.