Karl - Fischer Titrations.

The determination of water is one of the most common analytical determinations. Because of its widespread importance, many procedures for the determination involving physical and chemical methods have been developed. physical methods generally involve separation of the water by oven drying, absorption or azeotropic distillation. Many chemical reactions generally involving the hydrolysis of reactive agent, acetyl chloride and acetic anhydride have been suggested.

In 1935 an extremely, simply convenient and precise method was introduced by Karl Fischer. Water in a sample dispersed in an anhydrous solvent such as methanol is titrated directly with a reagent containing I2, SO2, pyridine and methanol. The method is reasonably specific for water and is commonly used for its determination.

Preparation of Karl - Fischer reagent:
The solution contain component in the ratio 1 mole I2 : 3 moles SO2 : 10 moles C5H5N : 129 moles CH3OH. The dilution with methanol should be such that 1cm3 of the resulting reagent reacts with 3 to 6 mg of water. However the composition may be altered in certain special cases.

Pyridine is required to combine with the acid products formed by the reaction. In addiction pyridine pyridine combines with I2 and SO2 (lewis acid base interaction) to reduce their vapor pressure and thus makes the reagent more stable. Methanol is a good solvent for the various products of the reaction end point. The chemicals used for the preparation of the KF reagent should not contain more than 0.1% water.

For preparing the reagent a known weight of iodine is taken in a dry glass stoppered bottle and required amount of anhydrous pyridine is added. The mixture is stirred till all iodine is dissolved. Required amount of anhydrous methanol is then added. The resulting stock solution is stable and may be kept almost indefinitely. Several days before the reagent is required. The stock solution is cooled in an ice bath, and liquid SO2 cooled in a solid CO2 bath is carefully added to the chilled stock solution. The mixture stoppered and allowed to return to room temperature. The reagent is kept for several days before using. Low temperature are to be maintained since the addition of SO2 to the stock solution is highly exothermic. Commercially available KF reagent generally consists of two stock solutions. One of I2 in methanol and the other SO2 in pyridine. These solutions are mixed several days before use.

Stability of the Reagent:

Freshly prepared KF reagent shows a marked decrease in strength within the first 2-3 days followed by a more gradual decrease with time. After the first week the strength decreases about 1% per day. The freshly prepared solution should be allowed, therefore to stand at least a week before being standardized. It should also be re standardized periodically, at least once each day of its use.

It has been observed that freshly prepared reagent generally has about 80% of its theoretical strength which decreases to about 40%after 3 months. Part of initial loss in strength comes from the moisture content of chemical, but part is most likely caused by side reactions leading to reduction of iodine to iodide. The rate of decrease in strength depends on how well the reagent is protected from moisture in air. Temperature doesn't markedly influence the decomposition of the reagent, but is slightly quickly. On the other hand the reagent prepared without SO2 is practically completely stable.

Studies have been made to substitute other nitrogen bases for pyridine (e.g., aniline, dimethylaniline, etc) and other inert solvent, for methanol (benzene, dioxan, ethanol, etc) in the KF reagent. However methanol and pyridine were found most suitable for various reasons.

The Titration method:

The Kf reagent is a very active desiccant. The apparatus used to store and dispense the reagent, and the titration flask assembly must be designed to protect the reagent from the moisture in air. The reagent solution can be used conveniently in all-glass automatic burettes having the solution reservoir and burette in an integral unit which can be protected with drying tubes.

The solution to be titrated should be protected from exposure to moist air. Very often the titration flask is attached to the burette by ground glass joint (or special plastic stoppers or sheets ) magnetic stirrer is used to mix the contents of the flask during titration.

The end point can be detected visually. The KF reagent has the dark brown color of iodine, which is changed to chromate yellow color by reduction with water. At the end point the color change from yellow to reddish brown of unused iodine is sharp and reproducible. However, visual end point is not useful for colored solution.

The electrometric method of end point detection can also be used. The "dead-stop" method is commonly used. A potential of 10-25mv is impressed between two platinum wire electrodes immersed in the titration solution. When iodide only is present in solution (i.e., before the end point ) the system is polarized and no current flows. When free iodine present at the end point there is an equilibrium between iodide and iodine and the system becomes polarized and galvanometer, register a flow of current in the circuit.

For a back titration, current flows through a galvanometer when the solution contains excess KF reagent (i.e., before the end point when iodide & iodine are in equilibrium) and drops to zero when the equivalence point is reached (i.e., when only iodide is present in solution)

Standardization of KF Reagent:

Because of the gradual decomposition of KF reagent, it must be standardized on each day of use. This is accomplished readily by titrating a known quantity of water with the reagent. several methods have been suggested for obtaining standard samples of water. These include pure water, standard solution of water in methanol and stable crystalline hydrates. The strength of the Kf reagent is normally expressed as water equivalent of KF reagent i.e., mg of water per cm3 of KF reagent.