Titrasi Karl FischerMustofa Ahda

DefinitionThis method determines the amount of water present in the test sample. It is reported as milligrams of water per 100 milligrams of sample, or percent water This method is based on the stoichiometric reaction of Karl Fischer reagent with water present in the test sample. The amount of water equivalent to each milliliter of Karl Fischer reagent is determined using a methanol water solution as the standard.

PrincipleGenerally :

The Karl Fischer reaction is a reaction between water,iodine, sulfur dioxide, methanol and an organic base

H2O + I2 + SO2 + ROH + 3RN (RNH)SO4R + 2(RNH)I where:RN = an organic base such as pyridine, AmineROH = alcohol as 2-methoxyethanol, methanol, etc.

Principle

The water content determination is based on the reaction described by R. W. Bunsen :I2 + SO2 + 2 H2O 2 HI + H2SO4

Karl Fischer discovered that this reaction could be used for water determinations in a nonaqueous system containing an excess of sulfur dioxide. Methanol proved to be suitable as a solvent. In order to achieve an equilibrium shift to the right, it is necessary to neutralize the acids that are formed during the process (HI and H2SO4). Karl Fischer used pyridine for this purpose. Smith, Bryanz and Mitchell formulated a two-step reaction:1. I2 + SO2 + 3 Py + H2O 2 Py-H+I + PySO32. PySO3- + CH3OH Py-H+CH3SO4

According to these equations, methanol not only acts as a solvent but also participates directly in the reaction itself. In an alcoholic solution, the reaction between iodine and water takes place in the stoichiometric ratio of 1:1. In an alcohol-free solution, the reaction between iodine and water takes place in the stoichiometric ratio of 1:2. The reaction such as : 1. I2 + SO2 + 3 Py + H2O 2 Py-H+I + PySO32. PySO3 + H2O Py-H+HSO4-

End point TitrationColor End point: The visual end point is not accurate and precise. Weakness of visual end point cant determine for highly colored sample The water content above 0,1 0,2 % in a relativelly colorless system

So the end point with electrometric end point as potensiometric

InterferenceSubstances that interfere by water-forming reactions include the following: esters, active carbonyl compounds, basic oxides and hydroxides, and inorganic carbonates. Aldehydes and ketones react with methanol to form acetals and ketals:

Karl Fischer Instrument

Determination of water contentDirect Titration: 1. Take 25 ml metanol in dried titration flask 2. Titrate with KF reagent (as Hidranal) to end point 3. Transfer the sample (weigh accurately) into the titration flask and striring to disolved 4. Titrate with KF reagent to end point

or

Back Titration: 1. Take 20 ml methanol in dried titration flask 2. Titrate with KF reagent (as Hidranal) to end point 3. Transfer the sample (weigh accurately) into the titration flask and striring to disolved 4. Add an excessive of KF reagent and striring for 30 min 5. Titrate the solution with metanol standar solution to be end point

Where: f is the number of water coresponding to 1 ml KF reagent f is the number of water in 1 ml of methanol standar solution