Water content determination by Karl Fischer titration Ivo

Water content determination by Karl Fischer titration Ivo Leito University of Tartu

Karl Fischer (KF) titration • The primary method for determination of water content • Based on the oxidation-reduction reaction:

KF Titration • Two types: – Volumetric • Classical titration medium: Methanol (or solvent mixture) • Classical titrant: I 2, SO 2 and pyridine in methanol – Coulonometric • Titration medium: SO 2, pyridine and some iodide in methanol • Iodine is generated electrochemically • We will look at volumetric KF titration only

End point determination • End point is determined by a small amount of unreacted iodine in solution – Usually voltammetrically

Titratnt titer (water equivalent) • Titrant water equivalent (milligrams of water per ml of titrant) is redetermined frequently – Every day or every second day • It is determined by titrating known amounts of water

Equipment • Karl Fischer titrator: – Airtight titration vessel – Electrode for end-point determination – Solvent bottle – Titrant bottle – Piston burette for dispensing titrant – Septum for sample injection

Sequence of work (ISO 8534: 1996 ) 1. The solvent is titrated to the end-point 2. 5 -20 g of oil sample is weighed in a syringe to nearest 0. 01 g. 3. The sample is injected into the titration cell 4. The empty syringe is weighed again to determine the mass of the oil sample. 5. When the sample has dissolved then the sample is titrated to the end-point 6. Concentration of water in the sample is calculated from the titration data.

ISO GUM Uncertainty estimation: Measurand, measurement equation • Measurand: Cwater, concentration of water in oil sample [mg/kg] – Vt: volume of titrant used for titration [ml] – Tt: the titer (water equivalent) of the titrant (mass of water per unit volume of titrant) [mg/ml] – ms: the sample mass [g] – 1000 [g/kg] is the unit conversion factor

Sources of uncertainty • Mass measurement • Volumetric measurement • Titrant concentration • Precision of determination of the equivalence point • Contamination by atmospheric moisture • Interferences

Measurement equation for uncertainty calculation: (5) • fr: Repeatability factor [unitless] – Value: 1 – Uncertainty: (6)

Uncertainty components and combined standard uncertainty (5)

Data