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Determination of Water by the Karl Fischer Titration: Theory

Motivation Volumetric KF titration one an two-component reagents resolution and detection limits Coulometric KF titration cell with or without diaphragm resolution and detection limits Indication, control algorithm, termination parameters KF titration: important points Support. P

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Determination of Water by the Karl Fischer Titration: Theory

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    5. Karl Fischer Titration: Why? Fast (e.g. 1...2 minutes) Selective for water Accurate and precise (0.3% srel) Wide measuring range : ppm to %

    6. Karl Fischer

    7. KF Titration KF Reaction SO2 + RN + ROH ------> (RNH)SO3R a sulfite compound (RNH)SO3R + H2O + I2 + 2RN ------> (RNH)SO4R + 2(RNH)I a sulfate compound Summary H2O + I2 + SO2 + 3RN + ROH ----->(RNH)SO4R + 2(RNH)I The solvent (generally methanol) is involved in the reaction A suitable base keeps the pH 5 - 7

    8. Solvent pH range

    9. Volumetric / Coulometric Titration

    10. Volumetric KF Titration Iodine is added by burette during titration. Water as a major component: 100 ppm - 100 %

    11. Volumetric KF Titration One - component reagent Titrant: I2 , SO2, imidazole, methanol and diethylene glycol monoethyleter Solvent: Methanol Two - component reagent Titrant: I2 and Methanol Solvent: SO2, Imidazole, Methanol -> fast reaction, chemically stable, higher cost

    12. Volumetric KF Reagents Titrant Concentration 1-2-5 mg H2O/mL Titer stability -----> Check by Standardization Standardization materials Water 100% Sodium tartrate 15.66% Standard solution 5 mg/mL Water Standard 1% (10 mg/g)

    13. Air humidity: Air Humidity

    14. Drift determination

    15. Volumetric Karl Fischer Titration Resolution and Detection Limit

    16. Coulometric KF Titration Iodine is generated electrochemically during titration Water in trace amounts: 1 ppm - 5 %

    17. Coulometric KF Titration Titration cell and reagents

    18. Coulometric KF Titration Same reaction as volumetric KF Titration but Iodine is produced just in time from iodide

    19. Coulometry Theory One Coulomb C is the quantity of charge transported by an electric current of one Ampere (A) during one second (s). 1 C = 1 A 1 s Absolute method, no standardization!

    20. Filling the Titration Cell

    21. Filling the Titration Cell

    22. With or Without Diaphragm

    23. With Diaphragm

    24. Without Diaphragm

    25. Without Diaphragm

    26. Without Diaphragm A little bit less accuracy for very small water content (< 50 g/sample). Not recommended for easily reducible samples: nitro compounds, unsaturated fatty acids, etc.

    27. Resolution and Detection Limit

    28. Coulometry versus Volumetry Repeatability

    29. KF Indication Principle (1/2) Bivoltametric indication constant current at the double platinum pin electrode ==> polarization current (Ipol) During titration: I2 reacts with water no free I2 in the solution high potential

    30. KF Indication Principle (2/2) At endpoint all water has reacted with I2 After the endpoint free I2 in the solution I2 is reduced to I- at the cathode ionic conductivity occurs and the measured potential drops potential change = endpoint

    31. KF Control: Titrator Algorithm Karl Fischer Fuzzy Logic Control DL31/38 No control band required (typical 300 mV) The titrant addition rate depends on: the distance to the endpoint EP the potential change/increment Advantages: Simpler control: Only two control parameters ?Vmin , ?Vmax (smallest/largest increment) Faster, more accurate, and better precision even at low water content (toluene: n = 5, 115 ppm, srel 0.17% )

    32. KF Control: Termination Parameters (1/3) Delay time the actual potential is lower than the EP for a defined time after the last titrant increment typical delay : 15 - 20 sec Note: Adapt the smallest increment to the drift and to the concentration of the titrant

    33. KF Control: Termination Parameters (2/3) Absolute drift stop the actual drift is less then the predefined value typical value : 30 ?g/min Note: Adapt the value to the initial drift

    34. KF Control: Termination Parameters (3/3) Relative drift stop the sum of the initial and the relative drift has been reached typical value : 15 ?g/min independent from the initial drift and of titrant concentration ideal with side reactions that cannot be suppressed otherwise

    35. Karl Fischer Titration : Checks Relevant points to be checked System tightness : Check carefully Ambient moisture : Drift determination Stability of titrant : Standardisation Side reactions : Check literature Sample handling : Accuracy, precision Free water only : Sample preparation

    36. Complete Solution : Solutions and Support Application brochures Internet databases

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