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Coulometric Karl Fischer Performance Check

You are here:   Metrohm USA   >>   Support   >>   FAQs   >>   Titration - Coulometric


 

Introduction

The coulometric KF technique is used to quantify low-level moisture in the range of 10 µg–200 mg. The chemistry involved is identical to volumetric KF, except for the way iodine (titrant) is introduced into the titration vessel. The basic premise relies on the fact that for every mole of iodine consumed in the reaction, one mole of water reacts:

H2O + I2 + 2RN + [RNH]SO3CH3 = [RNH]SO4CH3 + 2[RNH]I .

The iodine required for the reaction is generated directly in the titration vessel by electrolysis at the generator electrode. The progress of the reaction is monitored by the double platinum indicator electrode. The quantity of electricity required for iodine generation during the titration is used to calculate the water content of the sample. The coulometer keeps a record of the current used at the generator electrode and the titration time, and it uses these values to calculate the quantity of electricity required for the reaction. By applying Faraday’s law, the quantity of electricity can be converted to the amount of water in the sample. In order to get accurate and reliable results, the following conditions must be maintained:

  • Generator electrode must be kept clean, and it must operate at 100% efficiency, i.e., all the electricity must be used for iodine generation for reaction with water only.
  • The conductivity of the vessel solution must be maintained as this affects the operation of the generator electrode. If you add co-solvents (toluene, xylene, etc.) to help with solubility, be sure that the co-solvent does not exceed 10% of the total vessel solution. When the conductivity is compromised, the generator electrode will cease to function and the error message “Check Gen. Electrode” appears.
  • The double platinum indicator electrode monitors the presence (or absence) of water in the vessel. Be sure to keep the electrode clean and keep the pins parallel to each other and to the body of the electrode.
  • All efforts must be made to keep the cell sealed so that atmospheric moisture does not seep in. Pay attention to the following:
    Be sure drying agent is fresh and loosely packed
    Plastic sleeves must be inserted at all glass-on-glass connections
    Change septum at injection port as often as necessary

Instrumentation and reagents

  • Metrohm® 756 or 831 Coulometer
  • Gas-tight glass syringe (5 ml or 10 ml) fitted with a needle
  • Analytical, 4-place balance close to the coulometer
  • Hydranal® Coulomat AG (Brinkmann Catalog No. 019100618) or other suitable coulometric reagent
  • Hydranal Standard 1.0 (Brinkmann Catalog No. 019101169)

Performance check

Parameter settings on Metrohm Coulometer

Control parameters:
  EP at U 50 mV
  Dynamics 70 mV
  Max rate 1,800 µg/min
  Min. rate 15 µg/min
  Stop crit Rel. drift
  Rel. drift 10 µg/min
Titration parameters:
  Pause 0 s
  Extr. time 0 s
  Start drift 20 µg/min
  Temperature 25°C
  Time interval 4 s
  Max. titr. time off
Statistics:
  Status on
  Mean n = 5
  Res. tab original
Preselections:
  Drift corr auto
  Req. ident off
  Req. smpl. size value
  Request and titr off
  Smpl. unit g
  Cell no diaph (or diaphragm)
  Generator I 400 mA (or auto for cell with diaphragm)
  Oven no
  Activate pulse off

Calculations (under key #2 def)

Formula:
  Water = H2O/C01/C00
  RS1 decimal places 3
  RS unit mg/g
C-fmla:
  C01 = 1,000

Procedure

Preparing the cell

A Metrohm titrator will dry the cell and measure a drift after “Start” is pressed once. Drift is the rate of addition of KF reagent needed to keep the cell at the dry point; it is measured in microgram/minute. With a Metrohm autotitrator and a brand new or newly cleaned cell, a stable drift should be obtainable within 15 minutes. An acceptable stable drift level is 10 µg/min or less. If the autotitrator maintains this drift for 5 minutes, standardization may commence. These times will be much shorter once the KF cell has been in use.

Injecting the standard

  1. Break open an ampoule and pull up ~1 ml standard into the syringe.
  2. Pull the plunger back to rinse the entire syringe.
  3. Flush standard into the waste container.
  4. Pull up the remaining standard into the syringe.
  5. Wipe the needle then close it off with a silicone septum.
  6. Place the syringe on the balance and press the tare key, wait until the display is stable and it reads 0.0000 g.
  7. . Press “Start” on the coulometer, wait until the display shows “add sample.”
  8. Remove the septum and expel ~1 ml standard into the vessel solution, preferably below the liquid level.
  9. Remove the syringe, wipe the needle, replace the septum and place the syringe on the balance. The weight of the standard injected will appear as a negative number.
  10. Type the absolute value of the weight on the coulometer keypad and press “Enter” or the balance can be connected directly to the coulometer, and the weight transferred directly by pressing the appropriate key on the balance—thus avoiding typing errors!
  11. The titration will be run, and the result displayed and/or printed.

Common KF problems and their remedies

  1. Problem:
    Conditioning takes longer than usual, even with fresh vessel solution.

    Remedy:
    a. The conditioning step should be complete within 30 minutes for a brand new or newly cleaned cell (less for a cell already in use). Check/replace drying agent; check/replace septum at sample port. When display flashes “Ready,” swirl solution against the vessel walls to speed up the conditioning.
    b. If the drying reagent does not rectify the problem, clean both the double platinum pin and generator electrodes. Clean the electrode by soaking it in 50% HNO3 for 10 minutes, rinse thoroughly with deionized water to remove all traces of acid and then rinse with vessel solution or dry methanol.
  2. Problem:
    Results are high and erratic; vessel solution looks darker than normal.

    Remedy:
    This is indicative of overtitration most commonly caused by a slow responding measuring electrode. Clean the electrode by soaking it in 50% HNO3 for 10 minutes, rinse thoroughly with deionized water to remove all traces of acid and then rinse with vessel solution or dry methanol.
  3. Problem:
    Error message “Check Gen. Electrode.”

    Remedy:
    The generator electrode is not functional. Check the following:
    • Proper cable connection
    • Change the vessel solution; conductivity might be low
    • Clean electrode as outlined above for the indicator electrode
    • Replace electrode if there is sign of physical damage (broken mesh etc.)
  4. Problem:
    Titration is unusually slow.

    Remedy:
    Acid in your sample will lower the pH of the vessel solution and slow down the titration. Add ~3 g–5 g imidazole per 100 ml of vessel solution to control pH. If samples are basic, add ~3 g–5 g benzoic or salicylic acid. You can purchase these as ready-to-use solutions—Hydranal acid buffer and Hydranal basic buffer.

 

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