Wednesday, October 21, 2015

Process Wednesday: Karl Fischer titration

From the second edition of "Practical Process Research and Development" by our mentor-by-literature Neal Anderson, a lovely little section on the determination of water content by Karl Fischer:
The Karl Fischer titration (or KF titration or simply KF) is the classic analytical method used to detect water, and is convenient to use. The basis of the Karl Fischer titration is the reaction of water with iodine and sulfur dioxide. In the early develop of this analytical technique, the solution containing water was titrated with a solution of I2 in benzene or MeOH until the I2 color remained, providing a sharp, reproducible endpoint to determine the water content. Today of course benzene is avoided as a solvent for the laboratory and scale-up, and less toxic solvent or solvent mixtures are used. The accurate and rapid coulometric assay, which can detect down to 10 micrograms of water, is generally preferred, I2 is generated elctrolytically at the cell anode, and the amount of water is determined by the current required for electrolytic oxidation of HI. 
My favorite thing about Karl Fischer is that it's a machine that spits out a number (assuming your molecules are compatible with the reagents, that is.) How much water is in there? You can find out, instead of the whole "I dunno, it's wet, I think." I highly recommend them. 

5 comments:

  1. CJ, one of the things that I think is so important is doing a water check. I can't tell you how many times people are willing to put their sample in and read the number off. We had a walk-up titrator that wasn't an "analytical R & D" instrument. You have to keep the concentration of the titrant up-to-date on a volumetric titrator.

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    1. Also have experience with people just walking up, putting their stuff in, and blindly accepting whatever number the magic box spits out. I was instructed by an analytical chemist to do "bracketing standards". The first standard gives you a read on whether or not the machine/titrant is functioning properly, while the second can give you an indication of your sample negatively interacting with the titrant. Aldrich sells standards in vials, though a known amount of water in an anhydrous solvent works just as well.

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  2. The problem with our Karl Fisher being used on THF moisture content titration in single digits of ppm (2-4 ppm, we need our anionic polymerization solvents ultra dry) is that the volume of sample (few mL) needed to get a meaningful readout (above 10micrograms of water) is already big enough that the dilution of the titration mix itself affects the readouts, and from a freshly changed and stabilized re-fill I get only few self-consistent values, before the repeated readings from the same sample start climbing steadily - a baseline artifact from too much dilution. So this is annoying and labor intense, and tedious, and expensive (fresh re-fill) so nowadays I do this only infrequently - after we exchange the drying columns, and then with intervals of few weeks (or until something craps up), to re-confirm that the activated alumina drying columns are still holding up. Usually they last about a year, depending on the usage. We use kegs of pre-dried THF, around 50ppm, to put through four large activated Al2O3 columns.

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    1. Did you let the instrument's manufacturer know about this issue? Even though your complaint would probably not result in an immediate fix, the next iteration of the KF titrator may be more sensitive. The last thing the manufacturer wants to hear is that you don't want to use their instrument.
      I have had some positive interactions with a couple of companies and their support engineers.

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