KF Moisture Sample weight

[StephenO]

Hi All

I am looking to know how to calculate the amount of sample I should weigh for moisture analysis using a karl fischer titraitor. Or could someone point me in the direction of a forum or literature that could.


thanks in advance

[DR]

Assuming an Iodine reaction based unit, you generally want to use at least 1mL or titrant in order to get an accurate measure.

If you're using a "5" titrant (Aquastar Composite 5, for example), that means you are quenching approximately 5mg of water for every 1mL of titrant used. If your sample is approximately 5% water (you need to make an initial guess, guessing a little high and having to run a larger sample later is quicker and cheaper than guessing too low initially and having to wait for the thing to pump lots of titrant), your sample size should be at least 100mg. If you suspect a much wetter or drier sample, adjust your sample size accordingly.

[StephenO]

Thanks DR this is what I was looking for.

[Consumer Products Guy]

For samples very high in water content (80%) we use 0.1 gram sample.

[HippyLabRat]

I hate to resuscitate this old thread...

I'm using 50μl injections for my samples that are 25% water is that sufficient? The titration time is about 6½-7 min on my MT C20.

[Johnny Rod]

That works out at about 12ul water per injection or 12 000 ug which is a lot. You should see at least 100ug for a valid result, getting over 1000 though is going to eat up the reagents quickly, and I found the precision drops off a bit as well. If you can accurately measure in less sample then you should do so really.

[HippyLabRat]

I was doing 20ul injections previously, but my RSDs over 3 replicates were between 2 and 4%. Acceptable, but not as good as it could be. I think part of the issue may have been the syringe I was using. I'll have to try it again when the new syringes get here.

[Shinan]

i have a similar query about limits for RSD for duplicate samples. Is there a well defined criteria on a global scale for formulations or API for RSD limits of samples.

[Consumer Products Guy]

I'm using 50μl injections for my samples that are 25% water is that sufficient?

Hippy: if I had samples with 25% H2O, I'd use a 250µl gas tight syringe with a metal needle, and weigh at least 0.14 grams sample, to get good repeatability. I suck up sample into the syringe barrel, wipe off the needle with a disposable tissue, and weigh. I also wear disposable gloves to minimize any contamination weight. I just did over 25 competitive products this way.

The titration time is about 6½-7 min on my MT C20.

That's a lot, sounds like the old pyridine-solvent days. We use Aquastar Comp-5 or similar titrant, our titrations are like 2 minutes. Are you positive that you are not getting "drift" from atmospheric moisture? I'd check ALL my seals, then double check.