Chromatography Forum

I know that not only is this off topic but has nothing to do with this forum at all but I thought I would give it a try. Can anyone here give me a simple definition as to what the drift in the Karl Fisher is, I am finding it difficult to explain what it is. We run in ug/min.

And maybe a real simple way to explain the whole process of karl fisher titrations.

Thanks so much,
EtoH

Atmospheric water creeping into the system is typically the main cause of drift in a Karl Fischer titrator.

Thanks, that's what I keep trying to tell people but you know how people can be

The general reactions behind Karl Fischer titration are as follows:

CH3 OH + SO2 + RN → [RNH]SO3CH3

H2O + I2 + [RNH]SO3CH3 + 2 RN → [RNH]SO4 CH3 + 2 [RNH]I

(RN = Base)

This reaction consumes water and iodine in a 1:1 ratio. In order to assure a stoichiometric course of the Karl Fischer reaction, certain fundamental requirements must be met and several potential interferences must be avoided. The choice of working medium and pH range are the most critical considerations.

Then it it becomes more complicated

Use 1 or 2 component reagent system

Classic base as pyridine or (I hope people use) less toxic ones

Specail reagents for ketones and aldehydes

and so on..

http://www.sigmaaldrich.com/Area_of_Int ... heory.html

Gives some basics but just google it and you will find more

In my experience, it's not just atmospheric water, and most drift is associated with tramp water and on-going reagent degradation, especially if titration systems are left empty and only used occasionally.

If drift was mainly atmospheric, then leaving the system sealed should result in a continual downward drift to near zero, but I find the equilibrium time doesn't change much whether I add a sample or not, and if I put a dry Nitrogen purge in place, the drift only deceases by about 30%. I suspect most is tramp moisture, along with some inefficiencies of the desiccant.

My volumetric titration Mettler DL31 sits on a drift of about 4-5 ug/min, as it's used fairly infrequently. If I warm glassware and use a small nitrogen positive pressure, rather than just molecular sieves, it can drop to about 3 ug/min, and the Hydranal booklet say that 0.01 ml of reagent ( my reagent is 5 mg/ml )/ is the maximum drift expected from a "perfectly dry" titration cell. That would be 5 ug/min for my instrument, which is similar to what I had for a Mettler DL18 system, using the same composite 5 reagent...

The Hydranal book also says a coulometric system should have a maximum drift of 10 ug/min, with 4 ug/min as ideal.

The advantage of most modern reagents and systems is the speed of titration, which then minimise the effect of drift.

I highly recommend getting the free Hydranal CD from Sigma-Aldrich, it's full of good stuff about the reaction and the reagents, as well as plenty of hints on how to minimise issues, such as drift, stirring, and titration speed..

Please keep having fun,

Bruce Hamilton

As well as the Hydranal CD which is great I would also recommend you ask Metrohm for their free Karl Fischer monograph. This is a smallish booklet which gives a lot of unbiased information on the chemistries involved in KF titrations as well as applications.

Matt

As well as the Hydranal CD which is great I would also recommend you ask Metrohm for their free Karl Fischer monograph.

You may order this Monograph under http://www.metrohm.com/applications/lit ... order.html.

You got some problems with it or what?

Hi there,

I just would like to highlight issues related to Karl Fischer technique or method.

We are utilizing that method for our the moisture in the product we produce which is LPG (Propane and Butane).

Btw, we are following a method correlated to ASTM but frankly saying we are facing some problems with the method it self.

We sample the LPG in 150cc or 300cc bombs, we weight the bomb + the sample, then we introduce the sample thru regulated valve into the karl fischer vissel for about 10 minutes.

What should be done after the 10 minutes completion is to re-weight the bomb again and calculate the difference b/w the two weights and do the calculation for the total moisture in that bomb. But, reversly we are doing that by releasing the whole sample in the bomb then we re-weight the bomb w/o the sample and by that we get the total weight of the sample in the bomb and we calculate the moisture for the whole sample in the bomb?!!!!!!!! we have doubt here

By conclusion, we've done the both directions and we got that the second approach is the most reliable. Any comment here??

I would be very thankful if someone could emphasize or elaborate.

regards,

alchemist