We have all seen it. We’re running a test to see how much moisture is in our sample when inexplicably the liquid inside the Karl Fischer vessel starts to turn from a normal light-yellow color to a dark burnt-red looking color. Sometimes the titrator screen will inform us of the problem with a digital readout stating the dreaded
Sometimes the digital readout says nothing at all. In either case the operator knows something has gone wrong because the Karl Fischer Titrator is no longer giving moisture results. A panic to figure out the problem and get testing underway again becomes the immediate priority.
As it turns out “over-titration” is probably one of the top 2 or 3 complaints or issues we hear about from operators. So what is causing this problem to occur? How can we determine the source of this problem, fix it, and more importantly how can we avoid it?
This is a critical question for operators and managers working in a production or QC environment who are concerned with keeping their Karl Fischer Titrator 100% “in-service”, day-in and day-out. Having the ability to identify the problem correctly so appropriate measures can be taken quickly is very important.
So how do we approach the problem of “over-titration”? By knowing the facts. Having a clear understanding of the process can help operators correct the problem faster when time is of the essence.
✓ A damaged Detector Electrode
✓ A "Tricked" or "Fooled" Detector Electrode (no joke)
Since coulometric and volumetric Karl Fischer Titrators handle the delivery of iodine differently it’s worth describing the two methods separately.
In a coulometric system the reagent is a complete system where it is designed to release iodine when the generator electrode delivers an electrical current to it. So what causes the generator electrode to deliver too much current causing the over production of iodine? Another way to say it is, “who or what” is telling the generator electrode to continue to generate a current when it’s not needed?
The detector electrode! So why would the detector electrode do this?
Without getting into too much of the electronics the detector electrode is designed to “detect” conductivity in the vessel. Depending on the amount of conductivity detected the detector electrode will send a message to the titrator telling it to continue producing a current - enough to release the appropriate amount of iodine to counter and neutralize the water present in the vessel. As long as this process is working during a titration an eventual endpoint will be found and a result will be produced.
So it’s really a problem of misinformation. If the Karl Fischer Titrator is not getting the right information from the detector electrode then over-titration is possible.
If it’s determined that the problem is not the detector electrode then we need to look at the stirring action inside the vessel. If the iodine being released is not mixing well because the stirrer is off or set too low, then the detector electrode will not realize there is iodine already released inside the vessel. This will cause the detector electrode to continue telling the titrator to produce more current via the generator electrode up to the point where the detector electrode senses a reduction in the conductivity level inside the vessel. Conductivity only reduces as the iodine interacts with the water. So it is important for the detector electrode to sense the true and most accurate “mix or state” of iodine and water during the titration process. If it does not know the true state of the mix it will be fooled into telling the titrator to keep going – causing OVER TITRATION.
1. Don’t abuse the detector electrode! Be very careful with the detector electrode and do not handle it unnecessarily. Small bumps (clanks) here and there can lead to a crack. Do you really need to remove the detector
2. Don’t turn up the titrator’s stirrer speed to high. This will only cause the stirrer bar to bounce around uncontrollably and possibly hit and damage the detector electrode (crack).
3. Do inspect all connections and connectors on the detector electrode cable and Karl Fischer Titrator to ensure they are dry and clean.
4. Do be careful with the detector electrode cable. Try not to bend it unnecessarily.
5. Do make sure there is enough stirring action inside the vessel to mix the iodine around effectively. A small vortex should be visible. But not too fast to cause the stir bar to bounce around.
6. Do introduce some moisture - Sometimes when you are in an over-titration situation and the vessel is already very dark you can introduce a little moisture to bring the vessel back to equilibrium. This sometimes works and immediately the vessel turns from a dark burnt-red color to a light-yellow.
7. Do have a spare detector electrode on hand. This little electrode seems to get over looked but plays a huge role inside the Karl Fischer Titrator vessel.