Titer determination in Karl Fischer titration
11.5.2020
Article
Without a titer determination, you will not obtain correct results. The same applies for volumetric Karl Fischer (KF) titrations. In this blog post, I will cover the following topics (click to jump directly to each):
- Why should I do titer determinations?
- How often should I perform titer determinations?
- What equipment do I need for a titer determination?
- How to carry out a titer determination
- Acceptable results
- How long can the reagent be used?
If you are looking for information about titer determinations for potentiometric titrations, then please read our article below.
Why should I do titer determinations?
Why is a titer determination necessary? The answer is quite simple. Without knowing the titer of a KF titrant, the water content of the sample cannot be calculated correctly. In Karl Fischer titration, the titer states how many mg of water can be titrated with one mL of titrant. Therefore, the KF titer has the unit «mg/mL».
You might say: “Now, ok, let’s determine the titer. That isn’t too much work and afterwards, I know the titer value and I don’t need to repeat the titer determination.”
I agree this would be very nice. However, reality is somewhat different. You must carry out titer determinations on a regular basis. In closed bottles, KF titrants are very stable and the titer does not change appreciably. Once you open the bottle, the KF titrant starts to change significantly. Air will enter the bottle, and considering that 1 L of air contains several milligrams of water, you can imagine that this moisture has an influence on the titer. To prevent moist air from getting into the titrant, the bottle must be either tightly closed after use with the original cap, or should be protected with an absorber tube filled with a molecular sieve (0.3 nm).
Temperature changes also have an influence on the titer. A temperature increase of the titrant by 1 °C leads to a titer decrease of approximately 0.1% due to volume expansion. Consider this in case the temperature in your laboratory fluctuates during the working day.
Do not forget: if your titration system is stopped overnight, the reagent in the tubes and in the cylinder is affected and the titer is no longer comparable to the titrant in the bottle. Therefore, I recommend first running a preparation step to flush all tubes before the first titration.
How often should I perform titer determinations?
This question is asked frequently, and unfortunately has no simple answer. In other words, I cannot recommend a single fixed interval for titer determinations. The frequency depends on various factors:
- the type of reagent (two-component titrants are more stable than single-component titrants)
- the tightness of the seals between the titration vessel and the titrant bottle
- how accurate the water content in the sample must be determined
In the beginning, I would recommend performing a titer determination on a daily basis. After a few days, it will become apparent whether the titer remains stable or decreases. Then you can decide to adjust the interval between successive titer determinations.