As usual when we pose a question, like:
Should you use Coulometric or Volumetric Karl Fischer to measure moisture in your product?
We often get a the reaction, “So Who Cares?” We'll try to answer both questions.
When you really need to know the water content of your raw material, in-process status, final QC or shipment test comparisons our old friend Karl Fischer is often your best alternative. The technique has the benefit of detecting only water (and not other volatiles). Because it works with dissolved samples the Karl Fischer Method often gets to and measures bound water.
The Karl Fischer Technique
This technique uses a reaction of sulfur dioxide and iodine with water in the presence of a lower alcohol such as methanol and an organic base. This reaction changes the electrolytic properties of the sample material such that the completion of the reaction (or end point) can be detected through conductivity techniques.
A combination of these chemicals is included in a Karl Fischer reagent. The reagent permits the iodine to react with water in the sample. Either the reagent is:
Added to a sample until an end point is detected. The amount of water in the sample is determined by the amount or volume of reagent (i.e. Iodine) added to get an end point. This is the Volumetric Karl Fischer Method. or
The iodine is created by electrolysis from a special Coulometric Karl Fischer Reagent. This is a mixture of Karl Fischer reagent and a solvent into which the sample is introduced. An electrical current is applied until the end point is detected. The quantity of electricity needed to perform the electrolysis for an end point is measured. This is known as the Coulometric Method.
Range and Sensitivity of the Coulometric Method
The sensitivity of a typical Coulometric Karl Fischer instruments can detect amounts of water as small as 1 micro-gram (1 micron). Consequently, this method is preferred for the highest precision needs. Because of this high resolution, low parts-per-million (PPM) water content can be detected using small samples.
The upper limit generally recommended for the Coulometric is about 2% water content or 200 micrograms of water. That would be a liquid sample size of 10 milliliters. The problem with larger samples is that they quickly fill up the Coulometric measuring cell; requiring a cleaning and a reagent refill. Preferred water content levels, for Coulometric Karl Fischer, are under 1% with 2 or less milliliters of sample. This results in water levels per sample of under 20 micrograms.
There are limitations to the range of Coulometric solvents available. In these cases, a Karl Fischer Oven can be use to drive off the water which can then be measured directly.
If the moisture levels are greater than 2%, the Volumetric Karl Fischer is typically the preferred method. In addition, Volumetric reagents allow a wider range of solvents. However, there are still circumstances where a Karl Fischer Oven is needed to get appropriate Volumetric results.
The Volumetric method is the method needed for samples that are difficult or impossible to dissolve in Coulometric reagents.
Rule of Thumb
The net result of these considerations is a rule-of-thumb that the Coulometric method is the better choice where moisture content is less than 1% and the sample will work with available solvents. Other samples are candidates for the Volumetric Karl Fischer method.
Did This Answer the Question -- When To Use The Coulometric or Volumetric Karl Fischer Method?
Hope this helped shed a little light on the comparative benefits of Coulometric and Volumetric Karl Fischer.
These complex factors constantly remind me that if we don't pay attention to the details of testing we can create unintended problems.
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