The Scientific*gear Blog

As a service provider of Karl Fisher testing apparatus, we see different moisture testing issues that many operators, managers, and even companies face. We have come to realize that helping operators become more knowledgeable about "the little things" can help boost confidence, improve performance and efficiency, and ensure accurate testing. 

Are you new to Karl Fisher Titration and just beginning to learn about moisture testing or has it just been a while since you had to pull the Karl Fisher Titrator out to run some tests?  Regardless of your reason we know how important it is to get up to speed quickly so you can be running tests and providing moisture test results to those who need them.

To help with this we have compiled a list of the 20 most critical questions to help operators navigate through the learning curve and gain a better understanding of Karl Fisher Titration.

Some examples of issues you will discover include:

1.  "Our Karl Fisher says 'OVER TITRATION' and the reagent is turning really dark. Why?”

2.  “Why won’t the instrument go into “Ready mode”?”

3.  “Results seem "all over the place", what should I do?”

4.  "We use a solids evaporator and we are getting ZERO moisture results. Why?"

5.  "How many tests can I run and when should I change out my reagents?"

Avoid unnecessary surprises by getting a copy of the complete list of questions and learn what the issues are and why they are important.

Simply put, drift is background moisture that the Karl Fisher titrator is detecting.  What is background moisture?  Well, it is moisture that the Karl Fisher titrator (specifically the detector electrode) is detecting inside the vessel -that’s not coming from your sample.  Drift or "background moisture" can be the result of having the titration vessel sitting idle for some time where moisture has slowly infiltrated and accumulated inside the vessel, or it may be the result of a leak that is allowing a small amount of moisture to enter the vessel continually.  Although we might like to think that the Karl Fisher titrator vessel is air-tight/moisture-tight, it is not.  Depending on how well the vessel is sealed there may be a little or there may be a lot of background moisture interference.  All Karl Fisher titrators deal with the drift issue.  Unfortunately drift cannot be completely eliminated but the good news is that it can be reduced, measured, isolated, and discarded from your test results.

Before a single test is run on a Karl Fisher titrator it must go into a “ready” mode.   But before the titrator can go into a “ready” mode it most likely will go through a “pre-titration” mode.  During the “pre-titration” mode excess drift (moisture) is detected and removed by the reagent inside the vessel.  A “ready” mode ideally will occur when the drift being measured is low and steady/stable – usually below .1 micro grams per second.  Once the drift becomes low and stable the Karl Fisher Titrator records the drift level and goes into a “ready” mode and will allow the operator to introduce a sample into the vessel.  Upon completion of the test the Karl Fisher titrator adds up all of the moisture detected over the duration of the test and subtracts out the known drift level that was also measured during the test.  This process of knowing what the drift was before the test allows the Karl Fisher Titrator to then determine and backout the drift -leaving only the moisture detected from the sample as your result.

There are multiple methods of moisture determination, including loss on drying, Karl Fischer titration, piezoelectric sorption, spectroscopy, and chilled mirrors among others. However, it is advantageous to use Karl Fischer (KF) titration in moisture analysis for the following reasons:

1. It is highly accurate and precise (Part Per Million Accuracy).
2. KF is specific to water determination. This specification is different from the other popular moisture analysis method, loss on drying (LOD), because LOD can detect the loss of any volatile substance. However, this specification is advantageous because it allows KF titration to work independent from volatile substances present in the sample
3. The process does not require large samples, which is typically truer with Loss on Drying where more sample is required to achieve higher accuracy and repeatability - which introduces another entirely different problem.
4. It does not require much time to perform an analysis since the samples are easy to prepare and the analysis itself is short in duration.
5. The method has a nearly unlimited measurement range (from 1ppm to 100%).
6. Karl Fischer titration can determine the moisture content of a sample in any state, whether it is a solid, liquid, or gas.

We hope the above advantages show some of the benefits that Karl Fischer titrators can provide.  Even today with technological advancements Karl Fisher Titration remains very popular not only because of the advantages we mention, but also because it is widely accepted as a standard for moisture detection and measurement.  

This is a popular question for most operators using a coulometric Karl Fischer titrator.  So let's get started.  There are two things to consider.  First, you have the chemical limitations of the reagents themselves.  Second, you have the user/operator variable. Sometimes changing the reagent has more to do with the condition of the reagent sitting in the vessel.  How full is the vessel after running numerous test? How long has the reagent been sitting in the vessel? How messy is the reagent and sample residue inside the vessel? Sometimes the user may simply want to replace the reagents because they look dirty/messy or their vessel is too full from adding samples during previous tests.

Setting aside those factors just mentioned, if we look at the reagents themselves and their capacity to measure moisture, we can come up with a general guideline as follows:

Note:  This example describes a Coulometric Karl Fischer Titrator with dual reagent setup (using Anolyte and Catholyte)

1.  In general and with regard to reagent brand, 100mL of Anolyte (AKA Anode- the reagent used in the vessel) reagent analyzes up to 1gram (1 million micro grams) of water.  20mL of Catholyte (AKA Cathode- the reagent used in the generator electrode/inner buret) reagent analyzes up to 1gram (1 million micro grams) of water.  The relationship according to the amount of water each reagent can analyze has a relationship of 100mL Anolyte to 20mL Catholyte – a 20% relationship of catholyte to anolyte.

2.  Anolyte is commonly purchased in 500ml bottles, Catholyte is commonly purchased in10x5mL ampules.

3.  A typical coulometric Karl Fischer titrator Vessel is charged with 75mL of anolyte and 1ea 5mL catholyte ampule.  Based on the 20% relationship it says that 3x5mL catholyte ampules would be used with each 75mL vessel charge of Anolyte.

4.  A 500mL bottle of Anolyte can charge the Coulometric Karl Fischer titrator vessel 6.6 times (round to 6 times to account for spillage).  3x5mL Catholyte ampules per charge of the vessel times 6 charges of the vessel = 18x5mL catholyte ampules.

Typically users do not expire the entire useful life of the reagents moisture measuring capability because of some of the factors I mentioned initially.  Another factor that I have to mention is that ambient moisture will require the coulometric Karl Fischer Titrator to maintain a dry vessel.  This process of keeping or getting the vessel in a ready to go mode can use some of the reagents useful life.  e.g. it’s not counting the moisture in your samples but that of the outside ambient moisture – for the most part this should be a small amount, but something to keep in mind and know about.

So, with all of this information, the question you may have is how long will my reagents last?  Well, that depends.  But, if you want to continue using the math we have already discussed, then, 1 charge of the vessel (75mL of anolyte with 3x5mL catholyte ampules) can measure 750,000 micro grams of water.

And, for those of you who think in Parts Per Million (PPM) you can translate into the  following:

PPM = micro grams of H2O detected / Your Sample Size (in grams)

A simple method is to run a water standard through the Karl Fischer Titrator like a normal direct injection test. Depending on the water standard you use, the result should equal a pre-determined level of moisture plus or minus a margin for error. These water standards are certified by the manufacturer (a certificate is included) to equal a precise level of moisture.

We use Hydranal water standards. There are two kinds we typically use for coulometric Karl Fischer Titrators.

1) 0.1 normal

2) 1.0 normal.

The 0.1 normal administered at about 1mL should result in 100ppm (Parts Per Million) of moisture when measured. The acceptable result for this standard for the Karl Fischer titrator is +/- 10%. So your Karl Fischer Titrator should produce a reading between 90ppm and 110ppm to be in the acceptable range. If it is, you know your Karl Fischer Titrator is performing correctly.

For the 1.0 normal everything is the same except the standard should result in 1,000ppm and your acceptable range is smaller at +/-3%. So your Karl Fischer Titrator should produce a reading between 970ppm and 1,030ppm.

We get this question a lot.  "How do you go about testing for moisture if the sample is in liquid form?..What about solid form?"

Well basically, moisture testing using the karl fischer method is a standard in the industry that measures down to the Parts per million (PPM) level.  The theoretical accuracy is down to 1 part per million level.  I say theoretical because usually any variance is due to atmospheric conditions and operator repeatability.  Specifically, and for this example, “coulometric” Karl Ficsher is best when you are using a small sample and expect and are trying to measure less than 1% (1%moisture =10,000PPM) of water (moisture/humidity) in your sample.  [Note: there is a volumetric Karl Fischer method vs. coulometric Karl Fisher method but for this discussion I am speaking from the coulometric Karl Fischer standpoint]

With this in mind,

A. If you are testing a liquid sample you only have to use the karl fischer titrator and directly inject the liquid sample with a syringe (usually around 1mL) into the vessel.

B. If you are testing a solid sample (that cannot be “broken down sufficiently with solvents like Xylene for example) you will use both the karl Fischer AND an evaporator oven.  The evaporator oven heats up the sample (usually the sample size is less than 1gram…we typically might use 1/10th of a gram..but then again we might use 3 grams -it really depends on how much moisture you expect to find). The evaporator is connected with a nitrogen gas source that is used to deliver the moisture via a heater tube on the evaporator into the titration vessel.

To see an actual demonstration of the Karl Fischer Titrator and the evaporator oven during an actual test please view the video below and watch the short 2 to 3 minute video.

To be sure there are many more things I could mention but this is a high level summary of the two approaches.

Hope this helps.