As you know the Karl Fischer Method of moisture analysis has a reputation of being water specific. The method works through the use of a special Karl Fischer Reagent.
Basics of Karl Fischer
As a quick review, the material to be tested is dissolved in a solvent. The water is released and converted by the reagent. This process happens inside an enclosed airtight titration cell.
The amount of reagent needed to make full conversion is a measure of moisture. Note that the material to be tested is dissolved in the presence of the reagent.
When Basic Karl Fischer will Not Work
As with many testing methods there are complications. Some substances are difficult to dissolve and require solvents that operate on the Karl Fischer reagent to cause side reactions that distort the water content calculation. Other materials only release the water at high temperatures. In these cases the simple process of dissolving the sample in the presence of the Karl Fischer reagent won't work.
Solution to the Problem Samples
The answer to side reaction problems or high temperature exigencies is an instrument known as a Karl Fischer Oven or Evaporator. A Karl Fischer Oven consists of a heating tube in which the temperature can be controlled between 60ºC to 300ºC. Provision is made for a carrier gas to flow through this heating tube and move the escaping water into the titration cell. When the sample is ready to be tested, it is placed into the heating tube (operating at the appropriate temperature for the sample). As the moisture is released, the vapor is transported by the gas (usually dry Nitrogen) to the titration cell where it is bubbled into the reagent. The Karl Fischer process is completed and the moisture content calculated. Some of the materials that need to be processed in an oven are plastics and salts. These Karl Fischer Oven/Evaporators are used with standard Karl Fischer Titrators. When the moisture content is low (<1%) the Coulometric technique is recommended, otherwise a Volumetric Karl Fischer titrator is used.
For more information on the Karl Fischer Moisture Method click here.
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Another case where the simple has been complicated by reality.
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As I've said many times measuring Moisture Content, Surface Tension and Particle Size often confounds me.
Moisture measurements are nothing compared to the measurements needed to check out the Universe. Came across this video and thought you'd fnd it interesting and fun.
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.
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.
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 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
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.
If this was useful, feel free to share this with any colleagues who my find this interesting or helpful.
I remain your confused correspondent,
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Last week, as I was reflecting on a recent moisture content problem, I recalled our series “Loss-on Drying and Other Moisture Mysteries.” In that series I examined moisture chemistry in products. However, I did little to define moisture content.
In the world of material testing, moisture content is different things to different people. However, there is a common concern. Manufacturers, distributors and retailers each are concerned with how moisture content affects what they are making, shipping, storing or selling. This will be today's focus.
My experience with checking for moisture in numerous materials and in different environments makes me conclude that moisture levels, measurement precision and absolute moisture amounts are moving targets. Moisture content standards for individual products, services or environments vary greatly.
What is important involving moisture content? Sometimes it is process yield. Sometimes it is the economics of water vs. other components of products. It could also be health and safety or a unique measure of final quality. Sometimes it is what takes place over time.
Considering all that, we wonder how best to determine total water. When the complexity of that task becomes apparent, we are tempted to question whether we care.
Let us indulge in a short digression to the main techniques we use to get closer to some answers. We dry materials until they stop losing weight and tentatively assert that the difference between the starting and ending weights is that of the water. Stop! Are we certain that other volatiles did not leave with the water or decompose with the heat?
We have of necessity devised other ways of accounting for these possibilities. Chemical techniques like Karl Fischer Titration, desiccation, freeze-drying, distillation and chromatography are suitable alternatives.
Some users are oriented toward water activity, a technique that integrates temperature, vapor pressure, dew point and relative humidity to obtain accurate data
Frequently the results from each of these techniques differ from one another. It would seem, then, that determining “how much water” is technically challenging. An easy, catch-all method to get at water content levels remains elusive.
In the end, we must compromise rather than seek absolutes, focusing on what actually works for each product, process and end result desired.
Hope this stimulates some new solutions to your individual moisture content problems.
I continue to prove to myself, how little I know about the enigmas of determining moisture content.
I remain a mystified,
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“Why are my Moisture Test results inconsistent?”
That is an issue for many of you who test for moisture. We discussed the complexities and multiplicity of issues involved with moisture content determination in our “Loss-on Drying Moisture Analysis and other Moisture Mysteries” series.
In addition to intrinsic properties of test samples that may adversely affect moisture testing systems, automatic equipment parameter set-up, operator oversights and sample handling contribute to seemingly intractable moisture test result inaccuracies.
Common test sample vagaries include:
- Volatiles other than water that are released at close to the same vapor pressure as water evaporation.
- Strange conditions of entrapped moisture that release water at capricious times.
- Samples not representative of the principal batch
It is important to determine if these test sample quirks are responsible, because test protocols may require changing. Variations can also be minimized by running more tests to statistically reduce variation effects. In some cases, changes in test methods may be needed (i.e. Karl Fischer rather than Loss-on-Drying).
While trouble-shooting such problems, it is important to check automatic setting level and operation, which determine when a test is completed. With the Loss-on Drying method, these settings relate to measuring sample weight changes. If the instrument is set to stop too soon, the weight-loss curve will slope steeply and the moisture result will be subject to irregular variations from test to test. If the end-of-test calculation is based upon too small of a weight change, there is a potential for burning the sample -- another cause of inconsistency.
Similar problems can arise if widely different test-to-test sample weights are used in a timed test environment.
Another source of inconsistent analysis can be that a small amount of moisture requires detection. A small amount of sample and a very sensitive balance are frequently used for this type of test. Operators must carefully follow test procedures or the balance’s high sensitivity will yield wide result variations from test to test ( Often A switch to the Karl Fischer Method will Solve this Problem).
As equipment manufacturers, we will ultimately consider the possibility of instrument malfunctions. Our experience with instrument service leads us to either clear instrument failure or consistent high or low results signaling equipment problems and not just inconsistent results.
Our troubleshooting protocols require duplicating clients’ problems in our lab. When we cannot, experience leads us to consider environmental conditions at our customers' facilities. Frequently, we find electrical power conditions to be responsible. Special power-conditioning equipment will usually solve this problem.
On occasion, we cannot find the sources of variation.
In summary, issues relating to test sample properties and/or or incorrect test parameters normally yield inconsistent results. Occasionally it is operator error. However, on rare occasions, test site environmental conditions are responsible.
Sometimes it requires painstaking investigation to find the cause. When found, we can usually then develop workable solutions.
I hope this sheds some light on the sources of inconsistent test results.
As usual, there is still a bit of witchcraft and folklore needed to solve the more elusive measurement problems.
Still trying to get answers, I remain a puzzled,
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When People are first introduced to the Karl Fischer Moisture Determination Method, eyes glaze over and we can perceive a mental “Why did I Ask?”
If you have any history with moisture analysis, you will have found, that for some applications, the Karl Fischer Titration Method is the best and sometimes the only way to get an accurate moisture measurement.
Most non-chemists react to the thought of titrations with “Not for me – do I have to do this?” Even the chemical formula for the Karl Fischer reactionis mind blowing.
However, these days most of the pain is removed with automatic Karl Fischer instruments such as our Aquapal III, that make conducting Karl Fischer tests quite simple. There are still times when your not sure if the equipment is reading correctly or when it gives arcane messages like "Over Titration" that you feel panic. A host of other subtle problems bring on the question “What does this Mean?”
Recently Hank Levi, a colleague of ours at Scientific Gear, developed a list of 20 questions that are periodically asked. He also developed concise answers. They are questions like;
- Why won't my instrument get to the Ready Mode?
- What kind of reagent should I use?
- Is my instrument giving me correct results?
- How much sample should I use?
We would like to share these with you. If you click on the button you can down-load the full list of Karl Fisher questions and answers.
Hank tells us that some of his customers post this list in the lab where it is available for reference at any time to the staff conducting Karl Fischer moisture determinations.
Maybe this will help reduce trepidation about Karl Fischer and help get good repeatable results.
We hope that you check out this list and that it is useful to you. Again thanks for visiting.
I remain a wary respectful friend of Karl Fischer,
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Loss-On-Drying moisture analysis seemed like a simple process until, in a state of naïve bliss, I promised to look at evaporation, vapor pressure and bound water. While I was otherwise occupied with these realities, I offered to enter the world of witchcraft and folklore; water activity.
I offer what will hopefully be an uncomplicated definition:
The energy or escaping tendency of water.
I would be happy if I could leave it at that, but I am compelled to relate water activity to good old Loss-On-Drying. Unfortunately, the concept begins in the complex world of Boyle, Charles and Dalton and their gas laws. These populate the Ideal Gas Law with considerations of pressure, partial pressure, temperature (at Kelvin no less) volume, molecules and moles.
Herein is my attempt to integrate these physics/chemistry phenomena to formulate a comprehensible description of water activity.
Let us first reflect upon a question I recently asked:
“Should we care about the presence and amount of bound water?”
The answer is very often yes and the reason frequently involves water activity.
There are many reasons water activity (aw) is important. If it is too high, it can cause spoilage, browning, mold growth, clumping and a host of other unpleasant effects. In fact, excessive aw can screw up a perfect blend of fruit and breakfast cereal (dried up fruit and soggy corn flakes).
It seems that water has an energy quotient that can lead it to enhance chemical reactions, cause bad things like bacteria growth or mix with other materials to mess up a good combination of components. Moisture content alone is not a predictor of this energy, however.
Each material has a natural relationship between moisture content and water activity, called its Moisture Sorption Isotherm (MSI) defined as:
“The relationship at equilibrium between water content and the equilibrium humidity of a material.”
This is effectively a moisture fingerprint. These isotherms change with temperature so it is not a static attribute.
The implications of water activity in the food industry are related to shelf-life, contamination, health, texture and taste issues. Thus, the aw measurement is becoming an ever-increasing factor in food product design and food process quality control.
Also, water activity is becoming a serious consideration in the development and production of pharmaceutical products. It is water activity and its relationship to moisture content [not moisture content alone] that determines whether microorganisms can access water in a system, adding an important dimension to production process control.
The relationship of aw to moisture content is likewise of growing importance in other products where water action affects either the production process or a product’s physical characteristics.
Water activity measurement is a relative humidity technique; a comparison of a sample’s vapor pressure at equilibrium to that of pure water. The measurement consists of placing a sample in a closed space, waiting for equilibrium to be reached and then measuring the resulting relative humidity in the air space. This is done using a calibrated capacitance cell or a chilled mirror (a technique that gets a dew point and converts that to relative humidity). The aw number is the percentage of relative humidity divided by 100.
Knowing the MSI of a product, you can convert moisture content measurements to water activity. Loss-On-Drying results can be converted to water activity for many products. When the Loss-On-Drying process removes only -- and all -- of the water, a conversion of the moisture percentage to awcan be made with the MSI for the product.
In my musings and reflections on the science of the gas laws and mystical scientific witchcraft of water activity, I decided that to get you past a superficial understanding of water activity, you need a more expert source.
To get in-depth understanding about the action of water activity and how the measurements are used, I recommend Dr. Ted P. Labuza (firstname.lastname@example.org) at the University of Minnesota. He is an internationally recognized expert on water activity. You can find a bio of Dr Labuza and a link to his publications at this site.
I hope this helped in some way to cultivate an appreciation of the implications of water activity and the relationship of aw measurement to loss-on drying.
And I tried one more. Take a look.
As usual I remain a confounded,
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In my previous missive about Loss-On drying, we discussed Vapor Pressure -- because logically it was next. As we continue to explore moisture, we learn how vital vapor pressure is when regarding the quirky issues of free and bound water.
The easy part of the loss-on drying concept is free [or unbound] water. This is water in or on the surface that will evaporate with a moisture balance.
Things get tricky when we consider bound water, which may be caught in capillaries, fibers or held onto via chemical reactions.
You may recall that when water vapor pressure is at equilibrium, nothing evaporates. When the sample is heated, the vapor pressure rises and the water will begin to evaporate -- which does a neat job of reducing the sample’s weight. This weight stops changing when all of the water is gone, allowing us to determine the amount of water via the attendant weight change.
But wait! All of the water may NOT be gone. Bound moisture -- which, it turns out, has a lower Vapor Pressure than the pure water we just removed -- did not evaporate. A paradox perhaps; we seemingly got rid of all the water but maybe did not because the bound water still remained.
How do we know if there is any of this bound water left? The answer depends upon the test material’s hydro-characteristics. Hygroscopic materials attract water and find ways to bind the moisture while non-hygroscopic [or hydrophobic] materials keep water out of the capillaries, the fibers and the chemical reactions.
Some examples of hygroscopic are biological materials, sugar, and some engineered polymers. Hydrophobic examples are hydrocarbons, fats and lipids.
There are exotic ways to determine if and how much bound water remains. There also are intricate techniques to remove most of this residual.
Before attacking these, a question should be asked: should we care about the presence and amount of bound water?
I need a rest before dealing with these complexities, so I'm deferring further ‘enlightenment’ until another time.
Now we know that my original assumption that Loss-On drying was an easy means toward moisture testing is wrong. It is further complicated by abstruse manifestations such as vapor pressure as well as moisture binding to material via chemical reactions, capillaries and fiber.
Again we find that certain things are not as simple as they might seem.
I tried one more - Scary. Check it out
Until next time I am a bewildered,
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A short ramble.
Most of us think about measuring moisture by...
- Drying the material and measure the weight loss
- Using a calibration and electronic instrument
- Using a titration method such as Karl Fischer
When using these processes, we take for granted that the results will be the product's actual moisture.
Not always so.
Recently, I was reading about a procedure the Brazilian Agriculture Ministry (BAM) developed to get a very, very tight tolerance moisture determination for coffee beans.
The BAM procedure specifies placing the coffee beans in an oven for a long time -- 104° C for 24 hours. This test was the standard against which all other coffee moisture determination methods were compared.
However, it was found that significant amounts of other volatiles were being released during the 24-hour process, which resulted in higher-than-expected moisture readings. The oven test standard thus over-stated the actual moisture.
The BAM also used an oven method using ground beans and an infrared loss-on-drying instrument. As you may know, the grinding process frequently causes moisture loss.
In addition to two oven methods and the infrared loss-on-drying instrument, a Karl Fischer Titration was used. The Karl Fischer Titration measures only moisture and eliminates the effect of volatiles. Because the coffee beans need to be dissolved in a solvent media, the beans had to be ground -- thus inducing the aforementioned grinding error to this moisture analysis.
(If an evaporator oven had been used in concert with the Karl Fischer Titration, the whole beans would have been heated to a high temperature. The volatiles and moisture would then have been extracted and the residual gas collected in the Karl Fischer unit. Only the moisture would be measured using this method, thus eliminating the grinding error.)
The moisture range in the beans tested was 4.2% to 13.5%. Based upon these analyses, two significant inferences were made:
1.The effect of volatiles evaporating is more significant as the moisture levels decrease. This indicates that volatile material amounts tend to be the same at all moisture levels, thus representing a greater weight loss proportion for lower moisture samples.
2.Grinding causes moisture loss, which becomes a greater proportion of the total for lower moisture samples. In these lower moisture samples, it was found to be more significant than the volatiles.
Most of our solids moisture measurement applications are low volatile situations. When grinding is needed, the processes usually have a wide enough tolerance to not require grinding loss corrections. Even though most of you do not have to contend with this kind moisture test adjustment, I thought you would be interested in the implications of these factors on the true moisture level in a sample.
Let me know if you have questions or comments.