FAQ - Karl Fischer Titration - Water determination according to Karl Fischer explained
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FAQ Karl Fischer Titration

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Water determination according to Karl Fischer explained in this FAQ Karl Fischer Titration

FAQ Karl Fischer Titration
FAQ Karl Fischer Titration

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In the following FAQ - Karl Fischer Titration, our experts are giving the answers to the most common questions concerning titration with METTLER TOLEDO Compact titrators and Karl Fischer titration in general.


This FAQ - Karl Fischer Titration page will be expanded periodically. So revisit often!

 


What is the difference between volumetric and coulometric Karl Fischer Titration?

The titrant can either be added directly to the sample by a burette (volumetry) or generated electrochemically in the titration cell (coulometry).
The coulometric titration is mainly employed for the water determination according to Karl Fischer when the content is very low, e.g. smaller than 50-100 ppm (0.005-0.01%).


 

When should I use the cell with or without diaphragm ?


The C20 and C30 are available with two different coulometer cells – with or without a diaphragm. For most applications, we recommend the cell without the diaphragm because it is almost maintenance-free. Due to its innovative design, this diaphragm-free cell from METTLER TOLEDO can even be used for the determination of water in oils.
The version of the cell with a diaphragm is recommended for applications such as the determination of water in substances containing ketones. It is also recommended if the best possible accuracy is required.


 

How often do I need to standardize my titrant?


Naturally, this depends on the stability of the titrant and on what measures have been taken to protect the titrant from the typical contaminants that could cause a reduction in concentration. The most common examples of this titrant protection are the storage of light sensitive titrants in dark bottles e.g. iodine solutions, the protection of Karl Fischer titrants from moisture using e.g. molecular sieve or silica gel, and the protection of certain strong bases e.g. sodium hydroxide, from absorption of carbon dioxide.


 

How do I know when to replace the molecular sieve in the drying tubes on my Karl Fischer titrator?


The most practical solution to this question is to add some blue silica gel to the top of the drying tube to serve as an indicator. As soon as the first trace of pink appears in this gel layer, it is time to change or regenerate the molecular sieve. Naturally, an increase in background drift can also indicate that it is time to replace the molecular sieve.


 

How do I go about validating a method on my automatic titrator?


When validating a titrator method one needs to check things like accuracy, precision, reproducibility, linearity, systematic errors, robustness, ruggedness and limits of determination. For detailed recommendations on how to go about this validation please refer to our section on Quality Control, Validation or refer to the METTLER TOLEDO applications brochure 16 - Validation of Titration methods.


 

What is the best way to standardize my Karl Fischer titrant?


For standardizing Karl Fischer reagents one would expect the best standard to be pure water. Water, however, does not satisfy the requirements of a primary standard in that it is not stable during weighing and does not have a high enough molecular weight. The second point raises the problem of accurately weighing a sufficiently small sample so as to give a reasonable titrant consumption.

As an alternative to pure water, certified standard solutions can be obtained in various concentrations ranging from 0.1mg to 10mg water per g (or per mL). This allows the weighing out of a more suitable sample size.

A third possibility is to use a solid sample with exactly known water content. The most common standard here is sodium tartrate dihydrate. This standard contains two waters of crystallization giving it a water content of exactly 15.66%. The advantage of this standard is that it is available as a finely ground powder with a stable and guaranteed water content. Since the water content is only 15.66% as opposed to the 100% pure water, one is able to weigh a reasonable sample size with sufficient accuracy to ensure a good titer determination. The only drawback of this standard is thatit has limited solubility in methanol, which is the most common Karl Fischer solvent used. As a rule, approximately 0.15g of standard will dissolve in 40mL of methanol. Increasing results for the concentration determination on successive samples indicates incomplete dissolution. Provided this limited solubility is taken into account, the sodium tartrate dihydrate is the standard of choice for Karl Fischer reagent concentration determination.


 

What resolution should my balance have to ensure that I get accurate and precise results?


The answer to this question depends on many things such as expected result and homogeneity of the sample, both of which will determine the optimal sample size, required number of decimal places for the final result, and off course the required accuracy of the final result. As a general rule however, one should have at least 4 significant figures for the sample weight. Below are some recommendations:

Sample size Minimum number of decimal places
1-10g ..................................3
0.1 - 1g ...............................4
0.01 - 0.1g .........................5


 

How often do I need to replace the solvent in the titration beaker of my Karl Fischer titrator?


The first and most obvious answer to this question is that the solvent should be replaced as soon as the sample no longer dissolves. This, however, is only one of the reasons for changing the solvent. A second less obvious reason applies in the case of two component reagent where the titrant contains iodine and the solvent contains all the other components necessary for the Karl Fischer reaction. One of these other components is sulfur dioxide and this can become depleted long before the dissolving capacity of the solvent is exceeded. As a general rule the solvent in these two component systems has an approximate water capacity of 7mg of water per mL of solvent. This means that in theory 40mL of solvent can accommodate 280mg of water before the solvent need be changed. As the typical titrant has a concentration of 5mg/mL, 280mg of water would require 56mL of titrant.
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