What is the difference between volumetric and coulometric Karl Fischer titrations?
The titrant can be added directly to the sample through a burette (volumetric method) or generated by an electrochemical process in the titration cell (coulometric method).
Columetric titration is predominantly used for the determination of water (mass) composition, where the water content is less than 50-100 parts per million (ppm) or 0,005-0,01 % according to the Karl Fischer titration methods.
The titrant can be added directly to the sample through a burette (volumetric method) or generated by an electrochemical process in the titration cell (coulometric method).
Columetric titration is predominantly used for the determination of water (mass) composition, where the water content is less than 50-100 parts per million (ppm) or 0,005-0,01 % according to the Karl Fischer titration methods.
When should I use a coulometric titration cell with a diaphragm and when without?
The C20 and C30 titrator models are available with two different coulometric cells, with and without diaphragm. In most applications, we recommend using a cell without a diaphragm because it has no or very low maintenance costs. The innovative design of the cell without diaphragm from METTLER TOLEDO allows it to be used for the determination of water content in oils.
The diaphragm cell model is recommended for the determination of water content in solids containing ketones. Another reason for using the diaphragm cell is for increased precision measurements, especially when required or necessary.
How often should the titrant be changed?
Basically, it depends on the stability of the titrant itself and how the titrant is protected from the presence of possible impurities that can both alter the composition of the titrant and reduce the concentration of the active substance in the titrant.
Most often, as an example of titrant protection, it is the light-sensitive titrants that need to be kept in a closed and dark storage room or stored in dark bottles, an example being iodine-containing liquids.
Karl Fischer titrants and certain basic substances such as sodium hydroxide (NaOH) are protected from moisture by molecular sieves or SiO2 gels, which also protect against absorption of carbon dioxide (CO2).
How can I know when to change the molecular sieve for drying the titrant?
The most practical solution would be to add a blue SiO2 gel at the very top of the drying tube to serve as an indicator. As soon as the gel layer shows signs of pink or similar shade it is time to add a new or renewed molecular sieve. Certainly an increase in background drift may also indicate a good time to change the molecular sieve.
How is the method validation done in the automatic titrator?
When validating a titrator method there are a number of parameters that must be checked, such as -precision (the result obtained versus the theoretical value of the result obtained), accuracy (the result between the same measurements of the method), reproducibility (the possible repetition of the results obtained by the method), linearity of the results, system errors, the influence of side effects on the method results, limits to the determination of the results.
What is the best way to obtain a Karl Fischer titrant standard?
For the Karl Fischer method, the best reagent standard would be pure water. However, water does not satisfy the primary requirement of stability at the time of weighing and does not have a large enough molecular weight. Due to the low molecular weight, there is a major problem in weighing a low mass sample accurately to obtain a numerically reliable titration result.
As an alternative, the use of certified standards with a known amount of pure water content is proposed. The water composition can range from 0,1 mg to 10 mg per gram (or 1 mL) of the whole standard. This allows a more suitable sample for titration.
A third option is to use a solid sample with a known water content. The most common standard for this method is Sodium Tartrate Dihydrate (C4H4Na2O6). This standard contains two crystalline water molecules (per mole), giving a water content of exactly 15,66 % for the whole standard. The advantage of this standard is its availability in a homogeneously ground powder with a stable water content value. Knowing that the water content is only 15,66% compared to 100% pure water it is possible to make a reasonable weighting of the sample mass and ensure sufficient precision and good titration results. The only disadvantage of this standard is its limited dissolution in methanol, the most commonly used Karl Fischer titration solvent. In reality, about 0,15 g of this standard will dissolve in 40 mL of methanol. It was observed in the experiments that increasing titration concentrations resulted in incomplete dissolution of the finished samples. In view of this limited solubility in titration, it is Sodium Tartrate dihydrate that is the best choice of standard for determining the concentration of Karl Fischer reagents.
What should be the resolution of the scales to ensure consistently accurate results?
The answer to this question depends on many different things, such as the expected results and the homogeneity of the sample. These two factors together determine the recommended sample volume, the number of decimal places for the sample weight and finally the accuracy of the final results. The main rule is that the number of decimal places should be at least 4 significant digits before the sample weight. Examples of the recommendation are given below:
Sample weight Minimum number of decimal places for the result
1-10g ...................................................................................3
0.1 - 1g ................................................................................4
0.01 - 0.1g ...........................................................................5
How often do I need to change the solvent in the titration cup of the Karl Fischer titrator?
The first and most obvious answer is: the solvent should be removed as soon as the sample is no longer soluble in it. This is basically the only reason to change the solvent. The second and slightly less important reason concerns the two-component reagent, in which one of the titrants is iodine and the solvent consists of all the components necessary for Karl Fischer titration. One of these components is sulphur(IV) dioxide SO2 and its concentration can decrease long before the dissolution capacity of the solvent is exhausted.
In particular, it should be noted that the water capacity of these two solvent components is 7 mg of water per 1 mL of solvent. This means that theoretically 40 mL of solvent can accumulate 280 mg of water before the solvent has to be changed. A typical solvent concentration is 5mg/mL, which means that reactions with 280 mg of water would require 56 mL of such a titrant.
The C20 and C30 titrator models are available with two different coulometric cells, with and without diaphragm. In most applications, we recommend using a cell without a diaphragm because it has no or very low maintenance costs. The innovative design of the cell without diaphragm from METTLER TOLEDO allows it to be used for the determination of water content in oils.
The diaphragm cell model is recommended for the determination of water content in solids containing ketones. Another reason for using the diaphragm cell is for increased precision measurements, especially when required or necessary.
How often should the titrant be changed?
Basically, it depends on the stability of the titrant itself and how the titrant is protected from the presence of possible impurities that can both alter the composition of the titrant and reduce the concentration of the active substance in the titrant.
Most often, as an example of titrant protection, it is the light-sensitive titrants that need to be kept in a closed and dark storage room or stored in dark bottles, an example being iodine-containing liquids.
Karl Fischer titrants and certain basic substances such as sodium hydroxide (NaOH) are protected from moisture by molecular sieves or SiO2 gels, which also protect against absorption of carbon dioxide (CO2).
How can I know when to change the molecular sieve for drying the titrant?
The most practical solution would be to add a blue SiO2 gel at the very top of the drying tube to serve as an indicator. As soon as the gel layer shows signs of pink or similar shade it is time to add a new or renewed molecular sieve. Certainly an increase in background drift may also indicate a good time to change the molecular sieve.
How is the method validation done in the automatic titrator?
When validating a titrator method there are a number of parameters that must be checked, such as -precision (the result obtained versus the theoretical value of the result obtained), accuracy (the result between the same measurements of the method), reproducibility (the possible repetition of the results obtained by the method), linearity of the results, system errors, the influence of side effects on the method results, limits to the determination of the results.
What is the best way to obtain a Karl Fischer titrant standard?
For the Karl Fischer method, the best reagent standard would be pure water. However, water does not satisfy the primary requirement of stability at the time of weighing and does not have a large enough molecular weight. Due to the low molecular weight, there is a major problem in weighing a low mass sample accurately to obtain a numerically reliable titration result.
As an alternative, the use of certified standards with a known amount of pure water content is proposed. The water composition can range from 0,1 mg to 10 mg per gram (or 1 mL) of the whole standard. This allows a more suitable sample for titration.
A third option is to use a solid sample with a known water content. The most common standard for this method is Sodium Tartrate Dihydrate (C4H4Na2O6). This standard contains two crystalline water molecules (per mole), giving a water content of exactly 15,66 % for the whole standard. The advantage of this standard is its availability in a homogeneously ground powder with a stable water content value. Knowing that the water content is only 15,66% compared to 100% pure water it is possible to make a reasonable weighting of the sample mass and ensure sufficient precision and good titration results. The only disadvantage of this standard is its limited dissolution in methanol, the most commonly used Karl Fischer titration solvent. In reality, about 0,15 g of this standard will dissolve in 40 mL of methanol. It was observed in the experiments that increasing titration concentrations resulted in incomplete dissolution of the finished samples. In view of this limited solubility in titration, it is Sodium Tartrate dihydrate that is the best choice of standard for determining the concentration of Karl Fischer reagents.
What should be the resolution of the scales to ensure consistently accurate results?
The answer to this question depends on many different things, such as the expected results and the homogeneity of the sample. These two factors together determine the recommended sample volume, the number of decimal places for the sample weight and finally the accuracy of the final results. The main rule is that the number of decimal places should be at least 4 significant digits before the sample weight. Examples of the recommendation are given below:
Sample weight Minimum number of decimal places for the result
1-10g ...................................................................................3
0.1 - 1g ................................................................................4
0.01 - 0.1g ...........................................................................5
How often do I need to change the solvent in the titration cup of the Karl Fischer titrator?
The first and most obvious answer is: the solvent should be removed as soon as the sample is no longer soluble in it. This is basically the only reason to change the solvent. The second and slightly less important reason concerns the two-component reagent, in which one of the titrants is iodine and the solvent consists of all the components necessary for Karl Fischer titration. One of these components is sulphur(IV) dioxide SO2 and its concentration can decrease long before the dissolution capacity of the solvent is exhausted.
In particular, it should be noted that the water capacity of these two solvent components is 7 mg of water per 1 mL of solvent. This means that theoretically 40 mL of solvent can accumulate 280 mg of water before the solvent has to be changed. A typical solvent concentration is 5mg/mL, which means that reactions with 280 mg of water would require 56 mL of such a titrant.