Difference between revisions of "Karl Fischer titration"

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Karl Fischer titration, often shortened to KF titration, is an iodometric titration technique for the determination of water content in a sample. It is based on the oxidation of sulfur dioxide by iodine in the presence of water.

In aqueous solution, sulfur dioxide and iodine react as follows (Bunsen reaction):

2H₂O + I₂ + SO₂ ⇌ H₂SO₄ + 2HI

However, in KF titration, other reactions dominate due to solvent interactions. Depending on the solvent, the ratio H₂O : I₂ may vary from 2 : 1 to 1 : 1.

The titration endpoint is signalled by the presence of free I₂. This can be detected colorimetrically (for non-coloured samples) by the yellow-to-brown colour transition, or, preferably, by electrometric means.

Reagent composition

Karl Fischer reagents contain, as a minimum, iodine, sulfur dioxide, and a suitable organic base, dissolved in a solvent.

Sulfur dioxide

Serves as the oxidisable species.

Normally present in large excess to allow for loss due to the low vapour pressure.

Iodine

Serves as the reducible species.

The iodine may be present in elemental form directly (I₂), for volumetric titration using a burette.

Alternatively, I₂ may be produced in situ by the electrolysis of an iodide salt. By measuring the electrical charge delivered, the amount of iodine generated can be calculated. This method is called coulometric titration.

Base

A base must be present in order to buffer the solution to drive the equilibrium. Fischer's original reagent used pyridine, however modern KF reagents tend to use other bases like imidazole or pyridine derivatives, due to the volatility, toxicity, and unpleasant odour of pyridine.

Solvent

Normally, the reagent components are dissolved in a primary alcohol such as methanol, ethanol, or methyl cellosolve (ethylene glycol monomethyl ether). Alcoholic solvents like these give 1 : 1 reaction stoichiometry.

In these cases, sulfur dioxide reacts with the alcohol to produce the corresponding alkyl sulfite, which is an intermediate in the reaction. Non-alcoholic KF reagents are also possible, however the reaction mechanism and stoichiometry changes, and the overall system becomes much more sensitive to sample composition. For this reason non-alcoholic KF reagents are seldom used.

Ethylene glycol and its monomethyl ether give more stable titrants than methanol, and therefore the titrant requires less frequent standardization.^[1]

Additives / co-solvents

Other additives are sometimes added to improve sample solubility, for example, chloroform.

Theory

Fischer's 1935 paper^[2] discussed pyridine adducts being key to the reaction, and assumes that the methanol used served only as a solvent. The equation presented in his paper was:

2H₂O + SO₂.2Py + I₂ + 2Py → H₂SO₄.2Py + 2(HI.Py)

where Py = pyridine = C₅H₅N.

The left hand side of this equation gives the ratio H₂O : I₂ : SO₂ : Pyridine = 2 : 1 : 1 : 4. However, this was later determined by Smith, Bryant, and Mitchell to be an incorrect ratio when methanolic solutions are used. They determined the ratio was H₂O : I₂ : SO₂ : Pyridine : Methanol = 1 : 1 : 1 : 3 : 1. This gives the following equations:

H₂O + SO₂ + I₂ + 3Py → SO₃.Py + 2(HI.Py)

SO₃.Py + MeOH → Py.HSO₄Me

Verhoef and Barendrecht later determined that the above equations were incorrect, and that the reaction actually proceeds via the monomethyl sulfite ion produced by the reaction of SO₂ and methanol:

2 CH₃OH + SO₂ ⇌ CH₃OH₂⁺ + SO₃CH₃^-

They also determined that pyridine's only role in the reaction is as a buffering agent. Therefore, pyridine can be replaced by other organic bases (denoted RN). Adding the base shifts the equilibrium of the above equation to the right:

CH₃OH + SO₂ + RN → [RNH]SO₃CH₃

The KF titration reaction can then be formulated as:

H₂O + I₂ + [RNH]⁺ SO₃CH₃^- + 2 RN → [RNH]⁺ SO₄CH₃^- + 2 [RNH]⁺ I^-

Notice that this equation follows the 1:1:1:3:1 ratio determined empirically by Smith, Bryant, and Mitchell.

However, the original Fischer stoichiometry, having H₂O : I₂ = 2 : 1 is correct in some circumstances when aprotic solvents are used. This probably explains Fischer's error, as he originally used benzene as a solvent. In some solvents, or mixtures of solvents, the reaction proceeds via multiple routes and therefore the overall reaction is non-stoichiometric.

This is a particular problem with non-alcoholic KF reagents because if a sample contains a protic solvent then the stoichiometry will change over the course of the titration since the solvent composition also changes. For this reason, KF reagents are typically made as alcoholic solutions. Long-chain alcohols form the intermediate alkyl sulfite less readily, so methanol is commonly used.

Practical notes

Preparation of the titrant

A simple Karl Fischer titrant can be prepared according to the USP <921> Method I (Titrimetric):^[3]

1. Add 125g iodine to a solution containing 670ml methanol and 170ml pyridine, and cool.
2. Place 100ml pyridine in a 250ml graduated cylinder in an ice bath, and pass in dry sulfur dioxide until the volume reaches 200ml. This solution will take on a yellow colour.
3. Slowly mix the two solutions and shake to dissolve any remaining solid iodine. This step is strongly exothermic.
4. Allow to stand overnight before standardisation and use

The methanol and pyridine used should first be dried over molecular sieves 3A. The sulfur dioxide can be generated by the reaction of sodium metabisulfite and an acid, and should be dried by bubbling through concentrated sulfuric acid.

The reagent should be stored in a glass container with a good lid to keep moisture out. Methanol based KF reagents like this deteriorate slowly over time, and must be re-standardised on each day they are to be used.

Standardisation of the titrant

Fischer's 1935 paper provides a simple method of standardisation of his reagent:^[2]

1. Dry some methanol thoroughly, for example, using molecular sieves. After drying, remove all of the drying agent.
2. Titrate 10ml of this methanol with the reagent to be standardised. This should be repeated several times and the average volume of titrant used.
3. Add a known concentration of water to a large volume of this methanol
4. Titrate this spiked methanol solution again multiple times

The difference in the two averages gives the amount of titrant that corresponds to the known amount of water added.

From this, the absolute water content of the spiked methanol should be determined (i.e. including the water that the methanol contained before adding the water) and recorded. Provided this methanol is stored carefully, it can be used to re-standardise the titrant in the future.

It is also possible to standardise the titrant against sodium tartrate dihydrate.

Performing the titration

The titration should ideally be performed in sealed apparatus to avoid ingress of atmospheric moisture. If a totally-sealed apparatus is not available, then the titration should be performed as quickly as possible.

One approach is to use a small flask fitted with a rubber septum through which samples and titrant can be injected using a syringe and needle instead of using a burette. The flask should have a drying tube to allow the pressure to equalise without allowing water ingress. The syringe and needle can be weighed using an analytical balance before and after addition to determine the quantities added with greater accuracy than volumetric measurements.

The use of strong magnetic stirring is advisable.

It is strongly preferable to perform the titration using methanol as the working medium, in order to keep the reaction stoichiometry constant. To the titration vessel, add a suitable volume of methanol (pre-dried over molecular sieves 3A), and perform a pre-titration to consume all remaining traces of water from the working medium. Then, add a measured quantity of sample and titrate again to determine the sample's water content.

The titration endpoint is indicated visually by the transition from yellow to brown. However, it can be very difficult to repeatably determine the endpoint by eye. A better approach is to suspend two small electrodes (ideally made from platinum wire) in the solution, and apply a voltage of about 250mV. By measuring the current, which increases dramatically when excess iodine is present in solution, the endpoint can be repeatably detected.

References

1. ↑ Peters, E. D. and Jungnickel, J. L. (1955) "Improvements in Karl Fischer Method for Determination of Water". Anal. Chem. 1955, 27, 3, 450-453. doi:10.1021/ac60099a041
2. ↑ ^{2.0 2.1} Fischer, Karl (1935) "Neues Verfahren zur maßanalytischen Bestimmung des Wassergehaltes von Flüssigkeiten und festen Körpern". Angew. Chem. 1935, 48, 26, 394–396. doi:10.1002/ange.19350482605
3. ↑ U.S. Pharmacopeia <921> "Water Determination", https://hmc.usp.org/sites/default/files/documents/HMC/GCs-Pdfs/c921.pdf

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