Karl Fischer titration

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.

Sulfur dioxide

Serves as the oxidisable species.

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.

Additives / co-solvents

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

Theory

Fischer's 1935 paper 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.