Petroleum products are mixtures of long-chain or aromatic hydrocarbons. They are hardly soluble in methanol. Water determination by Karl Fischer therefore requires the addition of solubilisers. For light oils, long-chain alcohols are suitable. For dissolving of heavier oils toluene, xylene or chloroform are added. For the volumetric titration specifica Karl Fischer solvents for oils are available. Due to the very low ater concentration titrants with a low factor (2mg/ml or 1mg/ml) are recommended.

During coulometric determination without diaphragm 20% solubiliser can be added to the working medium, or 40% solubiliser to the anolyte in the case of coulometry with diaphragm.

Note that oils are often heterogeneous compounds with uneven distribution of water and should thus be homogenised prior to Karl Fischer determination.

Additives in oils can cause side reactions during Karl Fischer determination. Here, the direct coulometric analysis is not possible, the volumetric titration only conditionally. As an alternative, the Karl Fischer oven technique can be utilised in combination with coulometry, whereby the release of water is best achieved at temperatures between 120 and 140degree Celsius.