Determination of Water Content in Biodiesel Using Karl Fischer Titration

Product Group

Esters of carboxylic acids, Petroleum products

General Information concerning the product group

Esters of Carboxylic Acids

Esters of carboxylic acid can be analysed analogously to the hydrocarbons according to standard Karl Fischer methods. Long-chain compounds require the addition of solubilisers, e.g. long-chain alcohols or chloroform. Unsaturated compounds can usually be titrated in the same way. Recommended methods are both the volumetric titration with one or two component reagents, as well as the coulometric analysis. The latter is predominantly applied for low water concentrations (< 0.1 %).

Petroleum Products

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 specific KF solvents for oils are available. Due to the very low water concentration titrants with a low factor (2 mg/mL or 1 mg/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 (e.g. with Ultra-Turrax) prior to KF determination. Additives in oils can cause side reactions during KF determination. Here, the direct coulometric analysis is not possible, the volumetric titration only conditionally. As an alternative, the KF oven technique can be utilised in combination with coulometry, whereby the release of water is best achieved at temperatures between 120 and 140 °C.

Special Information concerning the sample and the methods

Biodiesel is manufactured from vegetable oils and animal fats by esterification with methanol, hence it consists of fatty acid methyl esters. High contents of water lower its oxidation stability und therefore influence the quality of the product. Water determination can by carried out volumetrically or coulometrically. For coulometric water determination the addition of a solubiliser (e.g. Xylene) is necessary. As additives can be found in many biodiesel products, water determination with the KF oven technique is recommended in this case to avoid disturbing side reactions.

Titration One Component System

Reagents:

Titrant
188002 Aquastar^® - CombiTitrant 2 - One component reagent for volumetric Karl Fischer titration, 1 mL = approx. 2 mg water

Solvent
60 mL 188020 Aquastar^® - CombiSolvent oils - Solvent for volumetric Karl Fischer titration with one component reagents for oils
or
60 mL 188021 Aquastar^® - CombiSolvent fats - Solvent for volumetric Karl Fischer titration with one component reagents for fats

Titration Parameters:
Default titration settings, e.g.:
I(pol) = 20 - 50 µA, U(EP) = 100 - 250 mV
Stop criterion: drift < 15 µL/min

Sample size:
5 - 10 mL

Procedure:
The titration medium is first placed into the cell and titrated dry by means of the titrant. Then the sample is added with a syringe (exact sample weight determination by weighing of syringe before and after injection) or volumetric pipette and the titration is started.

Coulometry with Diaphragm

Reagents:

Catholyte
5 mL 109255 Aquastar^® - CombiCoulomat frit - Coulometric Karl Fischer reagent for cells with diaphragm

Anolyte
80 mL 109255 Aquastar^® - CombiCoulomat frit - Coulometric Karl Fischer reagent for cells with diaphragm
and
20 mL 108661 Xylene - as solubiliser

Titration Parameters:
Default coulometer settings for cell with diaphragm:
For end point indication, e.g.:
I(pol) = 5 - 10 µA, U(EP) = 50 - 100 mV
Stop criterion: drift < 10 µg/min

Sample size:
1 - 1.5 g (depending on water content of the sample)

Procedure:
The Karl-Fischer reagent is placed into the cathode and anode compartment of the titration cell with diaphragm. The coulometer is started and the solvent is titrated dry. After preliminary titration and stabilisation of drift the sample is injected into the titration cell with a syringe (exact sample weight determination by weighing of syringe before and after injection) and the water determination is started.

Coulometry Without Diaphragm

Reagents:

Working medium
80mL 109257 Aquastar^® - CombiCoulomat fritless - Coulometric Karl Fischer reagent for cells with or without diaphragm
and
20mL 108661 Xylene - as solubiliser

Titration Parameters:
Default coulometer settings for cell without diaphragm:
For end point indication, e.g.:
I(pol) = 5 - 10 µA, U(EP) = 50 - 100 mV
Stop criterion: drift < 10 µg/min

Sample size:
1 - 1.5g (depending on water content of the sample)

Procedure:
The Karl-Fischer reagent is placed into the titration cell without diaphragm. The coulometer is started and the solvent is titrated dry. After preliminary titration and stabilisation of drift the sample is injected into the titration cell with a syringe (exact sample weight determination by weighing of syringe before and after injection) and the water determination is started.