Determination of Water Content By Karl Fischer Method in Transformer Oil

Product Group

Hydrocarbons, Petroleum products

Section Overview

• General Information for Water Determination in Hydrocarbons and Petroleum Products
• Methods for Determining Water Content in Transformer Oil Sample
• Related Materials

General Information for Water Determination in Hydrocarbons and Petroleum Products

Water content in hydrocarbons

Saturated hydrocarbons can, in most cases, be titrated according to standard methods. To overcome solubility problems of unpolar or weakly polar substances, the addition of a solubilizer to the Karl Fischer solvent is necessary. In the case of long-chain and cyclic hydrocarbons, long-chain alcohols (e.g., propyl alcohol or decyl alcohol) or chloroform are thus recommended. Toluene, xylene, or chloroform improves the solubility of aromatic compounds. Unsaturated hydrocarbons can usually be titrated in the same way. Interferences due to double bonds only occur with some very reactive compounds. In the case of interferences (unstable end point or none at all), a methanol-free, alcoholic solvent (e.g., CombiSolvent or CombiSolvent Keto) should be utilized instead of methanol. 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%).

Water content in Petroleum Products

Petroleum products are mixtures of long-chain or aromatic hydrocarbons. They are hardly soluble in methanol. Water determination by the Karl Fischer method therefore requires the addition of solubilisers. For light oils, long-chain alcohols are suitable. For dissolving heavier oils, toluene, xylene, or chloroform are added. For the volumetric titration, specific Karl Fischer 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 a diaphragm, a 20% solubilizer can be added to the working medium, or a 40% solubilizer to the anolyte in the case of coulometry with a diaphragm. Note that oils are often heterogeneous compounds with uneven distribution of water and should thus be homogenized (e.g., with Ultra-Turrax) prior to KF determination. Additives in oils can cause side reactions during Karl Fischer water determination. Here, the direct coulometric analysis is not possible, the volumetric titration is only conditional. As an alternative, the Karl Fischer oven method can be utilized in combination with coulometry, whereby the release of water is best achieved at temperatures between 120 and 140 °C.

Methods for Determining Water Content in Transformer Oil Sample

Water determination can be carried out volumetrically or coulometrically. Addition of solubilisers (long-chain alcohols), or rather specific KF solvents for oils, is necessary.

One-Component Volumetric Karl Fischer Titration

Karl Fischer Reagents:

• Karl Fischer Titrant
  Aquastar^® - CombiTitrant 2 (1.88002) - One component reagent for volumetric Karl Fischer titration, 1 mL = approx. 2 mg water
  
• Karl Fischer Solvent
  50 mL Aquastar^® - Solvent oils & fats (1.88016) - Solvent for volumetric Karl Fischer titration with two component reagents for oils & fats
  or
  30 mL / 20 mL Aquastar^® - CombiMethanol (1.88009) / Decanol (8.03463) - solvent mixture for one-component titration

Titration Parameters:
Stirring time: 60 sec.
Default titration settings, e.g.:
I(pol) = 20 - 50 µA, U(EP) = 100 - 250 mV
Stop criterion: drift < 20 µL/min

Sample size:
5 mL

Titration 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 the syringe before and after injection) or volumetric pipette, and the titration is started. For complete dissolution of the sample, a stirring time of 60 seconds is recommended.

Two-Component Volumetric Karl Fischer Titration

Karl Fischer Reagents:

• Karl Fischer Titrant
  Aquastar^® - Titrant 2 (1.88011) - Titrant for volumetric titration with two component reagents, 1 mL = approx. 2 mg water
  
• Karl Fischer Solvent
  50 mL Aquastar^® - Solvent oils & fats (1.88016) - Solvent for volumetric Karl Fischer titration with two component reagents for oils & fats
  

Titration Parameters:
Stirring time: 60 sec.
Default titration settings, e.g.:
I(pol) = 20 - 50 µA, U(EP) = 100 - 250 mV
Stop criterion: drift < 20 µL/min

Sample size:
5 mL

Titration 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 the syringe before and after injection) or volumetric pipette, and the titration is started. For complete dissolution of the sample, a stirring time of 60 seconds is recommended.

Coulometric Karl Fischer Titration (With Diaphragm)

Karl Fischer Reagents:

• Catholyte
  5 mL Aquastar^® - CombiCoulomat frit (1.09255) - Coulometric Karl Fischer reagent for cells with diaphragm
  
• Anolyte
  80 mL Aquastar^® - CombiCoulomat frit (1.09255) - Coulometric Karl Fischer reagent for cells with diaphragm
  and
  20 mL Decanol (8.03463) - as solubiliser

Titration Parameters:
Stirring time: 60 sec.
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 mL

Titration Procedure:
The Karl-Fischer reagent is placed into the cathode and anode compartments of the titration cell with a diaphragm. The coulometer is started, and the solvent is titrated dry. After preliminary titration and stabilization of drift, the sample is injected into the titration cell with a syringe (exact sample weight determination by weighing the syringe before and after injection), and the water determination is started. For complete dissolution of the sample, a stirring time of 60 seconds is recommended.

Coulometric Karl Fischer Titration (Without Diaphragm)

Karl Fischer Reagents:

Working medium
80 mL Aquastar^® - CombiCoulomat fritless (1.09257) - Coulometric Karl Fischer reagent for cells with or without diaphragm
and
20 mL Decanol (8.03463) - as a solubilizer

Titration Parameters:
Stirring time: 60 sec.
Default coulometer settings for cells without a 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 mL

Titration Procedure:
The Karl-Fischer reagent is placed into the titration cell without a diaphragm. The coulometer is started, and the solvent is titrated dry. After preliminary titration and stabilization of drift, the sample is injected into the titration cell with a syringe (exact sample weight determination by weighing the syringe before and after injection), and the water determination is started. For complete dissolution of the sample, a stirring time of 60 seconds is recommended.