Determination of Water Content in n-Hexane Using Karl Fischer Titration

Product Group

Hydrocarbons

General Information concerning the product group

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 solubiliser to the 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 improve 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 utilised 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 %).

Special Information concerning the sample and the methods

Water determination can be carried out without problems according to standard methods.

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
50 mL 188008 Aquastar^® - CombiSolvent - Methanol-free solvent for volumetric Karl Fischer titration with one component reagents
or
50 mL 188009 Aquastar^® - CombiMethanol - Solubiliser mixture for one component titration

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

Sample size:
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.

Titration two component system

Reagents:

Titrant
188011 Aquastar^® - Titrant 2 - Titrant for volumetric titration with two component reagents, 1 mL = approx. 2 mg water

Solvent
50 mL 188015 Aquastar^® - Solvent - Solvent for volumetric titration with two component reagents

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

Sample size:
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
100 mL 109255 Aquastar^® - CombiCoulomat frit - Coulometric Karl Fischer reagent for cells with diaphragm

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 - 2 mL

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
100 mL 109257 Aquastar^® - CombiCoulomat fritless - Coulometric Karl Fischer reagent for cells with or without diaphragm

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 - 2 mL

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.