Determination of Water Content in Pyridine Using Karl Fischer Titration

Product Group

Amines

General Information concerning the product group

Amines

Weakly alkaline amines pose no problems in the water determination after Karl Fischer and can be titrated according to standard methods. For strongly alkaline amines the buffer capacity of the Karl Fischer solution is not sufficient. A shift of pH into the alkaline range leads to a side reaction of the iodine. This can be recognised by the dragging end points, or a continuous consumption of iodine solution, respectively. If turn-off criteria are reached nevertheless, the result is too high. This side reaction, caused by the disproportionation of iodine molecules above pH 8, can be avoided through additional buffering of the working medium with a weak acid. Aquastar^® buffer solution for strong bases, benzoic or salicylic acid are the most suitable because, unlike acetic acid, they do not show any tendency for esterification in the Karl Fischer solution. The acids are added to the solvent prior to titration and can even be employed in excess. Some amines react with the Karl Fischer solution. Measures to suppress or overcome this side reaction are, for example, titration in methanol-free working media, utilisation of small sample sizes, as well as titration to a fast end point.

Special Information concerning the sample and the methods

Heterocyclic amines, such as Pyridine usually show weakly alkaline properties. Water determination can thus be carried out according to standard methods.

Titration one component system

Reagents:

Titrant
188005 Aquastar^® - CombiTitrant 5 - One component reagent for volumetric Karl Fischer titration, 1 mL = approx. 5 mg water
or
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 - Solvent for volumetric Karl Fischer titration with one component reagents, max. 0,01 % water

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:
5 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
188010 Aquastar^® - Titrant 5 - Titrant for volumetric titration with two component reagents, 1 mL = approx. 5 mg water
or
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:
5 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 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 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.