Determination of Water Content in Toothpaste Using Karl Fischer Titration

Product Group

Cosmetics

General Information concerning the product group

Cosmetics

Cosmetics on an aqueous and alcoholic basis can be titrated without problems according to the Karl Fischer (KF) method. Due to the often high water concentration of the emulsions small samples are required. The accuracy of analysis is determined through exact sampling, weighing and adding. The coulometric method is unsuitable for most emulsions. Fatty and oily preparations are insoluble in methanolic KF solvents and require the addition of a solubiliser, or respectively titration with specific KF solvents for oils and fats.

Special Information concerning the sample and the methods

Toothpaste does not dissolve completely in methanolic KF solvents. The addition of formamide increases solubility. After two to three analyses the KF solvent should be changed, since the sample may easily cover up the indicator electrode. The use of a homogeniser is recommended.

Titration one component system

Reagents:

Titrant
188005 Aquastar™ - CombiTitrant 5 - One component reagent for volumetric Karl Fischer titration, 1 mL = approx. 5 mg water

Solvent
40 mL 188009 Aquastar™ - CombiMethanol - Solvent for volumetric Karl Fischer titration with one component reagents, max. 0.01 % water
and
20 mL 109684 Formamide - as solubiliser

Titration Parameters:
Stirring- / extraction time: 120 - 180 sec.
Default titration settings, e.g.:
I(pol) = 20 - 50 µA, U(EP) = 100 - 250 mV
Stop criterion: drift < 20 µL/min

Sample size:
0.03 - 0.05 g

Procedure:
The titration medium is first placed into the titration cell and titrated dry by means of the titrant. Then the sample is added with a 1 mL syringe without hollow needle (exact sample weight determination by weighing of syringe before and after injection) and the titration is started. For complete dissolution / extraction of the sample a stirring time of 120 - 180 seconds is recommended.

Titration two component system

Reagents:

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

Solvent
40 mL 188015 Aquastar™ - Solvent - Solvent for volumetric titration with two component reagents
and
20 mL 109684 Formamide - as solubiliser

Titration Parameters:
Stirring- / extraction time: 120 - 180 sec.
Default titration settings, e.g.:
I(pol) = 20 - 50 µA, U(EP) = 100 - 250 mV
Stop criterion: drift < 20 µL/min

Sample size:
0.03 - 0.05 g

Procedure:
The titration medium is first placed into the titration cell and titrated dry by means of the titrant. Then the sample is added with a 1 mL syringe without hollow needle (exact sample weight determination by weighing of syringe before and after injection) and the titration is started. For complete dissolution of the sample a stirring time of 120 - 180 seconds is recommended.

Materials

     
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