Determination of Water Content in Citric acid Using Karl Fischer Titration

Product Group

Carboxylic acids

General Information concerning the product group

Carboxylic acids

Most carboxylic acids can be analysed relatively easily following standard KF titration methods. Due to their weakly acidic properties they exhibit only a slight effect on the pH of the KF solution. Only the stronger acids require neutralisation or buffering of the solvent prior to measuring. This is achieved by the use of organic amines. In some cases esterification with methanol results in the formation of water. This potential interference should especially be taken into account with coulometric determination. In the event of solubility problems solubilisers may be added. With some substances, only the adherent moisture can be determined by volumetric titration, or alternatively KF oven technology may be employed.

Special Information concerning the sample and the methods

This substance is easily soluble in alcoholic media. For an improvement of solubility of larger samples a buffer can be added. Alternatively, titration is possible at increased temperature. Side reactions do not occur in the KF solution. During coulometry the substance is either dissolved externally or the water can be released by means of the KF oven technique.

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 188009 Aquastar™ - CombiMethanol - Solvent for volumetric Karl Fischer titration with one component reagents, max. 0.01 % water

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

Sample size:
0.5 - 5 g (monohydrate or anhydrous)

Procedure:
The titration medium is first placed into the cell and titrated dry by means of the titrant. Then the sample is added from a weighing boat (exact sample weight determination by weighing of weighing boat before and after addition) and the titration is started. For complete dissolution of the sample a stirring time of 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
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:
Stirring time: 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:
0.5 - 5 g (mono hydrate or anhydrous)

Procedure:
The titration medium is first placed into the cell and titrated dry by means of the titrant. Then the sample is added from a weighing boat (exact sample weight determination by weighing of weighing boat before and after addition) and the titration is started. For complete dissolution of the sample a stirring time of 180 seconds is recommended.

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:
solution of 5 g in 50 mL methanol

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 titrated dry. After preliminary titration and stabilisation of drift an aliquote of the sample is injected into the titration cell with a syringe (exact determination through weighing of syringe before and after injection) and water determination is started. The result must be corrected for the blank value of the solvent. Alternatively the water can be released by means of the KF oven technique at 100 °C and determined coulometrically.

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:
solution of 5 g in 50 mL methanol

Procedure:
The Karl-Fischer reagent is placed into the titration cell without diaphragm. The coulometer is started and the solvent titrated dry. After preliminary titration and stabilisation of drift an aliquote of the sample is injected into the titration cell with a syringe (exact determination through weighing of syringe before and after injection) and water determination is started. The result must be corrected for the blank value of the solvent. Alternatively the water can be released by means of the KF oven technique at 100 °C and determined coulometrically.