Determination of Water Content By Karl Fischer Method in Citric Acid

Product Group

Carboxylic acids

Section Overview

• General Information for Water Determination in Carboxylic Acids
• Methods for Water Determination in Citric Acid Sample
  
• Related Materials

General Information for Water Determination in Carboxylic Acids

Most carboxylic acids can be analyzed relatively easily following standard Karl Fischer titration methods. Due to their weakly acidic properties, they exhibit only a slight effect on the pH of the Karl Fischer solution. Only the stronger acids require neutralization 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, solubilizers may be added. With some substances, only the adherent moisture can be determined by Karl Fischer volumetric titration, or alternatively, the Karl Fischer oven method may be employed.

Methods for Water Determination in Citric Acid sample

This substance is easily soluble in alcoholic media. For an improvement of the 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 Karl Fischer coulometric titration, the substance is either dissolved externally or the water can be released by means of the Karl Fischer oven method.

One-Component Volumetric Karl Fischer Titration

Karl Fischer Reagents:

• Karl Fischer Titrant
  Aquastar^® - CombiTitrant 5 (1.88005) - One component reagent for volumetric Karl Fischer titration, 1 mL = approx. 5 mg water
  or
  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^® - CombiMethanol (1.88009) - Solvent for volumetric Karl Fischer titration with one component reagents, max. 0.01 % water
  

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 (monohydrate or anhydrous)

Titration 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 the 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.

Two-Component Volumetric Karl Fischer Titration

Karl Fischer Reagents:

• Karl Fischer Titrant
  Aquastar^® - Titrant 5 (1.88010) - Titrant for volumetric titration with two component reagents, 1 mL = approx. 5 mg water
  or
  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 (1.88015) - 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)

Titration 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 the 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.

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
  100 mL Aquastar^® - CombiCoulomat frit (1.09255) - 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

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, an aliquot of the sample is injected into the titration cell with a syringe (exact determination through weighing of the 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.

Coulometric Karl Fischer Titration (Without Diaphragm)

Karl Fischer Reagents:

• Working medium
  100 mL Aquastar^® - CombiCoulomat fritless (1.09257) - Coulometric Karl Fischer reagent for cells with or without diaphragm
  

Titration Parameters:

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:

solution of 5 g in 50 mL methanol

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, an aliquot of the sample is injected into the titration cell with a syringe (exact determination through weighing of the 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.