Determination of Water Content in Salicylic Acid Using Karl Fischer Titration

Product Group

Carboxylic acids, Phenols

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.

Phenols

In most phenols water determination can be carried out according to the standard methods after Karl Fischer. Some substances, however, are oxidised by the Karl Fischer reagent. This side reaction leads to unstable or dragging end points. During coulometry, interferences can accumulate to the point where analysis becomes impossible. Because the side reaction is dependent on the pH, its influence can be reduced through modification of the solvent. In a weakly acidic titration environment (e.g. through the addition of Aquastar^® buffer solution for strong bases, benzoic or salicylic acid) easily oxidisable substances can be determined nevertheless.

Special Information concerning the sample and the methods

Salicylic acid does not cause any side reactions in the Karl Fischer solution and is easily soluble in alcohols. Salicylic acid is often used as a buffer for the determination of water concentration in strong bases. Analysis can 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 g

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.

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 g

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.

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:

0.5 g

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 added from a weighing boat (exact sample weight determination by weighing of weighing boat before and after addition) and the titration 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:

0.5 g

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 added from a weighing boat (exact sample weight determination by weighing of weighing boat before and after addition) and the titration is started.