Determination of Water Content in Acids, Aqueous Using Karl Fischer Titration

Product Group

Inorganic compounds

General Information concerning the product group

Inorganic compounds

Salts contain water in different kinds of bonding states: adherent moisture - enclosed water - crystal water. For determination of the total water content the substance must be completely dissolved. If the salt does not dissolve in methanol sufficiently, formamide can be added to the recipient vessel and/or titration carried out at 50 °C. If adherent moisture alone is of interest the dissolution of the sample must be prevented through addition of a non-polar solvent (e.g. chloroform) to the KF solvent. A good alternative for salts to the direct volumetric determination is often the Karl Fischer oven technique. Through slow heating of the sample it is hereby possible to differentiate among the various types of water bonding. For salts that cause disturbing side reactions in the KF solutions (e.g. carbonates, sulphites, borates) the KF oven technique is the method of choice. The same is also true for oxides and hydroxides. These alkaline compounds react with the weakly acidic KF solution resulting in the formation of water, which is erroneously detected as water during the titration. Aqueous inorganic acids and bases are titrated volumetrically. Since stoichiometry and reaction rate of the Karl Fischer reaction are influenced by the pH, sufficient buffering of the KF solution is a prerequisite for reproducible and accurate results. Optimum conditions are obtained in the neutral pH range (pH 4 - 8). Thus, the determination of acids is performed through the addition of Aquastar^® buffer solution for strong acids, imidazole or a derivative of imidazole to the titration media. For the titration of bases the pH is adjusted by addition of Aquastar^® buffer solution for strong bases, salicylic acid or benzoic acid. With particularly strong, highly concentrated acids the propensity for esterification in methanolic solutions should also be noted. In such cases external neutralisation in a methanol-free medium is necessary.

Special Information concerning the sample and the methods

Aqueous acids, e.g. hydrochloric acid (37%), hydrofluoric acid (48%) or phosphoric acid, can be titrated directly by the volumetric method without problems. Due to the high concentrations of water only small sample sizes are required, which can be buffered during KF titration. Due to its better buffering abilities, Aquastar^® Solvent can also be utilised as solvent for the one component titration. Through the addition of imidazole or Aquastar^® buffer solution for strong acids the amount of sample that can be determined in the solvent is increased. The coulometric determination is only recommended after external neutralisation of the sample.

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
50 mL 188015 Aquastar^® - Solvent - Solvent for volumetric titration with two component reagents
or
40 ml 188009 Aquastar^®- CombiMethanol - and Aquastar^® - buffer solution for strong acids Art. 1.88035 10 ml

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:
0.1 g

Procedure:
The titration medium and (if necessary) buffer solution are first placed into the titration cell and titrated dry by means of the titrant. Then the sample is added with a 1 mL plastic syringe (exact sample weight determination by weighing of syringe before and after injection) and the titration is started. If cell drift rises strongly and titration time increases significantly the KF solvent must be changed.

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
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:
0.1 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 plastic syringe (exact sample weight determination by weighing of syringe before and after injection) and the titration is started. If cell drift rises strongly and titration time increases significantly the KF solvent must be changed.

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:
Aliquote of 1 mL sample solution

Procedure:
The sample solution is prepared by addition of 5 g of sample to 50 mL of pyridine and subsequently filled up with methanol to 100 mL. 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. The results are corrected for the blank value of the solvent mixture.

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:
Aliquote of 1 mL sample solution

Procedure:
The sample solution is prepared by addition of 5 g of sample to 50 mL of pyridine and subsequently filled up with methanol to 100 mL. 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. The results are corrected for the blank value of the solvent mixture.