Determination of Water Content in phosphoric acid 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

Concentrated phosphoric acid can be titrated directly by volumetric titration without problems. Due to the high concentration of water only small sample sizes are required, which can be buffered through the addition of Aquastar™ buffer solution for strong acids or imidazole.

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
30 ml 188009 Aquastar™ - CombiMethanol - Solvent for volumetric Karl Fischer titration with one component reagents, max. 0.01 % water
and
20 ml 188035 Aquastar™ - buffer solution for strong acids - Buffer solution for strong acids for Karl Fischer titration

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:
0,1-0,2g

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 syringe (exact sample weight determination by weighing of syringe before and after injection) 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

Solvent
30 ml 188015 Aquastar™ - Solvent - Solvent for volumetric titration with two component reagents
and
20 ml 188035 Aquastar™ - buffer solution for strong acids - Buffer solution for strong acids for Karl Fischer titration

Titration Parameters:
Default titration settings, e.g.: z.B.
I(pol) = 20 - 50 µA, U(EP) = 100 - 250 mV
Stop criterion: drift < 20 µL/min

Sample size:
0,1-0,2g

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 syringe (exact sample weight determination by weighing of syringe before and after injection) and the titration is started.

Materials

     
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