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.
Highly concentrated sulphuric acid reacts with methanol resultíng in the formation of water and ester. Preliminary external neutralisation of the sample is necessary.
188005 Aquastar® - CombiTitrant 5 - One component reagent for volumetric Karl Fischer titration, 1 mL = approx. 5 mg water
null 188009 Aquastar® - CombiMethanol - Solvent for volumetric Karl Fischer titration with one component reagents, max. 0.01 % water
Default titration settings, e.g.:
I(pol) = 20 - 50 µA, U(EP) = 100 - 250 mV
Stop criterion: drift < 20 µL/min
2 mL aliquot of sulfphuric acid in 50 mL pyridine / 50 mL methanol ( exact weight determination of sulfonic acid, pyridine and methanol)
50 mL pyridine (max. 0,01 % water) are placed in a 100 mL measuring flask. The sample is weighed in, and after cooling down the precipitate of pyridinium salt is dissolved by addition of methanol. The flask filled up with methanol (max. 0,01 % water). The titration medium Aquastar® CombiMethanol is first placed into the titration cell and titrated dry by means of the titrant. Then the 2 mL to aliquot is added with a syringe (exact sample weight determination by weighing of syringe before and after injection) and the titration is started. The result to must corrected by the blank value of the pyridine methanol mixture.