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Determination of Water Content in L-Histidine Using Karl Fischer Titration

Product Group

Carboxylic acids

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.

Special Information concerning the sample and the methods

L-Histidine, like other amino acids, is hardly soluble in alcohols. Therefore, only the adherent moisture can be determined by the direct volumetric method. The presence of formamide and salicylic acid accelerates the extraction of water. In addition, titration under warm conditions and utilisation of a homogeniser are preferrable. Due to the low water concentration titrants with a low factor are preferred. External extraction, or rather dissolution of the sample can only be recommended for water concentrations of more than 0.5 %. Hereby the sample is dissolved in an appropriate solvent (e.g. pure acetic acid). An aliquote of the solution is placed into the titration cell and titrated. The result should be corrected for the blank value of the solvent. Alternatively, the KF oven technique can be used in combination with coulometry. To release the water, a temperature of 180 °C is recommended. (Disintegration occurs at approx. 200 - 220 °C).

Titration one component system

Reagents:

Titrant
188002 Aquastar™ - CombiTitrant 2 - One component reagent for volumetric Karl Fischer titration, 1 mL = approx. 2 mg water

Solvent
40 mL 188009 Aquastar™ - CombiMethanol - Solvent for volumetric Karl Fischer titration with one component reagents, max. 0.01 % water
and
20 mL / 10 g 109684 / 100635 Formamide / salicylic acid - null

Titration Parameters:
Extraction 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:
3 - 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. To extract the water an extraction time of three minutes is recommended.

Titration two component system

Reagents:

Titrant
188011 Aquastar™ - Titrant 2 - Titrant for volumetric titration with two component reagents, 1 mL = approx. 2 mg water

Solvent
40 mL 188015 Aquastar™ - Solvent - Solvent for volumetric titration with two component reagents
and
20 mL / 10 g 109684 / 100635 Formamide / salicylic acid - null

Titration Parameters:
Extraction 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:
3 - 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. To extract the water an extraction time of three minutes is recommended.

Materials

     
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