|
|
|||||||||||||||||||||||||||||
|
Volumetric Karl Fischer Titration / One-Component Reagents Available Online
Titration Steps: Addition of Reagent The burette is filled with HYDRANAL®-Composite from the stock vessel of the burette for the reagent. It is imperative that the burette, stock vessel and their interconnection be absolutely dry. Residual water can cause local changes in the titre and lead to errors in the results. Stock vessels are protected from atmospheric moisture by the addition of drying tubes. Addition of Solvent Depending on the size of titration cell and on the size of the sample to be investigated, 20-40 mL of methanol, HYDRANAL®-Methanol Rapid or HYDRANAL®-CompoSolver E are added to the titration vessel. The titration cell is closed immediately after the addition in order to keep the intrusion of atmospheric moisture to a minimum. Pre-titration The solvent thus added to the titration vessel is then titrated to dryness with the reagent. This "pre-titration" removes not only the residual water that was in the solvent, but also the adherent moisture in the cell, on the walls of the cell and the electrode. The atmosphere of the cell is also dried of moisture. The pre-titration must be carried out very carefully as any error will influence the subsequent determination of the water content of the sample. A titration to a stable end point is the prerequisite for a reliable analysis. A perfectly dried titration cell has a maximum drift consumption of 0.01 mL of KF reagent per minute. Weighing the Sample A determined quantity of the sample to be investigated is administered to the pre-dried working medium. The cell is opened for as short a time as necessary and then closed again immediately after the sample was added. Titration of the Water Content 
The titration should be started immediately. The dosing rate of the titration agent should be adjusted
to the amount of water likely to be present. The initial stages of the titration should be rapid.
The titration rate must be reduced when the end point is near.
Modern instrumentation automatically adjusts the titration rate to the amount of water still remaining
in the titration vessel.Generally, one titrates to an end point of 10 seconds stability. A stable end point is a significant indication of the course of a titration with no complications. A vanishing end point indicates the water of the sample is released slowly or there is an interference by a side reaction. Replacement of the Reagent Following the completion of a titration, the spent solutions should be removed from the titration cell. Usually the titration cell is immediately re-filled with fresh solvent and dehydrated by a pre-titration. For "subsequent titrations", the solvent mixture of the previous titration is immediately used as the working medium for the titration of the next sample. In this way, the use of a fresh solvent and a pre-titration for each titration is obsolete. Automatic titration instruments are usually programmed for continuous titrations. Such a series of titrations prescribes the determination of the same type of substance being investigated. Furthermore, it must be ensured that the methanol content in the working medium does not fall below 25% as otherwise the end point can be shifted. |
|||||||||||||||||||||||||||||
|
|
|||||||||||||||||||||||||||||
