Titration Steps:
Addition of Reagent
The burette is filled with HYDRANAL®-Composite, i.e. the stock vessel. 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 HYDRANAL®-Methanol dry, 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 in the titration vessel is then slowly 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 pre-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 "sequential 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 not required. When performing
‘sequential titrations’ the substance being investigated should
be the same type. 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.
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