Titration Steps:
Filling the anodic compartment
(with HYDRANAL®-Coulomat A/AG/AD/AG-H/AG-Oven/AK/Oil/E)
If the cell is dismantled, it must be dried at 50°C in a drying cabinet prior
to use. The drying of the diaphragm requires special care. A slight excess of
iodine present during the manufacture of HYDRANAL-Coulomat A and E types removes
traces of moisture from the reagent. This solution is added to the anodic compartment
of the coulometric titration cell using a dry funnel. The amount of reagent
used (usually 100-150 mL) depends on the instrument used. The titration cell
is closed immediately after the addition of reagent.
Filling the cathodic compartment
(with HYDRANAL®-Coulomat CG/CG-K or HYDRANAL®-Coulomat E)
The cathodic compartment is filled with HYDRANAL-Coulomat CG/CG-K/E (usually
5 mL). The level of the catholyte in the cathodic compartment should be lower
than that of the anolyte in order to prevent diffusion into the anodic compartment.
The cathodic compartment must also be anhydrous.
Drying the titration cell
The titration cell is self-drying. The instrumentation determines and indicates
the degree of dryness automatically as the "drift" or "background"
in µg/min or µg/sec of water. The drift value for a freshly replenished
titration cell should not exceed 10 µg/min (ideally, 4 µg/min) of water. The
drift can increase during a series of determinations. As long as the drift rate
is stable, this does not appreciably influence the accuracy of the analyses
since the instrumentation automatically compensates for the inherent drift value.
Slow release of moisture that adheres to the walls can cause drift - this residual
moisture can be quickly removed by gently swirling the filled titration vessel.
Sometimes, moisture from the cathodic compartment can also result in high drift.
Reductive substances produced in a catholyte, spent or unsuitable, have the
same effect; hence, it is recommended that the catholyte be replaced at least
once a week.
Addition of the sample
Titration is initiated by pressing the start button. The liquid sample to be
analyzed is then injected through the septum into the anolyte. Solid samples,
dissolved in suitable solvents, are added in dissolved form. Also, the moisture
can be evaporated from the solid sample in an oven, collected and added as vapor
to the titration cell. Gaseous samples are introduced into the anolyte via a
gas inlet tube. The size of the sample chosen depends on the anticipated water
content and the desired accuracy. The sample size should not exceed 10 mL for
practical reasons. The coulometric titration cell will hold approximately 50
mL – a single titration of 10 mL of sample means a total of 5 determinations
possible per reagent fill. Generally, 0.5-5.0 mL samples in liquid form are
administered. Gaseous samples range from 100 mL to 10 L.
Calculation of the results
Coulometric titration equipment indicates the amount of water determined digitally
in mg or µg. Most instruments will show the results in mass or volume units
(% or ppm) if the sample size was entered.
Replacement of the reagents
The spent reagent can be removed from the anodic cell with a plastic suction
bottle; certain instruments are equipped with pumps. The anodic compartment
is subsequently refilled with HYDRANAL-Coulomat A/AG/AD/AG-H/AG-Oven/AK/Oil/E,
and the cathodic compartment with HYDRANAL-Coulomat CG/CG-K/E. Determining when
the reagent is spent or requires changing is not always obvious. Usually, a
full titration cell necessitates replacement of the reagent. Also, successive
dilution reduces the conductivity and necessitates replenishment of the reagent.
The capacity limit of HYDRANAL-Coulomat (anolyte) is seldom reached. The capacity
of the catholyte must be determined empirically.
A drift increase often means the reagent must be replaced. An increase in the
drift can have different causes. In case of doubt, the recovery rate of water
standard should be checked - this method is a reliable way of determining whether
the equipment is functioning properly.
Performance Tip
The coulometric cell should always be dry and operational. This can be achieved
if the machine remains permanently switched on. Moisture penetrating the cell
will be immediately eliminated this way. If the machine is switched off, water
collects in the cell; after start up, water elimination takes up time and reagent.
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