Metrohm 875 KF Gas Analyzer - Manual (Page 44)

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6 Troubleshooting
A low and constant drift is a prerequisite for correct and precise water
content determination in the trace range. In the case of a carrier gas-
flooded coulometric titration cell, this drift consists of the measuring cell's
own basic drift (cell drift) and the water contained in the carrier gas.
Therefore, to the extent possible, the nitrogen used for prerinsing and
postrinsing must be water-free. Molecular sieve is capable of reducing the
residual water content to approx. 1 to 2 µg/L, which is sufficient for the
operation of the 875 KF Gas Analyzer. If the water concentration of the
inert gas used for rinsing is higher, then the gas has to be dried with
molecular sieve. A molecular sieve cartridge is located on the front plate
of the Gas Analyzer before valve 1. With 15 mL, however, its capacity is
rather limited, and therefore the cartridge only serves as a safety measure.
In an equilibrated state, the cell drift lies in a range between 1 and
3 µg/min. If a volumetric stream of 1 L/min of nitrogen that has been
dried through the molecular sieve is set for the titration cell, the cell drift
increases to approx. 2 to 4 µg/min.
A drift rise is attributable either to an increase in cell drift or an increased
water infeed via the carrier gas (see Table 5, page 36).
The carrier gas' share in the total drift can be determined with the Drift
diagnosis method. This share should not exceed 2 µg/min.
Table 5 Possible causes for a drift rise
Cause Remedy
Cell drift rise due to the infeed of
reactive matrix components Exchange the anolyte
Cell drift rise due to the accumula-
tion of water and H
2
S in the cath-
olyte
Exchange the catholyte
Water concentration rise in the
rinsing gas due to exhaustion of
the molecular sieve Check the nitrogen quality,
exchange the molecular sieve car-
tridge
Retardation of the water due to
accumulation in the vaporizer and
the oil filter Rinse the gas-carrying system with
solvent