Metrohm 757 VA Computrace - Manual (Page 177)

8.6 Voltammetric problems
757 VA Computrace – Software
167
8.6 Voltammetric problems
Low background current or unstable baseline
With all types of electrodes:
1. Check electrolyte concentration and pH of the solution.
2. Check
Start potential and End potential of the sweep.
3. If the ion concentration in the solution is too high: dilute the
electrolyte.
4. Has the sample been degassed? Degassing with nitrogen for
at least 5 min is recommended, for alkaline solutions approx.
10 min is recommended.
5. Is the reference electrode sufficiently full (inside and outside,
see Hardware Manual)?
6. Electrolyte solution too old: make up a new one. Its working
life with organic additives may be as short as 1 day or less.
With
DME/SMDE:
1. If the electrode drops irregularly: check the MME.
Adjust sealing needle. If necessary, change capillary or re-
place sealing needle (see Hardware Manual).
2. Check tapping mechanism on VA Stand. If tapping strength is
too weak, turn corresponding slotted screw on the valve block
during operation in an anticlockwise direction until a drop falls
each time the tapper is triggered (see Hardware Manual).
3. Is the gas pressure correctly set (1 bar)?
4. If the concentration to be determined is considerably lower
than expected: increase sample volume or change the elec-
trode mode (e.g. HMDE).
With
HMDE:
1. If the electrode drops or the drops do not remain hanging:
check the MME. Change capillary if necessary or replace
sealing needle (see Hardware Manual).
2. If the concentration to be determined is considerably higher
than expected: reduce sample volume or change the elec-
trode mode (e.g. from HMDE to SMDE or DME).
With
RDE/SSE:
1. The electrode surface must be repolished.
2. Has the correct RDE type been used?
3. Exchange the RDE.