The reference electrode is not yet equilibrated: After maintenance of the reference electrode, wait for at least 20 min before starting a measurement.
The platinum electrode tip is contaminated: Condition the working electrode in pure VMS.
The platinum electrode tip has been used for a very long time: Immerse the platinum electrode tip in c(NaOH) = 0.5 mol/L for approx. 10 min and then thoroughly rinse with distilled water. Replace the platinum electrode tip if necessary.
The VMS is contaminated: Check whether the reagents are clean. Only reagents with degree of purity "analytical grade" or better should be used.
The water quality is poor: Use distilled water, type-II grade (ASTM D1193-91) or higher.
The measuring solution is contaminated by diffusion: No infeed tubing other than the four-way micro dosing tip (6.1824.000) may be immersed in the measuring solution.