Hi Fritz, 3% drift isn't that much for electrochemistry. In my microelectronics lab we used to read the exponent of the conductivity, not the mantissa.
Rather than an explanation, here are suggestions for trials:
- After a week of run, rub the probe with DI water, check where the measure restarts. The hard back of a sponge is great to clean copper, should work on platinum too.
- Measure the initial conductivity with two or more probes. Run with one for a week, then check if the other probe's reading has drifted too.
- Has the solution's pH changed? Any odour after using the higher concentration?
- Take a bigger flask. Is the drift smaller? After a week, move the electrolyte gently, check if the reading changes. Or isolate somehow a smaller portion of the electrolyte around the probe, and mix again after a week.
Depending on the flask's size, it is numerically conceivable that one week worth of small current changes the electrolyte's composition, more so if the measurement frequency is low.