This is a bit more complicated than connecting the electrode to the ADC. Conductivity measurements based on AC voltage drop measurements. So what you need is an AC current generator and measure the voltage on the electrode or you need precise resistors and measure as a voltage divider. Both cases you need a rectifier between the electrode and the ADC. If it is possible use sinusoidal generator to avoid high freqvency components. The resistance you need to measure is between 10 Ohm and 1 MOhm which is not possible in one range, you need to handle range changes, too.
on a Raspberry B+ with whezzy, two Zero with Jessie Light