many moons ago i won a science fair doing a project on water conductivity. I had one of those multi-project circuit board kits, in it was a layout for a current metering circuit. Had a 3 or 4 way rotary switch so you could choose between tap, distilled, salt, and i think i did soapy water too. Couple 'C' cells and you could measure the current across each type of water. Was a good question and answer project at the time. With all the parts we have today u could probably spice up the circuit a good bit.