using sine or square wave to obtain conductivity of solution?
I am in the process of designing an EC (Electrical conductivity) meter that works with the Raspberry PI and I am using square waves.  It is much easier to generate a square wave than a sine wave.  In my case, the RPi actually controls the timing of the square waves.

Typically, an an op amp is used to drive an AC signal to the EC probe.  In this case, the edge rate is limited by slew rate of the op amp.  Unless you are using a VERY fast op amp, you will not need to worry about issues from fast-edged square waves.

Simple EC meter designs rectify and low pass the AC signal output of the EC probe and measure the resulting voltage, but this is not extremely accurate.  A better way is to use an A/D converter to measure the actual voltage output from the EC probe.

The design which I am currently debugging samples the EC sensor output after the rising edge of the square wave.  It averages the actual voltage with a sample-and-hold stage driving an RC low pass filter to accurately measure the voltage with a low frequency A/D converter.

I have found that the main issue with accurately measuring the conductivity is that for high conductivity solutions, the ions in solution migrate very quickly, so you need to measure the effective solution resistance very close to the edge the square wave.