I just got the pH portion of the circuit working. There actually wasn't anything wrong with the circuit. It turns out that the probe that I was initially was using was bad. I tried a different brand and it worked perfectly. The working probe is listed in the Components section of the project. It's quite reasonably priced.

I also verified the operation of the EC circuit using resistors in place of an EC probe. In order to test the conductivity of actual solutions, I am in the process of making a DIY EC probe using, believe it or not, ear piercing studs. They are gold plated stainless steel, only about $1 each, and have a pointed end that plugs very nicely into a gold plated Mill-Max machined pin contact (see image below). Correctly waterproofed, I should be able to make a cheap EC probe with replaceable contacts. Once I have it working, I will add a separate project on how to make your own EC probe. In the true MacGuyver spirit, perhaps I'll waterproof it with chewing gum.