Inkwell: Authors and Artists

    

For what you have in mind, Gary, it's probably worth considering moving up
to more professional-calibre gear. Those ion specific electrodes are not
cheap, AFAIK. Patrick can say more, I think.
  

    
The hand held meters are an arm and a leg also. Part of why these
little guys looked intriguing. 

Companies like Vernier and Ocean Optics have been putting out little
portable spectrometers that are basically an optical compartment hooked
to their unit by fiber optic cable and USB out. A job for Arduino?
  

    
In the case of ion specific electrodes and the like, a lot (maybe
half) the cost is in the sensor itself.  So you might safe half the
cost by connecting a specific sensor to an Arduino. 

One elaborate Rube Goldbergian approach might be to have a small
arduino controlled micro pump extract a small sample of water, which is
then injected (via another arduino controlled micro pump) with a small
amount of chemical reagent, which turns color based on the level of
whatever it is your measuring.  This would be imaged by a small color
sensor, which returns an RGB reading of the color of the liquid inside
the test tube.

The whole thing might cost as much as an ion specific probe, and be
prone to more frequent breakdowns, but think of the glory!