This has been on my "Hey, this would be cool to do..." list for a very long time so I thought I would share some links:
http://www.ph-meter.info/pH-meter-construction http://damien.douxchamps.net/elec/ph_meter/
The two above came from this site...do not know how up to date it is though:
http://www.home.zonnet.nl/rsetteur/a...h/index_ph.htm
And personally, the one I would build would be this one:
http://www.66pacific.com/ph/ph_1.htm
I understand that you want this to be for a computer, and that last one would basically output to voltage that you would read as pH. If you could have that, then just basically write a program to start logging it.
Hope this helps!
intermediate_noob