how to setup a ph sensor probe
http://www.all-about-ph.com/images/simple-construction-of-a-ph-meter.jpg  The internet has lots of schematics that are way complicated, can't work, or need parts you won't easily find.  

This should get you started.  Any op-amp with a bias current in the pA range and will operate on 5 volts will work.  This circuit does not include temperature compensation, but if you work anywhere near room temperature the errors will be small.  Any questions?  Forget the 1N126 and the bridge circuit.