Acording to a 1-wire wizzard on a koi forum here in the netherlands you would need these parts to build a 1-wire PH sensor
pH-Sensor 7120B Vernier
http://www.vernier.com/accessories/access....late=basic.html
electrode amplifier pH, Redox, mV
http://www.vernier.com/probes/ea-bta.html
and...