pH sensors put out about 60 mV per pH point - negative for alkaline solutions, positive for acidic solution, with an output impedance of typically in the order of 10-100 MOhm. This can not be read directly by an Arduino, so there's a hardware driver in between. Usually an offset is applied so you don't have to deal with negative voltages.
So you have to scale your output voltage (do you have a DAC?) to whatever offset the commercial meter applies (or not) plus/minus that 60 mV/pH point.