I built a circuit with an Arduino board that integrated a pH and pressure sensor. Unfortunately, I need to change the system to accommodate new instrumentation and software. Would there be a way to wire these sensors to a DB9 female connector? I suppose the question is less to deal with Arduino and more to deal with how to communicate an analog signal through a DB9 connector. Each of the sensors are only connected to the Arduino for the power source, signal, and ground.
Would it be possible to rewire those sensors and 3 wires to a DB9 female connector?
Well DB9/25 are digital serial interface you can't communicate analog signal over DB9.
Bullshit. Of course the analog signal can be connected using a DB9 connector. It might not be the best connector for that task but the length of the cable and it's quality is much more important for the analog signal than the used connector.
Would it be possible to rewire those sensors and 3 wires to a DB9 female connector?
Yes, given you have no other restrictions and the loose in accuracy is acceptable.
The main issue with connectors is preventing something being damaged through plugging into the wrong thing, hence the plethora of connectors available. 9-pin D connectors are commonly used for serial connections, but
not exclusively, any D-series connector could be used for practically anything, just label it well!
There is a restriction if you are connecting/disconnecting the DB9 many, many times. That series of connectors is not designed for multiple(thousands) of operations like that. The pins will wear and eventually fail.