Can I measure free chlorine content in treated water using ORP sensor?
In my experience ORP/pH sensors are not really suitable for other things. Also they need to be calibrated quite often (lab practice is 8 hour intervals). Given that an ORP sensor is essentially a redox indicator it will probably have a huge cross sensitivity to other elements (probably bromine, iodine and such)
Chlorine in water AFAIK in lab is usually measured by titration but probably doing that automatically is even more expensive.