This refillable 10" x 2.5" cartridge contains [strong acid] cation exchange resin. It captures cations and releases hydrogen (H⁺⁾ ions.  Used in separate bed deionization systems.  

This cation resin will change from purple to tan. When you see roughly half of the cartridge change over to tan, you'll know its time to replace. See cation refill resin here: 1 refill or for multiple cartridges, see bulk refills here