A cartridge for dispensing a pre-measured buffering agent together with a medical fluid such as dental anesthetic. The cartridge includes a sharp piercing element at one end, a frangible barrier proximate the piercing element, and a traveling closure. The buffering solution is in a chamber between the barrier and the closure. When loaded into a syringe and subjected to operating pressures, the barrier is pierced, enabling the buffering solution to mix with the medical fluid. Continued pressure propels a desired, controlled mixture into a tissue of a patient. A dye may be included for visual confirmation of mixing. The invention may be regarded as the cartridge, a syringe having the cartridge, or a kit, with or without a syringe, or cartridges having closures of different types.