A free chlorine measurement system and sensor are provided. In accordance with one aspect of the invention, the sensor has a porous working electrode disposed in an electrolyte proximate a porous membrane. The membrane allows free chlorine therethrough where it is reduced and generates a current. The current is related to the free chlorine concentration. The internal electrolyte solution is pH stabilized with a long-term pH stabilizer that has a solubility in water at room temperature between about 1.2 moles/liter and about 0.001 moles/liter. The stabilizer can be an acid or a base depending on whether the pH is to be stabilized at a relatively low value or a relatively high value respectively.