A system and method for determining a concentration of total chlorine in dialysis water are provided. The system comprises a main unit housing an iodide/water sample chamber and a reducing agent chamber. An electrode pair bridges the two chambers and generates tri-iodide proportional to the amount of total chlorine in the dialysis water. The electrode pair detects the amount of tri-iodide generated in proportion to the amount of active chloride in the dialysis water. The system is suitable for use in connection with, or for incorporation into, a water purification system for generating dialysis fluid, and may include a display that alerts the user to stop or prevent a hemodialysis treatment if the total chlorine level exceeds a predetermined level.