A system and method for determining a concentration of total chlorine in dialysis water are provided. The system comprises a main unit housing a KI/water sample chamber and a sodium sulfate chamber. A first electrode pair bridges the two chambers and generates tri-iodide proportional to the amount of total chlorine in the water sample. A second electrode pair in contact with fluid in the KI/water sample detects an amount of tri-iodide generated by the first electrode pair. 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.