Chemical control system for a power plant including at least one coolant electrochemical indication sensor of a flow type electrically connected to the measurement data processing and transmission unit with its outlet connected to a central computer (CPC) controlling the actuator for injection of hydrogen and chemical reagents. The hydraulic inlet of the electrochemical sensor in use of the system is connected by a sampling tube to the power plant process circuit and its hydraulic outlet is hydraulically connected to the first heat exchanger and the first throttling device with a coolant supply circuit in series. The sampling tube is configured to pass a coolant sample to the coolant electromechanical sensor and the coolant supply circuit contains tubes and valves configured to reverse the flow of the coolant sample through the first throttling device.