Abstract: A chemical sensing system senses a concentration of a chemical of interest in a fluid. A portion of the chemical of interest diffuses across a barrier into an electrolyte. The barrier can be one or more polymeric hollow fibers. Electrodes or other sensing devices are disposed within the electrolyte. The electrolyte is selected such that it undergoes regenerative chemical reaction as it is exposed to the chemical of interest and the electrodes. The concentration of the chemical of interest can be determined by measuring a property of the electrolyte, such as current flowing through the electrolyte.