Reagentless optical recognition and parts-per-million (ppm) quantification of FeCl3 in CH3CN was demonstrated using a redox-active Os(II)-chromophore-based monolayer on glass. The Fe3+-induced oxidation of the monolayer is fully reversible and can be monitored optically with a conventional UV/vis spectrophotometer (260-800 nm). The system can be reset with water within <1 min. Selectivity of the sensor toward FeCl3 is not affected by the presence of representative alkali metals, alkaline earth metals, and other transition-metal salts. Sensing of Fe3+ and concurrent generation of Fe2+ can be also observed with the naked eye by adding 2,2 '-bipyridyl (bipy) to the solution to generate [Fe(biPY)(3)](2+). Validation of the analytical performance characteristics of the sensor was performed including reversibility, reproducibility, stability, and the detection range (0.5-162 ppm of FeCl3 in CH3CN, 100-1000 ppm in water). The monolayer is sensitive and specifically responsive to its target ion. In addition, a blind test was conducted to probe the reproducibility and reproducibility variances of the system. The reaction of the monolayer with a CH3CN solution containing 5 ppm of FeCl3 follows pseudo first-order kinetics in the monolayer with Delta G(298K)(double dagger) = 21.6 +/- 0.1 kcal/mol, Delta H-double dagger = 10.2 +/- 1.5 kcal/mol, Delta S-double dagger = -38.3 +/- 4.9 eu.