Electrochemical devices, methods, and systems for detecting and quantifying analytes are disclosed. A chemical detection reagent is locally generated in a test solution by electrochemical reaction of a precursor compound caused to migrate into the test solution from a precursor solution separated from the test solution by a cell separator. This approach provides precise metering of the reagent, via the charge passed, and avoids the need to store a reagent solution that may be chemically unstable. In one embodiment, the starch concentration in a colloidal solution can be measured via spectroscopic detection of a blue complex formed by the interaction of starch with iodine produced, on demand, by electrochemical oxidation of iodide ion. The approach may also be used to characterize certain types of analytes. The invention is amenable to automation and is particularly useful for on-line monitoring of production processes, including the inclusion of feed back loop mechanisms for process control.