We introduce an original chemical reaction between two transparent reactant solutions (fluorescin and potassium ferricyanide), producing a fluorescent product in water (fluorescein). The reaction has a tunable kinetics, allowing the quantitative investigation of the interplay between molecular diffusion and reaction kinetics in various reactant field topologies. We document the chemical reaction kinetics and its sensitivity to ambient pH, temperature, reactants concentrations, etc., and we implement it in two simple reactant-contacting geometries in a Hele-Shaw cell. The resulting reaction-diffusion zones and product formation rates exhibit either the reaction-controlled or diffusion-controlled regimes, as well as a new "diffusio-chemical" regime which we show to be inherent to the early time transient dynamics of any bimolecular reaction between initially segregated reactants. The potential interest of this reaction, opening prospects for the study of reactive mixing, is underlined.

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