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Flow systems have been mainly used for quantitative determination of analytes in solution and are chiefly employed to increase precision and analytical frequency. This view was focused in some previous educational papers (1-3). However, flow injection analysis (FIA) can also be applied in fast experiments to determine physical chemistry parameters such as diffusion coefficients (4, 5), viscosity (6, 7), and complexing capacity of ligands (8,9). Other less usual FIA systems have also been proposed to determine reaction stoichiometries (10) and investigate temperature effects on dispersion (11).The aim of this work was to use a flow injection apparatus to evaluate the effect of ionic strength on the rate of reduction of hexacyanoferrate(III) by ascorbic acid. A mechanism was proposed (12) for this reaction that involves formation of an intermediate ascorbate anion (AH ᎑ ) by ionization of ascorbic acid (AH 2 ). The ascorbate anion transfers one electron to hexacyanoferrate(III) in a slow step that is followed by a fast step in which the ascorbate free radical (AHؒ) transfers one electron to hexacyanoferrate(III) to generate dehydroascorbic acid (A):Fe(CN) 6 ] 3᎑ → ← AHؒ + [Fe(CN) 6 ] 4᎑ (slow) AHؒ + [Fe(CN) 6 ] 3᎑ → ← A + [Fe(CN) 6 ] 4᎑ + H + (fast)The rate-determining step of this second-order reaction involves the collision between two anionic species. Thus, it is influenced by the ionic strength of the medium, which causes an alteration of ionic atmosphere and changes the charge densities around anions.The kinetics of the reduction of hexacyanoferrate(III) can be followed spectrophotometrically. When dissolved in aqueous medium, this compound generates a yellow solution. Ascorbic acid and reaction products are colorless. This study was previously made using a manual spectrophotometric procedure (13). That experiment was conducted in a medium containing 10 ᎑2 M nitric acid to decrease the reaction rate and to make practicable the manual procedure, since the rate constant is inversely proportional to the acid concentration (14).In the flow diagram designed to carry out this study, flow is stopped by commutation when the center of the sample zone reaches the flow cell, and the gradual decrease in signal, related to the redox reaction, is recorded as a function of time. A direct examination of this record allows determination of the half-time (t 1/2 ), and this parameter is used to calculate the rate constant and to estimate the charge product of the two reacting species.The proposed experiment can be completed in a 4-hour laboratory class. All reagents are nontoxic. Obtaining kinetic data without clock experiments is more compatible with the modern instrumentation that students will find in laboratories. Treatment of DataFor a second-order reaction, if the initial amounts of reagents (C o ) are equal and the half-time (t 1/2 ) is known, the rate constant (k) at the ionic strength I can be calculated by the following expression:After obtaining the rate constant, the Brønsted-Bjerrum equation (2), based on the limiti...

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