Effects of the Starch Indicator on the Buoyantly Unstable Iodate‐Arsenous Acid Reaction Front

Abstract

Buoyantly unstable chemical fronts, which propagated in the iodate arsenous acid reaction system, were studied experimentally in 3D space and numerically in vertical 2D space. The fronts invaded the reaction solution from the left top corner in horizontal and vertical directions. Before the starch effects were examined condition under which friction did not influence the front propagation were found both experimentally and numerically. The critical solution depth and the critical distance between lateral walls, above which the front speed is not influenced by friction, were determined. Moreover, the results also show that the 2D simulations correspond to a vertically oriented plane placed the 3D experiments in 1/2 of the walls distance and along the x axis of the reactor. Importantly, those results also show that friction caused by the presence of solid side walls exclude pseudo 2D experiments to be compared with 2D simulations, when flow is involved. The effects of starch on the IAA fronts were studied experimentally at conditions, at which the friction did not influence the front propagation. Although the starch is used in very low concentrations as an iodine indicator, the experimental results show that starch presence strongly affects the dynamics of the liquid IAA reaction system. The presence of starch decreases the front velocity independently on the direction of the front propagation. Starch also affects the pattern formation. A possible mechanism is discussed.

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“…The addition of starch reduces the effective diffusion coefficients of some of the reacting species, […] slowing down the velocity of propagation of the fronts, with a greater reduction in speed as more starch is used in the reaction mixture. [22] In 2017, Sebestikova et al [23] reported a similar tendency for a non-gelled IAA reaction system, in which the fronts propagated due to the reaction-diffusion-convection coupling. A decrease of both vertical and horizontal velocities was observed for increasing starch concentration up to 0.15%, then the velocity became independent from starch concentration, suggesting that only a certain amount of the produced iodine is available for the complexation reaction (Fig.…”

“…Fig.3Evolution of experimentally observed patterns in the IAA reaction for different starch concentrations. Reproduced with permission from[23] …”