Periodical redox changes

Figure 2, (a) Periodical redox changes of the miniature cubic poly(NlPAAm-co-Ru(bpy)j) gel and (b) the swelling-deswelling oscillation at 20 C. Transmitted light intensity is expressed as an 8-bit grayscale value. Outer solution [MA] = 62.5mM [NaBrOs] = 84mM [HNOsJ = 0,6M. 

Ru(bpy)3) solution, (b) Periodic redox changes of the miniature poly(NIPAAm-co-Ru(bpy)3) gel (lower) and the swellingdeswelling oscillation (upper) at 20°C. (c) Time course of peristaltic motion of poly(NIPAAm-co-AMPS-co-Ru(bpy)3) gel in a catalyst-free BZ reaction solution. 

Acid labile moieties inside polyanhydrides, poly(lactic/glycolic acid) (PLGA), and poly(b-amino esters) (PbAEs) induce redox responsiveness. Another typical redox-responsive pol mier is poly(NiPAAm-co-Ru(bpy)3), which can generate a chemical wave by the periodic redox change of Ru(bpy)3 into an oxidized state of lighter color. This redox reaction alters the hydrophobic and the hydrophilic properties of the potymer chains and results in swelling and deswelling of the potymer [103]. 

The BZ oscillating reaction has been discovered by B.P. Belousov, in 1951. In his early attempt, he wanted to make a chemical model for Krebs cycles (an energy pathway). For experimentation, he used KBr03, acidified solution of citric acid and a metal catalyst (Cerium (TV) metal ions). A redox indicator has also been used for observing the end of the reaction-phase. They found that the citric acid is oxidized into CO2, and B1O3 is reduced to Br ion. He observed a periodic color changes for duration of 10 min. He wanted to publish this report but editor refused his proposal and explanation because, it seems to be contrary with 2nd laws of thermodynamics. 

In miniature gels sufficiently smaller than the wavelength of the chemical wave (typically several miUimeters), the redox change of ruthenium catalyst can be regarded to occur homogeneously without pattern formation [15]. Because of the redox oscillation of the immobihzed Ru(bpy)3 +, mechanical sweUing-desweUing oscillation of the gel autonomously occurs with the same period as for the redox... 

Typically, the oscillation period increases with a decrease in the initial concentration of the substrates. Further, in general, the oscillation frequency (the reciprocal of the period) of the BZ reaction tends to increase as the temperature increases, in accordance with the Arrhenius equation. The swelling-deswelling amplitude of the gel increases with an increase in the oscillation period and amplitude of the redox changes. Therefore, the swelling-deswelling amplitude of the gel is controllable by changing the initial concentration of the substrates as well as the temperature. 

The oscillations in most demonstration experiments produce periodic color changes. However, other properties of the solution, like the electrical potential, oscillate as well. This is due to changes in the concentrations of the redox active species. The electrical potential changes can be observed by measuring the potential of a platinum electrode versus a reference electrode. The voltage oscillates in phase with the color changes. The range of oscillations in the classic Belousov-Zhabotinsky reaction is about 200 mV. If the solution is poured in a petri dish and left unstirred, mosaic patterns appear as spatial oscillations. 

The redox behaviour of Th, Pa and U is of the kind expected for d-transition elements which is why, prior to the 1940s, these elements were commonly placed respectively in groups 4, 5 and 6 of the periodic table. Behaviour obviously like that of the lanthanides is not evident until the second half of the series. However, even the early actinides resemble the lanthanides in showing close similarities with each other and gradual variations in properties, providing comparisons are restricted to those properties which do not entail a change in oxidation state. The smooth variation with atomic number found for stability constants, for instance, is like that of the lanthanides rather than the d-transition elements, as is the smooth variation in ionic radii noted in Fig. 31.4. This last factor is responsible for the close similarity in the structures of many actinide and lanthanide compounds especially noticeable in the 4-3 oxidation state for which... 

A short induction period is typically followed by an oscillatory phase, visible by the alternating colour of the aqueous solution due to the different oxidation states of the metal catalyst. Addition of a coloured redox indicator, such as the Fe,llZ,hl) phen)n couple, results in more dramatic colour changes. Typically, several hundred oscillations with a periodicity of approximately one minute, gradually die out within a couple of hours and the system slowly drifts towards its equilibrium state. 

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