The kinetics of the anodic dissolution of bismuth in acid solutions
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. A potentiostatic sweep technique was used to study the kinetics of the anodic dissolution of bismuth from a rotating disc electrode into acid solutions. The current density was measured as a function of potential, anion concentration and pH, at constant ionic strength which was maintained using aqueous perchlorate media. The anions studied, (NO3-, HSO4-, Br- and I-), accelerate the dissolution process in the order I->Br->HSO4-=NO3->ClO4-. The electrode dissolved actively except at [I-]<0.2M, where an insoluble BiI3 film was precipitated on the bismuth surface. Reproducibility of current density between the forward and reverse potential sweeps was used as a criterion for steady-state kinetics, which were observed in all solutions except when [HSO4-]<0.5M. Corrections were made to the experimental data for the effects of reference electrode junction potential, electrolyte resistance (ohmic drop), and the mass transport of halide ions to the surface of the electrode. A Tafel slope, b=60±4mV was observed in HClO4 solutions; it decreased as [NO3-] or [HSO4-] in the solution increased. In 1.0M HNO3 and 1.0M H2SO4, b=38±3mV, and the NO3- and HSO4- reaction orders were both one. The current density observed in the mixtures, HClO4-HNO3 and HClO4-H2SO4, was described by a liner equation, i=A+B[L], where L=NO3- or HSO4-; A is the contribution to the total current from the perchlorate path and B[L] is the current from a NO3- or HSO4- catalysed path. Two mechanisms were found which could account for the results. The first involves the formation of an adsorbed Bi(I) intermediate according to the equilibrium: Bi+H2O ? BiOH(ads)+ H+ + e-. The rate determining step is the desorption of this intermediate from the surface, accompanied by a simultaneous 2-electron transfer. The NO3- and HSO4- catalyse this final desorption step. The changing Tafel slopes are accounted for by a changing coverage of BiOH(ads), which is described by a Langmuir isotherm. The second mechanism involves the formation of an adsorbed Bi(II) intermediate in two fast one electron transfer steps at equilibrium: Bi+H2O ? BiOH+(ads)+ H+ + 2e-. The fractional coverage of BiOH+(ads) is described by a Temkin isotherm and NO3- and HSO4- catalyse the rate determining desorption of this species from the surface: BiOH+(ads) + HSO4- ? BiSo4+(aq) + H2O + e- Under Temkin conditions this desorption step has the rate equation: i=3Fk[HSO4-]exp(?FE/RT)[exp(?f?)–1] where ? is the transfer coefficient and f is the Temkin heterogeneity parameter. In ClO4- solutions the fractional coverage of BiOH+(ads)=1. The reaction proceeds via the equilibrium: BiOH+(ads) ? BiOH2+(ads) + e- followed by the rate determining step: BiOH2+(ads) ? BiOH2+(aq) Kinetic parameters for both mechanisms have been evaluated, and the experimental results agree well with the data calculated from the kinetic expressions. In bromide solutions, b increases from 22.5±0.3mV to 44±1mV with increasing potential. The Br- reaction order decreases from 5 to approximately 2.5 with increasing [Br-]. In iodide solutions the I- reaction order is 2.4±0.2; b varies in the range 60±10mV with potential and [I-]. The mechanism proposed, involves the formation of an adsorbed Bi(II) intermediate according to the equilibrium: Bi+4L- ? BiL42-(ads) + 2e- where L=Br- or I-. The fractional coverage ?, of BiL42-(ads) was described by a Temkin isotherm. The rate determining step is: BiL42-(ads) + L- ? BiL52-(aq) + e-. Under Temkin conditions it has the rate equation: i=3Fk[L-]exp(?FE/RT)[exp(?f?)–1] The variation in anion reaction order and Tafel slope is accounted for by the changing coverage of the adsorbed intermediate. Excellent agreement was found between the experimental and calculated results using derived values of the kinetic parameters.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1972