| pH value is one of the key physico-chemistry parameters of solution, therefore the pHdetermination is significant to many fields, such as chemistry, metallurgy, food, biology andmedicine, etc. Currently, the pH value is mainly measured with glass pH electrode. Theglass electrode is prohibited from applying in some situation, resulted from its shortage,such as being easy to be broken, hard to miniaturize, without high mechanical strength, andbeing not suitable to application in F- containing solution. Metal/Metal oxide pH electrodewas seen as one of alternatives to glass electrode, having high strength, and exhibits perfectresponse behavior in wide pH range. Ir/IrOx was prepared with a method of oxidization in carbonate melt at hightemperature in this paper. The fabricated electrode shows good pH response characteristicsin pH 1-13 with a response sensitivity of 57.2mV/pH, and the response time of it is less than1min. The Ir/IrOx electrode is of high selective to H+, as well as having little hysteresiseffect in the course of pH determination. The coated ion selective film on the electrodeimproves the resistance to the interference of oxidative/reductive negative ions. SEM wasused to observe the bore diameter distribution on the surface and cross-section of the coatedselective film on the electrode, for the purpose of optimize the prepared process of the film.The ion selective mechanism was presented based on the calculation of positive/negativeion transport coefficient, i. e. t+ and t-, with the method of film potential measurement. Thedecisive factors to response time and sensitivity were discussed in this paper. It is found thatthicker oxide film and higher solution temperature will shorten the response time, and theresponse sensitivity is also related to the film thickness. Cyclic voltammetry experiment wascarried out to calculate the intergral charges in the course of potential scanning from 0V to1V(VS.SCE), and a model was built to determine the composition of outer and inner surfaceof the oxide film. The effect of oxidized process to the response activity of the film wasexplained according to the above cyclic voltammetric charge model. It was found that thehydration effect takes place in the aging process, and the aging makes an effect to theresponse behavior. EDX, XPS were used to analyse the rate of Ir:O and valence of Irelement in the oxide film. It is found that the Ir:O is 1:6.78 in the oxide surface, while isnearly 1:2 in the bulk of the oxide. The valence of the Ir element is mainly composed ofIr+4,while it partly transfers to Ir+3 in the reaction course. The hydration equation wasdescribed as: IrO2 + 4H2O → IrO2 ?4H2O ,IrO2 ?4H2 O→[IrO2(OH)2 ?2H2O]2 + 2H+ ? The response mechanism was suggested for the thermo-oxidization prepared electrode,which took the hydration effect into consideration in this paper. The thermodynamicresponse equation could be written as: 2 IrO2(OH)2 ?2H2O]2 ?2Hf + 2e? + 2Hs → {[ ? +} + [Ir2O3(OH)3 ?3H2O]3? ?3Hf +3H2O + IIIEIS was used to investigate the dynamic course of the H+ response, and EIS equivalent circuitcould be described as: Rs(R1Q1)(RctQ2). It is pointed out that the control step of the H+ response isthe electrochemical reaction on the electrode surface. It also gave an explanation of the hysterisiseffect in the H+ determination loop of pH3→9→3 and pH9→3→9. Preparation of W/WO3 pH electrode with sol-gel method was presented here. TheW/WO3 electrode shows good pH response characteristics in pH 2-11 with a response timeof less than 1min. The response sensitivity is affected by heat treatment temperature, and itvaries between 37-52mV/pH according to different heat-treatment temperature. The besttreated temperature w... |
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