Sysnthesis Of Lead Dioxide Thin Layer Electrode And Its Application On Determination Of Chemical Oxygen Demand

Chemical Oxygen Demand (COD) is one of the important indices for evaluating water quality. At present, the standard COD determination method is potassium dichromate method or permanganate method which need more chemical reagents and the structure of this kind of analyzers is also complex. What worse is that they result in secondary pollution and long test time. Optical or electrochemical methods developed in recent years is an important way to realize portable COD rapid detection because they can avoid a lot of disadvantages involved in above methods, but there is a deviation problem of the data as different detection mechanism.This text propose a electrochemical detection method of COD which is based on PbO2 electrode, the mechanism of this method is that the hydroxyl radicals generated on PbO2 electrode surface are consumed through oxidizing organic matter in water and anode response current, which is caused by regeneration of hydroxyl radical on electrode surface is proportional to the concentration of organic substances. However, the oxidation ability of the hydroxyl free radical is different from the chemical oxidants such as potassium dichromate. So, analyzing the consistent between the results of COD value detected by electrochemical and the national standard method and the reason of the deviation between them, it can provide us important theoretical basis to realize electrochemical detection of COD.Two kinds of electrodes have been prepared and applied to the detection of COD in water, one is β-PbO2/graphite which was prepared on graphite substrate by electrochemical deposition method; In addition, F-β-PbO2/Sb-SnO2/Ti electrode was prepared by combining sol-gel and electrochemical deposition two-steps synthesis methods. The main research results include the following several aspects:(1) The results of XRD and SEM to β-PbO2/graphite indicated that the presence of crystalline structure on electrode surface is β-PbO2 with uniform grain size distribution and high purity crystalline phase. A finding of research, the oxygen evolution potential of β-PbO2/graphite electrode in the neutral support electrolyte is 1.6V (vs Ag/AgCl). Electrode surface produced a large number of adsorbed hydroxyl free radicals before the oxygen evolution during the constant potential electrolysis (such as 1.45 V); hydroxyl radicals are consumed by organic matter with reducibility, anode response current which is caused by regeneration of hydroxyl radicals is proportional to the concentration of organic matter. The factors such as the electrolytic potential, concentration of supporting electrolyte, pH and stirring rate have been investigated in paper. The results which were measured under the optimum test condition shown that, good linear relationship between the current intensity and the concentration for 8 kinds of organic matter and the proportionality coefficient of them is different with each other. The linear regression co-efficient between the net current response value and the concentration of the simulated water samples containing five kinds of organic matters is 0.997. The deviation between the result of electrochemical determination and the potassium dichromate method is below 5% to the actual water samples.(2) In order to overcome the disadvantage of the graphite substrate, such as lack of electrical conductivity and short of mechanical strength, we try to use titanium plate as substrate and prepared the Sb-doping SnO2 layer on it by sol-gel method to inhibit the growth of titanium surface oxide layer to increase the electrical conductivity and to improve the adhesion of PbO2 coating. The research indicated that it can obtain smaller grain size; (101) crystal plane preferential growth of pure F-β-PbO2 coating in modifying solution which contains F-. The calibration plot between the current intensity and the concentration of glucose obtained by constant potential electrolysis under 1.5V is y=0.0056x+0.1423; linear range is 50-2500mg/L; detection limit is 17.8mg/L. Compared with β-PbO2/graphite electrode, it has lower detection limit and higher detection sensitivity.