| Chemical oxygen demand (COD) is an important parameter for the evaluation of water quality and it is also one of the four reduction indicators during the twelfth five-year plan. The most common COD determination method is dichromate titration. The on-line COD analyzer based on dichromate method results in secondary pollution and the structure of this kind of analyzer is also complex. The electrochemical method developed in recent years can avoid many disadvantages involved in the conventional method. In the past research, the environmentally friendly material, boron doped diamond (BDD), was used as a sensor to determine COD under manual operating condition. In this dissertation, a practical BDD film electrode is fabricated and applied to a homemade on-line COD analyzer, and then the relative measurement parameters are optimized. Additionally, two kinds of composite electrode materials, Ti/BDD/PbO2 and Ti/Sb-SnO2/PbO2, are prepared, enhancing the detection limit of COD measurement by electrochemical method. The application of both electrodes would be extremely attractive in the field of COD determination for surface water or other clean waters with low COD values. Around what mentioned above, the following work has been done:(1) A microwave plasma chemical vapor deposition system (MPCVD) was used to prepare BDD films. The source gas (flow rate 100 sccm) with a mixture of methane (1 sccm), hydrogen (89 sccm), and B2H6 (10 sccm) was employed as the reagents. Temperature of the substrate was 800℃and pressure of the reacting chamber was in the range of 4000-6000 Pa. The fabricated BDD film was composed of well faceted diamond crystals in the micron range. The characteristic peak of diamond in Raman spectrum appeared clearly at 1334 cm-1 and the diffraction peaks of diamond (111) and (220) in XRD pattern were observed, indicating that the quality of the BDD film was perfect. Then the BDD film was assemble into a practical BDD film electrode with good sealing performance. The assembled BDD film electrode would be an important part of the homemade on-line COD analyzer.(2) Based on the assembled BDD film electrode, a homemade on-line COD analyzer was developed. The factors such as the instrument framework, the main components, the measurement process and the model of the calibration were discussed. The optimum B/C ratio was 10000 ppm, the diameter of electrode was 8 mm, the cleaning applied potential was 4 V and the cleaning time was 90 s. Under the optimized conditions, the analyzer was successfully employed to determine the COD values of glucose samples with different concentration and the resulting relative standard deviation (RSD) was below 5%. As compared with the standard method, the homemade analyzer could be carried out rapidly and overcome the shortage of secondary pollution. Therefore, the analyzer was promising to realize market operation.(3) A novel Ti/BDD/PbO2 composite electrode was prepared for COD determination via chemical vapor deposition (CVD) combined with electrodepositition method. Under the potential of 1.45 V, a linear range of 0.5-175 mg/L and a detection limit of 0.3 mg/L were achieved. The real samples from sewage plant, food and dyeing factories were applied to test the feasibility. The results measured by Ti/BDD/PbO2 composite electrode agreed well with those determined by the standard method. The proposed method with lower detection limit showed a respect in COD detection of slight polluted water.(4) A Ti/Sb-SnO2/PbO2 electrode was fabricated and applied in the area of COD determination. The optimum applied potential, pH range and electrolyte were 1.4 V,3-9 and 25 mM Na2SO4. Under the optimized testing conditions, the relationship between the net current and COD,inet=2.059COD+0.3762, was obtained. The linear range and the detection limit were 0.5-200 mg/L and 0.3 mg/L (S/N=3), respectively. The COD measurements for real water samples from reservoir, municipal sewage plants, and dyeing factories could be well completed and the resulting largest RSD was lower than 12%. So the proposed method would promote the development of a fast, in situ and on-line COD analysis.In conclusion, the structure of the on-line COD analyzer based on the BDD film electrode was simple and the measurement range is wide. More importantly, the analyzer could be carried out rapidly and overcome the shortage of secondary pollution. Meanwhile, the prepared Ti/BDD/PbO2 and Ti/Sb-SnO2/PbO2 composite electrode were sensitive enough to meet the requirement of pollution monitoring, which was hoped to promote the development of COD determination by electrochemical method.
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