Potentiometric Sensor For Total Residual Chlorine In Electrolytically Treated Ballast Water: Methods And Principles

Electrolytic treatment by in-situ generation of chlorine is adopted as one of the most economic and effective technologies for ballast water (BW) treatment due to its low cost, safe operation and good sterilization effect. BW electrolytic treatment system developed by our country has reached the international level, however, an in-situ device for detecting total residual chlorine (TRC) to make automatic control on the electrolytic apparatus still remains a challenge.To date, many methods have been developed to detect TRC, including iodometric titration, colorimetric methods, optical methods, and electrochemical methods based on amperometric measurement. They are all ex-situ methods with sampling and sample processing procedures and cannot fulfill the demand of automatic control on the electrolysis equipment. On the contrary, the potentiometric method is more suitable to monitor TRC in the electrolytic treatment of BW. Unfortunately, only a few researches on the detection of TRC by the potentiometric method have been reported so far. The electrodes such as Pt electrode coated with zephiran chloride and polyvinyl chloride (ZephCl-PVC-Pt), the electrode based on Ca(C10)2 active membrane and oxidation reduction potential (ORP) electrode which have been adopted for analyzing TRC are not suitable for detection of TRC in BW due to their poor stability, selectivity, reversibility and life time. Thus it is essential to develop an in-situ and quick potentiometric sensor to detect BW in TRC. To develop such a sensor is not only of important theoretical significance but also of broad application prospect.In this thesis, the research progress of the detection of TRC was reviewed firstly, with emphasis on the potentiometric sensor and its response mechanism. Then the performance of different substrates of ORP electrode was investigated. Finally, the performance of ORP modification electrode, redox-intermediated solid-contact ion-selective electrode (RI-SC-ISE), and ferrocene-based P-cyclodextrin (β-CD) inclusion complex electrode with enhanced stability of ferrocene were studied. The main research contents of this thesis include:(1) The response of different ORP electrodes to TRC was studied using open circuit potential measurement, cyclic voltammetry (CV) and Tafel polarization. The results show that the bare ORP electrode is not suitable for measuring TRC due to the continuous and irreversible oxidation of the electrode surface. Among these electrode materials, the glassy carbon is the most suitable electrode substrate. It was found that modification of the glassy carbon electrode with ferrocene-multi-walled carbon nanotubes (Fc-MWCNTs-GC) can greatly improve the selectivity of the electrode to TRC.(2) The preparation and performance of RI-SC-ISE for detecting TRC in BW were studied. A film-coated glassy carbon electrode containing ferrocene (Fc-ZephCl-PVC-GC) was successfully prepared using dripping-coating method. The electrochemical performance of the electrode was investigated using open circuit potential measurement, cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Compared with the performance of the ZephCl-PVC-GC electrode, the Fc-ZephCl-PVC-GC electrode showed good response speed, reproducibility and stability, demonstrating the good effect of Fc addition. The response time of the Fc-ZephCl-PVC-GC electrode was less than 50 s. The potential of the Fc-ZephCl-PVC-GC electrode was in linear proportion to the logarithm of TRC within the range of 1 mg/L to 20 mg/L. The possible co-existing ions in seawater had no evident interference on TRC measurement. The service life of the electrode was about one week. This suggests that the Fc-ZephCl-PVC-GC electrode is a promising sensor for the in-situ detection of TRC in BW. The response mechanism of the Fc-ZephCl-PVC-GC electrode was also discussed with a novel response mechanism proposed.(3) The effect of β-CD inclusion on the stability of ferrocene was studied. The electrochemical performance of P-CD-Fc-PVC-GC electrode demonstrated that β-CD could improve the stability of Fc but deteriorated the reversibility of the electrode. The poor reversibility is the result of the difficulty in reaction of Fc with TRC and desorption of TRC from β-CD.