Metal-based PH Electrode And Its Application In Seawater PH Monitoring

Since the industrial revolution,the extensive use of coal,oil,natural gas,and other fossil energy sources releases a large amount of carbon dioxide,disrupting the Earth’s original carbon balance,leading to global warming and ocean acidification.Ocean acidification seriously affect the security of marine environment,and further threatening the sustainable development of human.Therefore,it is of great significance to pay attention to the safety of Marine water environment.Seawater pH value is one of the basic parameters to reflect the safety of the marine ecological environment.It is of great significance to accurately monitor seawater pH value.Glass composite electrode is the most commonly used pH sensor.Its application in the field of marine environment monitoring is limited by the shortcomings of glass shell,such as low mechanical strength,fragility,and not high temperature and high pressure resistance.Metal-based pH sensors have the advantages of high mechanical strength,high temperature and high pressure resistance,long service life,stable pH response performance,and others,making them very suitable pH sensors for marine environment monitoring.The purpose of this paper is to develop a metal-based pH sensor suitable for marine environmental monitoring,to achieve continuous,accurate,and real-time monitoring of seawater pH value,to increase our understanding of marine pH dynamic value,early warning of possible harm caused by "carbon emissions",and safeguarding human health and sustainable development.The main research contents of this paper are as follows:(1)pH electrodes with excellent performance of silver-antimony binary alloy were developed.The silver-antimony alloy electrodes were prepared for the determination of pH value by one step high temperature melting casting method according to the phase diagram of a silver-antimony binary alloy.The alloy electrodes were characterized by XRD,SEM,and EDS,and the response mechanism of the pH electrodes was studied by the Tafel plot.It was confirmed that the response mechanism of the electrodes was the corrosion of the Sb phase,while the Ag3Sb phase was relatively stable.The results of electrochemical impedance spectroscopy(EIS)showed that the corrosion resistance of the alloy electrodes were enhanced with the increase of silver mass fraction,which displayed certain advantages over the pure antimony electrode.The response sensitivity,response time,stability,reversibility,hysteresis effect,and temperature coefficient of the silver-antimony alloy pH electrode prove that the alloy electrode has excellent pH response performance and has a wide application prospect.(2)An all-solid-state antifouling pH sensor based on a silver-antimony alloy electrode was developed and successfully applied to pH monitoring in the marine environment.We prepared an all-solid-state pH sensor by combining a silver-antimony alloy electrode with an all-solid-state reference electrode.Seawater Tris-HCI was used for multi-point calibration and comparasion with the glass composite electrode.The results demonstrated that the all-solid-state sensor has the similar accuracy as the glass electrode,and the result of in-situ monitoring in marine environment is good.Its successful application in seawater monitoring avoids the problems of easily fragile,poor anti-pollution ability,and large electrode maintenance of glass electricity,and provides instrument support for online monitoring of marine environmental safety and deep-sea exploration of marine mineral resources in the future.(3)The renewal and regeneration method of metal antimony-based pH sensors are studied to extend the service life of the sensor,reduce the operation and maintenance cost,and achieve the reuse the sensor.Mechanical polishing and electrochemical cathodic reduction methods were proposed to renew the antimony-based sensor with performance decreased or even pH response ability lost.The reason for the degradation of the sensor performance is analyzed,and it was concluded that the adsorption of the Sb2O3 layer is the main reason.The regeneration mechanism is the removal of the old oxide layer adsorbed on the surface of the antimony-based electrode.Considering the maintenance of the sensor for long-term monitoring,the remote updating of the sensor can be achieved by the cathode reduction method.Our results are of great significance for reducing the operation and maintenance cost of long-term monitors,realizing the renewal and regeneration of sensors,and promoting the development of metal-based sensors.(4)The open circuit potential drift of metal-base pH electrode during the testing process limits its wider application.We explore the feasibility of using electrochemical polarization(linear sweep voltammetry and Tafel polarization methods)to measure pH value with metal electrodes(platinum,iridium,and rhodium).The results show that the polarization overpotential decreases with the increasing pH value,and there is a good linear relationship,confirming the feasibility of the LSV and Tafel polarization methods for pH analysis and measurement.By analyzing the microstructure of the metalsolution interface,it is determined that the mechanism of pH analysis by polarization methods is based on an oxygen evolution reaction.This part of the research is an exploration besides the open circuit potential method to measure pH value.The purpose of this part is to avoid the problem of inaccurate test caused by the easy drift of open circuit potential of metal-based electrodes.However,there are still many problems that we hope to solve gradually in the subsequent study.