Novel Method Of Pollution Detecting Based On System Of Liquid/liquid Interface For Electrolyte Dropping Electrode

With the development of environmental monitor, the demand of online monitoring isincreasing. Due to the high sensitivity and simultaneous determination of various metalions, electrochemical analysis is widely used in the environmental monitoring. Howeversolid electrode surface is easy to be deposited and contaminated by electrolytic products; itis inevitably to be prepared by complicated pretreatment repeatedly, which limits thepractical application of electrochemical analysis.By opposite, the liquid electrode such as mercury electrode is good at its reliability.The continuous renewal of the mercury electrode surface thoroughly avoids the depositionand contamination. However mercury is high toxic. With the development of organicsynthesis, people pay more attention to green orangic solvent Room Temperature IonicLiquids （RTILs）. RTILs own high ionic conductivity, wide potential window, thermal andelectrochemical stability. It is possible to construct a novel system of electrolyte droppingelectrode.In this work, RTILs1-butyl-3-methyl imidazolium hexafluorophosphate （Bmim[PF₆]） was used to be organic electrolyte, and precise roller pump was used to control thesize of aqueous electrode. A novel electrolyte dropping electrode was constructed. Theions transferred across the liquid/liquid interface of electrolyte dropping electrode wasstudied by three-electrode system.The main experimental results are as follows： （1）The electrolyte dropping electrode was construct and the Activated carbon fiber（ACF） was used to be electrochemical probe. The size of electrode was controlled byflow-injection system. The electrolyte dropping electrode was used to determine peakcurrent of Cd²⁺transfer across the water/MIBK interface, and platinum wire and Ag/AgClwere used for the conter electrode and reference electrode respectively.The form of ligand in liquid/liquid interface was investigated. With the changes of pH, theneutral form of DzH₂was predominant at pH lower than4.9, on the contrary the ligandanion involved interface compexation was DzH-when the pH wasis higher than4.9. Forthe phen, the neutral form of phen wasis predominant at the pH higher than4.1, and whenthe pH was varied from2.2to4.1, the ligand cation involved interface was HL+, and HL2+was predominant at pH lower thand2.2.（2）The metal ions assisted by ligand transferred across the water/MIBK interfacewere studied. The concentration of metal ions would be determined rapidly. The detailwork represents as follow:The electrolyte dropping electrode was constructed, in water/methyl isobutylketone（MIBK）-DzH₂system Cd²⁺assisted by DzH₂transferred across the water/MIBKinterface. The transfer process was irreversible and controlled by the diffusion. The spikepotential appeared at-0.773V （vs.Ag/AgCl）. The metal ligand species Cd（DzH₂）₂²⁺formed in the water/MIBK interface. The peak currents were proportional to theconcentrations of Cd²⁺within the broad range from1×10^-6mol.L^-1to1×10^-4mol.L^-1, andthe detection limit was8×10^-7mol.L^-1. The practical sample of Ni-Cd battery wasmeasured and the result was compared with ICP-OES.In water/MIBK^-1,10-phenanthroline （phen） system, Pb²⁺assisted by phen transferredacross the water/MIBK interface was investigated; and it was irreversible and controlledby the diffusion. The spike potential appeared at-0.331V （vs.Ag/AgCl）.The metal ligandspecies formed in the water/MIBK interface. In order to simplify the experimental procedures the auto-sampler was performedbased on two-way pipe of flow-injection system. The detail work of new electroderepresents as follow:The concentration of Pb²⁺was measured, and the peak current was proportional to theconcentrations of Pb²⁺within the broad range from8×10^-6mol.L^-1to5×10^-4mol.L^-1andfrom5×10^-4mol.L^-1to1×10^-3mol.L^-1. Different concentrations of Pb²⁺were measuredeighth times and the relative standard deviation was from1.38%to5.80%.Different concentration of Cd²⁺was determined online. If the injector speed wasslower,45minutes later the peak current would reach equilibrium.5×10^-4mol.L^-1，1×10^-3mol.L^-1，1.5×10^-3mol.L^-1Cd²⁺was measured and the relative standard deviation oftransfer peak current was0.714%，2.16%，1.26%respectively. The concentrations ofCd²⁺were proportional to the average peak current and the linear regression coefficientwas0.991. If the injector speed was quicker,10minutes later the peak current wouldreach equilibrium. The stability of electrode was good.（3）The organic dropping electrode was constructed.1,2-Dichloroethane and Bmim[PF₆] were organic phase and organic electrolyte respectively, and pt wire waselectrochemical probe. The three-electrode system was used to measure the pH ofsolution.The peak current of H⁺transfer across the water/DCE interface was measured byorganic dropping electrode. Supporting electrolyte of aqueous phase affected thedetermination results. The dependence of the transfer peak current on the concentration ofH⁺was well linear-fitting within the range from pH2to12. The sodium dodecyl sulfate（SDS） existed in organic phase effectively reduced the interference of other heavy metalions.The practical samples with the similar media, such as juicy and dye wastewater, weredetermined. The dependence of the transfer peak current on the concentration of H⁺waswell linear-fitting within the range from pH1.32to6.41, and the standard curve wereY=-4.4175-0.1583pH，Y=-7.8432+6.8546pH, Y=-2.6652-0.6030pH. A variety of muddy even colorful sample could be determined directly by organic dropping electrode whichwas a good complement for glass electrode.（4）Cooperation with the Jiangsu electrochemical instrument Co., Ltd, commodityinstruments of electrolyte dropping electrode have been constructed. The electricresistance of working electrode was more than10Ω. The electrolyte dropping electrodewas used to measure the concentration of Cd²⁺, and peak current of Cd²⁺transfer acrossthe water/MIBK interface was linear with concentration. The standard curve was Y＝7.1968+10221.4X and the linear regression coefficient was0.997. Electrolyte droppingelectrode was stability when the sample temperature was changed from7to41oC, whichmeet the demand of fieldwork. The concentration of Cd²⁺was linear with the averagetransfer peak current. the standard curve was Y=3.56+29956.02X and the linearregression coefficient was0.995The instrument can achieve automated determination of heavy metal ions more than24hours. The sample would be measured in20minutes and the stability of onlinedetermination has been tested.