Potentiometric analysis is used to measure theemfBased on the relationship between the electromotive force and theactivity(or concentration)Quantitative relationship(Nernst equation) to determine the activity or concentration of the substance to be testedElectrochemical analysis。It uses the liquid to be tested asChemical batteryOfelectrolyte solution, insert two electrodes, one iselectrode potential It changes with the activity or concentration of the ion to be measured in the test solution to indicate the ion to be measuredIonic activity(or concentration)Indicator electrode(often used as negative electrode), and the other one is basically stable at a certain temperature and does not change with the activity of the ion to be measured in the test solutionReference electrode(Usually used as positive pole), the content of the substance to be tested is determined by measuring the electromotive force of the battery.[1]
Potentiometric analysis uses substanceselectrochemistryA large class of analytical methods for analyzing properties.Electrochemical analysisIt mainly includes potential analysis methodCoulometric analysisandVoltammetryAndPolarographic analysisEtc.includeDirect potentiometryandPotentiometric titration。The direct potential method uses a special electrode to measure the ionactivityConvert toelectrode potential Then measure, if usedGlass electrodeDetermination of hydrogen in solutionIonic activity, with fluorineIon selective electrodeDetermine the fluoride ion activity in the solution (see ion selective electrode).The potentiometric titration method uses the sudden jump of the potential of the indicator electrode to indicateTitration end point。The difference between the two methods is that: the direct potential method only determines the free ions that already exist in the solution, and does not destroy the equilibrium relationship in the solution;Potentiometric titration is used to measure theTotal concentration。Potentiometric titration can be directly used for the titration of colored and turbid solutions.In acid-base titration, it can titrate but is not suitable for useindicatorOfweak acid。It can titrate weak acids with K less than 5 × 10-9.After sedimentation andRedox titrationDue to the lack of indicators, it is more widely used.Potentiometric titrationContinuous andAutomatic titration。
Use oneIndicator electrodeAnd aReference electrode, or use two indicator electrodes, andTest solutionThe method of forming a battery and then analyzing it according to the change of the electromotive force of the battery or the change of the potential of the indicator electrode is called potentiometry.
Coulometric analysis
Measure the electricity consumed in the electrolysis process, and pressFaraday's lawThe analytical method for calculating the content of the substance to be measured is calledCoulometric analysis。Coulometric analysis can also be divided intoCoulometric analysis of control potentialAnd constant current coulometric titration.
Chemical batteryIt is a device for mutual conversion of chemical reaction energy and electric energy,electrochemical reactionIt must be carried out in a chemical battery.Chemical batteries are divided into primary batteries andElectrolytic cell, the primary battery ischemical reactionThe electrolytic cell is a device that converts electric energy into chemical reaction energy.
Chemical battery
Primary battery
Convert chemical reaction into electric energy
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Electrolytic cell
Convert electric energy into chemical reaction energy
Primary battery
A device that converts chemical reactions into electrical energy.Zinc electrode is inserted into ZnSO4 solution, and CuSO4 solution is inserted into the same electrode. Porous barrier orSemipermeable membraneThe separation constitutes a primary battery.When zinc and copper electrodes are connected withExternal circuitWhen the load (with electrical appliances) of is connected, the electrons flow from the zinc electrode to the copper electrode through the load (the current flows from the copper electrode to the zinc electrode through the load).The primary battery occurs as followselectrode reaction andBattery reaction。
Electrode reaction:
Zinc electrode (anode) (-)
Potentiometry
Copper electrode (cathode) (+)
Potentiometry
Battery reaction:
Potentiometry
The zinc electrode of the primary battery is called anode, also called negative electrode;Copper electrode is called cathode, also called positive electrode.
Potentiometry
Electrolytic cell
A device that converts electrical energy into chemical reaction energy.
Electrolytic CuSO4 solution
Ionization: 2CuSO4=2Cu2++2SO 4H2O4H++4OH -
Electrolysis: cathode: (ions in the solution get electrons flowing into the C rod, and Cu is folded out)
Anode: 4OH － － 4e2H2O+O2(OH ions in the solution lose electrons, and electrons flow out from the C rod, releasing O2)
Under reversible conditions (andInfinite approachEquilibrium stateWhen the current passing through the battery is infinitely small)Potential differenceIt is called electromotive force.The electromotive force of the primary battery is equal to all the components of the batteryPhase interfaceAlgebraic sum of potential difference.
The mark "|" or "," indicates the phase interface.There are four phase interfaces on the primary battery, namely Cu | Zn, Zn | Zn2+, ZnSO4 | CuSO4 and Cu2+| Cu.There are interfacial potential differences on the four phase interfaces.
electrode potential
After the metal is inserted into the solution, the atoms in the metal have the tendency to lose electrons and leave the metal surface in the nature of ions to enter the solution, which is the dissolution of metal;In solutionmetal ionThere is also a tendency to get electrons on the metal surface into the metal, which is the deposition of metal ions.When the dissolution of metal in the deposition of metal ions reachesdynamic equilibriumThe potential difference on the contact interface between the electrode and the solution is called electrode potential.This isMetal electrodeThe cause of the electrode potential of.
Electrons of different metalsEscape workThis is the reason for the metal contact potential.The so-called metal electron work refers to the energy required for electrons in the metal to leave the electron and enter the vacuum.Due to the different work of electrons escaping from different metals, the contact between different metals is mutual escapeNumber of electronsDifferent, metal contact potential is generated at the contact interface of two metals.However, the metal contact potential is very small, which can generally be ignored.
NonionicMigration rateDifferent, this is the cause of liquid connection potential.Liquid connection potential exists at the following two interfaces:
1. Contact interface between solutions of two different solutes
2. Contact interface between two solutions with the same solute but different concentrations
The size of liquid connection potential depends on the composition and concentration of solutions on both sides of the contact interface. When the composition concentration of solutions on both sides is constant, the size of liquid connection potential is also constant.useSalt bridgeSubstituted for two solutionsdirect contact The liquid connection potential can be basically eliminated.The salt bridge is composed of high concentrationelectrolyte solutionloadU-tubeComposition, of which,electrolytePositiveanionThe migration rate of is basically the same.becausePositive and negative ionsThe migration rate is basically the same, and the size of the liquid connection potential at both ends of the salt bridge (liquid connection potentialabsolute value) are basically the same;And because the migration direction of positive and negative ions is opposite, the direction of the liquid connection potential at both ends of the salt bridge is also opposite (the symbol of the liquid connection potential), so the algebraic sum of the liquid connection potential at both ends of the salt bridge is approximately zero.
The basic principle of potentiometric analysis is to use two electrodes and the solution to be measured to form a working cell (galvanic cell)emfTry to find out the content of the substance to be tested.
The two electrodes that make up the working battery are calledIndicator electrodeandReference electrode。The indicator electrode refers to theelectrode potential Determined with the content of the substance to be testedFunctional relation, i.e. ψ indicates=f (a) (where a is theactivity）。The so-called reference electrode means that the electrode potential of the electrode has no relationship with the content of the substance to be measured, and always keeps constant during the measurement process, that is, ψ reference=constant.
If the liquid connection potential is not considered and the indicator electrode is used as the positive electrode and the reference electrode as the negative electrode, the relationship between the electromotive force E of the working battery and the content of the substance to be measured is
Measure the working batteryemfAccording to the above formula, the content of the substance to be tested can be calculated.
It should be noted that known electrodes act asIndicator electrodestillReference electrodeIt is not absolute. The same electrode may act as an indicator electrode in some determinations and a reference electrode in other determinations.Theoretically, as long as the potential of the electrode and the content of the substance to be measured are certainFunctional relationCan be used as an indicator electrode;The potential of the electrode is independent of the content of the substance to be measured and keeps constant, so it can act as a reference electrode.
withinglass tubeLower layer placementCalomel（Hg2Cl2）The paste with mercury is placed on the upper layer, and a platinum wire (wire) is inserted into the mercury. The lower end of the inner glass tube is porous material to prevent calomel and mercury from leaving the glass tube (ions can pass through porous material);The internal solution of KCl with a certain concentration is squared in the outer glass tube, and the contact part between the lower end of the outer glass tube and the solution to be measured is also porous material (sintered ceramics orGlass sandCore).
Place a certain concentration of KCl internal solution in the glass tube;Silver filigreeSurface platingSilver chlorideThe silver silver chloride electric wire is formed, and the electric wire is immersed in the internal solution of KCl;The contact part between the electrode and the solution to be measured is porous material.
