PolarographyElectrolytic processThe current potential (or potential time) curve of the polarized electrode obtained in is used to determine the concentration of the measured substance in the solution.In 1922 byCzech RepublicchemistJ. HelovskyEstablish.Polarography andVoltammetryThe difference betweenPolarized electrodeIs different.Polarography uses mercury dropping electrode or other liquid electrode whose surface can be periodically renewed as polarization electrode;Voltammetry uses a liquid or solid electrode with static surface as the polarization electrode.
Chinese name
Polarography
Foreign name
polarography
Purpose
It can be used to measure most metal ions, etc
Classification
DC, AC, single scanning square wave polarography, etc
Polarography and voltammetryBothElectrochemical analysisDetermine the concentration of the measured substance in the solution by measuring the current voltage (or potential time) curve obtained during electrolysis.The difference between them and other electrochemical analysis methods in the same kind is that a polarization electrode and a depolarization electrode are used in the electrolytic cell.The difference between polarography and voltammetry lies in the polarization electrode.Polarography refers to the use of mercury dropping electrode or other liquid electrode whose surface can be periodically renewed;Those who use liquid or solid electrodes with static surface are called voltammetry.
Both polarography and voltammetry are based on the electrolysis process. The electrolysis process is divided into two categories, and polarography and voltammetry are correspondingly divided into two categories: control potential polarography and control current polarography.In the control potential polarography, the potential of the polarized electrode is the controlled object and the excitation signal;The current is the measured object and the response signal.In the control current polarography, the current is the controlled object and the excitation signal;The potential is the measured object and the response signal.
The basic device of polarography is shown in Figure 1. The electrode that electrolysis takes place is a drop mercury electrode. The upper end of this electrode is a mercury storage bottle. The mercury in the bottle enters the capillary tube (with an inner diameter of about 0.05mm) through a plastic tube, and then drops regularly into the solution of the electrolytic cell, so that the surface of the drop mercury electrode is constantly updated to obtain good reproducibility and accuracy.The other electrode mostly uses saturated calomel electrode (SCE), and occasionally uses Ag AgCl electrode.The potentiometer circuit is composed of DC power supply B, variable resistance R and sliding resistance DE.Through the potentiometer circuit, the potential (+0m5 ～ - 2V) applied to the electrolytic cell C can be continuously changed, and the potential value can be indicated by the voltmeter V.If the current changes during the voltage change, check
The current voltage curve recorded by current meter G is called polarographic wave.Taking the dilute solution of electrolytic cadmium chloride as an example, the process of polarographic wave generation is described below.Set the concentration to 5 × 10-4mol·L-1CdCltwoAdd the test solution into the electrolytic cell and add about 0.1mol · L at the same time-1KCl solution as supporting electrolyte.Nitrogen or hydrogen is introduced to remove the oxygen dissolved in the solution, and then mercury drops are dropped at the rate of 3-5 seconds per drop.Move the sliding resistance to make the applied voltage on the two poles gradually change from 0V to - 1V.Record the corresponding current value, draw with voltage as abscissa and current as ordinate to get Cd2+The current voltage curve of is the polarographic wave of cadmium ion, as shown in Figure 1.It can be seen from Figure 1 that when Cd is not reached2+Only a very small current (residual current) passes through the solution at the precipitation potential of.When the applied voltage increases to Cd2+At precipitation potential (between - 0.5 and -0.6V), Cd2+Start electrolysis on the dropping mercury electrode;At this time, Cd2+Reduction to cadmium amalgam, mercury oxidation to Hg on anodetwo2+, and mixed with Cl in solution-Generate HgtwoCltwo：
At this time, the applied voltage increases slightly, and the current increases rapidly. Due to the concentration polarization, Cd2+It diffuses to the surface of dropping mercury electrode for reduction and generates electrolytic current.The current generated by electrode reaction due to continuous diffusion is called diffusion current.However, when the voltage changes to a more negative value, due to the limitation of the diffusion speed of the measured ion, the current no longer increases, but reaches a limit value. The current at this time is called the limit current.After subtracting the residual current from the limiting current, it is the limiting diffusion current2+The concentration is proportional to
The expression can be used as the basis for quantitative analysis by polarography.
Another feature on the polarogram is the half wave potential, that is, the potential of the mercury dropping electrode when the diffusion current is half of the limiting diffusion current.When the composition and temperature of the solution are fixed, the half wave potential of each substance is fixed and does not change with its concentration, which can be used as the basis for qualitative analysis.[2]
also calledPotentiostatic polarography。By measuringelectrolysisDuring the process, the current potential curve is obtained to determine the concentration of the measured component in the solution.Its characteristic is that the rate of electrode potential change is very slow.It is a widely used rapid analysis method, which is suitable for the determination of substances that can be reduced or oxidized on the electrode.
Alternating current polarography
A low frequency with small amplitude (several to tens of millivolts)sineThe voltage is superimposed on theDC voltageAbove, by measuringElectrolytic cellThe AC polarographic wave is obtained by the branch current of, and the peak potential is equal to the half wave potential E of the DC polarography1/2, peak current ipIt is proportional to the concentration of the measured substance.The characteristics of this method are as follows: ① AC polarographic wave has a peak shape, its sensitivity is higher than DC polarographic wave, and the detection limit can reach 10-7② It has high resolution and can distinguish the adjacent two polar spectral waves with peak potential difference of 40mV. ③Strong anti-interference ability, frontReducing substanceIt does not interfere with the polarographic wave measurement of post reducing substances. ④The superimposed AC voltageElectric double layerRapid charging and discharging, large charging current, limit the further reduction of the minimum detectable concentration.
Single sweep polarography
At the later stage of the growth of a mercury drop, when its area is basically constant, quickly apply aImpulse voltage, while usingOscilloscopeObserve the current generated on a drop of mercury?Voltage curve.The characteristics of this method are: ①Polarographic wavePeak shape, sensitivity ratioDirect current polarography1~2 orders of magnitude higher, and the lower detection limit can reach 10-7② High resolution, strong anti-interference ability.When the peak potential difference between the adjacent two poles is 50 mV, the concentration of the pre reducing substance is 100~1000 times greater than that of the post reducing substance, and does not interfere with the determination. ③Rapid applicationpolarizationVoltage, which generates large charging current, so effective measures to compensate charging current shall be taken. ④Irreversible processThere is no polarographic peak, which is reduced or even completely eliminatedOxygen waveInterference.
Square wave polarography
Square wave polarography is the polarographic method that superimposes a low frequency, small amplitude (≤ 50 mV) square wave voltage on the ordinary, slowly changing DC voltage, and records the AC current passing through the electrolytic cell at the moment before the square wave voltage changes direction.It is a kind of polarography and one of the methods with high sensitivity in polarography.Under appropriate conditions, the minimum concentration can reach 10-7mol/dmthreeThe detection limit of individual ions reaches 10-8mol/dmthree。It has high sensitivity because it records the current when the charging current disappears, and there is no charging current in the polarographic current. Therefore, the sensitivity can be improved by amplifying the current.Square wave polarography has good resolution and can separate Cd wave and In wave (the difference between them is 45mV).The current in front of the square wave polarography has little influence on the current in the back, so Sn is measured2+Front Fe3+Wave pair Sn2+Low interference, Fe3+:Sn2+It can reach 20000:1.Square wave polarography is a powerful tool for trace component analysis.[1]
Pulse polarography
Polarography is one of the most sensitive polarographic methods.It is several times more sensitive than square wave polarography, and the concentration of supporting electrolyte used is also lower than square wave polarography (as low as 0.02mol/dmthree）。There are two kinds of pulse polarography: conventional pulse polarography and differential pulse polarography.There are differences in the way of applying voltage between the two, and the polarographic patterns are also different.The way in which the differential pulse pole increases the voltage is similar to the square wave polarography, and the polarographic wave obtained is also similar.The difference between differential pulse polarography and square wave polarography is that at a certain time of a mercury drop, only one square wave voltage is applied, Δ E is 2-100 millivolts, and the square wave voltage lasts for a relatively long time, some as long as 40-80 milliseconds. Only one reduction current is measured on a mercury drop;Square wave polarography adds many square waves to a mercury drop, and each square wave lasts for a short time. The current is measured twice in a cycle, including both oxidation current and reduction current.The capillary noise current is fully attenuated by pulse polarography and voltage, thus improving the sensitivity.Pulse polarography can detect 10-9G/ml (1 ppb) of cadmium.Good resolution. The difference between the two peaks is 25 millivolts.The front polarographic current has little effect on the rear current.
Oscillopolarography
In general, useCathode-ray oscilloscopeThe polarographic method for observing or recording the polarographic curve can be called oscillopolarography.There are two kinds of oscillopolarography.One is called linear displacement oscillopolarography (single sweep polarography), and the other is called alternating current oscillopolarography (oscillopolarography).AC oscillopolarography is a kind of polarographic method, which belongs to controlled current polarography.
frequently-usedOscillopolarographyAs shown in Figure 2, the oscillopolarographic curve obtained from this circuit is the dE/dt-E curve.The upper half is the curve obtained by metal ion reduction (called cathode branch), and the lower half is the curve obtained by metal oxidation (called anode branch).The potential at the tip of the notch is equivalent to the half wave potential of the general polarographic wave.The notch of the upper and lower branches is symmetrical, indicating that the electrode reaction of the metal is reversible.The depth of the incision reflects the concentration of ions.The oscillographic patterns of substances can be used to identify substances, but the sensitivity is not very high.The appearance or disappearance of the notch on the oscillopolarographic curve can indicate the titration end point of volumetric analysis. This titration method is called oscillopolarographic titration.[1]
Figure 2
characteristic
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The polarographic method has the following characteristics due to its special working electrode and analytical test method.
Wide scope of application
The overpotential of hydrogen on the mercury electrode is very high. Even in the acidic medium, the potential of the dropping mercury electrode becomes negative to - 1.0 V, which will not cause the interference of oxygen ion reduction.When dropping mercury electrode is used as anode, mercury itself will be oxidized, so its potential change cannot exceed+0.4 V.Within the above appropriate potential range, substances that can be reduced or oxidized on the electrode, including inorganic and organic substances, can be determined by polarography. It is also a means to determine the equilibrium constant of chemical reaction and study the reaction mechanism of the electrode.
Wide range of determinable component content
Generally, the concentration range of the measured component in the electrolyte is 10-5～10-2mol/L，It is suitable for the determination of trace components.In recent years, it has also been used to measure constant components.If some modern polarographic analysis methods are used, the concentration of the components that can be measured in the electrolyte can be as low as 10-10～10-7mol/L，It can be used to determine ultramicro components.
High accuracy and good reproducibility
As the mercury drops are constantly updated, the working electrode is always kept clean, so that the experimental data obtained is more accurate and reproducible.Generally, the relative error is about 1%, and the relative error can be reduced to 0.5% with very precise instruments.
Good selectivity and continuous measurement
Since the potential of the working electrode is completely controllable, all kinds of metal ions with potential difference of more than 50mV can be reduced to metal under different electrodes to generate non overlapping polarographic waves.It does not interfere with each other, and can realize the continuous determination of some coexisting metal ions in the same electrolyte.
It should also be pointed out that the mercury vapor used in the working electrode of polarography is toxic. In the experiment, care should be taken to prevent the scattering and evaporation of mercury, and the mercury used should be recovered in time.The laboratory should pay attention to ventilation, and should regularly check the mercury content in the air. If it exceeds the allowable amount, measures should be taken to use burning iodine for disinfection.Because of this defect of polarography, many scholars have been committed to looking for other microelectrodes to replace the drop mercury electrode for many years, but the effect is not as good as that of mercury electrode.However, as long as careful operation is carried out in the experiment and attention is paid to improving the working environment, this disadvantage of mercury electrode can be completely overcome.[3]
