Similarities and differences between polarography and electrolysis

polarography

An electrochemical analysis method determines the concentration of substances in solution by measuring the current-potential (or potential-time) curve of polarized electrode obtained during electrolysis.

It was founded by Czech chemist Heyrovsk, Jaroslav in 1922.

The difference between polarography and voltammetry lies in the difference of polarized electrodes.

Polarography uses mercury dropping electrode or other liquid electrode whose surface can be periodically updated as polarization electrode; Voltammetry uses liquid or solid electrodes with static surfaces as polarized electrodes.

The basic device of principle polarography is shown in figure 1.

Polarization electrode (mercury dropping electrode) is usually connected to the negative end of polarization voltage, and reference electrode (calomel electrode) is connected to the positive end of polarization voltage.

When the DC voltage applied to the two electrodes reaches the decomposition voltage enough to reduce the electroactive substance to be measured on the mercury droplet electrode, the current passing through the electrolytic cell is always very small (this tiny current is called residual current). When the decomposition voltage is reached, the substance to be measured begins to be reduced on the mercury dropping electrode, resulting in polarographic current, which then increases sharply with the increase of applied voltage and gradually reaches the limit value (limit current), but does not increase with the increase of applied voltage.

The current-voltage curve thus obtained is called polarographic wave.

Polarographic wave's half-wave potential E 1/2 is the characteristic value of the measured substance, which can be used for qualitative analysis.

The diffusion current depends on the diffusion speed of the measured substance from the solution to the surface of the mercury dropping electrode, and its magnitude is determined by the concentration of the measured substance in the solution, so that quantitative analysis can be carried out.

Classified polarography can be divided into two categories: controlled potential polarography and controlled current polarography.

In the controlled potential polarography, the electrode potential is the controlled excitation signal and the current is the measured response signal.

In controlled current polarography, current is a controlled excitation signal and electrode potential is a measured response signal.

Controlled potential polarography includes DC polarography, ac polarography polarography, single sweep polarography, square wave polarography and pulse polarography.

Control current polarography includes oscillopolarography.

In addition, there are polarographic catalytic waves and stripping voltammetry.

DC polarography is also called potentiostatic polarography.

The concentration of that measure component in the solution is determined by measure the current-potential curve obtained in the electrolysis process.

Its characteristic is that the electrode potential changes very slowly.

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.

Ac polarography is to superimpose a low-frequency sinusoidal voltage with a small amplitude (tens of millivolts) on the DC voltage of DC polarography, and obtain the AC polarographic wave by measuring the branch current of the electrolytic cell. The peak potential is equal to the half-wave potential E 1/2 of DC polarography, and the peak current ip 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 that of DC polarography, and the detection limit can reach10-7mol/L.

(2) The resolution is high, and the spectral waves of adjacent poles with a peak potential difference of 40mV can be distinguished.

③ Strong anti-interference ability, and the pre-reducing substance does not interfere with the polarographic wave measurement of the post-reducing substance.

④ The superimposed AC voltage makes the electric double layer charge and discharge quickly, and the charging current is large, which limits the further reduction of the minimum detectable concentration.

Single-sweep polarography is to apply a pulse voltage to the two electrodes of the electrolytic cell quickly when the area of the mercury drop is basically unchanged in the late growth stage of the mercury drop, and at the same time observe the current generated on the mercury drop with an oscilloscope. Voltage curve.

The characteristics of this method are as follows: ① polarographic wave has a peak shape, the sensitivity is 1 ~ 2 orders of magnitude higher than that of DC polarography, and the detection limit can reach10-7 mol/L.

② High resolution and strong anti-interference ability.

Adjacent polarographic wave with 50mV difference in peak potential can be distinguished, and the concentration of the substance before reduction is 100 ~ 1000 times higher than that after reduction, which does not interfere with the determination.

(3) The rapid application of polarization voltage produces a large charging current, so effective measures should be taken to compensate the charging current.

④ There is no polarographic peak in the irreversible process, which reduces or even completely eliminates the interference of oxygen waves.

Square wave polarography is to superimpose a square wave voltage with low frequency and small amplitude (≤50mV) on the DC voltage which usually changes slowly, and record the AC current component passing through the electrolytic cell instantly before the square wave voltage changes direction.

The square wave polarographic wave has a peak shape, the peak potential Ep is the same as E 1/2 of DC polarography, and the peak current is proportional to the concentration of the measured substance.

The characteristics of this method are as follows: ① Record the current when the charging current is fully attenuated, and there is no charging current in the polarographic current. By amplifying the current, the sensitivity can be improved, and the detection limit can reach10-8 ~10-9 mol/L.

② High resolution and strong anti-interference ability.

The spectral waves of adjacent poles with a peak potential difference of 25mV can be distinguished. When the amount of the substance before reduction is 104 times that of the substance after reduction, a small amount of the substance after reduction can still be effectively determined.

③ The peak current of oxygen wave is very small, so it is not necessary to remove oxygen when analyzing high-content substances.

(4) In order to reduce the time constant and fully attenuate the charging current, it is required that the internal resistance of the tested solution is not more than 50 Ω and the supporting electrolyte concentration is not less than 0.2 mol/L, so the reagent is required to have a particularly high purity.

⑤ The capillary noise current is large, which limits the further improvement of sensitivity.

Pulse polarography is to superimpose a small amplitude (10 ~ 100 mv) pulse square wave voltage on the DC voltage when the mercury drops grow to a certain area, and measure the current generated by the pulse voltage at the later stage of the square wave.

Pulse polarography can be divided into differential pulse polarography and conventional pulse polarography according to the different application modes of pulse square wave voltage.

The former is that a square wave pulse with equal amplitude is superimposed on the DC linear scanning voltage, and the polarographic wave obtained is in a peak shape, while the amplitude of the square wave pulse applied by the latter increases linearly with time, and the current-voltage curve of each pulse obtained is similar to the DC polarography.

The characteristics of this method are as follows: ① The sensitivity is high, and the Faraday current is amplified on the basis of fully attenuating the charging current ic and the capillary noise current iN, so that the detection limit reaches10-8 ~10-9 mol/L.

② Good resolution and strong anti-interference ability.

Adjacent bipolar spectral waves with 25mV difference between E 1/2 or Ep can be distinguished, and the amount of pre-reduced substance is 5× 104 times higher than the measured substance, which does not interfere with the determination.

(3) Due to the long pulse duration, iC and iN can still be fully attenuated when the supporting electrolyte with lower concentration is used, thus reducing the blank value.

④ Irreversible reactions with long pulse duration and slow electrode reaction speed, such as the electrode reaction of many organic compounds, can also achieve quite high sensitivity, and the detection limit can reach10-8 mol/L..

Oscillopolarography A controlled current polarography in which an oscilloscope is used to observe or record polarographic curves.

See Figure 2 for the oscillopolarograph and polarographic curve.

Commonly used polarized electrodes are suspended mercury electrode and mercury film electrode, and reference electrodes are silver-plated mercury electrode, mercury cell electrode or tungsten electrode.

220V AC sinusoidal voltage is adjusted to 2V through high resistance R (about 105 ~ 106ω) and added to the electrolytic cell.

An adjustable DC voltage is superimposed on the AC voltage to provide a fixed potential in the range of 0 ~-2V.

The high voltage of alternating current almost falls on the high resistance, and the amplitude of alternating current passing through the electrolytic cell is constant, mainly to measure its current change.

The vertical deflection plate of the oscilloscope tube is connected with two electrodes, and the horizontal deflection plate is scanned by sawtooth wave.

When the scanning voltage is synchronized with the AC voltage and fixed microelectrodes are used, a stable potential-time curve appears on the screen.

According to different recording methods, three types of potential time curves can be obtained: E-t curve, curve and curve.

The AC Et curve of the supporting electrolyte has a sinusoidal shape, which is truncated by a horizontal constant potential. When there are electroactive substances in the solution, there is a gradual time delay at the potential corresponding to the electrode reaction on the curve. The turning point potential of the delay is equal to E 1/2 of DC polarographic wave, and the length of the delay depends on the concentration of the substance to be measured.

The hysteresis on the E-t curve is shown as a notch on the sum curve, and the notches on the anode and cathode correspond to reversible reactions.

With the increase of substrate concentration, the depth of incision deepens, which can be used for quantitative analysis, but the sensitivity is not high, and the detection limit can only reach10-5 mol/L.

Oscillopolarography can distinguish the adsorption phenomenon on the electrode from the reversibility of the electrode reaction.

use

Polarography can be used to determine most metal ions, many anions and organic compounds (such as carbonyl, nitro, nitroso compounds, peroxides, epoxides, thiols and compounds with * * * yoke double bonds, etc.). ).

In addition, it is also widely used in electrochemistry, interfacial chemistry, complex chemistry and biochemistry.