Polarographic classification

Polarography can be divided into 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. When the mercury drops grow to a certain area, a pulse square wave voltage with a small amplitude (10 ~ 100 mv) is superimposed on the DC voltage, and the current generated by the pulse voltage is measured at the later stage of the square wave. According to the different application methods of pulse square wave voltage, pulse polarography can be divided into differential pulse polarography and conventional pulse polarography 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: ① high sensitivity, amplifying Faraday current on the basis of fully attenuating the charging current ic and the capillary noise current iN, making the detection limit reach 10-8 ~ 10-9mol/l② 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. The depth of notch deepens with the increase of substrate concentration, 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.