Change the superimposed AC sine wave into a square wave, use a special time switch, and take advantage of the characteristic that the charging current decays quickly with time, and record the AC polarization current signal in the later period after the square wave appears.

A square wave voltage with an amplitude of 10~30mV and a frequency of 225~250Hz (pulse width of several ms) is superimposed on the linear scanning voltage, and the electrolytic current is recorded at the moment when the square wave voltage changes direction.

At the moment when the voltage changes direction, the capacitor current decays the most. At this time, the electrolytic current also attenuates, but the attenuation speed is faster than that of the capacitor. The current decays slowly. At this time, recording the electrolysis current can Overcome the influence of capacitive current, thereby improving sensitivity.

Square wave polarography does not need to add surfactant to suppress the polarogram. On the contrary, surfactant will cause the electrode to react It should be blocked and change the electric double layer capacitance of the electrode and solution surface, thus affecting the measurement.

The reversibility of the electrode reaction has a great influence on the sensitivity of the assay. In the square wave polarogram, due to the relatively large superposition High-frequency voltage, that is, the speed of adding polarization voltage is quite fast, so the electrode reaction speed is relatively fast. For slow substances, the peak height obtained will be greatly reduced.

In order to effectively eliminate the capacitive current, the RC value of the electrolytic cell circuit should be made much smaller than the half-cycle value of the square wave. For a square wave frequency of 225Hz, the half period is 0.002s, and the R value is generally required to be no greater than 50Ω.

Capillary noise. Noise due to capillary tubes. This noise is several times higher than the noise of the entire instrument.

At the end of the growth period of the dropping mercury electrode, superimpose a given DC voltage or a linearly increasing DC voltage. The curve obtained by gradually increasing the amplitude or equal amplitude of the pulse voltage and recording the electrolytic current at the end of each pulse.

Voltage scanning method

Polarographic curve

Voltage scanning method

Polarographic curve

When the applied voltage does not reach the decomposition voltage of metal ions, a tiny current flows through the electrolytic cell in the solution, which is called residual current. It is generated by the reduction of a small amount of impurities in the electrolyte and the trace oxygen that has not been removed on the electrode.

When the potential reaches the precipitation potential of metal ions, the metal precipitates and an electrolytic current is generated.

The metal ions on the surface of the dropped mercury tend to zero, the concentration polarization reaches the limit, the diffusion current no longer increases with the external voltage, and the curve appears a plateau.

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The potential of the mercury-dropping electrode changes completely with the applied voltage, and it is a polarized electrode.

The electrode potential of the saturated calomel electrode can be considered unchanged and it is a depolarizing electrode.

The ions to be measured reach the electrode surface from the solution mainly by three mass transfer methods:

The concentration of the substance to be measured should be small, the solution is dilute, and the electrolysis current generated is small. Ensure that the calomel electrode is a depolarizing electrode and the dropping mercury electrode is a polarizing electrode.

Electrolysis is performed stationary, without stirring, and a large amount of supporting electrolyte is added to the solution

The electrode reacts quickly, and the reaction rate is much higher than the diffusion rate. It is shown that any point on the polarographic wave is controlled by the diffusion rate, and the Nernst formula is applicable.

The rate of electrode reaction is slower than the diffusion rate. Polarographic waves are controlled by both the diffusion rate and the electrode reaction rate. Due to the existence of overpotential, the Nernst formula is not applicable and the waveform is poor.

Electrolysis current, caused by trace impurities, very small

The charging current is caused by the charging process of the double electric layer at the interface between the mercury-dropping electrode and the solution.

Elimination method

There is no quantitative relationship with the concentration of the measured substance

Elimination method

Dissolved oxygen is reduced at the dropping mercury electrode, producing two polarographic waves that interfere with the measurement.

Elimination method

The diffusion current rapidly increases to a maximum value as the potential of the mercury-dropping electrode decreases, and then decreases and stabilizes at the normal limit diffusion current value. This prominent current peak is called "polarographic maximum"

cause

Elimination method

Try to reduce the charging current or increase the electrolysis current to improve the signal-to-noise ratio and improve the sensitivity of the measurement.

iR drop can be overcome using a three-electrode system

Polarography