One of the characteristics of classical polarography is that the voltage on the polarographic cell is constant (or changes very slowly).We can call it DC polarography (short for DCP).The other method is to study the relationship between voltage, current and time on the polarographic cell when voltage or current changes with time, which is called alternating current polarography (ACP).
Description: a control potentialPolarography。A small amplitude sinusoidal AC voltage is superimposed on the DC polarization potential of the DC polarography. Its amplitude is 10-50 mV, and the frequency is less than 100 Hz. The resulting AC current passing through the electrolytic cell is measured to obtain the peak shaped polarographic wave.The peak height has a linear relationship with the concentration of the substance to be measured in a certain range.For reversible systems, the sensitivity of AC polarography is 10-5-10-6 mol/L, while for irreversible systems, it is lower.The resolution is higher than that of DC polarography, and the two peaks can be separated when the potential difference is 40mV.In addition to analyzing the above applications, it is also used for the study of electrode reaction kinetics.
A low-frequency sinusoidal voltage with small amplitude (several to dozens of millivolts) is superimposed on the DC voltage of the DC polarography, and the concentration of the measured substance in the electrolytic cell is determined by measuring the AC current of the electrolytic cellElectrochemical analysisIt is a kind of control potential polarography.
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Figure 1
stayDirect current polarographyThe AC voltage is introduced into the circuit ofR1. After being amplified by the amplifier, measure with a vacuum tube voltmeter.capacitanceCCut off the influence of DC current in the electrolytic cell (Figure 1).
Figure 2
Obtained on AC polarographi-EThe dc (AC current – DC potential) curve is called AC polarogram (Figure 2)
A low frequency with small amplitude (several to tens of millivolts)sineVoltageSuperimposed on the DC voltage of the DC polarographyElectrolytic cellA tributary ofelectric currentGet communicationPolarographic wave, peakpotentialEqual to DC polarographicHalf wave potentialE1/2，The peak current ip is proportional to the concentration of the measured substance.
The characteristics of this method are:
① CommunicationPolarographic waveIn peak shape,sensitivityIt is higher than DC polarography, and the lower detection limit can reach 10-7mol/L.
③ Strong anti-interference ability, the pre reducing substance does not interfere with the post reducing substancePolarographic waveMeasurement.
④ The superimposed AC voltage makes the electric double layer charge and discharge rapidlyelectric currentLarge, which limits the further reduction of the lowest detectable concentration.
This kind of method has developed greatly in the past ten years and is divided into many branches.We can summarize it as follows:
one
In general, the characteristics of all the above AC polarography compared with DCP can be summarized into two points:
1. It is applied to analytical chemistry with high sensitivity.
2. It can explain more electrode reaction mechanisms.
Each of the above methods has its own unique advantages and disadvantages.However, the commonly called alternating polarography generally refers to Berea alternating polarography.
Berea alternating polarography
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The basic form of Berea alternating polarography is as follows:
The series DC and AC voltages are superimposed on the polarographic cell, and the current passing through the polarographic cell can be divided into two parts. The DC component of the current is caused by the DC voltage, and the AC component, or AC current, is generated by the AC voltage.
Alternating current polarographic theory
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AC polarography theory is generally considered from the following two aspects:
1. The electrode system is regarded as an equivalent circuit composed of several electrical components.Then the AC current through the equivalent circuit is studied.
2. The diffusion current generated by the electrode interface during the periodic concentration polarization in the AC electric field is studied.[1]
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Figure 3
Compared with DC polarographic wave, AC polarographic wave has two characteristics: ① AC polarographic wave has a current peak, similar to the first differential curve of DC polarographic wave.This is due to the magnitude of the AC polarographic current and the d of the DC polarographic wavei/dEThe slope d of a point on the DC polarographic wavei/dEThe greater the AC current, the greater the corresponding AC current;At the half wave potential of the DC polarographic wave, the AC current is the largest, so the polarographic wave has a current peak (Figure 2).
② The shape of AC polarographic curve is independent of the state of depolarizer.In DC polarography, when there is only oxidation state in the solution, the reduction wave is obtained;When there is only reduced state in the solution, the oxidation wave is obtained;When there are both oxidation and reduction states in the solution, a comprehensive wave is obtained;The shapes and properties of polarographic waves vary with the existence of depolarizers.However, in AC polarography, only one AC polarographic wave can be obtained under all the conditions of generating the three different DC polarographic waves mentioned above (Fig. 3).This is because no matter what kind of DC polarographic wave, the AC polarographic current is the largest at the half wave potential.
For reversible electrode reaction:
O+ne
Reversible electrode reaction
In formula R, O is in oxidation state, R is in reduction state,
Relationship between AC current and DC potential
Both O and R are dissolved. The relationship between AC current and DC potential in AC polarography is as follows:
WhereAIs the electrode area;DIs the diffusion coefficient;ωIs the angular frequency of alternating current;Oxidation in main solution
Molar concentration of state;VIs the amplitude of AC voltage;nIs the transfer number of electrons in the electrode process;FIs Faraday constant;RIs the gas constant;TIs the thermodynamic temperature;tIs the time.
Peak current of AC polarographic waveiP is:
Peak current iP of AC polarographic wave
The above formula is the theoretical basis for quantitative analysis by AC polarography.
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Figure 4
The resolution of AC polarographic wave is better than that of DC polarographic wave (the difference between the two peaks of AC wave is 40mV, and the difference between the two peaks of DC wave is 90-100 mV), the sensitivity is slightly higher (1 × 10-5 μ), and the interference of oxygen is less.Figure 4 is the AC polarogram of several inorganic ions.Organic matter generates highly sensitive AC wave, but its peak potential is often the same as that of DC waveE1/2 is inconsistent, and the wave height is often not in a linear relationship with the concentration. This is because most organic substances or their products on the electrode are easily adsorbed on the electrode.The adsorption reduces the capacitance of the electric double layer, thus reducing the AC current;Desorption increases AC current.
Figure 5
Some substances do not produce DC polarographic waves, but produce AC polarographic waves.This is because they are surface active substances that can be adsorbed and desorbed on the electrode surface, changing the double layer capacitance on the electrode surface.This current peak is not from the electrode reaction but from the tension change caused by adsorption, so it is called tension current.The tension current peaks are generally in pairs, and the distance is unequal on both sides of the zero charge potential (Figure 5).The peak height of tension current peak is inversely proportional to I.
