Introduction

Polarography is a type of electrochemical analysis method to determine the concentration of the substance in solution by obtaining the current-voltage curve of the polarization electrode in the process of electrolysis. It was founded by Jaroslav Heyrovský, a Czech chemist, in 1922. ^[1]Polarography can be considered as a special case of voltammetry. The difference between polarography and voltammetry lies in the polarization electrode. Polarography uses dropping mercury electrode (DME)  or other liquid electrode whose surface can be renewed periodically as polarization electrode. Voltammetry uses liquid or solid electrode whose surface is still as polarization electrode.^{[2, p.206-214]}

Principle

The basic device of polarography is shown in Figure 1. DME is used for electrolysis, and the upper end of the electrode is a mercury storage bottle. The mercury in the bottle enters the capillary tube and then drops regularly into the solution in the electrolytic cell, so that the surface of DME is constantly updated, so as to obtain good reproducibility and accuracy. In polarography, DME serve as working electrode.  The other electrode is  mostly saturated calomel electrode (SCE), working as reference electrode. Sometimes, other types of electrodes are used as reference electrode, depending on the electrolyte, such as, Ag-AgCl.^[1]

Figure1. Scheme of polarographic device (Figure: Lu Wang)

The circuit of a potentiometer typically includes a power supply (B), a variable resistance (R) and sliding line resistance (DE). With such a circuit, the voltage applied to the electrolytic cell can be changed continuously and the value of the voltage can be indicated by a voltmeter (V). The change of current as the voltage change is measured by a galvanometer (G). The recorded current-voltage is referred to as polarographic wave that is the basis for quantitative and qualitative analysis.^[1]

Figure 2 shows a typical polarographic wave, which has a shape of S. ^[2] At the very beginning, when the precipitation potential is not reached, only a very small current passes through the solution which is called residual current. As the voltage increases, the current increases rapidly, thus producing diffusion current due to the diffusion phenomenon. Due to the limitation of ion diffusion velocity, the current would reach a limit value at some point and no longer increase with voltage.This limit is called limiting current.

Figure 2 . A typical polarographic wave (Figure: Lu Wang)

Quantitative and qualitative analysis

Ilkovic Equation provides a method to quantitatively calculate the concentration of the depolarizer.^[1][2]

Where I_d is the diffusion current, k is a constant, F is Faraday constant (kF is evaluated to 708 at maximal current and 607 for average current), n is the number of electrons transferred in the electrode reaction, D is the diffusion coefficient, m_r is the mass flow rate of Hg through the capillary, t is the drop lifetime. C is the concentration of depolarizer.

The difference between limiting current and residual current is limiting diffusion current, which is proportional to the concentration of the solution. It can be used as the basis for quantitative analysis.

Another feature of polarogram is half wave potential, i.e. the potential of DME  when the diffusion current equals to half of limiting diffusion current. If 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.

Classification

Polarography can be divided into DC (Direct current) polarography, AC (Alternating current) polarography, single scan polarography, square wave polarography and so on. ^[3]

Direct current polarography

By measuring the current-potential curve obtained in the process of electrolysis, the concentration of the component to be measured in the solution can be determined. The feature of this method is that the electrode potential can change very slowly. It is a rapid analytical method widely used for the determination of substances that can be reduced or oxidized on the electrode.

AC polarography

A low-frequency sinusoidal voltage with small amplitude (several to tens of millivolts) is superimposed on the DC voltage of DC polarography. The AC polarography wave is obtained by measuring the branch current of electrolytic cell. The obtained polarographic wave is has the shape of a peak. There is a linear relationship between the peak height and the concentration of the analyte in a certain range. The features of the method are as follows: (i) The sensitivity is higher than that of DC polarography, and the detection limit can be as low as 10^-7 mol / L. (ii) It has high resolution and can distinguish two adjacent waves with peak potential difference of 40 mV.

Single scan polarography

At the later stage of a mercury drop growth, when its area is basically constant, a pulse voltage is rapidly applied to the two electrodes of the electrolytic cell, and the current-voltage curve generated on a mercury drop is observed with an oscilloscope. The features of this method are as follows: (i) the polarographic wave is peak shaped, the sensitivity is higher than that of DC polarography, and the detection limit can reach 10^-7 mol / L. (ii) High resolution and strong anti-jamming ability. (iii) The polarization voltage is applied rapidly, thus generating a large charging current. It is necessary to take effective measures to compensate the charging current.

Square wave polarography

A square wave voltage with low frequency and small amplitude (≤ 50 mV) is superimposed on the normal and slowly changing DC voltage, and the AC current component passing through the electrolytic cell is recorded immediately before the square wave voltage changes direction. Under suitable conditions, the detection limit of some ions can reach10^-3 μ.

Benefits and limitations

Classic polarographic analysis generally has the following benefits.^[3]

1. The relative error is small, generally ±2%.
2. Several substances (such as Cu²⁺, Cd²⁺, Ni²⁺, Zn²⁺, Mn²⁺) can be determined simultaneously without pre-separation.
3. The analysis speed is fast. It is suitable for the analysis of a large number of a single sample.
4. Because the current passing through electrolysis is very small, the composition of the analyzed solution basically does not change, and the analyzed solution can be used continuously.
5. It has a wide range of applications. All the substances that can undergo redox reactions on DME, including metal ions, metal complexes, anions and organic compounds, can be determined by polarography.

However, polarography also has some limitations.^[3]

1. Due to the existence of charging current, the sensitivity is low, with limit of detection of only around 10^-5M.
2. The resolution is poor. Qualitative and quantitative analysis can be carried out only when the half wave potential difference between two substances is greater than 100 mV.
3. The mercury vapor used in the experiment is poisonous and needs to be treated carefully.

A practical example in research

Azimi, et al, developed an effective method for extracting galegine from natural samples with Molecularly Imprinted Polymer (MIP). To show the good effectiveness of MIT as adsorbent, they used polarography to carry out quantitative and qualitative electrochemical measurements of galegine extracted, in order to evaluate this method and compare it with previous method. The table below shows the comparison.

Table 1. Comparison of the results of previous studies to determination of galegine with the result of the proposed method (License: CC BY 4.0). ^[4]

References