Polarography is a type of
voltammetry
Voltammetry is a category of electroanalytical methods used in analytical chemistry and various industrial processes. In voltammetry, information about an analyte is obtained by measuring the current as the potential is varied. The analytical data ...
where the
working electrode
The working electrode is the electrode in an electrochemical system on which the reaction of interest is occurring. The working electrode is often used in conjunction with an auxiliary electrode, and a reference electrode in a three electrode sys ...
is a
dropping mercury electrode
A liquid metal electrode is an electrode that uses a liquid metal, such as mercury, Galinstan, and NaK. They can be used in electrocapillarity, voltammetry, and impedance measurements.
Dropping mercury electrode
The dropping mercury electrode ( ...
(DME) or a static mercury drop electrode (SMDE), which are useful for their wide
cathodic ranges and renewable surfaces. It was invented in 1922 by
Czech
Czech may refer to:
* Anything from or related to the Czech Republic, a country in Europe
** Czech language
** Czechs, the people of the area
** Czech culture
** Czech cuisine
* One of three mythical brothers, Lech, Czech, and Rus'
Places
* Czech, ...
chemist
Jaroslav Heyrovský
Jaroslav Heyrovský () (December 20, 1890 – March 27, 1967) was a Czech chemist and inventor. Heyrovský was the inventor of the polarographic method, father of the electroanalytical method, and recipient of the Nobel Prize in 1959 for his ...
, for which he won the Nobel prize in 1959. The main advantages of mercury as electrode material are as follows:
1) a large voltage window: ca. from +0.2 V to -1.8 V vs reversible hydrogen electrode (RHE). Hg electrode is particularly well-suited for studying electroreduction reactions.
2) very reproducible electrode surface, since mercury is liquid.
3) very easy cleaning of the electrode surface by making a new drop of mercury from a large Hg pool connected by a glass capillary.
Polarography played a major role as an experimental tool in the advancement of both
Analytical Chemistry
Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separati ...
and
Electrochemistry
Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...
until the 1970s, when it was supplanted by other methods, that did not require the use of
mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
.
Principle of operation
![Heyrovského polarograf 1](https://upload.wikimedia.org/wikipedia/commons/6/6c/Heyrovsk%C3%A9ho_polarograf_1.jpg)
Polarography is an electrochemical voltammetric technique, that employs (dropping or static)
mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
drop as a working electrode. In its most simple form polarography can be used to determine concentrations of
electroactive species in liquids by measuring their mass-transport
limiting currents. In such an experiment the
potential
Potential generally refers to a currently unrealized ability. The term is used in a wide variety of fields, from physics to the social sciences to indicate things that are in a state where they are able to change in ways ranging from the simple re ...
of the working mercury drop electrode is linearly changed in time, and the electrode current is recorded at a certain time just before the mercury drop dislodges from a glass capillary, where a stream mercury comes from. A plot of the current vs. potential in a polarography experiment shows the current oscillations corresponding to the drops of Hg falling from the capillary. If one connected the maximum current of each drop, a sigmoidal shape would result. The limiting current (the plateau on the sigmoid), called the diffusion-limited current because diffusion is the principal contribution to the flux of electroactive material at this point of the Hg drop life. More advanced varieties of polarography (see below) produce peaks (which allow for a better resolution of different chemical species) rather than waves of the classical polarography, as well as improve the detection limits, which is some cases can be as low as 10^-9 M.
Limitations
![Heyrovského polarograf 2](https://upload.wikimedia.org/wikipedia/commons/1/17/Heyrovsk%C3%A9ho_polarograf_2.jpg)
There are limitations in particular for the classical polarography experiment for quantitative analytical measurements. Because the current is continuously measured during the growth of the Hg drop, there is a substantial contribution from capacitive current. As the Hg flows from the capillary end, there is initially a large increase in the surface area. As a consequence, the initial current is dominated by capacitive effects as charging of the rapidly increasing interface occurs. Toward the end of the drop life, there is little change in the surface area which diminishes the contribution of capacitance changes to the total current. At the same time, any redox process which occurs will result in faradaic current that decays approximately as the square root of time (due to the increasing dimensions of the Nernst diffusion layer). The exponential decay of the capacitive current is much more rapid than the decay of the faradaic current; hence, the faradaic current is proportionally larger at the end of the drop life. Unfortunately, this process is complicated by the continuously changing potential that is applied to the
working electrode
The working electrode is the electrode in an electrochemical system on which the reaction of interest is occurring. The working electrode is often used in conjunction with an auxiliary electrode, and a reference electrode in a three electrode sys ...
(the Hg drop) throughout the experiment. Because the potential is changing during the drop lifetime (assuming typical experimental parameters of a 2 mV/s scan rate and a 4 s drop time, the potential can change by 8 mV from the beginning to the end of the drop), the charging of the interface (capacitive current) has a continuous contribution to the total current, even at the end of the drop when the surface area is not rapidly changing. As such, the typical signal to noise of a polarographic experiment allows detection limits of only approximately 10
−5 or 10
−6 M.
Improvements
Dramatically better discrimination against the capacitive current can be obtained using the tast and pulse polarographic techniques. These have been developed with introduction of analog and digital electronic potentiostats. A first major improvement is obtained, if the current is only measured at the end of each drop lifetime (tast polarography. An even greater enhancement has been the introduction of differential pulse polarography. Here, the current is measured before the beginning and before the end of short potential pulses. The latter are superimposed to the linear potential-time-function of the voltammetric scan. Typical amplitudes of these pulses range between 10 and 50 mV, whereas pulse duration is 20 to 50 ms. The difference between both current values is that taken as the analytical signal. This technique results in a 100 to 1000-fold improvement of the detection limit, because the capacitive component is effectively subtracted.
Qualitative information
Qualitative information can also be determined from the half-wave potential of the polarogram (the current vs. potential plot in a polarographic experiment). The value of the half-wave potential is related to the standard potential for the redox reaction being studied.
This technique and especially the differential pulse anodic stripping voltammetry (DPASV) method can be used for environmental analysis, and especially for marine study for characterisation of organic matter and metals interactions.
Quantitative information
The Ilkovic equation is a relation used in polarography relating the diffusion current (''I''
d) and the concentration of the depolarizer (''c''), which is the substance reduced or oxidized at the dropping mercury electrode. The Ilkovic equation has the form
:
where:
* ''k'' is a constant which includes π and the density of mercury, and with the
Faraday constant
In physical chemistry, the Faraday constant, denoted by the symbol and sometimes stylized as ℱ, is the electric charge per mole of elementary charges. It is named after the English scientist Michael Faraday. Since the 2019 redefinition of S ...
''F'' has been evaluated at 708 for maximal current and 607 for average current
* ''D'' is the
diffusion coefficient
Diffusivity, mass diffusivity or diffusion coefficient is a proportionality constant between the molar flux due to molecular diffusion and the gradient in the concentration of the species (or the driving force for diffusion). Diffusivity is enco ...
of the depolarizer in the medium (cm
2/s)
* ''n'' is the number of electrons exchanged in the electrode reaction, ''m'' is the mass flow rate of Hg through the capillary (mg/s)
* ''t'' is the drop lifetime in seconds,
* ''c'' is depolarizer concentration in mol/cm
3.
The equation is named after the scientist who derived it, the Slovak chemist
Dionýz Ilkovič
Dionýz Ilkovič (18 January 1907 – 3 August 1980) was a Czechoslovak physicist and physical chemist of Rusyn ethnicity. Along with Nobel laureate Jaroslav Heyrovský, he helped to establish theoretical basis of polarography. In this field, he i ...
(1907–1980).
See also
*
Electroanalytical method
Electroanalytical methods are a class of techniques in analytical chemistry which study an analyte by measuring the potential (volts) and/or current (amperes) in an electrochemical cell containing the analyte. These methods can be broken down into ...
*
Hanging mercury drop electrode
A liquid metal electrode is an electrode that uses a liquid metal, such as mercury, Galinstan, and NaK. They can be used in electrocapillarity, voltammetry, and impedance measurements.
Dropping mercury electrode
The dropping mercury electrode ( ...
References
{{Authority control
Electroanalytical methods
Mercury (element)
Czech inventions