A liquid metal electrode is an

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...

that uses a

liquid metal A liquid metal is a metal or a metal alloy which is liquid at or near room temperature. The only stable liquid elemental metal at room temperature is Mercury (element), mercury (Hg), which is molten above −38.8 °C (234.3 K, −37.9&nbs ...

, such as

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 ...

Galinstan Galinstan (R) is a brand name for a alloy composed of gallium, indium, and tin which melts at and is thus liquid at room temperature. However, it is not a eutectic alloy but a near eutectic alloy. In scientific literature, galinstan is also used ...

, and

NaK In data networking, telecommunications, and computer buses, an acknowledgment (ACK) is a signal that is passed between communicating processes, computers, or devices to signify acknowledgment, or receipt of message, as part of a communicatio ...

. They can be used in

electrocapillarity Electrocapillarity or electrocapillary phenomena are the phenomena related to changes in the surface energy (or interfacial tension) of the dropping mercury electrode (DME), or in principle, any electrode, as the electrode potential changes or the ...

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 ...

, and impedance measurements.

Dropping mercury electrode

The dropping mercury electrode (DME) is a

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 ...

made of

and used in

polarography Polarography is a type of voltammetry where the working electrode is a 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 ...

. Experiments run with mercury electrodes are referred to as forms of

even if the experiments are identical or very similar to a corresponding

experiment which uses solid working electrodes. Like other working electrodes these electrodes are used in

electrochemical 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 ...

studies using three electrode systems when investigating

reaction mechanisms Reaction may refer to a process or to a response to an action, event, or exposure: Physics and chemistry *Chemical reaction *Nuclear reaction *Reaction (physics), as defined by Newton's third law * Chain reaction (disambiguation). Biology and m ...

related to

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...

chemistry among other

chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wi ...

phenomena.

Structure

A flow of

passes through an insulating

capillary A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...

producing a droplet which grows from the end of the capillary in a reproducible way. Each droplet grows until it reaches a diameter of about a millimeter and releases. The released droplet is no longer in contact with the

whose contact is above the capillary. As the electrode is used mercury collects in the bottom of the cell. In some cell designs this mercury pool is connected to a lead and used as the cell's

auxiliary electrode The auxiliary electrode, often also called the counter electrode, is an electrode used in a three electrode electrochemical cell for voltammetric analysis or other reactions in which an electric current is expected to flow. The auxiliary electrode ...

. Each released drop is immediately followed by the formation of another drop. The drops are generally produced at a rate of about 0.2 Hz.

Considerations

A major advantage of the DME is that each drop has a smooth and uncontaminated surface free from any

adsorbed Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...

analyte or impurity. The self-renewing electrode does not need to be cleaned or polished like a solid electrode. This advantage comes at the cost of a working electrode with a constantly changing surface area. Since the drops are produced predictably the changing surface area can be accounted for or even used advantageously. In addition, the drops' growth causes more and more addition of capacitive current to the

faradaic current The faradaic current is the current generated by the reduction or oxidation of some chemical substance at an electrode. The net faradaic current is the algebraic sum of all the faradaic currents flowing through an indicator electrode or working ele ...

. These changing current effects combined with experiments where the potential is continuously changed can result in noisy traces. In some experiments the traces are continually sampled, showing all the current deviation resulting from the drop growth. Other sampling methods smooth the data by sampling the current at the electrode only once per drop at a specific size. The DME's periodic expansion into the solution and hemispherical shape also affects the way the analyte diffuses to the electrode surface. The DME consists of a fine capillary with a bore size of 20–50 µm.

Hanging mercury drop electrode

The hanging mercury drop electrode (HMDE) is a

variation on the dropping mercury electrode (DME). Experiments run with dropping mercury electrodes are referred to as forms of

. If the experiments are performed at an electrode with a constant surface (like the HMDE) it is referred as

. Like other working electrodes these electrodes are used in

studies using three electrode systems when investigating

related to

chemistry among other

phenomenon.{{Cite book , edition = 5 , publisher = Brooks Cole , isbn = 978-0-03-002078-0 , last = Skoog , first = Douglas A. , author2=F. James Holler , author3=Timothy A. Nieman , title = Principles of Instrumental Analysis , date = 1997-09-03

Distinction

The hanging mercury drop electrode produces a partial

drop of controlled geometry and surface area at the end of a

in contrast to the dropping mercury electrode which steadily releases drops of mercury during an experiment. The disadvantages a DME experiences due to a constantly changing surface are not experienced by the HMDE since it has static surface area during an experiment. The static surface of the HMDE means it is more likely to suffer from the surface

adsorption Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a f ...

phenomenon than a DME. Unlike solid electrodes which need to be cleaned and polished between most experiments, the self-renewing HMDE can simply release the contaminated drop and grow a clean drop between each experiment.

References