Electrochemistry

Electrochemistry is the branch of chemistry that studies the relationship between electricity, as a measurable quantity, and chemical change. The electrochemical analysis techniques used in laboratories are mainly classic potentiometry, coulometry, voltammetry and polarography. A wide range of other techniques using electrical signals or associated effects have evolved recently such as scanning electrochemical microscopy and electrogenerated chemiluminescence.

How Does Electrochemistry Work?

In potentiometry, coulometry, voltammetry and polarography an electrical cell is set up and electro-active species detected based on their electrode potential. There are numerous variants of the basic techniques.

In potentiometry the difference in electrode potential between a pair of electrodes, placed in the sample solution, is measured.  This is used to monitor titrations or as a direct measure of concentration, e.g. with ion selective electrodes (ISE). 

Ion-selective electrodes are membrane electrodes that produce an electrical potential depending on the activity of ions dissolved in a solution. The potential can be measured using a pH meter or a voltmeter. The pH electrode is the most commonly used ISE. It includes a thin glass membrane that is sensitive to the H+ concentration in a solution. Enzyme electrodes use a special membrane that covers an ISE; an enzyme reacts with a specific substance, and the ISE detects the product of this reaction.

In coulometry the amount of matter transformed during an electrolysis reaction is determined by measuring the amount of electricity (in coulombs) consumed or produced.

In voltammetry, an electrical potential applied to a working electrode is varied and the resultant current is recorded. The transfer of an electron between an oxidised and reduced species occurs at a potential specific for each analyte at the electrode/solution interface. The sensitivity can be greatly increased by using an electrolysis step to concentrate the trace components of interest on the electrode before the analysis (stripping). The working electrode is often a glassy carbon covered by a thin film of gold or mercury. In polarography a mercury drop is used. 

A wide range of other techniques using electrical signals or associated effects are also available such as scanning electrochemical microscopy (SECM) and electrogenerated chemiluminescence (ECL). SECM is a technique within the broader class of scanning probe microscopy (SPM) that is used to measure the local electrochemical behaviour of interfaces at very high resolution. SECMs are used in microfabrication, surface patterning, and microstructuring.

In ECL, electrochemically generated intermediates produce an electronically excited state that then emits light upon relaxation to a lower-level state. It has found wide application in sensors for bio-related applications e.g. in Ru tagged antibody based immunoassays, Ru Tagged DNA probes for PCR etc., NADH or H2O2 generation-based biosensors for oxalate and organic amine detection.  ECL is heavily used commercially for many clinical lab applications.

What Is Electrochemistry For?

Analytical techniques based on electrochemistry are used to measure the concentrations of electro-active species in solution. These include many cations, such as mercury, nickel, cobalt, iron and arsenic, anions and cations with ISEs, pharmaceutical compounds such as aspirin and caffeine and neurochemicals capable of undergoing oxidation reactions. These include neurotransmitters such as serotonin and the catecholamines (e.g., epinephrine, norepinephrine, and dopamine).

Why Would You Use Electrochemistry?

The strengths of electrochemical techniques lie in their modest cost, good sensitivity, their suitability for direct species measurement in difficult matrices, such as seawater, and the scope they provide for direct speciation, e.g. As(III) and AS(V) and Se(IV), selenocystine and dimethyldiselenide.