Easily measuring ORP values in real-life applications
Oxidation-reduction potential (ORP), also known as redox potential, affects many of our daily routines such as simply opening the tap of a faucet. ORP (expressed in volts, V) is a measure of the tendency of a chemical species to acquire electrons from (or lose electrons to) an electrode and thereby be reduced or oxidized, respectively. This parameter can be used to predict the states of chemical species in various sample matrices, monitor water quality, control fermentation processes, and optimize wastewater treatment to prevent releasing higher concentrations of substances than regulation limits allow. The measurement of ORP values is an ongoing operation in many industrial sectors that require the improvement of the available instrumentation to facilitate their processes.
The concept of measurement with ORP sensors
ORP quantifies the ability of a substance to oxidize or reduce another substance. As an example, an oxidant prefers to steal electrons from another substance, making it more negatively charged and positively charging the other substance. This act generates a detectable potential between the two substances.
In practical terms, ORP is the direct measurement of electrons in transit during oxidation-reduction reactions. Therefore, ORP evaluates the capacity of a solution for electron transfer (oxidation or reduction) and is measured in millivolts (mV). This means that in oxidative conditions, the working electrode immersed in the solution loses electrons, creating a positive potential. On the contrary, in a reducing environment, electrons flow from the solution to the working electrode, producing a negative potential. While a reductant loses an electron, the oxidant can accept an electron. So, it can be said that strong reductants result in a more negative ORP value, while stronger oxidants lead to a more positive ORP value.?This concept is illustrated below.
The objective of an ORP sensor is to measure these small potential differences generated. This is made possible through a circuit formed by a working electrode (the positive pole of the circuit, usually made of an inert material e.g., platinum or gold) and a reference electrode (the negative pole) immersed in the solution. The amount of millivolt potential created is dependent upon the concentration of oxidants and reductants in the tested solution.
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Measuring ORP with screen-printed electrodes (SPEs)
ORP is a very useful parameter that must be controlled in many different situations. These include industrial fields where measuring large amounts of samples in open areas is necessary, to bench-scale analysis in the laboratory where minimal quantities of biological samples must be tested.
The uses of ORP measurement are widely varied and can be complex. Developing instruments to fulfill requirements in so many fields is not easy but is now much more possible thanks to miniaturization of equipment and the development of disposable sensors (e.g., SPEs).
The multitude of requirements from several industries that must measure ORP fit perfectly with the following advantages gained when using SPEs and miniaturized instruments. These include portability, accessibility, disposability, small sample sizes, and reliability.