Maintaining stable ORP levels also plays a special role in the correct functionality of our bodies as metabolism relies on a precise mechanism of interrelated redox systems. In the past decades, there has been a growing concern about the use of antioxidants to achieve a better quality of living. One example of this is of water that exhibits a less oxidant nature than that of normal tap water. The demand for this specialized water has grown in recent years for use in beverages, food, bathing, artificial body fluids, cosmetics, skincare products, and other purposes .
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 millivolts, mV) 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 currently available instrumentation to facilitate their processes.
ORP importance in the realm of water chlorination
A public health emergency arose in 1854 when more than 600 people died within a month as the result of a cholera outbreak in an area barely half a kilometer in diameter in London. John Snow, an English physician, talked to the local residents and created a map illustrating the occurrences of cholera in the area. He identified the outbreak’s source as the public water pump that supplied drinking water from a well contaminated with excrement. By removing the pump’s handle, the contaminated well was prevented from further use, and the outbreak was ended. Due to his research, John Snow became one of the founders of modern epidemiology, and as a result the importance of water treatment for human consumption started to grow.
Drinking water quality is clearly a critical public health concern. If the quality of water from public wells and fountains cannot be relied upon, then how is it possible to offer germ-free drinking water for thousands (or millions) of people in cities? On the other hand, what about rural or wilderness areas where clean water is not always readily accessible? The answer to these questions lies in the process of water treatment by chlorination.
In its elemental form, chlorine (Cl2) is a toxic gas. When added to water, Cl2 causes alterations in bacterial cell walls, destroying proteins and DNA contained within. This is the mechanism by which chlorine kills microorganisms – it affects their vital functions until they die, rendering them incapable of spreading diseases. By adding chlorine to disinfect municipal water systems, the risk of catching contagious cholera, typhus, dysentery, and polio are minimized.
Chlorination of water can be done by using elemental chlorine gas, though it is much safer to use liquid sodium hypochlorite or solid calcium hypochlorite. These compounds chlorinate water by generating residual «free chlorine» that attacks disease-causing germs and makes the chlorine disinfection process more versatile and user-friendly.
You may wonder, since chlorine is a toxic element, is there any method to control how much of it is added to the water? Oxidation-reduction potential (ORP) measurement offers a reliable solution to this matter.
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.
Returning to the discussion of chlorination and ORP, adequate water disinfection is only possible when a certain amount of chlorine concentration is reached. Considering that high levels of chlorine can be toxic for human life, it is important to accurately control the ORP value of water during the chlorine disinfection process.
The presence of an oxidizing microbiocide (e.g., chlorine) creates an oxidizing environment, therefore inducing a high level of ORP. This is in contrast to reducing environments with lower ORP values, which is where germs usually proliferate. Maintaining the ORP under control in the water makes chlorination a safer procedure. In 1971, the World Health Organization (WHO) stated that «A redox potential of 650 mV (measured between platinum and standard calomel reference electrodes) will cause almost instantaneous inactivation of even high concentrations of virus» . This value was subsequently recommended as the minimum ORP level for human safety and was implemented in the legislation of public swimming pools and spas.
In recent years, electrolyzed water (EW) has gained popularity in the food industry as a sanitizer in many countries. Although this technology has existed for more than 40 years, companies producing such solutions have only recently approached the global market. This chlorine-based disinfectant is the product of the electrolysis of dilute sodium chloride (NaCl) solution that dissociates into acidic electrolyzed water (ORP value >1100 mV) and basic electrolyzed water (ORP value between -800 and -900 mV). By making this technology widely accessible, chlorine disinfection of the water supply continues saving lives in places where the technology is fairly developed .
Application examples – monitoring ORP in real life situations
The example introduced at the beginning of this article about drinking water quality and its effect on public health is just one of myriad processes that affect our daily life where the control of ORP values is necessary. A selection of these can be found below, followed by a graphic showing ORP control ranges for many kinds of industrial processes.
ORP readings in seawater are around 400 mV, while mineral water has a value of around 250 mV. Similar to the water in hot springs, spas, or swimming pools, lower ORP values in the sea can be related to uncontrolled microbiological activity, and higher values could be related to oxidant contamination. This is one reason why aquaculture and fish farming operations must take special care not only about the pH of the media but the ORP measurement as well.
Oxidative stress is a phenomenon provoked by an imbalance between the production and accumulation of reactive oxygen species in cells and tissues. Although biological systems can naturally detoxify these reactive products, illness or the presence of pollutants can disturb the balance. This disturbance can be monitored with ORP measurements, making it possible to detect oxidative stress and its effects. These can include problems like male infertility , development of brain lesions , and in patients with myocardial infarction, sepsis or multiple kinds of trauma .
Pollutants are not only responsible for promoting oxidative stress in cells and tissues, they can also be linked to many human and animal diseases. For this reason, ORP control is well known in the wastewater treatment industry where a significant number of biological processes must be controlled. Biochemical reactions performed by microorganisms must be maintained within a certain ORP range to promote the desired bioremediation reactions (e.g., those in the nitrification or fermentation processes).
Wastewater from the metal plating industry is another good example. To overcome the toxic effects of chromium and cyanide, ORP values in wastewater with these contaminants must be below 250 mV and above 450 mV, respectively .
Oxidation is also a common concern in food chemistry. When foods and beverages become oxidized, their quality deteriorates.
As mentioned before, ORP values are one of the parameters that can be adjusted to optimize fermentation. For example, monitoring ORP controls the addition of oxygen to wine, allowing winemakers to maintain its value above 215 mV during the entire fermentation process . This is necessary because the microorganisms responsible for creating alcohol from the sugars require some oxygen to perform their duties well. A similar control over fermentation has been applied during the processes of wet-aging beef  and cheese ripening .
Measuring ORP with screen-printed electrodes (SPEs)
ORP is a very useful parameter that must be controlled in many different situations, as shown in the previous sections. 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.
These advantages become abundantly clear when using the ORP kit from Metrohm DropSens. The ORP kit is a complete, all-in-one solution to measure oxidation-reduction potential. This kit contains all of the necessary components to proceed with an ORP analysis: ORPSTAT (the main instrument), ORPSEN (disposable sensors), and ORPSTD (redox standard solution).
- Portability is mandatory for field measurements. Thanks to miniaturization, potentiostats like the ORPSTAT offer a user-friendly interface where the ORP value of the sample can be easily checked on an LCD display. This portable equipment powered with a Li-ion battery is compact at 9.0 × 6.0 × 2.5 cm (L × W × D) and handy, only weighing 100 g.
- Accessibility to the data is necessary to be able to study a large number of ORP values, especially when screening in the laboratory or sampling in the field are required. The ORP kit from Metrohm DropSens allows users the ability not only to check results on the LCD display—it also offers internal data storage. All ORP values obtained are stored in the internal memory of the device and can be downloaded to a PC for further access and evaluation.
- Disposability makes the use of the instrument and the facilities easier when handling complex samples. The Metrohm DropSens ORP kit is the most suitable system to perform in situ ORP measurements with samples that are expensive, scarce, or hazardous (e.g., biological samples or wastewater). Measurement with disposable sensors (ORPSEN SPEs) gives users the advantage of avoiding cleaning the ORP probe after measuring such samples. This is especially useful in the industrial field where complex aqueous matrices are commonly tested.
- Small sample size requirements mean less sample is needed, which is particularly useful when testing biological fluids. Miniaturization not only offers users portability when discussing the ORP instrument, but thanks to the ORPSEN SPEs, only 60 µL of sample volume is required to run decentralized or «Point of Care» (PoC) testing assays.
- Reliability is a desirable characteristic in any type of sensor, but mandatory for ORP since accurate ranges must be precisely controlled. ORPSEN electrodes are capable of measuring ORP values with enough precision to fulfill requirements in several application fields. In addition, a redox standard solution (ORPSTD) is also included with these sensors for checking the accuracy of ORP measurements.
Working with a miniaturized potentiostat allows easier on-site ORP measurements, while using disposable screen-printed electrodes allows such measurements in unsanitary conditions or in circumstances where conventional electrodes or other systems cannot be adequately polished or cleaned. Metrohm DropSens presents a complete kit for reliable, easy-to-use, and reproducible measurement of ORP: the ORP kit.
Screen-printing technology gives users the chance to work in areas such as environmental testing, agri-food, biotechnology, and the quality control of industrial processes. Additionally, this technology excels for biomedical research studies where samples can be expensive, scarce, or hazardous, and when only a very small sample volume is required. Moreover, these sensors do not require any maintenance or cleaning procedures as they can be discarded after an assay is completed, facilitating their use in all kinds of research.
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