Similar to how measurements quantify alkaline or acidic solutions by pH electrodes, solutions can also be established as reducing or oxidizing based on measurements of ORP values (sometimes called “REDOX”).
Oxidation is a process during which a molecule or ion loses electrons, unlike the reduction that is when it wins. Oxidation is always linked to the decrease so that if one element is oxidized, the other is automatically reduced.
ORP measurements are based on the potential difference established between an indicator electrode and a reference electrode. The reference system is the same as that used for the pH electrode (Ag / AgCl), which is also used for redox measurements.
Redox electrodes are used to control many chemical processes, particularly those involving reversible reactions, such as the following:
Industrial Wastewater Treatment
The redox systems used in the treatment of water are the reduction of chromates and the oxidation of cyanide, mainly due to its toxicity to humans and the environment. The hexavalent chromium (Cr6 +) residual is reduced to trivalent chromium (Cr3 +) with the addition of sodium bisulfite or sulfur dioxide. In the case of cyanide, sodium hypochlorite or chlorine is used to oxidize cyanide, followed by hydrolysis of cyanogen chloride to form cyanate.
This parallel process of oxidation-reduction is known as redox and the ORP value is the measure of the activity of the electron compared to the activity of a reference electrode, which always maintains the constant potential.
Redox potentials are measured with an indicator electrode that is usually made of inert material and that is capable of absorbing or releasing electrons. The metals commonly used are platinum or gold. When the redox electrode is immersed in a solution containing a reversible chemical reaction system, the migration of electrons between the electrode and the system is established. This flow of electrons can be analyzed as an exchange current density and is extremely important for the rapid and reproducible measurement of the redox potential.
ORP measurements are increasingly being used as an effective measure of sanitation activity in drinking water, swimming pools and spas. The elimination time of the E.Coli bacteria in the water depends on the value of the redox potential. The ORP is a reliable indicator of the bacteriological quality of water. Water with an ORP value equal to or greater than 650 mV is within the parameters for swimming pool and spa waters.
Installing and Checking the Electrode
The ORP electrodes can be used in any pH / mV meter from Hanna.
1. After removing the protective cap from the electrode, soak the tip in warm tap water. This will increase the flow of the reference junction.
2. To check the function of the electrode, immerse the tip in the ORP solution (HI 7020). The value should be between 200 and 275 mV. Oxidation or reduction treatment with HI 7092 or HI 7091 will prepare the electrode surface and decrease the initial response time.