Free Residual Chlorine/ORP
Free chlorine residual means the chlorine concentration, in parts per million (ppm) of water, available for rapid and effective biocidal action. This is the chlorine, which remains uncombined with nitrogenous compounds after the initial chlorine demand of the water has been satisfied.
FRC is the amount of chlorine available at the end of a specified contact period in chlorinated water or wastewater. This residual may be in the form of dissolved gas (Cl2), hypochlorous acid (HOCl), or hypochlorite ion (OCl–), but does not include chlorine combined with an amine (ammonia or nitrogen) or other organic compound.
ORP is a less common analytical measurement, thus it is frequently misunderstood. ORP is an abbreviation for “Oxidation Reduction Potential”. It is also commonly referred to as a “Redox” as the sensor is used to measure redox reactions.
ORP is a measurement of the net voltage potential of excess oxidizers or reducers present in a liquid. Based on this description it is helpful to understand the chemistry behind these particles. An oxidizer is a molecule with an excess of electrons and has a negative charge. A reducer is a molecule with a deficit of electrons and has a positive charge.
The measurement electrode is the main differentiator between a pH sensor and an ORP sensor. ORP sensors do not have the specialized glass membrane used in pH sensors. Instead, the measurement electrode of an ORP sensor has a metal band or wire in contact with the process liquid. The metal contact point completes the sensor’s voltage circuit that includes the reference electrode and the liquid junction. There is no potential generated by the metal; however changes in the concentration of oxidizers or reducers will influence the overall voltage potential of the circuit. Normally a noble metal such as platinum or gold is used for the measurement electrode so that it will not get damaged due to the redox reaction. All Hamilton ORP sensors use platinum for their measurement electrode.
It is important to remember that the reference electrode generates 220mV. This means that in a truly neutral solution with no potential voltage, the ORP sensor would still output 220mV. For proper measurement this voltage is subtracted out by the pH transmitter electronics during calibration.
The ORP sensor is not specific to any single oxidizing or reducing molecule. Due to this, the sensor output is a raw millivolt value with no other unit of measure. Increasing voltage is associated with a higher concentration of one type of molecule or the other. Because of this, ORP is frequently used as a control parameter for a redox reaction. For example, chlorine is a strong oxidizer frequently used for disinfection.
When bleach is added to the liquid, the ORP value immediately increases. Over time, as chlorine reacts with biological organisms and other molecules, the ORP value will decrease. An ORP setpoint is used to control chlorine addition to optimize the disinfection. While ORP is technically not directly measuring chlorine, its measurement is a good indicator of the strength of the reaction.