ORP stands for “Oxidation-Reduction Potential.” Oxidation Reduction Potential is the measure in millivolts of the tendency of a chemical substance to oxidize or reduce another chemical substance. For hypochlorite manufacturing, the more positive the potential, the greater the oxidizing power. A technical paper entitled “The Control of Bleach Manufacture by Oxidation Potential,” by D.J. Pye proved a correlation between ORP measurement and excess caustic levels. The paper concluded that the use of ORP for the control and monitoring of a chlorination process is reliable, efficient, and practical.
ORP stands for “Oxidation Reduction Potential” while Redox stands for “Reduction/Oxidation.” Both terms are interchangeable. Both refer to a process in which one substance or molecule is reduced and another is oxidized. They can also refer to the measure in millivolts of the tendency of a chemical substance to oxidize or reduce another chemical substance.
Powell ORP probes produce a voltage signal (read in millivolts) that is inversely proportional to the amount of excess caustic in a hypochlorite solution. When plotted, this millivoltage curve has a sharp inflection near the reaction endpoint (when all the caustic is reacted). This makes it an excellent method for controlling this reaction process.
Typically, the ORP measurement is used to control the flow of chlorine into the reaction process. However, it could be used to control the flow of caustic into the reaction process in instances where the chlorine flow cannot be modulated.
More information can be found in the article, “The Control of Bleach Manufacture by Oxidation Potential,” by D.J. Pye.
Using pH measurement is not recommended for production control of sodium hypochlorite with excess sodium hydroxide levels in the range of 2-8 gpl (typical levels).
When a scrubber or hypochlorite production unit is charged with strong caustic (10%-22% w/w NaOH), the initial caustic concentration is off-scale (greater than 14 pH). The pH versus excess NaOH curves follows a strong acid/strong base titration curve, which results in a reading that only drops back on-scale near the point of complete depletion of the caustic. This is then followed by a sudden drop at the depletion point.
ORP control is very repeatable and does not need temperature compensation. It is very useful over a wide range of excess caustic levels, allowing it to be used not only in sodium hypochlorite production but also in chlorine scrubbers.
ORP control has an extremely fast response time, so with the use of a well-designed control loop, the tendency to over chlorinate at the end point is eliminated.
The two main types of ORP electrodes are direct immersion electrodes and wetted electrodes. Both operate under the same ORP measurement principles but differ in the way they read the ORP. Direct Immersion Electrodes are in direct contact with the solution for both the positive and negative electrodes.
Wet electrodes will contain a buffer solution, typically KCl (Potassium chloride), to protect the negative electrode (silver) from chemical attack.
A direct immersion electrode does not have a buffer solution. Instead, it is intended that the silver will dissipate over time due to the chemical attack from the hypo solution and will need to be replaced. It is standard that a wetted electrode comes with an integral pH sensor since both ORP and pH use the same reference electrode for measurement. In contrast, the direct immersion electrodes only generate a millivoltage specifically for ORP and do not give a pH measurement.
Powell electrodes do not need to be calibrated because the probes are silver and platinum electrodes with no buffer solution. The probes may need to be periodically cleaned if the electrode tips become coated. This is commonly seen in applications where hard water is used to dilute sodium hydroxide in the sodium hypochlorite manufacturing processes.
Under normal conditions, electrodes can last anywhere from several months to several years depending on the type of operation, rate of production, strength of product, and quality of the raw materials used in the process. Because each application is different, there is no average life expectancy.
An indication that the electrodes may need to be replaced include leaking at the electrode, erratic ORP control, or a significant change in set point to achieve the same excess alkalinity level.
Over chlorination (adding excess chlorine to sodium hydroxide) during the production of sodium hypochlorite will result in a chlorine release and destruction of the hypochlorite to chlorate, salt, and oxygen. A single ORP control loop including ORP electrodes, an ORP transmitter, a controller, and chlorine feed valve can fail for various reasons. Therefore, a second set of ORP Electrodes is always installed. This second ORP set includes ORP electrodes, ORP transmitter, and a separate automatic failsafe block valve. The second set of electrodes and alarms will close the automatic block valve if the primary set fails or the controls do not respond fast enough. The design of the control systems will vary, but the essential element of any chlorine feed system is to install two ORP electrodes as a basic part of the design.
If soft water, RO, or DI water is used to dilute the sodium hydroxide, the negative electrodes (silver tip) are virtually maintenance-free in the range of one year. The silvertip will be sacrificed by chemical reaction with the sodium hypochlorite during this time, and when the silver is depleted, the electrode will need to be rebuilt.
The positive electrode (platinum tip) will only need repair if there are signs of leakage at the terminal end of the electrode (3-5 years typical). This would signify an O-ring failure in which case the electrode could be returned to Powell for repair. Alternatively, repair kits can be purchased.
If city water is used to dilute the sodium hydroxide, the water hardness can react with the caustic and form a coating on the negative and positive electrode. In this event, there would be a shift of millivoltage relative to excess caustic indicating the coated electrode. The electrodes would be removed from the system and soaked in acid in a beaker. Detailed instructions can be supplied for this procedure.
No. The millivoltage from the ORP electrodes is an indication of the excess caustic in the solution, but it has no useful relationship to the bleach strength.
Under normal conditions, electrodes can last anywhere from several months to several years depending on the type of operation, rate of production, strength of product, and quality of the raw materials used in the process. Because each application is different, there is no average life expectancy. An indication that the electrodes may need to be replaced include leaking at the electrode, erratic ORP control, or a significant change in set point to achieve the same excess alkalinity level. Sometimes the electrodes will only need cleaning and not a replacement.
Occasionally, the ORP set point will need to be changed due to worn or dirty electrodes. Necessary adjustments can be determined by performing regular bleach tests. Other things that may cause a necessary change in the ORP set point include soft versus hard water, different grade of caustic, different caustic suppliers, or additives in the water or caustic.