The issue of free chlorine in the demineralized brine produced by the dechlorination tower

Didn’t you already say that ORP stands for oxidation-reduction potential? You’ll understand it if you read books on electrochemistry. ORP can be used to measure free chlorine; a value of -50 mV is generally fine. Please check the sampling location for ORP and whether it’s damaged

I agree with the comment above; the effect of normal physical chlorination can only ensure free chlorine levels in the range of dozens of PPm, and a level of -50mV should be a value obtained after chemical chlorination. It is recommended that the poster check the ORP value and compare it with the results of chemical analysis. An ORP meter is used to measure the redox potential; the potential is high in the presence of free chlorine, while it is low in the presence of reducing substances such as sodium sulfite, thereby indicating the effectiveness of dechlorination along with relevant data.

The instrument commonly used to measure free chlorine is an ORP meter; a value of less than 20 mV should indicate that there is no free chlorine present. Of course, this mainly depends on the instrument itself. It is necessary to check the electrode of the instrument, calibrate it, and verify whether there are any blockages in the pipes at the measurement points – poor flow in the pipes can result in lower readings. Thorough analysis is required, with the analytical results being the most reliable source of information. When the pipes and instruments are in good condition, it is advisable to take multiple samples under different free chlorine levels and compare those values with those shown by the instrument, in order to determine the control values suitable for one’s specific process

Keeping it below 100mV is sufficient; does it really need to be as low as -50mV? Last edited by yzhms on 2009-3-21 17:08.]

Either the electrode is damaged or there is a problem with the analysis. There’s no doubt about the control indicators. :)

What is the pressure in your dechlorination tower? How much sodium sulfite was added?

ORP I. Overview ORP is the abbreviation for O*dation-Reduction Potential in English, and it represents the redox potential of a solution. The ORP value is a measure of the redox capacity of an aqueous solution, with the unit being mV. It consists of an ORP composite electrode and a millivoltmeter. An ORP electrode is a type of electrode that can absorb or release electrons at the surface of its sensitive layer, which is an inert metal, usually made of platinum or gold. The reference electrode is a silver/silver chloride electrode, the same as the pH electrode. 　　In natural water bodies, there are various valence ions and dissolved oxygen. When industrial wastewater is discharged into such waters, a large amount of ions and organic substances end up in them. Due to the differences in the properties of these ions, redox reactions occur within the water body, leading to an equilibrium state. As a result, natural water bodies are not composed of a single redox system, but rather of a mixed redox system. The potential measured by the electrode is also a mixed potential, which has significant experimental errors. Additionally, the pH value of the solution also affects the ORP value. Therefore, it is meaningless to emphasize the absolute potential of the solution during actual measurement. We can say that the ORP value of a solution indicates its degree of reduction or oxidation at a certain value, or it reflects some property of the solution (such as its cleanliness level), but this value can vary significantly, and it is not possible to determine it quantitatively; this is a different concept from the accuracy in pH testing. Additionally, the temperature coefficient that affects the ORP value is also a variable that cannot be corrected; therefore, ORP meters generally do not have a temperature compensation function. 　　II. Scope of Application 1. Industrial wastewater treatment Used in oxidation-reduction systems for water treatment, mainly for the reduction of chromic acid and the oxidation of cyanides. Adding sodium disulfide or sulfur dioxide to wastewater can convert hexavalent chromium ions into trivalent chromium ions. Adding chlorine or sodium hypochlorite can be used to oxidize cyanides, followed by the hydrolysis of cyanogen chloride to form cyanates. This chemical reaction process is called an oxidation-reduction reaction system. The redox potential is a measure of electron activity, which is similar to the method used to measure hydrogen ion activity. 　　2. Disinfection and Application of Water Redox electrodes can measure the effectiveness of disinfection in swimming pool water, mineral water, and tap water. Since the sterilization effect of E. coli in water is affected by the redox potential, the redox potential is a reliable indicator of water quality. If the redox potential value in pool water and mineral water is equal to or higher than 650 mV, it indicates that the bacterial content in them is acceptable. 　　III. Installation and Inspection of Electrodes 1. Redox electrodes can be used with any pH/mV meter. 　　2. The ORP meter does not require calibration and can be used directly. Only when there are doubts about the quality of the ORP electrode or the test results, can an ORP standard solution be used to check whether the potential falls within the range of 200–275 mV, in order to determine the condition of the ORP electrode or the instrument. Instruction Manual for Redox Electrodes Instruction Manual for Redox Electrodes Instruction Manual for Redox Electrodes 3. The ORP measuring electrode (platinum or gold) should have a smooth surface; a rough or contaminated surface can affect the electrode’s potential in mv. It can be cleaned and activated using the following methods. 　　(1) For inorganic contamination, the electrode can be immersed in 0.1 mol/L dilute hydrochloric acid for 30 minutes, washed with pure water, then immersed in a 3.5 MOL/L potassium chloride solution for 6 hours before use. 　　(2) For organic oil and oil film contamination, the platinum or gold surface can be cleaned with detergent, followed by rinsing with pure water; after that, it should be immersed in a 3.5 MOL/L potassium chloride solution for 6 hours before use. 　　(3) If the surface of platinum is severely contaminated and an oxide film has formed, the surface can be polished using toothpaste, then cleaned with pure water, and after that immersed in a 3.5 MOL/L potassium chloride solution for 6 hours before use.

Our company’s ORP set value is 200 mV. Tests conducted by the quality control department show that the free chlorine level in the desalinated water is zero, and the sodium sulfite content is also very low – sometimes a few ppm, other times around ten ppm. The consumption of sodium sulfite is extremely low. The original poster has reached -300mV; free chlorine should not be detected, so there is likely an issue with the testing analysis.

Let’s check if there’s a problem with the equipment; our ORP value is usually around 150, and then there’s no more free chlorine. What about the amounts of dechlorination acids and bases you use, as well as the pressure in the dechlorination tower? Alas! I guess there’s a problem with the gauge!

It’s likely there’s a problem with the instrument.

Previously, it was kept below 50 mv; for safety reasons, it was kept below -50 mv. However, under conditions of adding acid, alkali, and sodium sulfite, with normal vacuum levels, the laboratory was able to detect free chlorine at -320 mv, but not at -190 mv. The instrument technician said there was no problem with the ORP. This post was last edited by limingshuguang on 2009-4-25 19:35.]

ORP is an oxidation-reduction potential meter; if there are Cl ions in the deionized water, its potential (i.e., conductivity) will be relatively high. This can be seen in potentiometry; last edited by limingshuguang on 2009-4-25 19:36]

The free chlorine ORP meter in lightly saline water shows -300 mV on-site, but free chlorine is still detected; this could be due to an issue with the analysis or with the instrument itself. It is possible to stabilize the PH312 and PH314 instruments, then add a certain amount of sodium sulfite, and monitor the value indicated by the ORP meter. After that, the flow rate can be increased or decreased to observe any changes in the ORP value, thereby determining whether there is a problem with the instrument. At the same time, analytical testing should be conducted to identify where the issue lies.

The instrument commonly used to measure free chlorine is the ORP meter. Under normal conditions, the value in our company should be below 300 mV; however, recently it has been kept below 100 mV, yet free chlorine can still be detected frequently. I’m not sure why If the ORP is kept below 0, I think it’s a bit of a waste of sodium sulfite, but yet it has to be kept below 0 anyway, which is really troublesome.

(1) Check the electrodes to see if there is a shortage of the standard solution (potassium chloride). (2) View the display system. (3) Conduct manual analysis by sampling, requiring multiple samples. (4) The ORP value should generally be kept below -150 MV, with a stable vacuum level. By adjusting the pH to 1.3–1.8 before dechlorination and to 10–11 after dechlorination, the ORP value will remain within the controlled range and will not exceed the specified limit. Also, there’s no need to use that much NA2SO3; 30–50 ML/H is sufficient.

We have also encountered the problem you mentioned. It’s the redox potential that’s faulty; the process gas blows the gases from the anode discharge tank over, so naturally the potential exceeds the normal level. . .

It seems that the control parameters required for using redox potentiometers vary from one company to another; is this due to differences in the instruments used? Logically speaking, everyone’s systems are similar; there shouldn’t be such a big gap, right?