The effect of excess sodium carbonate on electrolysis during the purification of saturated salt water

An excess of sodium carbonate undergoes electrolysis, which is also the source of CO2 in chlorine gas; the electrolysis reaction is: Na2CO3 + H2O ====> 2 NaOH + CO2

In the later stage of the brine purification process, before it enters the electrolyzer, it is also necessary to adjust the pH value of the purified brine; after this adjustment, carbonates have no effect on the electrolysis process. Where did the reaction equation provided by the person upstairs come from?

Just think about where the CO2 contained in chlorine comes from, and consider the relationship between the CO2 content and the excess amount of soda ash during production; the fact that this is not mentioned in manuals or books does not mean that this reaction does not actually occur.

Since you believe it is caused by an electrolytic reaction, I would like to ask: during the electrolysis process, where or in which element can the gain or loss of electrons for the various components in your reaction equation be reflected? I would appreciate your advice.

Anodic side reactions: 2 CO3 = -4e ==> 2 CO2 + O2. There’s no need to go into detail about the migration of Na+ ions. Therefore, the crude chlorine gas produced by electrolysis (after cooling and water separation) generally contains < 0.5% CO2 and < 1.0% O2. When the excess amount of Na2CO3 added is well controlled (0.3–0.4 g/L), the CO2 content in the crude chlorine gas is generally around 0.2 V%.

This post was last edited by qugd on 2022-9-30 12:46. Are there any variable-valence elements in this reaction equation? It seems you really don’t understand the mechanism of carbon dioxide generation in the anode chamber during the chlor-alkali process. The small amount of oxygen in chlorine arises from electrolytic side reactions or other factors, but the residue of carbon dioxide is by no means due to the electrolysis process as you described. You really don’t understand electrochemical reactions, and the equation for an electrochemical reaction that you’ve written would be laughed at by those who truly understand electrochemistry – there is balance in terms of materials in your equation, but there is no balance in terms of electron transfer. I’ll be direct; please bear with me.

Since you know so much about this, please tell everyone directly where the approximately 200 tons of CO2 produced per year by a 200,000-ton caustic soda plant comes from! It can’t be that C is separated from Cl atoms and combined with the O2 produced by the electrolysis of H2O to form CO2, right? !

This post was last edited by qugd on 2022-10-2 at 12:22. Of course I understand it, but when telling you, I’m worried whether you can truly comprehend it. Judging from the posts you make here, it’s clear that you tend to interpret things based on subjective assumptions, and you even invent chemical reaction equations and electrochemical reaction equations that simply do not exist. What follows is the content related to what you need to learn; I hope you can understand it. The concentrated brine that enters the anode chamber of the electrolyzer is brine that has undergone secondary purification. Before this secondary purification, it is necessary to precisely adjust the pH value of the primary brine, in order to ensure that calcium and magnesium ions can be removed during the secondary purification process as it passes through the chelating resin bed. In other words, the level of alkalinity must be kept below a certain threshold, with the pH value being around 8 (with slight variations depending on the specific process), in order to achieve effective removal of calcium and magnesium. The alkalinity of the brine after secondary purification is already low, and the carbonate ions present are mainly in the form of bicarbonate ions. Before the secondary brine enters the electrolyzer, its pH value must be adjusted using hydrochloric acid to ensure that the electrolysis efficiency remains within the normal range. The range within which hydrochloric acid is used to control the pH value may vary depending on the specific process setup, but it is generally kept between weakly alkaline and neutral levels; in some processes, the secondary brine is even adjusted to a weakly acidic state. In the secondary refined brine thus treated, the carbonate ions exist mainly in the form of bicarbonate and carbonic acid. In the anode chamber of the electrolyzer, chloride ions are discharged at the anode to form chlorine gas. However, in a weakly alkaline saline solution, a certain amount of chlorine gas dissolves, and upon dissolution hypochlorite ions are formed. Since the temperature in the electrolyzer is around 80 degrees, the hypochlorite ions decompose to some extent due to this temperature; simultaneously, a disproportionation reaction may occur, resulting in the formation of chlorate ions. After the decomposition of hypochlorite ions, oxygen and hydrochloric acid are produced. Oxygen constitutes a by-product of the electrolysis process (when the voltage is not properly controlled, hydroxide ions also discharge at the electrodes, producing a small amount of oxygen, which is another side reaction of electrolysis). Hydrochloric acid causes the pH value of the saline solution in the anode chamber to drop further, prompting bicarbonate ions to convert into carbonic acid. Carbonic acid and bicarbonate ions decompose under slightly higher temperatures, releasing carbon dioxide, which then escapes from the anode chamber along with chlorine gas and trace amounts of oxygen. In saline solution, carbonate, bicarbonate, and carbonic acid (carbon dioxide and water) are involved; no electrochemical reactions occur during electrolysis. In other words, there is no gain or loss of electrons for carbon, which means that the valence of carbon does not change. You should have learned in high school that redox reactions require a change in valence, and electrochemical reactions are a special case of redox reactions. Therefore, the chlorine gas produced by ion-exchange membrane caustic soda is not resulting from the electrolysis of carbonate ions in the saline solution. You do not understand the chemical reactions involved in this process, nor are you aware of the electrochemical mechanisms behind it; that is why there are errors in your reply. There are quite a number of experts in the industry who understand the essence and mechanism of this process; it’s just that they aren’t as meticulous as I am. Regarding the content and my evaluation of you, please bear with me if there are any inaccuracies.

In the process of producing chlorine, hydrogen, and sodium hydroxide by electrolyzing saturated salt water, an excess of sodium carbonate solution must be added during salt water purification to remove calcium ions from the salt water. In that case, will the excess sodium carbonate in the brine be electrolyzed during the brine electrolysis process? @hnhg2008 provides a formal response to the original poster’s question: An excessive amount of sodium carbonate is added, and a certain level of excess alkalinity is maintained. The purpose of this is to remove as much calcium and magnesium as possible through precipitation during the first purification step. An appropriate level of excess alkalinity is retained for the second purification stage, so that the remaining calcium and magnesium ions can be removed under suitable conditions, ensuring that the saltwater entering the system meets the required standards. Before the concentrated brine enters the anode chamber of the electrolyzer, its pH level needs to be adjusted (using hydrochloric acid to adjust the pH). At this point, the remaining alkali forms bicarbonates or even carbonic acid, which escape along with chlorine gas at certain temperatures. Carbonate ions are not electrolyzed because their discharge voltage is higher than that of chloride ions, and their concentration in saltwater is not high, so there are no conditions for competitive reactions with the discharge of chloride ions.