Under what conditions do calcium ions not react with sodium carbonate?
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This post was last edited by Yinozhang on 2019-1-13 at 10:55. As I recall, the first chemical precipitation reaction learned in junior high school class was the one where carbon dioxide gas is introduced into lime water solution, resulting in the formation of calcium carbonate precipitate. In actual chlor-alkali production, sodium carbonate is usually added to calcium-containing saline solutions, with a certain excess of sodium carbonate, to produce calcium carbonate; thereafter, filtration equipment is used to remove this calcium carbonate in order to eliminate calcium ions from the saline solution. However, recently we encountered a situation where there was an appropriate excess of sodium carbonate, yet calcium ions were still present after filtration. Please, expert professionals, help analyze what could be the reason for this In other words, under what conditions do calcium ions (other than in acidic environments) not react with sodium carbonate? For example, in the forms of calcium sulfate, calcium sulfite, calcium hypochlorite, calcium hydroxide, etc.?
When the calcium ion concentration is below the minimum level required to form calcium carbonate precipitate in sodium carbonate solution, it no longer reacts with sodium carbonate; this concentration can be calculated using the solubility product data for calcium carbonate. In practical applications, when there is a sufficient concentration in water of chelating agents that can form stable calcium complexes with calcium, calcium does not form calcium carbonate precipitates at certain sodium carbonate concentrations. This situation is actually a special case of the one mentioned above, and it is subject to strict limitations.
Calcium carbonate also has a certain degree of solubility; even if you put calcium carbonate into water, some of it will dissolve. It’s not the case that precipitation occurs just because carbonate ions and calcium ions are present. The dissolved portion of calcium carbonate will ionize to release calcium ions
The amount of sodium carbonate in the solution is now 0.6 g/l in excess. Based on the solubility product, the calcium ion concentration should be below 1 mg/l, but the actual measurement shows around 10 mg/l. As for the chelating agents that can form complexes with calcium in the solution, it’s not yet clear; everything was normal before, but recently we added sodium hypochlorite to the solution first, and then sodium sulfite (to react with the excess sodium hypochlorite). Could it be that the excess sodium sulfite has affected the reaction involving calcium carbonate?
I agree with your view, but even if it does dissolve, the amount that dissolves should not be very large; it cannot be detected using the titration analysis we use currently (below 1 mg/l). Yet actual analyses show levels around 10 mg/l, which is truly puzzling.
The calcium carbonate concentration product is calculated based on a pure calcium carbonate system, but in actual media, factors such as complexation effects and salt effects that influence the dissolution of calcium carbonate can cause the actual dissolution concentration of calcium (note that this is not necessarily the concentration of calcium ions) to be much higher than the concentration calculated from the concentration product. It is better to make judgments and provide explanations based on the actual composition data of the medium.
Has the original poster solved their problem? Could you share some experience?