The issue of free chlorine in high-purity hydrochloric acid
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The high-purity hydrochloric acid produced by us is mainly used in the ion-exchange membrane process, and it is required to be free of free chlorine. The method we use at present is as follows: 50 ml of hydrochloric acid is added to a 250 ml iodometry flask containing 100 ml of distilled water. 10 ml of potassium iodide solution is then added, the flask is covered with a water seal and left in the dark for 5 minutes. After that, starch indicator is added; the sample turns blue and the color is quite intense. When the flask is shaken for three or four seconds, the blue color disappears and the solution becomes colorless. This indicates whether the sample contains free chlorine or not. We also add hydrochloric acid between the outlet of the resin tower and the brine tank; sampling analyses show that there is no free chlorine at the outlet of the resin tower, whereas it is present at the pump outlet of the brine tank. Is it the hydrochloric acid that contains free chlorine, or is there some other reaction taking place?
When testing free chlorine in hydrochloric acid using the iodometric method, it is best to conduct a blank test at the same time. Because under strongly acidic conditions, dissolved oxygen in the air can also oxidize iodide ions to iodine. As for what was mentioned later, it is likely that chlorate ions in the refined salt water reacted with chloride ions to produce chlorine. However, the reaction rate of chlorate with hydrochloric acid is not very fast, and it is significantly affected by acidity. The brine at the outlet of the chelating resin column is neutral or slightly alkaline; in such conditions, chlorate does not react with chloride ions due to insufficient acidity ； After adjusting the pH with hydrochloric acid, it should be between pH 3 and 4, or even more acidic; at this point, chlorate ions will react with hydrochloric acid.
Reply to 2# qugd: Could you explain the method in more detail? Is the blank solution prepared using an acid free of free chlorine, or pure water? Before entering the brine buffer tank, we add acid to adjust the pH to 6–7. This is done to remove carbon dioxide and reduce side reactions. Additionally, the chlorate content at the outlet of our resin tower is around 2 g/l, which is quite low.
I assume the blank sample is prepared without acid, but to maintain a consistent sample volume, it is best to use pure water for dilution. I personally think it is free chlorine generated by the disproportionation reaction of chlorates with hydrochloric acid; you can test whether high-purity hydrochloric acid contains free chlorine.
Reply to 4#: I’ve talked to Muziyuan; we’re not sure about the judgment regarding the final outcome
Measure in accordance with GB320-2006
The problem remains unsolved. I followed the methods specified in GB320-2006. Could you please explain what happens: after adding starch, a blue color appears, but this blue color disappears when the mixture is shaken. Does this indicate the presence of free chlorine, or not? If there is no free chlorine, then why does a blue color appear?
It should be contained; it’s already present in hydrochloric acid