Analysis method for sodium chlorate in brackish water by ion membrane electrolysis

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Our factory has newly installed a chlorate decomposition tank. We analyzed the chlorate levels at the inlet and outlet of this tank, and they were both around 20 g/L. Based on these results, it seems that this chlorate decomposition device isn’t working effectively. However, the chlorine content in the gas coming out of the tank is as high as 95%, which indicates that the chlorates have indeed been decomposed; otherwise, the chlorine level wouldn’t be so high. Now we suspect there may be an issue with the analysis method. I’ve written this down so that we can discuss whether there are any problems with our method for analyzing chlorates, and if so, where exactly the issues lie Or maybe there is some better way. The method is as follows: Take 25 ml of lightly salted water and dilute it to 250 ml in a volumetric flask; then take 10 ml of this solution for analysis. First, add 25 ml of 0.05 M ammonium ferrous sulfate standard solution, followed by 10 ml of (1+4) sulfuric acid. After boiling for 10 minutes, allow the mixture to cool, then add 15 ml of (1+1) phosphoric acid. Finally, perform back-titration using 0.01 M potassium permanganate solution. We have been using this method to measure the sodium chlorate content in weak saline water; the amount of potassium permanganate consumed each time was very small, sometimes just a few drops. We wondered whether insufficient amounts of ammonium ferrous persulfate were being added, so we tried using 50 ml in the experiments, but the results obtained were still similar. I’m not sure if free chlorine in saltwater has a significant impact on this measurement; what’s the best way to remove free chlorine? Let’s discuss it