One question per day; those who answer correctly receive 6 wealth points, while participants receive 2 wealth points； Question 514
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Fill-in-the-blank: When analyzing the purity of hydrogen chloride before it enters the VCM converter, ( ) solution is used; therefore, when the color of the solution changes to ( ), it indicates the presence of free chlorine, with a darker color corresponding to a higher concentration of free chlorine. Answer: Starch potassium iodide, blue
When analyzing the purity of hydrogen chloride before the VCM conversion mixer, since a (starch-potassium iodide) solution is used, a change in the color of the solution to blue indicates the presence of free chlorine; the darker the color, the higher the concentration of free chlorine.