“Problems in determining fluoride using the “acid dissolution-xylenol orange-zirconium colorimetric method”

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There is a slag of unknown composition that needs to have its fluoride content determined. The technician said that it’s sufficient to have an approximate idea of the amount; by \"approximate\", it means knowing whether the concentration is below 0.1, or whether it’s in the single-digit or double-digit range It’s just a way of checking,; the supplier doesn’t want to reveal it or maybe they don’t know the approximate composition of the material. In any case, for a slag (which can be turned into a fine powdered dry sample), it is necessary to determine the fluoride content. Someone recommended a method: acid dissolution-xylenol orange-zirconium colorimetry. The general principle is to distill the fluoride in the sample into fluorosilicic acid by adding an oxidizing agent, silica, and a strong acid; it is then absorbed by a base, and the fluoride ion content is determined using a spectrophotometer on the absorbent solution. I checked online, and indeed there is such a method, but I haven’t tried it in practice. The experimental equipment is simple: a distillation flask, a condenser, a colorimetric tube, and a spectrophotometer. Principle: Fluorine forms more stable compounds with zirconium, causing a change in the color of the solution containing xylene orange and zirconium ions. In the provided method, the fluoride standard curve ranges from 0 to 10 micrograms. However, in the method, the \"xylene orange-zirconium mixed solution\" provided by the other party is prepared as follows: 0.07 g of zirconium hydroxide is weighed, dissolved in hydrochloric acid, and the volume is adjusted to 2500 mL using 1+1 hydrochloric acid. Based on the molecular weight ratio, I calculated that the zirconium amount is approximately 20 micrograms/mL. 100 mL of a solution containing approximately 20 micrograms/mL of zirconium was taken, and 2–3 mL of a 0.5% xylene orange solution was added to prepare a xylene orange-zirconium mixed solution. When preparing the standard curve, fluorine standard solution is added to 50 mL colorimetric tubes in a concentration gradient, followed by the addition of 5 mL of the “xylene orange-zirconium mixed solution”. After bringing the volume to the mark and mixing well, the sample is analyzed using a spectrophotometer after 5 minutes to create the standard curve, which is then used to calculate the fluoride content in the sample. Question 1: The 5 mL \"xylene orange-zirconium mixed solution\" contains 100 micrograms of zirconium, while the maximum amount of fluorine that can be added is 10 micrograms. It is not known what compound is formed between fluorine and zirconium; even if it were zirconium difluoride, isn’t the amount of zirconium in the coloring solution too high? Question 2: Compared with the zirconium colorimetric method, which is more suitable – the fluoride reagent (alizarin amine carboxylate complexing agent) method? I’d like to hear the opinions and suggestions from fellow sea lovers. The reagents are still being shipped; the method has not been tried yet. I’m just thinking about whether there might be any issues with this method. You can give it a try once the reagents are purchased.