How to eliminate the interference of manganese when determining zinc content in zinc-containing waste
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This post was last edited by Yang Qing on 2018-11-27 at 16:12. General composition of the raw material: 20–40% zinc, 0.2–2.5% iron, 0–5% copper, 0.5–2% manganese. The testing method is an industry-standard protocol: The steps are as follows: 1. Weigh 0.5 grams of the sample into a beaker, add 1 gram of potassium chlorate, and moisten it with a small amount of water. 2. Add 20 ml of concentrated nitric acid, heat on an electric furnace until all yellow smoke has disappeared, add 5 ml of concentrated hydrochloric acid, and add 1 g of solid potassium sulfate. 3. Continue heating for a short while, then remove and cool. Transfer to a 250 ml volumetric flask. 4. Add 30 ml of concentrated ammonia, 10 ml of 10% potassium fluoride, and 10 ml of anhydrous ethanol to the volumetric flask, then bring the volume to the mark. 5. Filter, transfer 25 ml of the filtrate to a beaker, and heat it to remove ammonia until most of it has been driven off. 6. Add 1 drop of methyl orange indicator, and adjust the solution to a faint red color using 1+1 hydrochloric acid. 7. Add 10 ml of saturated thiourea solution and 20 ml of pH 5.4 acetate buffer; using xylene orange as an indicator, titrate with an EDTA standard solution of approximately 0.015 mol/L until a bright yellow endpoint is reached. 8. Calculation (omitted). During the aforementioned testing process, manganese interferes with the determination of zinc, resulting in higher zinc values in the test results. (The presence of interference can be confirmed by adding an appropriate amount of manganese sulfate to the sample. ) There are two issues now: 1. How to eliminate the interference of manganese in such samples? 2. Logically, under this method, manganese has been oxidized to manganese dioxide and should be filtered out so as not to interfere with the measurement. Or under strong ammonia conditions during filtration, divalent manganese is also precipitated. Why is there still interference? Could some experts analyze this and suggest whether there are suitable methods to eliminate the interference?
This post was last edited by qugd on 2019-8-20 at 12:10. I believe the source of interference is that after nitric acid is added in the second step, and after the nitric acid has decomposed before hydrochloric acid is added, some high-valent manganese ions give rise to a small amount of divalent manganese due to the reducing property of chloride ions. You might try using sulfuric acid instead of hydrochloric acid as a control; it could yield better results. The establishment of some analytical methods may not have taken all interfering factors into account at the time, so the means to eliminate these interferences are not necessarily entirely appropriate.
This post was last edited by Yang Qing on 2018-11-29 at 16:11. Thank you for your reply. I guess the addition of hydrochloric acid in this method is to achieve a \"king’s water effect\" by mixing nitric acid with hydrochloric acid. If it is the reducibility of chloride ions that leads to the formation of divalent manganese, I think that even if divalent manganese is formed, adding a large amount of concentrated ammonia in the fourth step should result in the precipitation of manganese hydroxide upon filtration, and it should not cause significant interference. The Ksp of manganese hydroxide is very small. By adding manganese sulfate to the raw material, it was confirmed that the measurement values indeed became higher after the addition of manganese sulfate. According to theoretical calculations based on the degree of influence, approximately 50% of the manganese ions participate in the complexation reaction with EDTA; moreover, the reproducibility is poor and the endpoint indication for titration is not clear. I’ll try sulfuric acid to see what happens.
Now that I think about it, under the experimental conditions mentioned, manganese ions are first oxidized to manganese dioxide; however, when hydrochloric acid is added, manganese dioxide reacts with the hydrochloric acid and is reduced back to divalent manganese. Divalent manganese forms manganese ammine ions with ammonia, which can interfere with the measurement.
This post was last edited by Yang Qing on 2019-8-7 at 12:04. As expected, I tried using pure zinc oxide plus manganese sulfate; when nitric acid was added, the presence of potassium chlorate caused the manganese ions to form manganese dioxide. After adding hydrochloric acid, the black substance disappeared! Your analysis is quite correct. Thank you.
“Change “Add 1g of solid potassium sulfate” to “Persulfuric an” and that’s it
The manganese ions that have been dissolved again are oxidized back to manganese dioxide; P