Determination of zinc sulfate containing ferrous ions
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This post was last edited by Yang Qing on 2015-8-20 at 12:10. Currently, methods involving the addition of ascorbic acid, ammonium fluoride, and thiourea are used, but the endpoint is not clear. It often happens that if the initial titration is fast, one cannot slow down near the endpoint either; it might be possible to reach a bright yellow color in one go (though the color will revert quickly). But if there is hesitation at the final point, if there is a slight pause during the titration, then the endpoint will never be reached. This is likely due to the sealing or stiffening of the xylene orange indicator. General characteristics of the sample: The zinc sulfate monohydrate content is around 90%, while the content of divalent iron is about 1.5% (calculated as Fe, not as ferrous sulfate). It is often difficult to determine the endpoint during testing, and only experienced analysts are able to identify it accurately. Such a method seems to need improvement. Seek ways to improve.
It is also said that first, about 2 g of the sample is weighed, and 10 drops of 1+1 sulfuric acid are added to dissolve it in a 250 mL volumetric flask. Then, 25.0 mL of this solution is transferred to a 250 mL beaker, 2 mL of **hydrogen solution is added, and the mixture is shaken. It is heated to a gentle boil in a fume hood and held at that temperature for 2 minutes. After cooling, sulfuric acid solution is added drop by drop until the solution becomes transparent. The solution is then diluted to 100 mL with water, 20 mL of sodium fluoride solution and 1 g of potassium iodide are added, followed by 2 drops of xylene orange indicator solution. Hexamethylenetetramine solution is added drop by drop until a magenta color appears. Finally, 10 mL of acetic acid-sodium acetate buffer solution is added, and the solution is slowly titrated using an EDTA standard solution; the endpoint is reached when the color changes from magenta to lemon yellow. The volume of EDTA standard solution used in this titration is recorded.
There are also opinions suggesting that iron can be removed by precipitation with ammonia, using the method employed for determining zinc concentrate; however, the resulting concentration tends to be low. It’s not clear whether this is due to incomplete complexation of zinc with ammonia ions Was it the formation of hydroxides that led to the lower results?