How to determine calcium ions in the presence of manganese
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There is an ore sample with a high content of silicon and manganese – around 30% – and the calcium content is estimated to be around 20%. How can one mask the large amount of manganese ions in order to measure the calcium ions, or how can they be separated?
Can it be removed by oxidation, using potassium chlorate to oxidize it and then filtering? The measurement method is EDTA titration in an alkaline ammonia buffer, right? You should explain your testing methods and then consider ways to eliminate interference.
This post was last edited by qugd on 2020-11-6 at 10:01. Acidic dissolution of the sample allows for an effective separation of manganese and calcium from silicon. An appropriate amount of base is added to the acidic sample; the amount of base required is calculated using the solubility products of manganese hydroxide and calcium hydroxide, so that manganese forms a hydroxide precipitate while calcium does not form one. This allows a large amount of manganese to be separated from calcium. The pH at which manganese is completely precipitated is approximately 9.8 to 10. By performing an EDTA complexometric titration under strongly alkaline conditions, the calcium content can be determined. The manganese content can be determined by EDTA complexometric titration under weakly acidic or neutral conditions.
Teacher Q’s approach is simpler and more feasible: calcium can be determined using the EDTA titration method under strongly alkaline conditions, with sodium carboxylate as an indicator, which also helps to eliminate the interference of magnesium. Since, some time ago, when using EDTA to measure zinc and aiming to eliminate manganese interference, the potassium chlorate oxidation method was employed, there was no consideration given to using pH to achieve separation of calcium from manganese; under normal testing conditions, the solubility of calcium should be sufficient.
This post was last edited by qugd on 2020-11-6 at 13:43. Thanks to @YangQing for the praise. If there is concern that calcium will not dissolve completely under strong alkaline conditions, the sample size can be reduced. Additionally, by adjusting different pH values, calcium and manganese can be measured separately using colorimetry; however, for routine samples, the sample volume may be smaller, resulting in a decrease in precision.