Chemical formula: MnCO3
Synonyms: manganese (II) carbonate, carbonate of manganese, manganous carbonate.
Amount: 200 g
Appearance and odour: fine, beige powder without appreciable odour.
Description and uses: often used in pottery glazes as a colouring agent/opacifier, it’s also one of the most versatile manganese (II) compounds and a stable and handy precursor to much of manganese’s complex chemistry. Water insoluble but readily soluble in acids, it is a gateway to many other manganese compounds.
Upon heating it starts losing carbon dioxide from about 200 C, in the presence of air yielding mixed oxides, mainly MnO2. In inert atmosphere (N2, Ar) or vacuum decomposition of the carbonate leaves behind MnO (manganese monoxide or manganese (II) oxide).
This technical grade MnCO3 dissolves completely in hot 1 M (1 mol/L) hydrochloric acid (HCl). The main contaminant is iron and it can be removed with little or no loss of product as follows.
Dissolve the amount you want purify in a precise amount of sulphuric acid but make sure there’s a little excess of MnCO3: this way the resulting (contaminated) manganese (II) sulphate will be almost neutral. Allow reaction until all fizzing stops and allow to clear. Don’t worry if it doesn’t clear completely. Decant or filter off, setting aside the excess MnCO3 for future use.
Take about one quarter of the solution and set the rest (the stock) aside. Add enough of an alkali (NaOH, KOH or ammonia solution are all good but not Na2CO3) to precipitate all the manganese and iron as hydroxides. Filter this and wash the filter cake of manganese and iron hydroxides with plentiful small aliquots of clean water until the filtrate is almost neutral. This washes out the soluble cations.
Now scoop out most of the precipitated hydroxides on the filter, add them to the stock solution and leave this to stand overnight (do not discard the filter, instead cover it with cling film to keep it moist). During standing overnight any contaminating iron will be converted to highly insoluble iron (III) oxide. Now filter the stock plus precipitates, using the same filter. The obtained solution is now essentially free of any iron and can be used to re-precipitate the manganese as purified MnCO3 or to crystallise it as quite pure MnSO4. This purification can also be carried out on manganese chloride: substitute sulphuric acid with hydrochloric acid.
Oxidation of manganous (II) salts in alkaline condition using hydrogen peroxide, hypochlorite or other results in manganese dioxide (MnO2) to precipitate.
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