QualitativeTests for Manganese Seeking chemically interesting tests for use in our qualitative analysis course, we have found two uncommon reactions of manganese which require no unusual reagents and illustrate interesting manganese chemistry. Texts have suggested that Mn(l1) cannot be oxidized to MniVII) in basic solution; however, our first test is a simple procedure for doing just that. Students rarely see Mn(II1) in the general course, and our second gives them this opportunity. It also involves aqueous Cu(ll1) complexes which have recently been found and may have biochemical importance ( I 1.
Oxidation to Perrnanganate in Basic Solution Test: Mn(I1) test solutions as low as 0.001 M can he used in 1-2-drop portions. These test solutions can be prepared from the MnO? produced in the qual separation by dissolving it in a minimum of H 2 0 2and HC1 and hoiling. 1) Prepare the oxidant: Mix 10 drops of saturated NaHCO3,l drop 0.05 M CuSOa, and 10 drops 6% NaOCl (bleach). 2) Add to the oxidant 1drop of Mn(I1) test solution, about 0.01 M. Heat in a boiling water bath for 2 min. along with a blank. If the test color is pale, add more of the test solution, and continue heating. Chloride, present in NaOC1, does not interfere. This method was suggested by Feigl (2);however, much precipitate (CuO, MnOz?) form when NaOH is used alone. Following a hint in Mellor ( 3 ) ,we found that no precipitate forms when a bicarbonate buffer and less Cu(I1) are used. A possible explanation of this result follows. Cu(I1) and Mn(I1) are held in solution as carbonate~bicarbonatecomplexes. OC1- produces a CuiIII) species which also remains complexed in this mixture (I). The Cu(II1) or some other transition metal catalyst is needed for some of the oxidation steps (possihly the final M n O F to MnOa- reaction), since OCI- does not produce appreciable M n O 4 by itself. We found that a pure green M n O i ' solution in 0.2 M NaOH was oxidized by the Cu(II1) oxidant in 2-3 min a t room temperature. This test in basic media is useCul because, in the presence of much chloride, the usual acidic oxidations (by persulfate, periodate, or bismuthate) require excess Agt or Hg" to prevent chloride reduction of permanganate which is rapid in acid (4).
The Trioxalatornanganate (Ill) Color Test This test illustrates the use of a chelating agent to stabilize an unusuhl oxidation state. Proper pH and ligand concentration are needed to shift this equilihrium to a preponderance of the trioxalato complex ( 5 , 6 ) . T e s l Start with fresh MnOz from the qual separation schemes (either by basic HzOzoxidation of Mn(ll) or by acidic KCIOj oxidation). Use 0.5-5 mg Mn. Wash the precipitate and remove most liquid. Add 5 drops of saturated oxalie acid. Shake the solution and quickly add 5 drops of 0.5 M ammonium oxalate. Cherry red MniCnOajs" forms and can last several minutes if the pH is favorable. Eventually, the Mn(II1) finishes oxidizing the oxalate to COz, and the color fades. In the first step, HzCzOnreduces Mn(IV) t o Mn(III), and then the higher oxslate concentration from the ammonium oxalate causes the formation of the red trioxalato complex. The d i and mono-oxalato complexes are yellow~brown(5).If the yellow-brawn color appears, the student can add more oxalate. Adding HC1 shifts the equilibrium by prutonating oxalate. Low pH also speeds the final redon reaction to COz and Mn(I1). Note that COz is evolved in both oxidations of oaalate.
Literature Cited F.A,, and W#lkinsun, G.,"Aduancedlnurganic Chernistry,('4th ed.,J. Wiley, New York, 1980, p. 819. (2) Feigl, F.,andAoger. V., "SpotTestsin lnorganicAndysis,"6thed.,ElsevierPuhlishingCo.,New York, 1972,p.299.
111 Cotton,
la1 M d m , .I. W., "A Comprehensive Treatise or>Inorganic and Theoreticrl Chemistry: Longmans, Green m d Cn., New Ynrk, 1932, Val. X11, p. 303-4. (41 Arorn.C.1. ...I. CHEM.Enuc., 14,302 (19771. Gwes clarsraum demonstrat~unsufthe colors of theoiidatiunrtateiof Mn. (51 Cartledge, G. H . , m d E ~ i c k sW. , P., J. Amer. Chem Soc., 58, 2061, (1936). 161 Rich. R.. I. CHEW EDLIC., :2V, 400, 11962). Details and conditions of the test n e not specified. Acetate wa* "3rd which may be a competing ligand.
William B. Guenther University of the South Sewanee. TN 37375
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Journal of Chemical Education
