edited by GEORGEL. GILBERT Denison University Granville, Ohio 43023
Manganese Color Reactions SUBMCTTED BY
Robert S. Pearson Univelslty Of Arkansas at Monticdlo Montlcello, AR 71655
CHECKED eV
MnO;
Arnold George
purple
Mansfield Unlverslty Mansfield, PA 16933
' Arora, C. L. J. Chern. Educ. 1977, 54, 302
orange
Mn02
Chemists generally are aware of the purple of permanganate ion, the brown of manganese dioxide, and the pink of manganese(I1) ion, but a much wider array of colors can be extracted from this element. The marvelous green of the manganate ion is easily prepared, and, when this green is mixed with the permanganate purple, optical cancellation creates other shades of purple and blue. Partial disproportionation of the manganate ion with acid can produce a neutral gray through optical cancellation, and complete disproportionation gives an orange-red mixture of permanganate and manganese dioxide. The orange-red color from the disproportionation is somewhat similar to the orange of manganese dioxide alone. Evidence that disproportionation has actually occurred, rather than just reduction to manganese dioxide, can be produced by adjusting the pH and again reducing the available permanganate to manganate. This time there is manganese dioxide present, and the green color is not as bright and clear as when manganate alone is produced. Classroom demonstrations using manganese compounds' can graphically illustrate the importance of the oxidation numher concept and help explain why teachers are so fastidious about its correct assignment. The purple one is manganese(VII), and the green one is manganese(VI)! The jawbreaker word disproportionation can be tamed a little with a simple and colorful demonstration. Oxidation and reduction can be shown to occur for the single species manganate MnOa2- just by changing the pH of the solution. I t seems likely that the sputtering resurgence of descriptive chemistry can be assisted by attention-holding demonstrations. Different parts of the demonstration described below can be incorporated into lectures at the appropriate times to help explain terms and concepts or &&iy to provide a change of pace from lecturing. Those with a flair for demonstrations should be able to devise several interesting sequences of color change for children's .erouos . or other nontechnical audiences. \luch reaction chemistry can he made interesting by careful choice uf lecturr demonstratiuns. Descri~nivechemistrv can be more exciting than reading the chicago telephone directory. The figure gives the general plan of changes. In all cases hydrogen sulfite ion is used as the reducing agent. Permanganate ion is acidified and reduced to manganese(I1) ion 2Mn04- + 5HS03- + 6Ht = 2Mn2++ 5HSO4-
t HSOj
+ 3Hz0
Flow chart of
manganese changes.
Neutral permanganate is reduced to manganese dioxide 2Mn04- + 3HS03-
= 2MnOds)
+ 2SOn2-+ HSOa- + HzO
and basic permanganate is reduced to manganate 2Mn01- + HSO again inverting several times. Reduction to MnOn requires only the addition of 5 drops of 0.10 M NaHS03 to a test tuhe of KMn04 solution and mixing. The orange (brown) color in the test tuhe looks like a true solution and solid M n 0 2 does not quickly settle out. The Mn042- ion is produced from MnOa-ion by adding 10 droos of 8 M NaOH. mixing. then addine 2 droos of 0.10 M N ~ H S Oand ~ mixing again: Too little base or too much NaHSO? will send this on to MnOg. S O ~ instead The addition of only one drop O ~ N ~ H solution of two will produce a blue coloration. The red part of the Mn04- purple is cancelled by the green of the MnOa2- leaving the blue of the MnO4- purple as the only visible color. Adding ahout a half drop of NaHS03 to the basic Mn04solution gives a still different blue purple coloration. Disproportionation is easily demonstrated with the MnOa2- solution. I t is best to save one test tube of the green solution as reference and make uw more test tubes of MuO42- to work with. Addition of about 15 drons of 6 M HCI to a ereen MnOd2solution produces a neutraigray. The red of the MnO4- and green of the MnOa2- cancel each other as described above and the remaining blue of the Mn04- is cancelled by the orange of the suspended MnOz. A perfect gray may require a drop more or less than the 15 drops indicated so a dropwise titration with HC1 with thorough mixing near the 15 drop suggested amount may be required. The red and green simultaneously present in this solution prior to mixing is also quite interesting and atest tube of this mixture can he saved. Addition of 25 drops of 6 M HCI to the green Mn04'converts it through disproportionation into an orange-red mixture of the purple MnO4- and the orange suspended MnO2. This same mixture can he seen as a stage in the neutral reduction of Mn04- to MnOz when only part of the required 5 drops of NaHS03 is added. T o prove that disproportionation, rather than simple reduction to MnO2, has occurred, repeat the disproportionation orocedure above. then make the solution basic with 10 dropsbf 8 M NaOH mix, add one drop of 0.10 NaHS03, and mix aeain. The ereen of MnOa2- will aeain - awwear, .. . but now modiEed by theorange of ~ n 0 2 . Anyone who can devise a way to extend this system to include Mn3+ could give us even more color variety.
S u s ~ m sv o
Flame Colors Demonstration
John R. Peyser and John R. Luomal Cleveland State Unlvsrslty Cleveland, OH 44115 CHECKED eV
Arnold George Manstleld Unlvsrrlty Mansfleld, PA 16933 Flame colors are discussed often in chemical education (I, 2) and many chemistry texts (3, 4) contain references to these colors. Fireworks (5-7) are mentioned as an application of these colors. We have adapted some of these concepts to give this dramatic and colorful lecture demonstration. Carefully conducted, this demonstration produces a simple and exciting visual display producing little smoke or ash. 452
Journal of Chemical Education
The filter paper disks, when properly prepared, burn rapidly hut not explosively. The demonstration may be conveniently conducted in class by simply igniting individual disks with a match or Bunseu burner while holding the disk with forceps. Alternately, the disks may be suspended from a ringsiand-supported metal rod. Using the latter method, the disks may be arranged in such a fashion as to ignite one off another's edge to produce a series of color changes as the flame proceeds from one disk to the next. We have found this sequential arrangement of colors to be particularly entertaining. We report here the preparation of small-diameter filter disks for use in small lecture rooms or laboratories. These disks can be made and stored so that the demonstration may be incorporated into lectures and employed by other lecturers in a very convenient manner. After the disks are made, the convenience of this demonstration is that the demonstration can be carried out without set-up, replenish, or tear-down time. Furthermore, this demonstration (with the small disks) can be carried out in almost anv educational settine since a fume hood or a Bunsen hurner is not required. T h i convenience stimulates wider use of this demonstration amone busv staff members. This Drocedure, with caution, may ge scaled up for large ~ecturkhalls and is discussed below. Methods Nitrocellulose Preparation Begin with a dry (see nitration caution note 9 below) 125X 65-mm crystallization dish that is ice-bath cooled and that can he magnetically stirred. To this dish is added 125 mL of concentrated nitric acid. Finally, to form the nitration mixture. 125 mL of concentrated sulfric acid is slowlv added to the chilled nitric acid while being stirred. The disdis covered with a large watch -glass,. the ice bath is removed, and stirring continues-until the nitrating mixture reaches room temperature. Twelve pieces of 4.25-cm, #1 qualitative filter paper disks are added individually. Each disk is individually suhmerged with the aid of a glass rod. The nitrating mixture is stirred to keep the disks separated. The nitration must proceed for 10 min after the last disk is immersed. The disks will aonear .. translucent a t this time. The white translucent disks are removed from the solution with Teflon-coated tongs and dropped into a large beaker containing distilled water. When the desired amount of disks has been treated,. thev. should be manuallv pressed free of excess water between paper towels and ta!% to the impregnation step while still wet. The above nitrating solution is sufficient to react with a total of 7.5 g of filter paper. The process may he repeated until the desired quantity of disks is nitrated. Alternatively a large scale nitration corresponding to doubling the acid quantities and using a larger crystallization dish permits the nitration of larger disks for demonstration in larger classrooms. It is recommended that the acid mixture be carefully disnosed of bv slowlv addine it to laree amounts of dilute sodi;m carhoAate schtion. %he neutralized solution may be disposed of by pouring it into large amounts of running cold water. We make reference here to a similar nitration with its attendant discussion of hazards and disposal techniques (8). Impregnation with Salts A variety of flame colors may be produced using various inorganic salts (9). We found that the following materials produced highly satisfactory results.
' To whom all correspondence should be addressed.
