edited by FRANK SCHULZ Johnsburg Central Schwl North Creek, NY 12853
The Stability of Potassium Permanganate Solutions Thomas Reesl Phillips Academy, Andover, MA 01810 Potassium permanganate is so commonly used in high school laboratory work that i t is often necessary to prepare large volumes of solution. I t would he very convenient if these solutions could he stored without danger of deterioration, hut it is widely reported that they deteriorate with the passing of time. Nearlv all the texthooks on ouantitative analvsis recommend lengthy nnd time-consumkg procedures t i retard the deterioration process. It is stated that the solution must he heated tu a boil, held at this temperature for an hour, cooled for 2.1 h, and either filtered thn~ughsintered glass, siphoned, or decanted. Data from our lahoraiury suggest that it id possihle to omit thew procedures and save ctmsiderable time in the preparation of KMnO, solutions. It isnot surprising thar the texthooks recommend a purification procedure. Potassium permangannte can react with uwtw to form oxygen and solid manganese dioxide:
Pure potassium permanganate reacts very slowly, but the reaction is catalyzed by solid MnOz. Sunlight also speeds up the reaction. In the past the reagent grade of KMn04 contained some Mn02. Possibly the KMn04 today has a higher degree of purity. Of course, if the distilled water contains traces of organic matter or other reducing substances, Mn02
' Present address: 35 Sunset Rd.. Hamden, CT 06514.
Latimer, W. M. Oxidation Potentials, 2nd ed.: Prentice-Hall: Englewood Cliffs, NJ. 1952; p 238. Andrews, L. V.: Brown, D. J. J. Am. Chem. Soc. 1935, 57,254. Walton. H. F. Elementary Owntitative Analysis; Prentice-Hall: Englewood Cliffs,NJ, 1958: p 264. Muir. M. M.; Diaz, J. A,; Figueroa, N. C. Laboratory Manual to Accompany Moore/Davis/Coilins CHEMiSlRTRY McGraw-Hill: New York, 1978; p 158.
'
1058
Journal of Chemical Education
Results from Analvsls of KMnO. Solutlon
Mass of NaE2Oq(g) Volume KMnO, (mL)
Norrnallw
0.7228 32.61 0.3308
0.8342 37.68 0.3304
0.7221 32.43 0.3323
0.8468 38.10 0.3317
will form and will catalyze the reaction of KMn04 with water. In a way i t is surprising that KMnOa solutions deteriorate so slowly. The f r e e k n e r 6 change corr&ponds to ED = 0.187 V2," or E = 0.295 V if the concentration is 0.02 M, the pH is 7, and the activity of the oxygen corresponds to 0.2 atm. Even though the solution is thermodynamicallv unstable, a carefully prepared solution is reported to have-lost only 1% of its normality after storage for four y e a n 4 In this laboratorv a solution of KMnO". in distilled (not ,~~.. ~~~~~~~~-~~ deionized) water w k analyzed' immediately after preparation, without heatina or filterine the solution. Another analysis nearly two years later showed virtually no change in concentration (see table). Prudence suggests that great care should he used in preparing a solution of KMn04 when the solution is t o he used in a long, continuing investigation. But when the solution is to be used for only a week or two in a general chemistw laboratory experiment, it is possible that the solution can b i prepared directly without purification. If the quality of the distilled water is doubtful, the solution can be standardized again with a stock solution of Na2C204in less time than it takes to filter a large quantity of KMnOa through a sintered glass funnel. The solution should he protected from daylight, of course. Solutions which are in laboratory reagent bottles should not be returned t o the stockroom bottle.
