A Bromate Clock Reaction: The Formation of Purple Tris[diphosphato)manganate[lll) Ronald L. Rich' Bluffton College, Bluffton, OH 45817 Richard M. Noyes University of Oregon. Eugene, OR 97403
T h e reaction presented here uses bromate to oxidize nearl y colorless Mn2+ t o a deep purple complex of Mn(III), t h e title species. It is interesting for several reasons: (1) A colorful clock reaction can always help provide a stimulating introduction to kinetics. (2) The reduction of hromate probably involves the same intermediates as in the fascinating oscillating reactions that use bromate ( 1 3 ) . The likely mechanism, then, depends on a spurt in the concentration of bromine dioxide for the final visible oxidation. No oscillation ensues in thiscase, although a small amount of sulfite can be added repeatedly as a bleach, followed by a later spontaneous re-oxidation of the manganese. (3) Thereaction was used analytically long ago (4) hut not reported as a clock reaction. (It was re-examined in the course of work (5)on the oscillating Morgan reaction.) (4) Speculating on the mechanism (see Results and Discussion) and balancing the tentative formulation (all equations are, by definition, balanced ( 6 ) )for the main overall process may be interesting challenges for students:
606
Journal of Chemical Education
Reagents The originally recommended (4) solution of pyrophosphoric (diphosphoric) acid is not highly stable, and the more stable tetrssodium salt is not verv soluble. Therefore. the salt is dissolved in enough sulfurre arid to form the more wluhle trisodium hvdnlgen salt. which is $till stable in wlution for munthr. Furthermure, ryanrde ( 4 ) is not needed here to eliminate the w r y slight cuior due to bromine, so it is omitted. Procedure Prepare the following aqueous solutions, using distilled water (some deionized water contains traces of a strong catalyst or inducing agent, such as formaldehyde): 1.0 M MnSOd (11g MnSOl. 4Hz0 per 50 mL); 6.0 M HnSO* . . .(100 mL eonc. H&Oa - . eorefullv . . added to 218 mL H 2 0to give 300 mL after mixing and ewling); 0.48 M NasHPlOi (214 g N a P 9 0 7 .10H20dissolved in 40 mL 6.0 M H2SOnplus water to yield 1000 mL);
'
Author to whom correspondence should be addressed. Present address: Department of Chemical Engineering, North Carolina State University. Raleigh. NC 27695.
+ Br- + 2Ht =HBrO, + HBrO HBrO + Br- + Ht t Br2+ HZO
Br0,Reagent S~lution Formula
Reagent Volume. Varylng
V,/mL 1-2% Varying Mn2+H+ 6IO;Pyr.
6,'
8 1 15 50 1 13 20
9 1 17 62 1 9 10
100 100
10 1 15 50 2 22 10 0.015 too
Refer now to the table. For experiment 1 combine the following in a 250-mL beaker: 1mL 1.0 M MnS04; 15 mL 6.0 M HZSOI; 50 mL 0.48 M NasHPa07; 0 mL 2 mM KBr (i.e., add KBr, if any, before the KBr03);
24 mL H20. (Addition of the pyrophosphate before the acid produces a temporary, harmless precipitate of manganese pyrophosphate.) When ready to proceed, quickly add 10 mL 0.25 M KBrO3 while simultaneously starting a timer. Check the temperature; it rises a degree or so depending partly on the Length and vigor of stirring. The reaction time is about 35 sat a final temperature of 24.5 'C. Results and Dlscusslon The end of the induction ~eriodis not as sham as in some other clock reactions. It see& better to define i i as the time not when the purple color first appears but when i t resembles that of areferencesolution. (Weused arecently finished sample, diluted about 1to 10.) The table shows some results. The reaction time is essentially independent of the concentration of Mn2+,likewise of pyrophosphate, if we allow for the concomitant changes in oxonium ;on concentration and ionic strength. (The latter was kept constant in an earlier kinetic study of the hromate-bromide reaction (7)) The time is shortened greatly by adding more acid or a little formaldehvde. while further additions of bromate have a smaller efEeci The addition of bromide ion, beyond that inevitablv .nresent in reaeent-erade KBrOq.-. delavs the endpoint significantly. The mechanism of the reaction may involve the following steps, which are nearly the same as those already published (3):
-
(2)
CHIO
0.25 M KBr03 (10.5g KBr03 per 250 mL); 2.0 mM KBr (24 mg KBr per 1M) mL).
.
(1)
HBrO,
+ BrF + H ' S ZHBrO,
-
2HBrO
+
Br03- HBrO + Ht
(5) (6)
Steps 3 and 4 introduce autocatalysis; until most of the bromide ion disappears, however, step 5 prevents an appreciable buildup of HBr02. Then, presumably, the reaction takes off. Supporting step 4 is the finding (8)that 1 0 2 oxidizes Mn2+ but not H202, while Mn(II1) can oxidize Hz02 even though its reduction potential is clearly not as positive as that of 10%. If stirring is not continued after the reagents are mixed, the purple color appears earlier near the surface. This may reflect the vaporization of bromine, which can be smelled, with the conseauent decrease in the amount of inhibiton, hrornidein equijibrium with it, or it may be due toa reaction of triplet Or with an intermediate. The early cessation of stirring, in fact, may even be preferred as a way of minimizine. this otherwise less controlled effect in the main body of th;! solution, and as a way of showing the interesting influence of the surface. The table, however, is hased on continuous moderate stirring. Commercial analytical grades of potassium bromate vary significantly in their small amounts of bromide impurity. (Our main source was Baker Analyzed Reagent, for which the bromide content was eiven as "
