Jack L. Lambert and Gary T. Fina Kansas State University, Manhattan, KS 66506 The iodine clock reactions have been favorites for demonstrations because of their sudden and vivid color changes. They also serve as experiments in elementary chemistry laboratory courses to illustrate solution kinetics. The simple iodine clock reaction involves a reaction sequence that (with initial coefficients doubled for stoichiometric reasons) probably is
-
r- H7+105 + HSO; or starch
(2e- step)
time to the appearance of the blue color, which was generated more sluggishly the lower the bisulfite concentration. No threshold concentration could be determined. One of the reactions prior to the formation of hypoiodous acid is slow. As soon as the triiodide ion amears, it is seauestered in the starch helix, and the co~or~essib~uti& turns blue. Only triiodide, of all the iodine species, produces the blue color with starch. (Closely similar higher polyiodides, such as 1;( I ) and 1;; ( Z ) ,have been suggested as the species that produce the blue color, hut the reaction sequence would be the same.) Given the fast sequestering reaction and the intense color of the chromogen, &duction of the color appears to be almost instantaneous. The "Old Nassau" modification of the iodine clock reaction bv Alvea (3)(see Appendix for both recipes) is identicaldown to the production ofiodide ion, which is complexed and precipitated hy mercury(I1) until the mercury(I1) is exhausted before resumine the oathwav leadine to the blue starchpolyiodide complex.
-
4 + OH-
2105
+ 2Hso; +linear starch
210,
i
+H+
+ 2HSO;
HOI + 103
linear starch-I5 (blue helical complex) The first two-electron reduction step to indite is speculative, as other iodine species may he involved, including iodine oxides in a more complicated reaction sequence. The second two-electron reduction-the reduction of hvnoiodous acid to iodide ion-could be accomplished by eithe; bisdfite or the secondam alcohol eroum of the starch. Secondarv alcohols are oxidiied to ketones Ly hypoiodite even with alcohols that fail to eive the iodoform test. Under the moderatelv acidic conditions of the reaction, hypoiodous acid ( K . = 2.3 2 lo-") would he lareelv - undissociated. In an experiment to determine if a minimum concentration of bisulfite is required for the iodine clock reaction, successively lower concentrations of bisulfite merely extended the
1 1
+ HSO3 or starch
I
HgIds) (orawe) (5)
i
+starch
linear starch-I3(blue +orange = black)
Volume 61
Number 12 December 1964
1037
-
Reactions (I), (2), (3), (4), and (5) take place sequentially. Mercury(I1) chloride, HgCI2, is largely undissociated in but it is complexed aqueous solution (82 = K1 X Kp more strongly by sulfite (P2= lop3)in [Hg(S0&I2- and still more strongly by iodide (KI 1013) in [HgI]+ and in precipi10-28) (4,5).The generated iodide first tation of HgI2 (K,, forms the colorless mixed iodosulfite complex, followed by precipitation of the orange mercury(I1) io&de. Alyea a t t r i b uted the rapid appearance of HgIz to supersaturation, which may play a part, but the himolecular reaction of iodide with the mercury(I1) iodosulfite complex probably would be rapid enough to account for its nearly instantaneous appearance. When the mercury(I1) has all been precipitated as HgI2, the extremelv rapid production of triiodide produces the blue starch-triiodihe complex, and the comple&entary orange and blue colors appear black. Orange and hlack are the school colors of princeton University and also of the House of Nassau which bestowed its name on Nassau Hall a t that University. Bisulf~teion is a two-electron reductant and thus a transient iodite, IO;, species must be assumed. In a colorimetric method for carbon monoxide in the parts-per-million range reported by Lambert and Wiens ( 6 ) ,iodate ion is reduced in a similar sequence by the generated, atomically dispersed, palladium metal in the presence of leuco crystal violet at pH 3.1,
-
-
HOI + 10, + HCVH+ (HCVH+is leuca crystal violet)
1
CV+ + I-
+ 2HC(CVCis crystal violet)
Evidence for the production of hypoiodons acid is observed in the oxidation of leuco crystal violet. Only hypoiodous acid and hypoiodite anion, of all the oxidizing species of iodine itself) oxidize the leuco form of the (including elemental 1% common triphenvlmethane dyes a t an observable rate. As the above reaction appears to h e ~ a u t ~ c a t a l ~tot iac considerable degree, a further reaction sequenw prnt~nl~ly takes place as the result of reduction of iodatc hy thr generated iodide.
dant, 103,and a two-electron reductant. Neither iodous acid nor iodite salts can be isolated, but their transient existence in solution is oossible. The rate-determining slow step in the clock reactions may involve the migration and reaction between iodite species that have been generated by reduction of iodate by bisulfite. One of the two two-electron reactions or the disproportionation step in the reaction sequence that produces iodide ion must be relatively slow, and the iodite-iodite interaction would seem to be a likely candidate. This slow step must account for the interval before the appearance of the blue color (iodine clock) and aportion of the interval before the appearance of the orange precipitate (Old Nassau). The interval between the appearance nf the orange precipitate and the black color in the latter is the time needed for all the mercury(l1) to react with steadilv eenerated iodide ion. Thc reaction of hv~r~iodite with to produce iodine, the reaction of iodrie with iodide iodide ion to oroduce oolviodide anion. the reaction of nolviodide anion with line& starch to produce the blue helicil complex, and the interactions of mercury(I1) with iodide ion all are known or presumed to he rapid. In the colorimetric method for carbon monoxide (including the continuing autocatalytic reaction sequence), iodate ion is considered to be reduced bv finelv dispersed palladium metal and, subsequently, by iodide ibn. ?;he production of hwoiodous acid is demonstrated bv its specific reaction to okidize leuco crystal violet to crystai violet. Reduction of iodate ion by iodide ion is inferred to explain the autocatalytic production of crystal violet. ~ l t h o u g hi t is a common ohservation that the reaction of iodate and iodide ions in acid solution results in the production of elemental iodide, that end product occurs only when an excess of iodide ion is present. When iodate ion is present in large excess, iodate apparently is reduced to iodite, and iodide oxidized to hypoiodite, in a simultaneous two-electron redox reaction
i 6
10;
+ I- + Hf
-
10;
+ HOI
Acknowledgmenl
Reactions contributing to the proposed explanation for the iodine clock reactions and colorimetric methods for the determination of carbon monoxide were part of the research supported by National Science Foundation Grant No. CHE7915217. Literature Cited (1) Gilbert, 0. A , and Marriott, J. V . R, Tmw. Farndoy Sac., 4684 (1948). (2) Thornmn, J. C., and Harnori, E..J. Phys. Chem., 75.272 (1971). (3) Alyla, H.N.,"Amehair Chami8try.A ProgrammedLaboratorykd."TOPS-Alyla, Prioccton, NJ, 1971, Experiment 28 (41 Sill&, L. G., and MarteU, A. E., "Stability Constants of MeM-IonComplcres,"S~cial Publicstirm No. 17, The Chemical Society, London, 1964,pp. 231,292,341. (51 SilYn, L. G..and Martell, A.E.."SfabilityConstants olMeM-Ion Complexes."Speial Publication No. 25, Tho Chemical Saeiety, London, 1971, p. 220.
(6) Lsmbert. J.L.,and Wienn,R.E.,Anol.Chem.,46,929(1974).
Appendix
Recipes for the iodine clack and the "Old Nassau" reactions are as follows: Solution A: 15 g of potassium iodate, KIO8, per liter of solution. Solution B: 15p: of sudi!rm hisulfite. NaHSO?,and 2 E of soluhle rwrh per liter. Note: diiwlve the wrch in hwling wnter. ruol. add N~HSOL ... and dilute to I I. 'I'hls mlutfon deteriorates raoidlv . " and should be orepared fresh as needed. Solution C: 5.4 g of mercuric chloride, HgC12, per liter of solution. Discussion
Several assumptions must he made to explain the sequential reactions. the most important of which is the production of the 10; species formed by reaction of the tw&electron oxi1038
Journal of Chemical Education
Iodine clock reaction: Add one volume of solution A to three volumes of water. Then with short vigorous stirring, add one volume of solution B. "Old Nassau" reaction:
Addone volumr ufuolution Aandone volume ofsolutiun Ctoone volume of water. Then with short vigorous sirring, add one volume of solution B.
