0
A CLOCK REACTION M. G. SURYARAMAN and ARCOT VISWANATHAN Madras Christian College, Tambaram, South India
T w o well-known, spectac(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) ular "clock reactions," for Ironsolution,ml. 30 50 70 50 50 50 50 50 50 50 50 50 lecture demonstration are 15rater to the Landolt reaction (1,S,6, iron solution, ml. 40 20 0 20 10 30 50 70 0 50 70 50 7, in which iodic and sulfuXaI retardant, ml. . . . . . . . . . . . . . . . . . . 0 10 20 . . . . . . . . . rous acids interact in the C,H,OH accelerator, ml. ........................... presence of starch, and the 0 o 20 Forbes' reaction (5,13,9) in Iodine solution, ml. 1 1 1 1 1 1 1 1 1 1 1 1 which arsenious oxide in Decolorization time, 35 24 19 28 17 14 32 61 92 strong hydrochloric acid resee. 18 14 7 acts with saturate:! t,hiosulfate. The sudden appearance of iodo-starch blue in the complex are simultaneously increased, the induction one case, and of a yellow colloid in the other, occur after period lengthens (the reaction rate decreases) ; appara n induction period ~vhichis lengthened by dilution of ently upon increasing the concentrations the acceleratthe reactants in conformity with the Mass Action law. ing action of the ferrous complex is less favored than is Also, the indurt,ion period for the Landolt reaction is the retarding action of Na13 and I- (the solution forms lengthened by addit,ion of ethyl alcohol (2) and other Na13from NaI Iz). inhibitors, and hy increase in pH (10). Experiments 7 to 9 show that NaI retards reaction, A third clock reaction is as follows. Ferrous ions in lengthening the induction period. Experiments 11 and an alkaline solution of a tartrate form a soluble chelate 12 show that ethyl alcohol accelerates reaction, shortencomplex (compare copper in Fehling's solution (4)), ing the induction period, while 10 and 11 are identical which reacts sluggishly with iodine: to 5 and 6. Separate experiments showed that with pure iodine 1. + H 2 0 + 2Fe (11) tartrate complex in alcohol, NaI being excluded, decolorization was pracZFP (111) t,art,mtecomplex + 2HI MATERIALS tically instantaneous even though a little iodide is formed as one of the products. ~ e t a r d i naction ~ of Water. Since the chelate complex is oxidized by air other halides showed NaI > NaBr > NaCl > NaF, (S), distilled vater used in the experiment must be being least with NaF. Sodium citrate, substituted for boiled free of dissolved air; also, the solutions must be Rochelle salt, gave slightly faster reactions but the stored in well-stoppered bot,tles. same relative orders. Iron Solution. Dilute to 1 liter a mixture of 3.77 g. On the basis of the experiments above, it is suggested of Rochelle salt (KNaCaHnOs.4H20),4.48 g. of sodium that in the method of Rupp (11)and of Tananaev (12) bicarbonate, 33.3 ml. of ferrous sulfate solution (55.6 g. for iodometric estimation of ferrous salts by the use of FeSOI.7H20 per liter of solution), and 47 ml. of a 1 this ferro tartrate complex, the time for titration can per cent solution of starch. be considerably shortened by adding some ethyl alcoIodine Solution. 12.7 g. of iodine plus 25 g. of sodium hol to thereactants. iodide in a liter of water. LITERATURE CITED Sodium Iodide Solution. 15 g. in a liter of solution. Ethyl Alcohol. 95 per cent solution. (1) ARTHUR,P., "Lecture Demonstrations in General Chemis-
+
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EXPERIMENTAL
The table shows the volumes of the above solutions which were mixed simultaneously a t 29 to 32' C., and the times required for the blue color to disappear. In experiments 1 to 3 the concentration of iodine is held constant, and the induction period shortens with increasing concentration of the ferrous complex; this is the same mass law behavior as in the Landolt and Forbes reactions. However, experiments 4 to 6 show that if the concentrations of both iodine and the ferrous
try," McGraw-Hill B w k Co., New York, 1939, p. 37. W. J., J. CHEM.EDUC.,17,398 (1940). (2) CONWAY, (3) EGGERT,J., Z e i f . Eleklro. Chem., 23,s (1917). ( 4 ) EMELEUS,H. J., A N D J. S. ANDERSON, "Modern Aspects of Inorganic Chemistry," George Routledge and Sons Ltd., London, 1938, p. 95. , Am. (5) FORBES,G. S., H. W. EETILL,AND 0. J . ~ - . ~ L K E R J. Chem. Soe., 44,97 (1922). H., Ber., 19,1317 (1886). (6) LANDOLT, H.,zbid.,ZO, 745 (1887). (7) LANDOLT, (8) MELWR,J. W., "A Comprehensive Treatise on Inorganic Chemistry," New Impression, Longmans Green and Co., Ltd., New York, 1927, Vol. I, p. 380.
JULY. 1951 (9) METER,M., J . Am. C h e m Soe., 44, 1498 (1922). (10) NERNST, \T., "Theoreti1~81 Ch~mi~try," The Marmillan Co., London, 1923, p. 657. (11) Rnrp,F:., Rw.,36, l64(1903).
387 (12) TAXANAEV,x. A,, Chem. Z . Blntl, 2, 418 (1925). (13) VAN KLOOSTER, H. S. "Lecture and Lahhoratory Fkperi-
ments in Phyniral Chemistry," 2nd ed., The Chemical Publishing Co., Easton, Pa., 1925, p. 54.
