3S7G
T. C. HOERING, F. T. ISHIMORI AND H. 0. MCDONALD [CONTRIBUTION FROM
THE
DEPARTMEXT O F CHEMISTRY, UNIVERSITY O F
The Oxygen Exchange Between Oxy-anions and Water. Perchlorate Ions1
L-01.
so
ARKANSAS]
11. Chlorite, Chlorate and
BY T. C. HOERING, F. T. ISHIMORI AND H. 0. AICDOXALD RECEIVEDNOVEMBER 14, 1957 The kinetics of the isotopic exchange reaction between chlorate ion and water have been measured.
In the region of
secI.? ( J . 1 t o 1.Ofchloric acid, the exchange follows the rate law H = k(H')2(C103-) where k equals 0.89 X iitids~-'nt 100' and ionic strength 1.34. T h e activation energy is 27,100 cal. per mole. T h e course of the reaction was not affected by added chlorine dioxide. T h e reaction proceeds faster in DaO with Rd/Rh equal t o 2.83. T h e exchange b e t w e i i ui(l water is not induced during the oxidation of halide ions. T h e rate of this exchange is compared to the rates anion exchanges and the rates of oxidation of halide ions by chlorate. Perchlorate ion does not n:easurably excliniigc its oxygen with water over long periods of time a t elevated temperatures and high acidities. Chlorite ion does not incasurald~-exchange its oxygen with mater under any condition where it is stable.
Introduction The exchange of isotopic oxygen between water and chlorite, chlorate and perchlorate ions has been studied qualitatively by several workers. No measurable exchange has been reported in neutral or slightly acidic solution. A measurable exchange has been reported for chlorate ion and water a t an elevated temperature and a high aciditya3 This paper is a report on the quantitative study of the rate of exchange between water and chlorate ions. Qualitative studies on the exchange of chlorite and perchlorate ions with water were performed. Previous work4 in this Laboratory on the bromate-water exchange has shown similarities between the kinetics of the bromate oxidation of halide ions arid the exchange reaction. 9 mechanism invnlving the bimolecular displacement of water by the attacking species was postulated. These experiments were performed to see if this inechanisin could be applied to reactions involving chlorate inn. X study of the rates of exchange between water and the series of ions CIOz-, C103-, C104-, was attempted in order to measure the effect of structure on the lability of the C1-0 bond. It is known qualitatively t h a t reactivity increases in the series C103- < Br03- < IO3- but there. are few quantitative data to attempt any correlation of reaction mechanisms with the nature of the center atom of these oxy-anions. This work was done with the purpose of extending the knowledge of the reactinn tncchatiisms o f this type of molecule. Experimental Materials.--Osygeii-eigliteeii tagged water was obtained rrom the Stuart Oxygen Co. of San Francisco. Oxygeneighteeii tagged potassium chlorate was prepared by the clispropi)riioiiatioii of C12 in a hot solution of KOH in tagged !rater. Sodium chlnrnte was prepared by treating tagged potassirirri chlorate with Dowes-50 ion-exchange resin iii the sodium forin. Tlie water used a s a solvent was rcdistilled f r o m an alkaline permanganate solution. Sodiuni chlorite, received from the Matheson Chemical Company, \vas purified by repeated recrystallizations. .ill other chemicals were reagent grade and used without further purification. ( I) This work mas supported by funds supplied by t h e National Pcicnce Pndndation. ( 2 i 13. 'l'onlje, "i\nnual Reviews of Kuclear Science," \-d.6, I%%?, gi 9 8 2 ; A f . Dole, C h e m . Revs., 60, 275 (1952). ( 3 ) E, R S.TVinter, hf. Carlton a n d H V. A. Briscoe, J r h r m SIX.. 131 ( I n l o ) . ( 4 ) 'I. C . IIoering. I < . C Bntler a n d €1. 0 , h l c n o n a l d T a X R J O I J H . NAI., 7 8 , I 8 2 9 (1856).
Experimental Procedures.--The chlorite-wtter cxcliatigc was studied by dissolving sodium chlorite of normal isotopic abundance in tagged water. Oxygen exchaiigc reactions are, in general, more rapid in acidic solutions. This cschange was studied in solutions which were 3s acidic as p i s sible, without causing complete decoinpositiivi of tlic chlorous acid. Solutions which were buffered Tvith bor:atc and acetate ions or were acidified with dilute perchlori were sealed into Pyrex ampules. T h e reactants m r e rated by precipitating Pb(C102),n. T h e dried precip were converted to 0 2 for isotopic analysis b y tliermal d c coniposition in a vacuum system. T h e evolved osygcii \v:ts trapped carefully with liquid air before introducing it iiito the mass spectrometer. The perchlorate-water exchange was expected tn lie very slow, and the exchange mas studied iii highly acidic solutions a t elevated terriperatures. Perchloric acid of norinal isotopic coniposition was dissolved iii tagged wxter. Tlie concentration of t h e acid v:Lried from 5.9 t o 9 . 5 j. The solutions were sealed into Pyrex ampules anti pl:icetf in ;L thermostat. For example, a solution OF 9.54 f €IC104 wzts held a t 100" for RS long as 63 days aiid at room tenipcrLtture for 296 days. T h e reactants mere separated by precipipting KCIOl. This precipitate was converted t o 0, for 1 ~ ) topic analysis b y thermal decomposition. T h e kinetics of the C10d--H20 exchange were studied iii solutions acidified with sulfuric or perchloric acid. Tlic ionic strength was maintained constant with added potassium sulfate or sodium perchlorate. T h e exchange of osygen hetrveeii sulfate inn and watcr is slow under tlie coiiditions used in this experiment .: The solutions were prcpared by dissolving t h e proper a i n u u n t s of taggetl chlorate and inert salt in t h e solutioii of acid. E:xh s:iniplc wiis sealed i n a Pyres ampule and placed in a t1ieriiiost:itic b a t l l held a t 100.0'. The exchange was stopped ljy riioling the sample t o room temperature arid by neutr:rlizitig it. 'l'lic hydrogen ioii conceiitration of each sain;)le WXL; tletcriiiiiictl by titrating an aliquot with NnOI1. In tlie esperiinetits where perchloric acid was used t o acidify the solutioii, n portion of the solveiit water was distilled from the neutralizcd saniple for isotopic analysis. T h e progress of the exchaiigc reaction was followed by observing the iiicrcasc of the oxygen-eighteen content of the solveiit. Tlic water w t s ;malyzed for oxygen-eighteeii cmltcnt b y ilie rricthod i i f Cohn and Urey.6 In the experiments where sulfuric :icicl w:is used to :icitlify the renciioti mixture, t h e exchange was stopped Iiy cooling tlie sample t o room temperature and by rientralizitig it witli potassium hydroxide. Much of the K2SOI wa$ precipit:ttctl by adding alcohol arid centrifuging. The rctii:tiiiiiig sulfate ion was removed Iiy adriitig lmriuin chloride. Tlie solutio~i was then evapor:ited t(i dryness atid the potxssiiiiii rhlririti and potassium clilornte collected, The clil(ir:~tci i i t ' mixture W:LS coiiverted t o osygcn gas for isotopic :iii:ilA by thermal decomposition in P ' I I C Z ~ O . Blank experiments showed this separ:itiiiII ciiulti 11c carried out without causing exchange. Since the experiments usiiig sulfuric acid were easier tri perform and gave a higher precision, they were used to determine the orders of the reaction. There are soiiie uncertainties in the corrections needed t o c:tlculatc aciditiei i i i ( 5 ) '1.. C. Hoering and J. W. Kennedy, ibid., 7 8 , 182Y (1!3-4) (6) M. Cohn and H. C. Urey, i b i d . , 60, 679 (1938).
Aug. 5 , 1958
sulfuric acid solutions due t o the weakness of t h e second dissociation. Some of the experiments were repeated using perchloric acid t o acidify the solution. T h e isotopic analyses were made on a Consolidated Mass Spectrometer Model 21-401. T h e mass spectra due t o the isotopic molecules were scanned enough times t o reduce the errors in peak height measuremeat t o less than 170. Calculations.-The chlorate-water exchange was followed 11y measuring the change in oxygen-eighteen content of either the water or the chlorate fraction a s a function of time. T h e half-time for the exchange was determined b y plotting the logarithm of one minus the fraction exchanged i ~ stime.' The fraction cxch:mged is defined by equation 1 0 18 - Ou18 F = (1) 0-18 - OOlY
~
=
I
I5L IO
The symbol 0 1 8 iridicates the atom fraction of oxygen-eighteen in the O2 or C 0 2 at tlie time indicated b y the subscript. The rate of the reaction ~vhichlcatls to isotopic exchange was theii calcul:ited by
