2842
J. Am. Chem. SOC.1989, 111, 2842-2845
a second electrostatic analyzer (ESA-11). The field-free regions (FFR), located between the source and ESA-I (1st FFR), and between the magnet and ESA-I1 (3rd FFR), are equipped with standard collision cells. An Ion Tech saddle-field atom gun (Ion Tech, Middlesex, England) was used for producing 7-to 8-keV Ar atoms for FAB desorption in a commercially available Kratos FAB source. For FAB-MS/MS experiments, an ion of interest was selected by using MS-I at a mass resolution of approximately 1000 (width at 10% height). Mass selected ion kinetic energy spectra (MIKES) were obtained by scanning MS-11. Twenty scans were averaged using software written in this laboratory. CA experiments were done by activating the mass selected ion in the third FFR by using a helium pressure that gave a 50% main beam suppression. For MS/MS/MS experiments, source-produced ions were activated in the first field-free region. The fragment ion of interest was then
transmitted to the third FFR by setting both the first ESA and the magnet at the appropriate values. The selected daughter ion was collisionally activated and the resultant second generation fragment ions were analyzed by scanning the final electric sector.29
Acknowledgment. Preliminary results were presented a t the 35th and 36th A S M S Conferences on Mass Spectrometry and Allied Topics; Denver, CO, 1987; and San Francisco, CA, 1988. W e are grateful to Dr. K. B. Tomer for assistance in the early stages of this research, and to Professor K. G. Standing and Professor D. H. Russell for preprints of recently published work. This work was supported by the Midwest Center for Mass Spectrometry, a National Science Foundation Regional Instrumentation Facility (Grant No. CHE-8620177).
An Algorithm for the Design of Propagating Acidity Fronts Lajos Szirovicza, Istvin Nagypil,* and Endre Boga Contribution from the Institute of General and Physical Chemistry, Attila J6zsef University, H-6701 Szeged, P.O. Box 105, Hungary. Received June 13, 1988
Abstract: On the basis of elementary chemical considerations, it is shown that proton-producing redox reactions of oxyanions are expected to be autocatalytic and to manifest propagating acidity fronts. The conditions of proton production in redox S2062-,and N2H5+ reactions are established and verified experimentally by the reactions of S2032-,S4062-,S2042-, with Br03-, IO3-, C103-, C102-, and S20g2-.Twenty new propagating acidity front reactions have been discovered among the possible combinations of these reactions.
Propagating reaction fronts in autocatalytic reactions were discovered by Luther] in 1906. His discovery, however, seems to have been forgotten until recently.2 Interest in the study of propagating acidity fronts and other forms of chemical waves was renewed after the discovery3 and interpretation4 of the fascinating Belousov-Zhabotinsky (BZ) reaction. The number of reaction systems exhibiting chemical-wave behavior, apart from the various modifications of the original BZ system5-” is less than 20.’2-2’ Three of them, the hydrolysis of
(1) Luther, R. Z. Z . Electrochem. 1906, 12 (32), 596. (2) Showalter, K.; Tyson, J. J. J . Chem. Educ. 1987, 64, 742. (3) Belousov, B. P. Sb. Ref. Radiats. Med. 1958; Medgiz. Moscow 1959, 145. (4) Field, R. J.; Koros, E.; Noyes, R. M. J . Am. Chem. SOC.1972, 94, 8649. (5) Busse, H . G. J . Phys. Chem. 1969, 73, 750. (6) Beck, M. T.; Viradi, B. Z. Nature Phys. Sci. 1972, 270C, 1049. (7) Winfree, A. T. Science 1972, 175, 634. (8) Field, R. J.; Noyes, R. M . J . Am. Chem. SOC.1974, 96, 2001. (9) Jorne, J . J . Am. Chem. SOC.1980, 102, 6196. (10) Showalter, K.; Noyes, R. M.; Turner, H. J . Am. Chem. SOC.1979, 101, 7463. (11) Orbin, M. J . Am. Chem. SOC.1981, 103,4311. (12) Lotka, A. J. J . Am. Chem. SOC.1920, 42, 1595. (13) Gribshaw, T.; Showalter, K.; Banville, D. L.; Epstein, I. R. J . Phys. Chem. 1981, 85, 2152. (14) Showalter, K. J . Phys. Chem. 1981, 85, 440. (15) Weitz, D.M.; Epstein, I. R. J . Phys. Chem. 1984, 88, 5300. (16) De Kepper, P.; Epstein, I. R.; Kustn, K.; Orbin, M. J . Phys. Chem. 1982, 86, 170. (17) Nagypil, I.; Bazsa, G.; Epstein, I . R. J . Am. Chem. SOC.1986, 108, 3635. (18) Rudakov, E. S.; Mastikhin, V. M.; Popov, S. G.; Rudakova, R. I. J . Org. Magn. Reson. 1973, 5, 343.
0002-7863/89/ 151 1-2842$01.50/0
alkyl sulfates’ and the iodate-arsenite2’ and the chlorite-thio~ u l f a t e ”systems ~ ~ ~ are characterized by propagating acidity fronts; ie., the diffusion of the proton produced in the reaction is coupled to its catalytic effect. The considerations below show that propagating acidity fronts are a generally expected phenomenon in proton-producing redox reactions of oxyions.
Chemical Considerations It is well-known that the structure of most oxyanions in deprotonated form is symmetrical; the bond order of the central atom-oxygen bonds is higher than 1. Breaking strong bonds in a symmetrical structure requires a high activation energy. Thus the oxyanions are expected to be kinetically stable against redox transformation in alkaline medium. In protonated form, however, the original symmetry is lost, the order of one of the central atom-oxygen bonds decreases to 1. In other words, protonation is expected to decrease the kinetic stability of the oxyanion, making redox transformations more facile. It follows from these qualitative considerations that protonproducing redox reactions of oxyanions may be expected to be autocatalytic and to manifest a propagating acidity front, if the reactants are mixed in alkaline solution and a drop of acid is used to initiate the reaction in an originally homogeneous unstirred solution. Proton production is easy to anticipate from the stoichiometry of the reaction. Oxidizing agents usually consume protons and electrons, while reducing agents liberate both species. Therefore, (19) (20) (21) (22)
Gowland, R. J.; Stedman, G. Chem. Commun. 1983, 1038. Bazsa, G.; Epstein, I. R. J . Phys. Chem. 1985, 89, 3050. Harrison, J.; Showalter, K. J . Am. Chem. SOC.1986, 108, 225. Nagypil, I.; Epstein, I. R. J . Phys. Chem. 1986, 90, 6285.
0 1989 American Chemical Society
J . Am. Chem. Soc., Vol. 111, No. 8, 1989 2843
A n Algorithm f o r the Design of Propagating Acidity Fronts
if the reducing agent liberates more protons per electron than the oxidizing agent consumes, then the net effect is the acidification of the reaction mixture. For a more quantitative treatment let us take a general oxyion AbO; which is reduced to AkOlm,while another ion, BP,h is oxidized to B,O,S. b, c, k , I, A g, p , and r are the appropriate stoichiometric numbers, d , m, h , and s stand for the charges of the ions. c, I, g, r, d , m, h , and s may be 0. The redox reaction in its most general form is
+ eB,O,h + i H z O = jAkOlm+ nB,O,S + t H +
aAbO;
Table I. Half-Reactions of the Oxidizing and Reducing Agents proton/electron half-reaction
+ + +
(1)
t / x = (1
+ (arB-aoB)/bB)
+
(arA-aoA)/bA)
(2)
where arB= h / j aoB= s / p ; arA= m / k ; aoA= d / b ; Le., the average charge of the ions for one central atom. r and o indices denote the reduced and oxidized forms, respectively. bA and 6, are the changes of oxidation number for each central atom when the jAkOIm aAbO$ and the nBpO,S eB,O,h transformations take place, respectively. (6, and bB are positive numbers.) The change of the oxidation number may be assigned to the central atom (e.g., S032- SO4,-) or to the oxygen atoms in the case of the peroxo compounds, e.g., 2S04,S20a2-. Let us take the oxidation of S406,- by chlorine dioxide as an example to illustrate the use of eq 2.
-
-
-
14c102 t / x = [I
+ 5s406'- + 22H20 = 14C1- 2os0d2+ ((-2/4) - (-2))/3.5] - [ l + (-1)/5] =
44H'
10/7 - 4 / 5 = 22/35 It is easy to see that the first term (1017) expresses the proton/electron ratio liberated by the reducing agent, while the second term (415) expresses the protonlelectron ratio consumed by the oxidizing agent. These data may be obtained from the appropriate half-reactions independent of the partner in the redox reaction
S406,-+ 1 0 H 2 0
-
4s042-
+ 14e- + 20H+
proton/electron liberation ratio = 10/7 CIO,
+ 5e- + 4H+
+
+
-(I
-
CI-
+ 2H20
proton/electron consumption ratio = 4 / 5
+ +
(3) Equation 3 indicates that the oxidation of reducing agents for which arB> aoB,;.e., for which the average charge decreases as a result of oxidation, produces more protons than electrons. All oxyanions of sulfur that contain S-S bond(s) belong to this group when they are oxidized to sulfate. On the oxidizing agent side, the relation a/ < aoAis favorable for proton production in the net reaction. All of the oxidizing metal ions belong to this group, but they form hydroxide precipitates in alkaline solution. There are only a few other oxygen-containing oxidizing agents, one of them is C102 in the example above, which fulfill this relation. The peroxo compounds also belong to this group, for example, S20a2-when it is reduced to sulfate. For all of the oxyhalogen ions (C103-, Br03-, CIOz- etc.) arA = aoA.Thus, as far as stoichiometry is concerned, propagating acidity fronts are expected in their reactions if the reducing agent
1 0
+ + -
Table 11. Front Characters of the Reactions Studied (Velocity of the Prouagatine. Aciditv Front Is Given in lo3 mm/sY S2O3'S4o6,- SZO4'- S032- S2O6'- N2H5' C F C C Br03C C 9.7 2.5 3.0 5.7 1.3 10~C10,-
c10,-
C
C
C
510
70
1.0
C 8.3
C 0.67
N
N
P
N
c
C
F
F
C
F
ref 17
108
N N
N
1 .o
87
P P P Pb P P 1.0 1.7 2.5 1.3 1.3 " C = complete front; P = partial front; F = fast reaction without front; N = no reaction. bobserved in a Petri dish.
S2Os2-
fulfills the condition a: > aoB. There is another possibility of acidification of the reaction mixture not directly seen from the stoichiometry. This is the case in which the reducing agent is a much stronger base than its oxidized product. One example is the oxidation of arsenite by iodate,21studied and explained by Harrison and Showalter. The same possibility arises in the case of the reducing nitrogen-containing bases, for example, hydrazine or hydroxylamine. Their solution is basic even in partly protonated form, thus they liberate protons when oxidized to nitrogen. To sum up, there are two types of reducing agents conducive to propagating acidity fronts: (1) all oxyanions of sulfur that contain S-S bond(s); (2) reducing agents that are significantly more basic than their oxidized counterparts, for example, H3As03 H3As04; H2N-NH3+ N2 H,O, or SO3,SO4,-. As to oxidizing agents, peroxo compounds and oxidizing metal compounds that are stable in alkaline solution (e.g., [Fe(CN)6I3-) are suitable for producing propagating fronts in their redox reactions. Starting from the above analysis, the oxidizing and reducing agents summarized in Table I have been studied experimentally in all possible combinations in order to see if they manifest propagating acidity fronts in their reactions. -+
Their algebraic sum gives the proton/electron liberation factor for the net reaction. It follows from eq 2 that the redox reaction liberates protons, if and only if
1
--
+
(If the reaction consumes protons, t and i are negative numbers.) Taking into account the charge and mass balances, ab = j k ; ef = np; ad eh = j m ns t ; ac e g + i = j l + nr; and t = 2i, the following relation can be derived for the ratio of the number of protons produced ( t ) and the number of electrons (x) transferred in reaction 1
+ +
-
liberation
Reducing Agents 2S042- + 10H' + 8e' S,032'+ 5H,O + 20H' 14e- S4062-+ 10H20 4SO-: S204'- + 4H20 2SOd2-+ 8H' + 6e2H' + 2eSO3'- + H20 SO-: 4H' 2eSzO>' + 2 H 2 0 2SO:N2 5H' 4eN2H5'
+ eRed2 + iHzO = j R e d , + nOxz + t H +
+
---
Oxidizing Agents 6eBr- + 3 H 2 0 6eI- + 3 H z 0 6eC1- + 3H2O 4e- -. C1- + 2H,O 2SO42-
Br0,6H' + IO3- + 6H' + 6H' + (210,C10,4H' + S20s2-+ 2e-
i.e. aOx,
-
-. consumption
-
+
-
Experimental Section The highest quality chemicals available have been used to make up slightly alkaline reaction mixtures, containing appropriate acid-base (phenolphthalein, methyl orange, or xylenol orange) indicators. The solution was transferred to a glass tube of internal diameter 2 mm. The wave was initiated at the end of the tube by a drop of 0.1 M H,S04, and its propagation was followed visually in the horizontal direction, except for the reactions of N2HS', where the position was vertical because of the.N2 gas formation. The reaction of N2H5' with S,0s2-was studied in a Petri dish only, because of N, formation. In other reactions of N2HS', N, formation did not disturb the velocity measurements as the density of the reaction products prevented undesired turbulence. As the aim of our studies was the detection of the wave only, no strict temperature control was applied.
Szirovicza et al.
2844 J . Am. Chem. SOC.,Vol. I l l , No. 8, 1989 Table 111. Concentration of the Reactants in mol dm-3
ox
S2032-
s40:-
S204’-
0.015
Br0
