1238
E. J. MEEHAN,I. &I.KOLTHOFF, ASD 13. KAKIUCHI
Vol. 66
REACTIOS BETWEEN FERRICYANIDE AND %MERCAPTOETHANOL1 BY E. J. MEEHAN,I. hI. KOLTHOFF, AND H. XAKIUCHI School of Chemistry of the University of Minnesota, Minneapolis 14, Minnesota Received September 18, 1061
The oxidation of Z-mercaptoethanol by potassium ferricyanide has been investigated in the pH range 1.8-4.1 at 0-25'. The reaction product is the disulfide. The reaction has ver complex kinetics, being in the initial stages second order to ferricyanide and becoming zero order to ferricyanide in the h e r stages. Addition of lead(I1) makes the reaction first order to ferricyanide. Addition of small concentrations of ferrocyanide retards, but large concentrations of ferrocyanide accelerate and make the reaction zero order to ferricyanide. T w o mechanisms which need further experimental substantiation have been postulated to account for the facts.
The reaction between ferricyanide and a mercaptan has been made use of for the initiation of emulsion polymerization. In order to aid in the interpretation of kinetics observed in such systems it was decided to study the kinetics of reaction between ferricyanide and a mater-soluble mercaptan. For this purpose 2-mercaptoethanol TT;as used. The reaction has a measurable speed at pH less than 4. I n a subsequent paper the oxidation of a water-insoluble mercaptan by ferricyanide in acetone-mater medium mill be presented. Quite generally the mechanism and kinetics of oxidations by ferricynnide are not accounted for by the simple stoichiometric reactions2 In our present work we have observed an effect of ferrocyanide on the kinetics of the mercaptan oxidation which is not accounted for by any simple reaction and which has not been mentioned hitherto. +4n attempt has been made t o interpret the phenomena. Admitkedly the proposed mechanisms are speculative but may serve as a guide iii further work. After completion of the present work, a study of the oxidation of 3-mercaptopropionic acid by ferricyanide was reported by Bohniiig and Weissa3 These authors also observed kinetic behavior which could not be accounted for quantitatively although some qualitative deductions could be made about the mechanism. They worked in the pH range 3.8-4.8. Since the pKl of 3-mercaptopropionic acid is 4.3, the effect of the varying ratio of undissociated acid and carboxylate ion must be taken into account. %Mereaptoethanol at pH < 4 is present virtually completely in the undissociated form, and the absence of the carboxyl group should make the reaction less complicated than with a mercaptocarboxylic acid. Experimental Chemicals.-2-Mercaptoethanol, denoted as ESH, was obtained from Union Carbide and Carbon,Co. It "as, distilled under nitrogen and the fraction boiling a t 69-70 , 23 mm., was collected. Results of amperometric titrations with 0.05 M silver nitrate corresponded to a mercaptan content of 99.379 of the theoretical value. From the measured pH after addition of known amounts of sodi2m hydroxide, pK, was calculated to be 9.56 i 0.02 at 25 , 1.1 = 0.001. A value of 9.5 in 0.15 N sodium chloride, 25', has been reported by Olcottc found that the rate of loss of ESH (presumably by air oxidation) in aqueous acidic medium is negligibly small compared to the reaction under consideration. The disulfide, ESSE, r a s prepared by This investigation was carried out under a grant from the NaScience Foundation. See, e.g., A. W. Adamson, J . Phys. Chem., 56, 858 (1952). J. J. Bohning and K. Teiss, J . A m . Chem. Soc., 83, 4724 (1960). (4) M. Calvin, U. S. Atomia Energy Comm., UCRL-2438, 3 (1954). (5) H. S.Olcott, Science, 96, 454 (1942).
(1) tional (2) (3)
oxidation with i0dine.O Potassium ferricyanide (Feic) was recrystallized from Merck reagent, and solutions were prepared fresh as needed. Potaelsium ferrocyanide (Feoc) and other reagent grade chemicals were used without purification. All solutions were made up in conductivity water and were made air-free with Linde "high purity" nitrogen. The pH, measured a t 26", was regulated in the range 1.8 to 4.1 with one of the following systems: acetic acidsodium acetate, hydrochloric acid-potassium chloride, potassium biphthalate-potassium chloride, or phosphoric acid-potassium chloride. Methods.-The reaction was studied at 0 and 25" in the absence of air. In moat cases the rate was followed spectrophotometrically with a Beckman DU spectrophotometer, using a thermostated reaction vessel with an attached I-em. absorption cell. After the air-free reaction mixture, except Feic, had reached the desired temperature, an air-free solution of Feic at the same temperature was injected through a self-sealing gasket. The vessel was shaken vigorously for a few seconds and the absorbance of Feic was measured at 420 m,u, at which wave length the absorbance due t o Feoc is negligible.2 In a few experiments the rate of reaction of Feic was determined polarographically using the dropping mercury electrode at a potential at which ESSE is not reduced. The polarographic and spectrophotometric methods gave the same results for Feic concentration. ESSE formed in a completed reaction with an excess of ESH was determined polarographically in a borate buffer a t pH 7 . 6 , at -1.6 v. 8s. s.c.e., a t which potential the diffusion current is obtained by the reaction ESSE 2e 2Hf = PESH
+ -+
Results Stoichiometry.-Prom the amount of ESSE formed with an excess of ESH it was established that the over-all reaction is 2Feic
+ 2ESH = 2Feoo + ESSE -+
2H+
Separate experiments showed that ESSE does not affect the reaction rate, and that ESSE does not react to a measurable extent with Feic under the experimental conditions. Reaction Kinetics.-The concentration of Feic was measured in the presence of an excess of ESH, at 0 and 2 5 O , at varied pH and ionic strength, and in the presence of Feoc and lead perchlorate. (a) In mixtures containing ESH and Feic, at any pH in the range 1.8-4.1, both a t 0 and 2j0, the initial rate is relatively large. Following this rapid reaction, the extent of which depends upon reactant concentrations and pH, a much slower, practically zero-order disappearance of Feic occurs. This is seen clearly from the curves of Fig. 1, which illustrate some typical results. The ionic strength was varied with potassium chloride from 0.002 to 0.2, with no effect upon the reaction rate. Figure 2 shows plots of (Feic)-I us. time at (6) I. M. Kolthoff, A. Anastasi, and B. H. Tan, J . Am. Chem. Soc., 80, 3235 (1958).
REACTIOX BETTVEEN FERRICYASIDE A S D 2-11'IERCAPTOETH-4SOL
July, 1962
various pH. From the initial linear portion of the curves it is evident that the reaction initially is second order to Feic. From a few experiments a t 25' using doncentra144 it aptions of ESH between 5 and 9.5 X peared that the initial rate, as measured by extent of reaction a t 1 min., is proportional to ESH. The iniiiial rate mas found to decrease with increasing acidity but no simple relation exists between rate and (H+). Between pH 1.8 and 3.2 the rate increased only sixfold; a t pH above 3.2 the ratc appeared to approach inverse proportionality with (H+). The dependence of the rate of the zero order reaction at 25' upon (ESH)could not be established, because with increase of (ESH) the initial rapid reaction persists longer and the "final rate" becomes too small to be measured (vide infra, results a4t0'). TvDical results a t 0' are summarized in Table I. The"kinetic features are the same as those discussed above. The rate constant Vf of the final zeroorder reaction a t 0' is approximately proportional to (ESH), The value of Vf/ [ESH] a t 0'. is about 8 X 10-6 min.-l (pH 2.0). At 25' Vr[ESH] is min.-l (pH 1.76). about 5 X REACTION BETWEEN
(Feic) X l0aa
TABLE I FEICAND ESH, o", pH 2.0 f 0.05
vf x
(ESH) X 109'
1.10 1.89 2.06 1.19 2.06 1.66 1.66 11.7 Initial molar concn. stant, mole 1.-1 min.-'. 0
x
106
7 9 1.8 9 10 8 Limiting zero-order rate con-
(b) Addition of Feoc to the reaction mixture causes remarkable changes in the kinetics (Fig. 3). With increase of (Feoc) the extent of the initial rapid second-order reaction is decreased arid the subsequent. zero-order reaction is accelerated. As a result, the entire reaction becomes rapid and zero order to Feic iii the presence of much Feoc. The final zero-order rate varies linearly with, but not in direct proportion to, (E'eoc) (Table 11). TABLE I1 EFFECT OF FEOC ON RATE M; (ESH) = 1.00 x (Feic) = 3.91 X (phosphoric acid); 25' Added (F'eoo) X 104
0
0 76 2 4.59 4 92 6.45
40.1 a
102 In % Feic per min.
Initial ratea
22 17 2.4 3.2 4.4
Series
a
Series
b
80
60
40
& 20 c
.i
d
2
.3
E o 3
.i
fo
k
&? 80
60 1.95
40
Vf/(ESH) 105b
1.4 1.9
*
L
1239
20
5
10 15 20 Minutes. Fig. 1.-Effect of pH, 25': (Feic) = .3.8 (i0.1) X M exceDt a t DH 3.55 and 4.03, in which (Feic) = 3.2 ( I t O . 1 ) X 10-4 M,: (ESH) = 9,0 >i 144 in series a and M in series b. Buffers: pH 1.76, hydrochloric; 4.0 X pH 2.1 and 1-98, phosphoric; pH 3.10, 3.20, and 4.05, biphthalate; pH 3.55, acetic. 0
M ; pH 1.9
Rate et 75% rea ctiona
1.3 1.8 1.3 1.7
...
1.8 2.7
6.7'
5.5
10.3b 10.3 After a brief induction period.
(c) Addition of lead perchlorate causes a pronounced acceleration (Fig, 4). While the rates are too large for exact kinetic studies it appears
01 0
10 15 20 25 30 Time, minutes. Fig. 2.-Plots of (Feic)-' os. time: Initial (Feic), 3.53.9 X M; curve 1 pH 4.05 (biphthalak), (ESH) = 3.95 X 10-3 M ; curves 2 and 3 pH 3.55 (acetate), (ESH) = 9.24 X lo-* M (2) and 6.37 X M (3); curve 4 pH 2.10 (phosphoric), ESH = 9.65 X 10-3 M ; curve 5 pH 1.80 (hydrochloric), (ESH) = 11.1 X 10-8 M; curve 6 pH 1.95 (phospbrio), (ESH) 3.83 X lO-3M. 8
that the reaction becomes about first order to Feic.
E. J. MEEHAN, I. M. KOLTHOFF, AND H. KAKIUCHI
1240
Vol. 66
100
depends on the ferrocyanide species, and the overall rate is not a simple function of (H+), as has been found experimentally. If ferricyanide reacted 80 with ES- rather than with ESH, a large ionic strength effect would be expected. bi 0 Reactions 2 and 3 are written instead of a simple .e dimerization of ES. to account for observations .g 60 in this Laboratory (unpublished) on the catalyzed E e0 addition of mercaptans to olefins. When persulfate, or persulfate and iron(IJ), is added to a solu& 40 tion containing mercaptan and olefin, a quantitative addition to the double bond occurs. However, when Feic is the oxidant the mercaptan is oxidized 20 quantitatively to the disulfide and no addition to the double bond occurs. It appears that ES. reacts more readily with Feic (reaction 2 ) than with 0 an olefin. 0 10 20 30 40 50 Quantitatively mechanism A accounts f3r the Minutes. Fig. 3.--Effect of ferrocyanide; (Feic), 3.91 x 10-4 M ; order with respect to Feic and to ESM,and for the ( E M ) ,1.05 X lQ-*M; pH 1.9; 2 5 O , and (Feoc) = curve 1, fact that both the rate and extent of the initial 0; curve 2, 0.76 X iW;curve 3. 2.00 X l o d 4I11: curve reaction are decreased upon addition of small 4,6.45 X l O - * X ; curve 5, 1.02 X 1 Q P i W . amounts of Feoc (vide infra). The acceleration observed upon addition of lead(I1) is accounted 100 for by the precipitation of lead ferrocyanide, which prevents the occurrence of reaction (-1). The 80 rate of disappearance of Feic then becomes first order to this constituent. Because of reaction (-1), mechanism A is of no consequence when much Feoc is added. To account for a reaction zero order to Feic and ac0 celerated by Feoc, mechanism B is postulated. ' E 40 (In the subsequent discussion, no distinction is si made between Feoc and HFeoc.) The ratedetermining step is a reversible substitution 20 of CN- in Feoc by ES.e
Feoc
0 6 8 10 12 Minutes. Fig. 4.--Effect of Pb(ClQ&; (Feic) = 1.10 X l o w 3M ; (ESH) = 1.88 X Jl; pW, 2.01; 0"; concn. of added Pb(C1Oj)Z: 1, 0 ; 2, 0.0033 '$1;3, 0.0065 ..W;4,0.0133 AI.
0
2
4
Discussion The kinetic results indicate the existence of a t least two reaction mechanisms. Mechanism A, which is proposed to account for the initial reaction when no Feoc is added, has a reversible rate-determining step Feic + ESH -+ HFeoc + E9' (1, - 1) Feic + ES'+ Hf--+FIFeoc + ES+ (2) ES' + ESN --+ESSE + H c (3) Ferricyanic acid is a strong acid, but ferrocyanic acid is strong only for the first two hydrogens. According to Kekrasov and Zotov7 K B = 1 X and Kq = 5 X (values obtained at 16-18', corrected to g = 0). Thus a t pH below 4, a negligible fraction of iron(I1) is present as Fe(CX)6-4. The major constituent is HFe(CN)a-3 or H2Fe(CN)s-2, depending upon pH, and the maximum fraction of H F ~ ( C N ) Goccurs -~ at about pH 3.6 ( p = 0). For the sake of simplicity HFeoc in eq. (1, - 1) represents the various forms in which ferrocyanide is present. The rate of reaction (- 1) (7) B. V. Kekrasov and G. V. Zotov, Zh. P~tb2.Khzm., 14, 264 (1941).
+ ESH
Feoc'
+ HCN
(4, -4)
in which Feoc' represents Fe(CN)5ES-4. Feoc' is then oxidized by Feic. However, this oxidation reaction cannot occur with the direct formation of ES*;if it did so occur, ES. would react according to reaction (- 1) and no acceleration by Feoc would be observed. It is plausible that the oxidation occurs as Feic
+ Feoc'
--f- Feoc
+ Feic'
(5)
in which Feic' represents Fe(CN)5ES-3, where the iron may be present in the divalent state and ES is uncharged, or iron in the trivalent state and ES as monovalent anion Feic'
+ Feic ( + CN-) +2Feoc + ES4 (6)
followed by (3). Reaction 5 undoubtedly is rapid, by analogy with the rapid electron exchange between Feic and Feoc.8 Apparently the ferricyanide oxidation of 2mercaptoethanol is at least as complex as other oxidations cited by Adamson.2 From the kinetics of the oxidation of monohydric phenols by alkaline ferricyanide, Waters9 had already concluded that the initial stage of the oxidation is a reversible reaction. Similarly, Bohning and W e k a assumed a reversible first step in the oxidation of 3-mercapto(8) A. C. Wahl and C. F. Deck, J . Am. Chem. Soc., 76,4054 (1954). (9) C. G . Haynes, A. H. Turner, and W. A. Waters, J . Chem. Soc., 2823 (1956).
JL~Y 1962 ,
CATALYTIC REACTIONS ON SEMICONDUCTORS
propionic acid. In our studies on 2-mercaptoethanol, the initial reaction in mechanism A is explained in the same way as by Bohning and Weiss. However, they observed second-order kinetics throughout; the pronounced change in order and the remarkable effects of Feoc were not observed in the oxidation of 3-mercaptopropionic acid. The mechanism of oxidation by ferricyanide is very cornplex and probably involves more reactions than those in mechanisms A and B. The complex nature of oxidation reactions with ferricyanide is encountered also in reduction reactions with ferrocyanide. I n this connection it is
1241
of interest to mention that the simple mechanism postulated by Boardmanlo for the reaction of cumene hydroperoxide with ferrocyanide to form acetophenone and methanol does not account for many complex characteristics which apparently are specific for this reaction. The marked retardation by cyanide observed in this Laboratory and many other effects summarized by Reynolds11 justify the conclusion that the mechanism cannot be accounted for by the simple stoichiometric reaction. (10) H. Boardman, J. A m . Chem. SOC.,76, 4268 (1953). (11) W. B. Reynolds, Ph.D. Thesis, University of Minnesota, 1955.
CATALYTIC REACTIONS ON SEMICONDUCTORS: HYDROGEN-DEUTERIUM EXCHANGE AND FORI1/IC ACID DECOMPOSITION ON CHEMICALLY DOPED GER&lANIU&fl BY GEORGEE. MOO RE,^^ HILTON A. SMITH,^^ AND ELLISON H. T A Y L O R ~ ~ Department of Chemistry, University of Tennessee, Knoxville, Tennessee, and Chemistry Division, Oak Ridge iYational Laboratory,' Oak Ridge, Tennesses Received October $0,1961
Hydrolzen-deuterium exchange and the decomposition of formic acid were studied from 100 to 400' over samples of germanium {dopedwith 1016 to 1020 atoms per cc. of n- and p-type impurities. Over these wide ranges of electronic chemical potential (Fermi level) and temperature the rate and activation energy of the exchange reaction vary definitely with the Fermi level. A minimum in the activation energy-Fermi level curve suggests a mechanism in which one step is rate-limiting on the n-type and a different one on the p-type side of intrinsic composition. Of the two paths for formic acid decomposition, only dehydrogenation seems to be markedly affected by doping.
Elemental semiconductors are well adapted to studies of the effect of electronic properties upon ca,talysis, since the concentration of charge carriers ca,n be altered by many orders of magnitude by the incorporation of amounts of impurity too small to produce detectable changes in other bulk properties. C:hemicrally doped germanium has been used for B number of such with somewhat inconclusive results from the standpoint of a general theory of catalysis. I n some eases, marked differences in activity were found between n- and ptype sa:mples, although the degree of doping was often without infl~ence.4J'~~~~ I n the case of HrDz exchange, no dependence of activity upon doping w,as ~bserved."~ The present experiments were undertaken in (1) Based on a thesis presented to the University o f Tennessee, Knoxville, Tennessee, by George E. Moore in partial fulfillment of the requirements for the Ph.D. degree, August, 1961. The work was carried out a t the Oak Ridge National Laboratory. ( 2 ) (a) Oak Ridge National Laboratory; (b) University of Tennessee. (3) Operated for the United States Atomio Energy Commission b y Union Carbide Corporation. (4) (a) G. RP. Schwab, in R. H. Kingston (ed.) "Semiconductor Surface Physics," University of Pennsylvania Press, Philadelphia, Pa., 19157, pp. 2!91-294; (b) G. M. Sohwab, G. Greger, St. Krawczynski, and J. Pennkofer, Z . physilc. Chen. (Frankfurt), 16,363 (1958). (5) V. M. Frolov, 0. V. Krylov, and S. 2. Roginskii, Dokl. Akad. Na,uk SSSR, 126, 107 (1959). I:6) V. L. Kuchaev and G. K. Boreskov, Probl. Kinetiki i Kataliza, Akad. Nauic 8SSR. 10, 108 (1960). (7) V. L. Kochaev and G. K. Boreskov, Kinetika i Katalis, 1, 356 (1'260).
(8) V. M. Frolov, 0. V. Krylov, and S. Z. Roginskii, Prob2. Kinetiki i Kataliza, Akad. Nauk SSSR, 10, 102 (1960). (9) W. €1. Watson, Jr., J . A p p l . Phus., 32,120 (1961).
order to explore this important question of the catalytic behavior of doped germanium over as wide a range of doping and of temperature as possible. Experimental Vacuum System.-A conventional vacuum system using a mercury diffusion pump, liquid nitrogen traps, and highvacuum stopcocks lubricated with Apiezon-N grease was employed. That portion of the apparatus accommodating the reaction vessels was separated from the rest of the system by dental gold foil and a liquid nitrogen trap. Reaction Ves:sels.-Quartz reaction vessels of about 3 cc. volume (and with an additional dead space of about 3 cc. in connecting tubing) were employed. The connection to the vacuum line was through 2-mm. vacuum stopcocks and standard taper connections. Catalysts.-Single crystals of germanium were obtained from the Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey, and from the Solid State Division of the Oak Ridge National Laboratory.lo These crystals were doped with Al, Ga, In, As, or Sb, in concentrationa varying from 10'6 to 2 X lozoimpurity atoms per cc. (2.5 X 10-8 to 0.5 atom % impurity). The samples were washed with acetone, etched in "CP-8" (a mixture of concentrated nitric, hydrofluoric, and acetic acids in the volume ratio 5: 3 :3), fractured into smaller pieces, crushed, and finally mechanically ground in an agate mortar and pestle. Approximately 1 g. of the powder in a reaction vessel was heated a t 675 to $00" for about 2 hr. in a stream of flowing (about 45 cc./min.) Matheson prepurified electrolytic hydrogen of 99.97, minimum purity and
