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The reduction of HgCl2 in gamma-irradiated aqueous-organic systems
Radiat. Phys. Chem. Vol. 20, No. 5--6, pp. 333-340, 1982 Printed in Great Britain.
0146-5724/g2/110333--08503.00/0 Pergamon Press Ltd.
THE R E D U C T I O N OF HgC12 IN GAMMA-IRRADIATED A Q U E O U S - ORGANIC SYSTEMS R. A. DELLA GUARDIA and FRANCIS J. JOHNSTON Department of Chemistry, University of Georgia, Athens, GA 30602, U.S.A. (Received 7 April. 1982; accepted 4 June 1982)
Abstract--In the presence of a variety of organic solutes, mercuric chloride is reduced to mercurous chloride and HC1 in aqueous solutions. G(HC1) values in dilute solutions (0.001 M HgC12 and 0.01 M organic) for a series of alcohols, acetaldehyde, acetone and acetonitrile lie within the approximate range of 3-6 molecules per 100 eV. The value depends upon the reactivity toward HgCI2 of the radical product formed by reaction of OH with organic component. The results show a decreasing reactivity as the radical site is farther displaced from the a-carbon atom. At high concentrations (0.1 M HgC12 and 1 M organic) chain reactions occur in most of the systems with G(HCI) values increasing with decreasing dose rate. Evidence is presented for an involvement of colloidal Hg2C12 in the reaction chains.
INTRODUCTION THE REDUCTION of mercuric chloride by UV light in the presence of oxalate ion is known as Eder's reaction and was the subject of extensive investigation through the 1930s.
and (4)
2 HgC1 ~ Hg2C12
occur with rate constants of 4.0x 101°, 1.0x 101° and 8 x 109M -1 s -a, respectively. In steady gamma radiolysis, however, reduction yields are low (0.02-< G(Hg2CI2)< 0.2) and dose dependent. This was attributed in part to oxidation of HgCI by OH
(1) hv
2 HgC12 4- C204 -2 --*Hg2C12 + 2 C1- + 2 CO2o
(5)
Cartledge Ct) summarized the results of these early studies and suggested that the oxalate radical ion, C204-, was the active reducing species in the initiation of a chain reaction. The first report of reduction of mercuric chloride by ionizing radiation was that by Roseveare t2) in 1930. Reaction (1) occurred upon absorption of X-radiation with an efficiency of approx. 6 x 105 reductions per ion pair formed. Subsequent experiments with this system were described by Rao. °-5) G-values for Hg2CI2 formation in aqueous mercuric chloride solutions containing oxalate were as high as 250 molecules per 100 eV absorbed. The high yield and reproducibility of the reaction suggested its use as a low level 3' ray dosimeter. Pulsed radiolysis studies with aqueous mercuric chloride have shown t*) that the reactions
and in part to an oxidation of Hg2CI2 of unspecified stoichiometry. In the presence of OH scavengers, reduction of HgC12 does occur upon gamma radiolysis with a G-value dependent upon whether or not the product of the OH scavenging reaction will reduce HgC12. With t-butanol, G(Hg2C12) approaches 2 at high alcohol concentrations (0.05 M) and in stlutions containing t-butanol and saturated with N20, the reduction of HgC12 is not observed. These results indicate that the radicals produced by the reaction of OH with t-butanol, c"
(2)
HgCI2 4- ea-q--->HgCI + CI-
(7)
(3)
H + HgCI2 --~HgC1 + H + + C1-
HgC1 + OH ~ HgC1 + + O H -
followed by (6)
HgCI + + CI- ~ HgCI2
OH + (CH3)3COH ~ (CH3)2COHt~H2, (CH3)3C0
RPC Vol. 20, No. 5-6---C
4- H 2 0 333
334
R. A. DELLA GUARDIA and FRANCIS J. JOHNSTON
do not reduce HgC12. With isopropyl alcohol as OH scavenger, G(Hg2CI2) was f o u n d t o be 3.1(G(HCI) = 6.2) corresponding to an apparently complete utilization of the radical yield from water in the reduction process. Evidently, the (CHD)2COH and, possiblyl (CHD)2CH0 radicals formed by reaction with OH do reduce HgC12. The reduction reaction is presumed to be an electron transfer process of the type
more than -+ 0.2 V, however, it does limit results of HgCI2 reduction studies to a relative comparison of radical reactivities. This report describes the results of experiments involving a variety of organic additives as sources of free radicals for possible reduction of HgCI2. In addition, a chain reaction that is observed at high organic and HgC12 concentrations is described and evidence presented for this being a heterogeneous process.
(8) (CHD)2COH + HgCI2 ~ (CHD)2CO + HgC1 + HC1 The analogous reaction of HgC12 with ethanol radicals has been shown 6 to be (9) CHDCHOH + HgCl2-, CHDCHO + HgCl + HC1. We wished to pursue the use of the HgC12 reduction as a possible additional discriminator with regard to the oxidation-reduction behavior of radicals (8) formed in irradiated aqueous-organic systems: The free energy change for the standard reduction reaction
(10) 1/2H2(g) + HgCl2(aq)--* HgCl(aq) + HCl(aq) is not known, however, it can be approximated from known free energies of formation (9) and an estimated AG~ for HgCl(aq). AG~ for HgCl(g) is 15.0 kcal/mole. With no other basis for estimation, AG ° for (11)
H g C l ( g ) - , HgCl(aq)
was taken as a mean of the corresponding values for Hg and HgCI2. F o r Hg, this is 1.8 kcal mole -~. To obtain the value for HgC12, AG~ for HgCl2(g) was required. This was evaluated from AG~ for HgCl2(s), - 4 2 . 7 kcal mole -~, and the free energy of vaporization, ~° 9.27 kcal mole -~. With AG~ for H g C l 2 ( a q ) = - 4 1 . 4 kcal mole -~ and AG~ for HgC12(g) = - 3 3 . 4 kcal mole -~, the standard free e n e r g y change for HgCI2 undergoing the reaction analagous to 10 is - 7.0 kcal mole -t. The estimated value of AG ° for reaction (10) is - 2.6 kcal mole -t. AG~ for HgCl(aq), based upon this approximation, is 12.4 kcal mole -~. AG ° f o r r e a c t i o n (10) becomes 22.4 kcal mole -~ with the corresponding reduction potential calculated to b e - 0 . 9 7 V, The uncertainty in this result is difficult to assess. That introduced by our approximation for reaction 11 can hardly be
EXPERIMENTAL The reduction reaction produces a strong electrolyte, HC1, from covalent mercuric chloride and is conveniently followed by measuring the specific conductivity of the irradiated solutions as a function of absorbed dose. A conductance-radiolysis cell of approx. 25 ml in volume was used and measurements were made with a Beckman RC-18A conductivity bridge at 25.0°C. The value of the cell constant was frequently checked with standard KC1. Reactant solutions were introduced into the cell and thoroughly flushed with N2, Ar or N20 before radiolysis. Irradiations were carried out in a Gammacell 220 in which the unshielded dose rate, as measured by Fe 2+Fe 3+ dosimetry, was 0.35-0.50x 1016 eV m1-1 over the course of the experiments. Radiolyses at reduced dose rates were carried out using lead shields and in another Gammacell in the Division of Biological Science. Reagent Grade or equivalent chemicals were used in the experiments without further treatment and triply distilled water was used in the preparation of all the solutions.
RESULTS AND DISCUSSION In the presence of a variety of organic additives, HgCl2 undergoes a radiation-induced reduction in aqueous, ~ oxygen-free systems at a reproducible rate. The overall reaction may be represented as (12) HgC12 + Red,~H20---> 1/2Hg2C12 + HC1 + Ox. Red and Ox are reduced and oxidized forms of the organic additive. The conductometric technique has the distinct advantages of convenience and sensitivity for following the reaction in the very early stages. Previously used gravimetric determination of Hg2Cl2 limited measurements to substantial extents of reaction. HCI yields in the radiolysis experiments were obtained from the measured specific conductances through the use of calibration curves constructed from conductivity measurements on HgC12 solutions in the aqueous-organic systems containing known quantities of added HCI. The
The reduction of HgCI2 in aqueous-organic systems rate of formation of HCI is (13)
d[HCll/dt =
(dK/dt)l(ddd[HCl])
dddt is the rate of increase in specific conductance of the irradiated system and dK/d(HCI) is obtained from the calibration plot for the particular HgC12 and organic concentration being studied. Within experimental variations, this latter ratio was 0.398 ohm -1 cm -~ M -1 for 0.001 M HgC12 and 0.01 M organic additive for all of the solutes studied. This may be compared with 0.423 ohm -~ cm -~ M -t (All000) measured in this work for 10-3M HC1 in 0.bl M ethanol at 25°C and in the absence of HgCI2. The difference is due to an involvement of HC1 in hydrolysis and complexation equilibria with HgC12. The most important of these are listed below along with values of the association constants at 25°C °1) (14) (15)
HgCI2+CI-~.-~HgC13- K = 10 HgCI3- + CI-,~-HgCL-2
K = 10°'~
(16)
HgOHC1 + HCI~HgC12 + H20
335
depend upon the reactivity toward HgCI2 of the radical formed by reaction of OH with the organic component. Figures 1 and 2 show typical experimental data. In Fig. 1, specific conductivity, as a function of time of irradiation is plotted for 0.001 M HgCI2 in 0.01 M acetone. The results are characterized by a short, apparent induction period followed by a
ooo~M H9CI~
% _
x
32C
'2
~ 24C ~J r"~16C to 0 h,~ 8C o
K = I09"s6. l
At higher HgC12. and organic concentrations, dK/d(HCD was markedly different from that in the absence of HgC12 and varied significantly from one organic component to another.
Time (s x I 0 "~1
FIG. 1. Specific conductivity-time of irradiation data for 0.001 M HgCI2in 0.01 M acetone and in an atmosphere of N2. The dose rate was 3.89 x 1015eV m1-1 S-l.
Radiolysis at low concentrations of HgCI2 and organic additive Under the usual initial experimental conditions (pH, 4.25; HgC12, 0.001 M and organic additive at 0.01 M), over 98% of the e~q react with HgC12. With increasing extent of reaction, the pH decreases and reaction of e~-q with H + with the formation of H becomes significant. Under these conditions, both of reactions (2) and (3) occur in the system. In systems containing acetone or acetaldehyde, with respective rate constants n for reaction with e~q of 5.3 x 109 and 3.5 x 109 M -~ s -1, there will be significant competition with HgCl2 for reaction with e2q. In dilute systems in which only e~q and H reduce HgC12 and with no other contribution or competition, a maximum value for G(HCI) of G(e~q)+ G ( H ) = 3.3 molecules per 100 eV would be expected. The rate constant for reaction of OH with HgCI2 has not been measured. It is assumed, however, that reaction by this mode does not compete with that involving the organic additives at 0.01 M and higher. The observed G(HCI) value will, then,
ISO
,o
% l
/
/
///
i.o ' 2 Io T i m e (S x 10-3)
3.0
FIG. 2. Initial specific conductivity-time of irradiation data for 0.001 M HgCI2 in (A) 0.01 M n-propanol (At), (B) 0.01 M n-butanol (N2), (C) 0.01M n-butanol (N20). The dose rate was 2.90X 1015eV ml-t s-t.
336
R. A. DELLA GUARDIA and FRANCIS J. JOHNSTON
linear increase in conductance with a limiting value corresponding to the stoichiometry of reaction (9). Representative data from other systems are shown in Fig. 2. G(HC1) values were obtained from these initial, linear portions of the plots. Results for the several systems studied are summarized in Table 1. Column 2 lists pseudofirst-order rate constants for the OH scavenging reactions at the solute concentrations used,
ethanol and isopropanol, the great majority of e~q, H and OH radicals that escape intra-track reaction and recombination reduce HgCI2 by reactions (2) or (3), or, by reaction (17), produce S radicals which reduce HgCI2. With ethanol as an example, the S species are the radicals CH~CHOH, CH2CH2OH and CH3CH2() formed in the ratios 1.0; 0.16; 0.03. (13) The typical reduction process is
(17)
CH3CHOH + HgC12~ CH3CHO + HgC1 + HCI. (18)
With rate constants of 107 s -I or greater, OH radicals are effectively scavenged by all of the alcohols used (a possible exception is t-butanol at 0.01 M). The third column lists the observed Gvalues for HC1 formation in N2 or At" saturated systems. With G(HC1) values of approx. 6 molecules per 100 eV, it is apparent that for methanol,
At the solute concentrations used in these experiments, some competition for e~-q and OH radicals that otherwise are consumed in intra-track reactions and recombinations must occur. Yields for total radical scavenging greater than 6 (14"t5) a r e possible and our results do not allow conclusions with regard to the effectiveness in the reduction
OH + S ~ S + H20.
TABLE 1. G(HC1) VALUES FOR THE REDUCTION OF 0.001 M HgCI2 IN AQUEOUS-ORGANIC SYSTEMS. UNLESS OTHERWISE INDICATED, THE ORGANIC COMPONENT IS 0.01 M Additive, S
klSl a
G(HCI) b
G(HcI)c
CH30H
1.2 x i07
5.93
5.83
CH3CH2OH
1.5 x 107
5.95
5-98
CH3CHHCH20H
7.7 x i07
5.52
4.81
CH3CHOHC ~
7.3 x i07
6.07
5.84
CH3(CH2)3OH
4.0 x I07
4.72
3.29
(CH3)3COH
5.2 x 106
2.55
0.75
(CH3)3COH(O.IM)
5.2 x 107
3.06
-
(CH3)3COH(I.0M)
5.2 × 108
3.17
5 x 106
6.11
6.hO
CH3CHO (CH3)2CO
1.2 x i06
3.06
0.84,
CH3CN
2.2 x 105
3.10
0.35
CH3COOH
2.0 ~ 105
1.8
0.32
p-dioxane
2.0 × I07
h.8
5.0
ethyl acetate
2.8 x 106
4.3
3.h
apseudo first-order rate constant for the reaction of OH with additive s. Values used for k were selected from reference 13. bN~ atmosphere CN20 atmosphere
The reduction of HgC12in aqueous-organic systems process of radicals formed in lowest yields from these alcohols. It is quite definite, however, that the major radical species formed by reaction 17 with these three alcohols, CH2OH, CH3CHOH and (CH3)2COH, are effective reducing agents for HgC12. For n-propanol, butanol and t-butanol, G(HC1) values are significantly below 6. Since OH scavenging by these alcohols is at least as effective as that by methanol, ethanol and isopropanol, it is evident that all of the products of the scavenging reaction are not completely effective in undergoing reactions analagous to 18. Using, as a basis for calculation, G-values for e~-q, H and OH that are characteristic of dilute solutions in the absence of track scavenging effects, it is possible to estimate the fractions of OH-produced radicals that do reduce HgClz. With G(e~q)= 2.7, G(OH)= 2.8 and G(H)=0.6 molecules per 100eV, and with f representing the fraction of OH-produced radicals that reduce HgC12, G(HC1) = 2.7 + 0.6 + 2.8f. For n-propanol, f = 0.79 and for n-butanol, 0.50. The results in N20 saturated solutions may be used for a similar estimate with a correction being made for an incomplete conversion of e~-qto OH. Using rate constants of 4 x 10~° and 8 x 109 M -~ s -~, for reaction of e~-q with HgC12 and N20, respectively and with corresponding concentrations of 0.001 and 0.024 M, it is estimated that approx. 83% of e~q is converted to OH. A similar estimation as before gives f =0.75 for n-propanol and 0.44 for nbutanol. Within the assumptions and limitations mentioned previously, these results are not inconsistent. Rate constant data °3) indicate that 54 and 40% of OH abstraction reactions occur at the a-hydroxy site in 1-propanol and in 1-butanol, respectively. Our results do not allow a quantitative correlation between reduction reactivity and location of the radical site at the a-hydroxy position, however, they strongly suggest a decreasing reactivity with positions farther removed from that position. For t-butanol the dominant species produced by reaction with OH is (CH3)2C(CH2)OH. It is evident that this radical does not reduce HgCI2 under our experimental conditions. Acetone and acetaldehyde compete with HgCI2 for reactions with e~-q and the expected reaction products are (CH3hCOH and CH3(~HOH, respectively. These are the same radicals as are formed by the reactions of OH with 2-propanol and ethanol and our results with the latter systems have shown them to be effective reducing agents for HgC12. With a pseudo-first-order rate constant of 1.2 x 106 s-1 at 0.01 M acetone for reaction with OH, most of these radicals will react with acetone
337
forming CH2COCH3 radicals. With G(HCI) -- 3.06 in a deaerated system and 0.8 in a N20 saturated system, our results indicate that this radical does not reduce HgC12. For 0.01 M acetaldehyde the pseudo-first-order rate constant for reaction with OH is approx. 5 x 106 s -1 and, as with acetone, most of the OH radicals will be utilized in reaction with the organic additive. The radical product, dominantly CH3(20, does effectively reduce HgC12. Acetonitrile will not compete with HgC12 for reaction with e~-q or H under the experimental conditions used (ke-= 2.5 x 107 M -~ s -~ and kn = 3.5 x 106 M -~ s-~).° ° The rate constant for reaction with OH is 2.2 × 10 7 M -~ s -~ which gives a borderline rate constant for scavenging of 2.2 x 105 s -~ at 0.01 M. The reaction product is the addition compound CH3C(OH) = lq 17and our results, G(HC1) = 3.1 in N2 and 0.4 in N20, indicate that it is not effective in reducing HgC12. Results with acetic acid, p-dioxane and ethyl acetate were mechanistically non-informative and are included without comment. In general, our results with the reduction reaction in these dilute aqueous systems are quite well explained within the framework of established aqueous radiation chemistry. Reactivity differences for HgCI2 reduction by several organic free radicals have been observed and these are consistent with expected behavior for these radicals. As reactant concentrations are increased, the behavior becomes more complex with reduction yields increasing and at 0.1 M HgCI2 and 1 M organic, chain reactions are observed in most of the system.
Reduction yields at high solute concentrations For 0.001 M HgC12, G(HC1) values are only slightly changed as the organic component is increased to 1 M. When both HgC12 and organic concentrations are increased, G(HC1) increases markedly for most of the systems studied. For example, with 0.001 M HgC12 and 1 M ethanol, G(HCI) = 6. With 0.01 M HgCl2 and 1 M ethanol, G(HC1) = 17. At still higher concentrations, G(HC1) becomes very large (103 molecules per 100 eV at 0.1 M HgCI2 and 2 M ethanol) and is dependent upon the dose rate. These are usual characteristics of a chain reaction. Table 2 summarizes results at the highest dose rates utilized for 0.10 M HgCl2 and 1 M organic additive. The complexity of the reaction system at such high reactant concentrations makes mechanistic speculation less meaningful. Reactions are occur-
338
R. A. DELLA GUARDIA and FRANCIS J. JOHNSTON TABLE
2.
G(HCI)
VALUES
FOR SOLUTIONS
CONTAINING
0. l M
HgCl,
AND
IM
ORGANIC
ADDITIVE
IN A N2
ATMOSPHERE
0r~anic
component
D(eV
ml-ls"1x10-15)
CH30H
3.27
63
CH3CH20H
3.75
h?
CH3CH2CH20H
3.27
37
(CH3)2CHOH
3.37
?h
CH3CHO
3.29
62
(CH3)3OOH
2.90
8.9
(CH3)2OO
2.90
3.9
ring within tracks and spurs and significant fractions of energy are being absorbed in the HgC12 and organic component. An interesting observation, however, is that chains do not occur in the acetone and t-butanol systems. These are the species that, in dilute solutions, formed radicals upon reaction with OH that did not reduce HgCI2. In the systems exhibiting chain behavior, G(HCI) values increased with decreasing dose rate. Figure 3 gives log-log plots of G(HCI) vs dose rate for 0.1 M HgC12 in 1 M acetaldehyde, methanol, 1-propanol and 2-propanol. In general, G(HCI) is proportional to D", where D is the dose rate and m has values of from - 0.23 to - 0.43. At the lowest dose rate, G(HC1) values of greater than 200 molecules pe r 100 eV were observed. As HgCI2 concentration s increased to 0.2 M and
A I~ 17 16 (.9 ~2.2,c 2 2C 19
-06 0 4 - 0 2
0
i
l
14020 ~
L9 t8 22
organic concentrations to 2 M, G(HCI) also increased. Only with ethanol, however, were experiments continued to even higher organic concentrations. In this case, an apparent maximum occurred at 8 M ethanol (250 molecules per 100 eV at 3.3 x I015 eV ml-' s-') with 0.2 M HgC12. An additional factor in the chain reacting systems became evident in experiments with 0.2 M HgC12 in 4 M ethanol solutions. When an irradiation series was interrupted for several hours and then continued, discontinuities were observed in the HCI production rate. Colloidal Hg2CI2 is produced in the reaction and only slowly coagulates. Upon standing, the colloidal material had precipitated and the solution became visually clear. Evidently, the chain reaction involves the surface of colloidal Hg2C12 particles. Figure 4 shows the results of 4 experiments at 0.2 M HgCI2 w
22 21
3
G(HCI) (molecules/lOOeV)
l
/
i
l
l
i
l
i
t
~
40 h L I l t
30
21
ZO 1.9 t8 0 OZ 04 06 -06-014 0'2 log [Dose Rate]
,20
Time (see)
0 0'2 04 06
FIG. 3. Log-log plots of G(HCI) as a function of dose rate for 0.10M HgCI2 in (A) I M n-propanol, (B) I M 2-propanol, (C) I M methanol and (D) I M acetaldehyde.
FIG. 4. Specific conductivity-time of irradiation data for several 0 . 2 M H g C l r - 4 M ethanol systems. The discontinuities indicate points at which the radiolyses were interrupted and the conductance-radiolysis cell allowed
to stand overnight before continuing the irradiations. Dur~ing this time, the colloidal HgzCl~ coagulated and settled out of solution. For clarity, the points for each series are displaced 10 sec along the time axis. The dose rate was 3.75 x 101~eV ml-~ s-].
The reduction of HgC12 in aqueous-organic systems and 4 M ethanol. After 40, 52, 52 and 65 sec of irradiation, respectively, each reaction system was allowed to stand for 12-18 hr before continuing the radiolysis. In each case, the Hg2CI2 had completely precipitated and a discontinuity in the reaction rate was observed. A rate was soon reestablished, however, that was, within experimental variations, the same as before interruption. Figure 5 shows the effect of a series of such interruptions in one experiment. Results for a continuously (except for measurement) irradiated sample are shown for comparison. Evidently, the chain reduction in HgC12-ethanol system does involve colloidal HgjC12. Electron transfer processes at the surface of colloidal gold and silver °8'~9~ have been described earlier, however, no such involvement of HgzCl2 has been reported. In considering possible chain propagating steps, a role for HgCl(aq) must be considered. Based upon our estimation of 12.4 kcal/mole as the standard free energy of formation for HgCl(aq), AG ° for the reaction (19) HgCl(aq) + 1/2Hj(g) ~ Hg(aq) + HCl(aq) is - 3 4 . 4 kcal corresponding to a standard potential of - 1.49 V. Based upon tabulated free energy data and the reduction potential (2°) of - 1 . 1 V for (20) CH3CHO(aq) + 1/2H2(g)--* CHj(~HOH(aq)
~o iz0 .~._x
A
c
¢-~ 0 8 0
#" 0(~
I
I 20
I
I 40
I Time
I 60
1
I 80
I
[ 100
I 120
(sec)
Fro. 5. Specific conductivity-time of irradiation data for 0.2M HgC12 in 4.0M ethanol. Curve A represents a sample subjected to continuous (except for times of measuremen0 irradiation. Points B, C, D and E on the curve for a second sample represent times at which the radiolysis was interrupted for overnight (B, D and E), and for 4 hr (C), to allow coagulation of the colloidal HgjC12. The dose rate was 3.75 x 1015eV m1-1 s-1.
339
and a ~ G ° for the reaction (21) CHjCH2OH(aq) ~ CHjCHO(aq) + 1/2H2 of 9.7 kcal, (2.) the standard free energy change for
(22) CH3CHjOH(aq) + HgCl(aq) --*Hg(aq) ÷ HCl(aq) + CHjCHOH(aq) is positive by 1 kcal. Within the uncertainties of the free energy estimations and considering a possible role for adsorption at the colloid surface in altering the energetics, the occurrence of reaction (2:) must be considered as a possible chain propagation step. The Hg formed in the reaction would react rapidly with HgC12 (23)
Hg(aq) + HgC12(aq) ~ Hg2Cl2(s). '
Some support for reactions, such as (22); in which HgCI plays the role of propagating the chain through reactions with t h e organic additive is found in the observation that chains do not occur when the additive does not contain an a-hydrogen atom. Radical formation for these species would be much more endoenergetic than (22) or analogous reactions in the s y s t e m s where chains are observed. Other possible chain propagating steps would include, of course, reactions b y intermediates produced in tracks and spurs at the high solute concentrations in the chain reacting systems.
REFERENCES 1. G. H. CARTLEDGE,J. Am. Chem. Soc. 1941, 63, 906. 2. W. E. ROSEVEARE,J. Am. Chem. Soc. 1930, 52, 2612. 3. P. S. RAO, Z. Physik. Chem. Neue Folge, 1959, 19, 226. 4. P. S. RA0, Bull. Chem. Soc. Japan 1965, 38, 137. 5. P. M. OZA and P. S. RAO, Z. Physik. Chem. Neue Folge 1968, 57, 265. 6. N. B. NAzntrr and K. D. ASMUS, J. Phys. Chem. 1973, 77, 614. 7. K. D. ASMUS, H. MOECKEL and A. ]-IENGLEIN, J. Phys. Chem. 1973, 77, 1218. 8. A. J. SWALLOW,Prog. Reaction Kinetics 1978, 9, 195. 9. NBS Technical Note 270-3; 4. Nat. Bur. Stand., U.S. Dept. of Commerce (1968-69). 10. D. CUBIOOOT'rt, H. EDING and J. W. JOHNSON, J. Phys. Chem. 1966, 70, 2989. 11. L. SILLEN and A. MARTELL (Eds.), Stability Constants of Metal-Ion Complexes Supplement No. 1, Special Publication No. 25, The Chemical Society, London, 1971. 12. M. ANnAR, M. BAM~ENEKand A. Ross, Nat. Stand. Ref. Data Ser. NBS No. 43 (1975).
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R. A. DELLA GUARDIA and FRANCIS J. JOHNSTON
13. FARHATAZIZand A. Ross, Nat. Stand. Ref. Data Ser., NBS No. 59 (1977). 14. T. I. BALKAS, J. H. FENDLER and R. H. SCHULER, J. Phys. Chem. 1970, 74, 4497. 15. R. H. SCHULER, A. L. HARTZELL and B. BEHAR, J. Phys. Chem. 1980, 85, 192. 16. M. ANBAR, FARHATAZIZ and A. Ross, Nat. Stand. Ref. Data Ser., NBS No. 51 (1975). 17. J. W. T. SPINKS and R. J. WOODS, An Introduction to
Radiation Chemistry, 2nd Edn, p. 330. Wiley, New York, 1976. 18. A. HENGLEIN, ./'. Phys. Chem. 1979, 83, 2209. 19. D. MEISEL, J. Am. Chem. Soc. 1979, 101, 6133. 20. M. BREITENKAMP, A. HENt3LEIN and J. LILIE, Bet. Bunsenges. physik. Chem. 1976, 80, 973. 21. W. M. CLARK, Oxidation-Reduction Potentials of Organic Systems. Williams and Wilkens, Maryland, 1960.
0146-5724/g2/110333--08503.00/0 Pergamon Press Ltd.
THE R E D U C T I O N OF HgC12 IN GAMMA-IRRADIATED A Q U E O U S - ORGANIC SYSTEMS R. A. DELLA GUARDIA and FRANCIS J. JOHNSTON Department of Chemistry, University of Georgia, Athens, GA 30602, U.S.A. (Received 7 April. 1982; accepted 4 June 1982)
Abstract--In the presence of a variety of organic solutes, mercuric chloride is reduced to mercurous chloride and HC1 in aqueous solutions. G(HC1) values in dilute solutions (0.001 M HgC12 and 0.01 M organic) for a series of alcohols, acetaldehyde, acetone and acetonitrile lie within the approximate range of 3-6 molecules per 100 eV. The value depends upon the reactivity toward HgCI2 of the radical product formed by reaction of OH with organic component. The results show a decreasing reactivity as the radical site is farther displaced from the a-carbon atom. At high concentrations (0.1 M HgC12 and 1 M organic) chain reactions occur in most of the systems with G(HCI) values increasing with decreasing dose rate. Evidence is presented for an involvement of colloidal Hg2C12 in the reaction chains.
INTRODUCTION THE REDUCTION of mercuric chloride by UV light in the presence of oxalate ion is known as Eder's reaction and was the subject of extensive investigation through the 1930s.
and (4)
2 HgC1 ~ Hg2C12
occur with rate constants of 4.0x 101°, 1.0x 101° and 8 x 109M -1 s -a, respectively. In steady gamma radiolysis, however, reduction yields are low (0.02-< G(Hg2CI2)< 0.2) and dose dependent. This was attributed in part to oxidation of HgCI by OH
(1) hv
2 HgC12 4- C204 -2 --*Hg2C12 + 2 C1- + 2 CO2o
(5)
Cartledge Ct) summarized the results of these early studies and suggested that the oxalate radical ion, C204-, was the active reducing species in the initiation of a chain reaction. The first report of reduction of mercuric chloride by ionizing radiation was that by Roseveare t2) in 1930. Reaction (1) occurred upon absorption of X-radiation with an efficiency of approx. 6 x 105 reductions per ion pair formed. Subsequent experiments with this system were described by Rao. °-5) G-values for Hg2CI2 formation in aqueous mercuric chloride solutions containing oxalate were as high as 250 molecules per 100 eV absorbed. The high yield and reproducibility of the reaction suggested its use as a low level 3' ray dosimeter. Pulsed radiolysis studies with aqueous mercuric chloride have shown t*) that the reactions
and in part to an oxidation of Hg2CI2 of unspecified stoichiometry. In the presence of OH scavengers, reduction of HgC12 does occur upon gamma radiolysis with a G-value dependent upon whether or not the product of the OH scavenging reaction will reduce HgC12. With t-butanol, G(Hg2C12) approaches 2 at high alcohol concentrations (0.05 M) and in stlutions containing t-butanol and saturated with N20, the reduction of HgC12 is not observed. These results indicate that the radicals produced by the reaction of OH with t-butanol, c"
(2)
HgCI2 4- ea-q--->HgCI + CI-
(7)
(3)
H + HgCI2 --~HgC1 + H + + C1-
HgC1 + OH ~ HgC1 + + O H -
followed by (6)
HgCI + + CI- ~ HgCI2
OH + (CH3)3COH ~ (CH3)2COHt~H2, (CH3)3C0
RPC Vol. 20, No. 5-6---C
4- H 2 0 333
334
R. A. DELLA GUARDIA and FRANCIS J. JOHNSTON
do not reduce HgC12. With isopropyl alcohol as OH scavenger, G(Hg2CI2) was f o u n d t o be 3.1(G(HCI) = 6.2) corresponding to an apparently complete utilization of the radical yield from water in the reduction process. Evidently, the (CHD)2COH and, possiblyl (CHD)2CH0 radicals formed by reaction with OH do reduce HgC12. The reduction reaction is presumed to be an electron transfer process of the type
more than -+ 0.2 V, however, it does limit results of HgCI2 reduction studies to a relative comparison of radical reactivities. This report describes the results of experiments involving a variety of organic additives as sources of free radicals for possible reduction of HgCI2. In addition, a chain reaction that is observed at high organic and HgC12 concentrations is described and evidence presented for this being a heterogeneous process.
(8) (CHD)2COH + HgCI2 ~ (CHD)2CO + HgC1 + HC1 The analogous reaction of HgC12 with ethanol radicals has been shown 6 to be (9) CHDCHOH + HgCl2-, CHDCHO + HgCl + HC1. We wished to pursue the use of the HgC12 reduction as a possible additional discriminator with regard to the oxidation-reduction behavior of radicals (8) formed in irradiated aqueous-organic systems: The free energy change for the standard reduction reaction
(10) 1/2H2(g) + HgCl2(aq)--* HgCl(aq) + HCl(aq) is not known, however, it can be approximated from known free energies of formation (9) and an estimated AG~ for HgCl(aq). AG~ for HgCl(g) is 15.0 kcal/mole. With no other basis for estimation, AG ° for (11)
H g C l ( g ) - , HgCl(aq)
was taken as a mean of the corresponding values for Hg and HgCI2. F o r Hg, this is 1.8 kcal mole -~. To obtain the value for HgC12, AG~ for HgCl2(g) was required. This was evaluated from AG~ for HgCl2(s), - 4 2 . 7 kcal mole -~, and the free energy of vaporization, ~° 9.27 kcal mole -~. With AG~ for H g C l 2 ( a q ) = - 4 1 . 4 kcal mole -~ and AG~ for HgC12(g) = - 3 3 . 4 kcal mole -~, the standard free e n e r g y change for HgCI2 undergoing the reaction analagous to 10 is - 7.0 kcal mole -t. The estimated value of AG ° for reaction (10) is - 2.6 kcal mole -t. AG~ for HgCl(aq), based upon this approximation, is 12.4 kcal mole -~. AG ° f o r r e a c t i o n (10) becomes 22.4 kcal mole -~ with the corresponding reduction potential calculated to b e - 0 . 9 7 V, The uncertainty in this result is difficult to assess. That introduced by our approximation for reaction 11 can hardly be
EXPERIMENTAL The reduction reaction produces a strong electrolyte, HC1, from covalent mercuric chloride and is conveniently followed by measuring the specific conductivity of the irradiated solutions as a function of absorbed dose. A conductance-radiolysis cell of approx. 25 ml in volume was used and measurements were made with a Beckman RC-18A conductivity bridge at 25.0°C. The value of the cell constant was frequently checked with standard KC1. Reactant solutions were introduced into the cell and thoroughly flushed with N2, Ar or N20 before radiolysis. Irradiations were carried out in a Gammacell 220 in which the unshielded dose rate, as measured by Fe 2+Fe 3+ dosimetry, was 0.35-0.50x 1016 eV m1-1 over the course of the experiments. Radiolyses at reduced dose rates were carried out using lead shields and in another Gammacell in the Division of Biological Science. Reagent Grade or equivalent chemicals were used in the experiments without further treatment and triply distilled water was used in the preparation of all the solutions.
RESULTS AND DISCUSSION In the presence of a variety of organic additives, HgCl2 undergoes a radiation-induced reduction in aqueous, ~ oxygen-free systems at a reproducible rate. The overall reaction may be represented as (12) HgC12 + Red,~H20---> 1/2Hg2C12 + HC1 + Ox. Red and Ox are reduced and oxidized forms of the organic additive. The conductometric technique has the distinct advantages of convenience and sensitivity for following the reaction in the very early stages. Previously used gravimetric determination of Hg2Cl2 limited measurements to substantial extents of reaction. HCI yields in the radiolysis experiments were obtained from the measured specific conductances through the use of calibration curves constructed from conductivity measurements on HgC12 solutions in the aqueous-organic systems containing known quantities of added HCI. The
The reduction of HgCI2 in aqueous-organic systems rate of formation of HCI is (13)
d[HCll/dt =
(dK/dt)l(ddd[HCl])
dddt is the rate of increase in specific conductance of the irradiated system and dK/d(HCI) is obtained from the calibration plot for the particular HgC12 and organic concentration being studied. Within experimental variations, this latter ratio was 0.398 ohm -1 cm -~ M -1 for 0.001 M HgC12 and 0.01 M organic additive for all of the solutes studied. This may be compared with 0.423 ohm -~ cm -~ M -t (All000) measured in this work for 10-3M HC1 in 0.bl M ethanol at 25°C and in the absence of HgCI2. The difference is due to an involvement of HC1 in hydrolysis and complexation equilibria with HgC12. The most important of these are listed below along with values of the association constants at 25°C °1) (14) (15)
HgCI2+CI-~.-~HgC13- K = 10 HgCI3- + CI-,~-HgCL-2
K = 10°'~
(16)
HgOHC1 + HCI~HgC12 + H20
335
depend upon the reactivity toward HgCI2 of the radical formed by reaction of OH with the organic component. Figures 1 and 2 show typical experimental data. In Fig. 1, specific conductivity, as a function of time of irradiation is plotted for 0.001 M HgCI2 in 0.01 M acetone. The results are characterized by a short, apparent induction period followed by a
ooo~M H9CI~
% _
x
32C
'2
~ 24C ~J r"~16C to 0 h,~ 8C o
K = I09"s6. l
At higher HgC12. and organic concentrations, dK/d(HCD was markedly different from that in the absence of HgC12 and varied significantly from one organic component to another.
Time (s x I 0 "~1
FIG. 1. Specific conductivity-time of irradiation data for 0.001 M HgCI2in 0.01 M acetone and in an atmosphere of N2. The dose rate was 3.89 x 1015eV m1-1 S-l.
Radiolysis at low concentrations of HgCI2 and organic additive Under the usual initial experimental conditions (pH, 4.25; HgC12, 0.001 M and organic additive at 0.01 M), over 98% of the e~q react with HgC12. With increasing extent of reaction, the pH decreases and reaction of e~-q with H + with the formation of H becomes significant. Under these conditions, both of reactions (2) and (3) occur in the system. In systems containing acetone or acetaldehyde, with respective rate constants n for reaction with e~q of 5.3 x 109 and 3.5 x 109 M -~ s -1, there will be significant competition with HgCl2 for reaction with e2q. In dilute systems in which only e~q and H reduce HgC12 and with no other contribution or competition, a maximum value for G(HCI) of G(e~q)+ G ( H ) = 3.3 molecules per 100 eV would be expected. The rate constant for reaction of OH with HgCI2 has not been measured. It is assumed, however, that reaction by this mode does not compete with that involving the organic additives at 0.01 M and higher. The observed G(HCI) value will, then,
ISO
,o
% l
/
/
///
i.o ' 2 Io T i m e (S x 10-3)
3.0
FIG. 2. Initial specific conductivity-time of irradiation data for 0.001 M HgCI2 in (A) 0.01 M n-propanol (At), (B) 0.01 M n-butanol (N2), (C) 0.01M n-butanol (N20). The dose rate was 2.90X 1015eV ml-t s-t.
336
R. A. DELLA GUARDIA and FRANCIS J. JOHNSTON
linear increase in conductance with a limiting value corresponding to the stoichiometry of reaction (9). Representative data from other systems are shown in Fig. 2. G(HC1) values were obtained from these initial, linear portions of the plots. Results for the several systems studied are summarized in Table 1. Column 2 lists pseudofirst-order rate constants for the OH scavenging reactions at the solute concentrations used,
ethanol and isopropanol, the great majority of e~q, H and OH radicals that escape intra-track reaction and recombination reduce HgCI2 by reactions (2) or (3), or, by reaction (17), produce S radicals which reduce HgCI2. With ethanol as an example, the S species are the radicals CH~CHOH, CH2CH2OH and CH3CH2() formed in the ratios 1.0; 0.16; 0.03. (13) The typical reduction process is
(17)
CH3CHOH + HgC12~ CH3CHO + HgC1 + HCI. (18)
With rate constants of 107 s -I or greater, OH radicals are effectively scavenged by all of the alcohols used (a possible exception is t-butanol at 0.01 M). The third column lists the observed Gvalues for HC1 formation in N2 or At" saturated systems. With G(HC1) values of approx. 6 molecules per 100 eV, it is apparent that for methanol,
At the solute concentrations used in these experiments, some competition for e~-q and OH radicals that otherwise are consumed in intra-track reactions and recombinations must occur. Yields for total radical scavenging greater than 6 (14"t5) a r e possible and our results do not allow conclusions with regard to the effectiveness in the reduction
OH + S ~ S + H20.
TABLE 1. G(HC1) VALUES FOR THE REDUCTION OF 0.001 M HgCI2 IN AQUEOUS-ORGANIC SYSTEMS. UNLESS OTHERWISE INDICATED, THE ORGANIC COMPONENT IS 0.01 M Additive, S
klSl a
G(HCI) b
G(HcI)c
CH30H
1.2 x i07
5.93
5.83
CH3CH2OH
1.5 x 107
5.95
5-98
CH3CHHCH20H
7.7 x i07
5.52
4.81
CH3CHOHC ~
7.3 x i07
6.07
5.84
CH3(CH2)3OH
4.0 x I07
4.72
3.29
(CH3)3COH
5.2 x 106
2.55
0.75
(CH3)3COH(O.IM)
5.2 x 107
3.06
-
(CH3)3COH(I.0M)
5.2 × 108
3.17
5 x 106
6.11
6.hO
CH3CHO (CH3)2CO
1.2 x i06
3.06
0.84,
CH3CN
2.2 x 105
3.10
0.35
CH3COOH
2.0 ~ 105
1.8
0.32
p-dioxane
2.0 × I07
h.8
5.0
ethyl acetate
2.8 x 106
4.3
3.h
apseudo first-order rate constant for the reaction of OH with additive s. Values used for k were selected from reference 13. bN~ atmosphere CN20 atmosphere
The reduction of HgC12in aqueous-organic systems process of radicals formed in lowest yields from these alcohols. It is quite definite, however, that the major radical species formed by reaction 17 with these three alcohols, CH2OH, CH3CHOH and (CH3)2COH, are effective reducing agents for HgC12. For n-propanol, butanol and t-butanol, G(HC1) values are significantly below 6. Since OH scavenging by these alcohols is at least as effective as that by methanol, ethanol and isopropanol, it is evident that all of the products of the scavenging reaction are not completely effective in undergoing reactions analagous to 18. Using, as a basis for calculation, G-values for e~-q, H and OH that are characteristic of dilute solutions in the absence of track scavenging effects, it is possible to estimate the fractions of OH-produced radicals that do reduce HgClz. With G(e~q)= 2.7, G(OH)= 2.8 and G(H)=0.6 molecules per 100eV, and with f representing the fraction of OH-produced radicals that reduce HgC12, G(HC1) = 2.7 + 0.6 + 2.8f. For n-propanol, f = 0.79 and for n-butanol, 0.50. The results in N20 saturated solutions may be used for a similar estimate with a correction being made for an incomplete conversion of e~-qto OH. Using rate constants of 4 x 10~° and 8 x 109 M -~ s -~, for reaction of e~-q with HgC12 and N20, respectively and with corresponding concentrations of 0.001 and 0.024 M, it is estimated that approx. 83% of e~q is converted to OH. A similar estimation as before gives f =0.75 for n-propanol and 0.44 for nbutanol. Within the assumptions and limitations mentioned previously, these results are not inconsistent. Rate constant data °3) indicate that 54 and 40% of OH abstraction reactions occur at the a-hydroxy site in 1-propanol and in 1-butanol, respectively. Our results do not allow a quantitative correlation between reduction reactivity and location of the radical site at the a-hydroxy position, however, they strongly suggest a decreasing reactivity with positions farther removed from that position. For t-butanol the dominant species produced by reaction with OH is (CH3)2C(CH2)OH. It is evident that this radical does not reduce HgCI2 under our experimental conditions. Acetone and acetaldehyde compete with HgCI2 for reactions with e~-q and the expected reaction products are (CH3hCOH and CH3(~HOH, respectively. These are the same radicals as are formed by the reactions of OH with 2-propanol and ethanol and our results with the latter systems have shown them to be effective reducing agents for HgC12. With a pseudo-first-order rate constant of 1.2 x 106 s-1 at 0.01 M acetone for reaction with OH, most of these radicals will react with acetone
337
forming CH2COCH3 radicals. With G(HCI) -- 3.06 in a deaerated system and 0.8 in a N20 saturated system, our results indicate that this radical does not reduce HgC12. For 0.01 M acetaldehyde the pseudo-first-order rate constant for reaction with OH is approx. 5 x 106 s -1 and, as with acetone, most of the OH radicals will be utilized in reaction with the organic additive. The radical product, dominantly CH3(20, does effectively reduce HgC12. Acetonitrile will not compete with HgC12 for reaction with e~-q or H under the experimental conditions used (ke-= 2.5 x 107 M -~ s -~ and kn = 3.5 x 106 M -~ s-~).° ° The rate constant for reaction with OH is 2.2 × 10 7 M -~ s -~ which gives a borderline rate constant for scavenging of 2.2 x 105 s -~ at 0.01 M. The reaction product is the addition compound CH3C(OH) = lq 17and our results, G(HC1) = 3.1 in N2 and 0.4 in N20, indicate that it is not effective in reducing HgC12. Results with acetic acid, p-dioxane and ethyl acetate were mechanistically non-informative and are included without comment. In general, our results with the reduction reaction in these dilute aqueous systems are quite well explained within the framework of established aqueous radiation chemistry. Reactivity differences for HgCI2 reduction by several organic free radicals have been observed and these are consistent with expected behavior for these radicals. As reactant concentrations are increased, the behavior becomes more complex with reduction yields increasing and at 0.1 M HgCI2 and 1 M organic, chain reactions are observed in most of the system.
Reduction yields at high solute concentrations For 0.001 M HgC12, G(HC1) values are only slightly changed as the organic component is increased to 1 M. When both HgC12 and organic concentrations are increased, G(HC1) increases markedly for most of the systems studied. For example, with 0.001 M HgC12 and 1 M ethanol, G(HCI) = 6. With 0.01 M HgCl2 and 1 M ethanol, G(HC1) = 17. At still higher concentrations, G(HC1) becomes very large (103 molecules per 100 eV at 0.1 M HgCI2 and 2 M ethanol) and is dependent upon the dose rate. These are usual characteristics of a chain reaction. Table 2 summarizes results at the highest dose rates utilized for 0.10 M HgCl2 and 1 M organic additive. The complexity of the reaction system at such high reactant concentrations makes mechanistic speculation less meaningful. Reactions are occur-
338
R. A. DELLA GUARDIA and FRANCIS J. JOHNSTON TABLE
2.
G(HCI)
VALUES
FOR SOLUTIONS
CONTAINING
0. l M
HgCl,
AND
IM
ORGANIC
ADDITIVE
IN A N2
ATMOSPHERE
0r~anic
component
D(eV
ml-ls"1x10-15)
CH30H
3.27
63
CH3CH20H
3.75
h?
CH3CH2CH20H
3.27
37
(CH3)2CHOH
3.37
?h
CH3CHO
3.29
62
(CH3)3OOH
2.90
8.9
(CH3)2OO
2.90
3.9
ring within tracks and spurs and significant fractions of energy are being absorbed in the HgC12 and organic component. An interesting observation, however, is that chains do not occur in the acetone and t-butanol systems. These are the species that, in dilute solutions, formed radicals upon reaction with OH that did not reduce HgCI2. In the systems exhibiting chain behavior, G(HCI) values increased with decreasing dose rate. Figure 3 gives log-log plots of G(HCI) vs dose rate for 0.1 M HgC12 in 1 M acetaldehyde, methanol, 1-propanol and 2-propanol. In general, G(HCI) is proportional to D", where D is the dose rate and m has values of from - 0.23 to - 0.43. At the lowest dose rate, G(HC1) values of greater than 200 molecules pe r 100 eV were observed. As HgCI2 concentration s increased to 0.2 M and
A I~ 17 16 (.9 ~2.2,c 2 2C 19
-06 0 4 - 0 2
0
i
l
14020 ~
L9 t8 22
organic concentrations to 2 M, G(HCI) also increased. Only with ethanol, however, were experiments continued to even higher organic concentrations. In this case, an apparent maximum occurred at 8 M ethanol (250 molecules per 100 eV at 3.3 x I015 eV ml-' s-') with 0.2 M HgC12. An additional factor in the chain reacting systems became evident in experiments with 0.2 M HgC12 in 4 M ethanol solutions. When an irradiation series was interrupted for several hours and then continued, discontinuities were observed in the HCI production rate. Colloidal Hg2CI2 is produced in the reaction and only slowly coagulates. Upon standing, the colloidal material had precipitated and the solution became visually clear. Evidently, the chain reaction involves the surface of colloidal Hg2C12 particles. Figure 4 shows the results of 4 experiments at 0.2 M HgCI2 w
22 21
3
G(HCI) (molecules/lOOeV)
l
/
i
l
l
i
l
i
t
~
40 h L I l t
30
21
ZO 1.9 t8 0 OZ 04 06 -06-014 0'2 log [Dose Rate]
,20
Time (see)
0 0'2 04 06
FIG. 3. Log-log plots of G(HCI) as a function of dose rate for 0.10M HgCI2 in (A) I M n-propanol, (B) I M 2-propanol, (C) I M methanol and (D) I M acetaldehyde.
FIG. 4. Specific conductivity-time of irradiation data for several 0 . 2 M H g C l r - 4 M ethanol systems. The discontinuities indicate points at which the radiolyses were interrupted and the conductance-radiolysis cell allowed
to stand overnight before continuing the irradiations. Dur~ing this time, the colloidal HgzCl~ coagulated and settled out of solution. For clarity, the points for each series are displaced 10 sec along the time axis. The dose rate was 3.75 x 101~eV ml-~ s-].
The reduction of HgC12 in aqueous-organic systems and 4 M ethanol. After 40, 52, 52 and 65 sec of irradiation, respectively, each reaction system was allowed to stand for 12-18 hr before continuing the radiolysis. In each case, the Hg2CI2 had completely precipitated and a discontinuity in the reaction rate was observed. A rate was soon reestablished, however, that was, within experimental variations, the same as before interruption. Figure 5 shows the effect of a series of such interruptions in one experiment. Results for a continuously (except for measurement) irradiated sample are shown for comparison. Evidently, the chain reduction in HgC12-ethanol system does involve colloidal HgjC12. Electron transfer processes at the surface of colloidal gold and silver °8'~9~ have been described earlier, however, no such involvement of HgzCl2 has been reported. In considering possible chain propagating steps, a role for HgCl(aq) must be considered. Based upon our estimation of 12.4 kcal/mole as the standard free energy of formation for HgCl(aq), AG ° for the reaction (19) HgCl(aq) + 1/2Hj(g) ~ Hg(aq) + HCl(aq) is - 3 4 . 4 kcal corresponding to a standard potential of - 1.49 V. Based upon tabulated free energy data and the reduction potential (2°) of - 1 . 1 V for (20) CH3CHO(aq) + 1/2H2(g)--* CHj(~HOH(aq)
~o iz0 .~._x
A
c
¢-~ 0 8 0
#" 0(~
I
I 20
I
I 40
I Time
I 60
1
I 80
I
[ 100
I 120
(sec)
Fro. 5. Specific conductivity-time of irradiation data for 0.2M HgC12 in 4.0M ethanol. Curve A represents a sample subjected to continuous (except for times of measuremen0 irradiation. Points B, C, D and E on the curve for a second sample represent times at which the radiolysis was interrupted for overnight (B, D and E), and for 4 hr (C), to allow coagulation of the colloidal HgjC12. The dose rate was 3.75 x 1015eV m1-1 s-1.
339
and a ~ G ° for the reaction (21) CHjCH2OH(aq) ~ CHjCHO(aq) + 1/2H2 of 9.7 kcal, (2.) the standard free energy change for
(22) CH3CHjOH(aq) + HgCl(aq) --*Hg(aq) ÷ HCl(aq) + CHjCHOH(aq) is positive by 1 kcal. Within the uncertainties of the free energy estimations and considering a possible role for adsorption at the colloid surface in altering the energetics, the occurrence of reaction (2:) must be considered as a possible chain propagation step. The Hg formed in the reaction would react rapidly with HgC12 (23)
Hg(aq) + HgC12(aq) ~ Hg2Cl2(s). '
Some support for reactions, such as (22); in which HgCI plays the role of propagating the chain through reactions with t h e organic additive is found in the observation that chains do not occur when the additive does not contain an a-hydrogen atom. Radical formation for these species would be much more endoenergetic than (22) or analogous reactions in the s y s t e m s where chains are observed. Other possible chain propagating steps would include, of course, reactions b y intermediates produced in tracks and spurs at the high solute concentrations in the chain reacting systems.
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R. A. DELLA GUARDIA and FRANCIS J. JOHNSTON
13. FARHATAZIZand A. Ross, Nat. Stand. Ref. Data Ser., NBS No. 59 (1977). 14. T. I. BALKAS, J. H. FENDLER and R. H. SCHULER, J. Phys. Chem. 1970, 74, 4497. 15. R. H. SCHULER, A. L. HARTZELL and B. BEHAR, J. Phys. Chem. 1980, 85, 192. 16. M. ANBAR, FARHATAZIZ and A. Ross, Nat. Stand. Ref. Data Ser., NBS No. 51 (1975). 17. J. W. T. SPINKS and R. J. WOODS, An Introduction to
Radiation Chemistry, 2nd Edn, p. 330. Wiley, New York, 1976. 18. A. HENGLEIN, ./'. Phys. Chem. 1979, 83, 2209. 19. D. MEISEL, J. Am. Chem. Soc. 1979, 101, 6133. 20. M. BREITENKAMP, A. HENt3LEIN and J. LILIE, Bet. Bunsenges. physik. Chem. 1976, 80, 973. 21. W. M. CLARK, Oxidation-Reduction Potentials of Organic Systems. Williams and Wilkens, Maryland, 1960.