Radiolysis of methanol and methanolic solutions. V. The acid effect

V. The acid effect. K. Suryanarayanan, and Norman N. Lichtin. J. Phys. Chem. , 1969, 73 (5), pp 1384–1391. DOI: 10.1021/j100725a036. Publication Dat...
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K. SURYANARAYANAN AND NORMAE N. LICHTIN

1384

The Radiolysis of Methanol and Methanolic Solutions. V. The Acid EffecW by K. Suryanarayanan and Norman N. Lichtin Departments of Chemistry, Brookhaven National Laboratory, Upton, New York Boston, Massachusetts 0 2 2 1 5 (Received October 1 , 1 9 6 8 )

11973,

and Boston University,

Extensive measurements of the effect of acid on yields of products from the W o y radiolysis of liquid methanol confirm previously reported enhancement of radiolytic yields. Product enhancement is discernible with apM acid and is fully developed with M acid: AG (H2) = 0.9; AG (CH4) = 0.0; proximately AG (CH20) = 0.3; AG (HOCH2CH20H) = 0.7. Measurements of G (Hz) as a function of solute concentrations in the presence of 3.76 X 10-2 M acid plus varying ( 0 2 ) , varying ( 0 2 ) in neutral solution, and varying (acid) plus lo-’ M ( 0 2 ) provide the basis for evaluation of the rate ratios k H + O J k H + C H n O H = 5.8 =t 0.7 x 10’ and “kea-+oz/kea-+~+” = 0.36 f 0.11 and of the yields (molecules/100 eV) G H = ~ 1.81 f 0.27, GH = 2.02 f 0.18, GH GeB- = 3.96 f 0.27, and Gea- = 1.45 f 0.30. Repeated pulsing with 1.9-MeV electrons up to a total dose of lozoeV ml-1 has no significant effect on the lifetime of the solvated electron nor does preirradiation with a dose of 1.5 X 1019eV of Gocoyrays followed by rectification: k&ay = 2.3 X lo5 sec-’. Possible explanations of the enhancement of yields by acid which are examined include competition of acid for the freely diffusing solvated electron with adventitious solutes or radiolytic products and interception by acid of various transient radiolytic species other than the solvated electron.

+

Introduction Acid, which converts solvated electrons into hydrogen atoms, and 02,which competes with methanol for both solvated electrons and hydrogen atoms, are convenient probes of these species in neat liquid methanol. Both reagents have been used for this purpose previously, oxygen in at least three s t ~ d i e s ~and - ~ acid in a number of investigations.6-12 Although prior workers agree that O2 reduces G(H2), agreement as to the quantitative aspects of this effect is p o ~ r . ~Similarly, ,~ several workers have reported enhancement of yields of several products by sulfuric acid but there is disagreement as to the magnitude and concentration dependence of this effect. A basic unresolved question is whether acid exercises its influence a t concentrations so low9 that only bulk solution interaction can be involved or is effective only a t concentrations so high7JJ2 that the question of intervention in spurs is raised. The research reported here was directed toward exploring the effect of added acid on product yields more accurately and more extensively. Quantitative interpretation of the data in the format of competition kinetics was made possible by determining G(H2) in the presence of acid and oxygen as a function of their concentrations. Interpretation of yield data was further refined with the aid of kinetic information derived from pulse experiments. Experimental Section Materials. Methanol was Eastman Spectral quality. Samples were taken for no more than 3 days after opening a bottle and were not purified. Analysis of a limited number of samples by a modificationl8of the chromotropic acid method of Bricker and Johnson14 revealed less than 5 X 10-6 M formaldehyde in all The Journal of Physical Chemistry

cases. The ultraviolet absorption of one sample, measured in a Cary 14 spectrophotometer with a 10-cm cell os. air, diminished smoothly with increaaing wavelength, presumably due to end absorption of the peak in the vacuum uv;15 E = 3.6 X 104 M-l cm-l a t 235 at 270, and -5 X lo4 a t 350. No nm, 4 X features ascribable to impurities were detectable. Analysis of occasional samples by gas chromatography on di-n-decyl phthalate a t 125’ in an F & M Model 300 instrument with a Model 1609 flame ionization detector revealed no impurities up to a retention time of 48 min. It is estimated that the procedure em(1) Research carried out under the auspices of the U. 8. Atomic Energy Commission, in part under Contract AT (30-1)2383. This report is based on a dissertation which was submitted by K. Suryanarayanan in July 1968 in partial fulfillment of the requirements for the Ph.D. degree. (2) For paper I V in this series, c f . N. N. Lichtin and J. W. Wilson, J . Phys. Chem., 69, 3673 (1965). (3) S. U . Choi and N. N. Lichtin, J. Amer. Chem. Soc.. 86, 3948 (1964). (4) E. Hayon and J. J. Weiss, J . Chem. SOC.,3970 (1961). (5) A. Habersbergerova, I. Janovsky, and J. Teply, Report No. 1457 of the Nuclear Research Institute a t fie;, Prague, Czechoslovakia, 1966. (6) G. E. Adams and J. H. Baxendale, J . Amer. Chem. SOC.,80, 4125 (1958). (7) J. H. Baxendale and F. W. Mellows, ibid., 83, 4720 (1961). (8) H. Seki and M. Imamura, J . Phys. Chem.. 7 1 , 870 (1967). (9) W. V. Sherman, ibid., 7 1 , 4245 (1967). (10) H. Matsuoka. K. Sato, T . Masuda, and M. Kondo, Nippon Kagaku Zasshi, 8 7 , 1320 (1966). (11) G. V. Buxton, F. 9. Dainton. and M. Hammerli, Trans. Faraday SOC.,63, 1192 (1967). (12) (a) I. A. Taub, D. A. Harter. M. C. Sauer. and L. M. Dorfman, J . Chem. Phys., 4 1 , 979 (1964);(b) K. N . Jha and G. R. Freeman, ibid., 4 8 , 5480 (1968). (13) N. N. Lichtin, J. Phys. Chem., 63, 1449 (1959). (14) C. E. Bricker and H. R. Johnson, Ind. Eng. Chem., Anal. Ed, 17, 400 (1945). (15) J. L. Weeks, G. M. A. C. Meaburn, and 8. Gordon, Radiat. Res., 19, 365 (1963).

RADIOLYSIS OF METHANOL AND METHANOLIC SOLUTIONS ployed could detect 2.5 X M of an impurity with a response similar to that of methanol. The following reagents were used in experiments of group A (cf. below): Lehigh Valley Chemical Co. ACS reagent sulfuric acid, Matheson Coleman and Bell 37% US!? formaldehyde, Eastman practical grade chromotropic acid, Matheson prepurified hydrogen, and Matheson C P methane. Reagents used in experiments of group B were from the same sources except as follows. Sulfuric acid was Baker and Adamson CP grade. Perchloric acid was Eastman 72%. Ethylene glycol was Eastman White Label while propylene glycol was a Matheson Coleman and Bell product. Hydrogen and argon were Liquid Carbonic pure and Air Products ultrahigh purity grades, respectively. Nitrogen and oxygen were Matheson prepurified and extra dry grades, respectively. Irradiation and Dosimetry. Steady irradiations were carried out in Schwarz-Allen type sources16 a t dose rates in the vicinity of 1017 eV ml-l min-’ a t temperatures in the vicinity of 23’. Source intensities were determined with the aerated 0.8 N sulfuric acid, millimolar ferrous ammonium sulfate dosimeter17 taking the molar absorbancy index of FelI1 a t 305 nm as 2195 a t 25’ with a positive temperature dependence of 0.701, per degree. G(FelI1) was assumed to be 15.6 and dose rates in methanol were calculated by multiplying those in the dosimeter by 0.783, the ratio of electron densities. Optical densities were measured with a Cary Model 14 spectrophotometer for experiments of group A and with a Beckman DU for experiments of group B. I n experiments of group A samples were irradiated in Pyrex ampoules which had been sealed under autogenous pressure a t -78’. Free volumes were about 10% of liquid volumes. I n experiments of group B samples were irradiated in completely filled 10-ml Clay and Adams “Golden Seal” or 20-ml B-D Yale syringes closed with ground-glass caps. Pulse experiments employed 1.90- or 1.95-MeV electrons from a Van de Graaff accelerator. Pulse lengths were 6, 8, or 16 psec. Integrated pulse current was in the range 2-3.5 X lo-* C. Accelerator and monitoring equipment have been described.lg Oscillographic traces were photographically enlarged to facilitate their measurement. Deaeration and Preparation of Solutions for Irradiation. Experiments of group A employed methanol which was degassed by pumping the liquid a t -78’ with a diffusion pump for 15-20 min, distilling the methanol at autogeneous pressure into a trap a t -78’, and repeating the pumping cycle before distilling the methanol into irradiation cells. Sulfuric acid solutions were prepared by distilling degassed methanol into cells containing known amounts of degassed methanolic sulfuric acid and pumping the resulting solutions a t -78’.

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I n experiments of group B deaeration was accomplished by rapid sweeping for 30 min with argon which had been passed through silica gel and then presaturated with methanol vapor. A sintered-glass disk dispersed the argon through a reservoir which was attached to three 10-ml syringes via Teflon stopcocks and groundglass joints. Each syringe was prerinsed with several aliquots of deaerated liquid and filled by suitable manipulation of the argon pressure by means of a stopcock a t the top of the reservoir. Solutions of sulfuric or perchloric acid were prepared by serial dilution prior to deaerating. Solutions of hydrogen (for calibration of its determination) were prepared by dispersing hydrogen through the sintered disk a t measured (atmospheric) pressure and temperature and calculating the concentration from its knowdBsolubility. Solutions of oxygen were prepared similarly by dispersing pure 0 2 or its mixtures of known composition with argon or nitrogen. Concentrations were calculated19 as for hydrogen. Methanol employed in pulse experiments was deaerated as described for group B experiments. It was transferred from a syringe to the irradiation cell and stored there under an argon atmosphere. Determination of Products in Group A Experiments. Formaldehyde was determined by a modificationla of the chromotropic acid method.14 A linear calibration plot was obtained with aqueous solutions prepared by serial dilution of an analyzedz0stock solution. Optical densities were measured a t 570 nm with a Bausch and Lomb Spectronic 505 spectrophotometer. Hydrogen and methane were determined together by in-line gas chromatography employing an 8-ft X +-in. i.d. column of Linde 5A Molecular Sieve and a Gow-Mac thermal conductivity detector. The gaseous products were stripped by automatic Toepler pumping while the methanol underwent distillation a t room temperature from the irradiation cell into a trap maintained a t -78” which was separated from the Toepler pump by a second trap maintained a t -78’. Nitrogen, flowing a t 20 cma min-’ was used to sweep the total gaseous product of each experiment through the gas chromatograph. Retention times were 5 min for hydrogen and 44 min for methane. Area factors were measured independently for each product determination with known amounts of hydrogen and methane. Peak areas were measured with a planimeter. Determination of Products in Group B Experiments. (16) H.A. Schware and A. 0.Allen, Nucleonics, 12, 58 (1954). (17) Cf. A. 0. Allen, “The Radiation Chemistry of Water and Aqueous Solutions,” D. Van Nostrand C o . , Inc., Princeton, N . J . . 1961, pp 20-22. (18) R. J. Hagemann and €1. A. Schwarz, J. P h y s . Chem., 71, 2694 (1967). (19) “Landolt-Btirnstein Zahlenwerte und Funktionen,” 6th ed, Book 11, Part 2, Springer-Terlag, Berlin, 1962, pp. 1-70 for H z and pp 1-75 for 0 2 . (20) “Scott’s Standard Methods of Chemical Analysis,” 5th ed, D. Van Nostrand C o . , Inc., Princeton, N. J.. 1939, p 2149. Volume YS, Number 6 May 106Q

1386 Formaldehyde was determined as described above for group A experiments except that optical densities were measured with a Beckman DU spectrophotometer for both calibration and determinations of radiolytic products. Ethylene glycol was determined by a gas chromatographic method using propylene glycol as internal standard which was developed during the course of this work. Accurately known amounts of 6.64 X 10-4M methanolic propylene glycol, chosen to match the anticipated yields of ethylene glycol, were added to precisely measured volumes of irradiated methanol. The resulting solutions were concentrated 15- to 20-fold by boiling off the solvent. Then 15- or 20-pl aliquots of concentrate were transferred by injection to an F & M Model 300 gas chromatograph equipped with a Model 1609 flame-ionization unit and an 8-ft X $-in. i.d. column of 10% di-n-decyl phthalate on F & M Haloport F which had been preconditioned by heating a t 150’ for 1 hr while passing helium. Helium carrier gas flowed a t 45 cm3 min-l during analyses. The column, injection port, and detector temperatures were 125, 150, and 120°, respectively. Hydrogen and air flow rates to the detector were 45 and 430 cm3min-l, respectively. Cleanly separated sharp peaks were obtained for the two glycols. Peak heights were used as measures of amounts. Response ratios were determined independently for each set of analyses using aliquots of analyzedz1 samples of the glycols. The ratio of response to ethylene glycol: response to propylene glycol always fell in the range 0.72-0.75. Hydrogen was determined by gas chromatography a t room temperature on a 6-ft X $-in. i.d. column of 30-60 mesh Molecular Sieve 5A in a Model 154 PerkinElmer vapor fractometer with a thermistor detector. Flow rate of carrier argon was 10 cm3 min-l. Area factors were determined independently for every experiment with known amounts of hydrogen. Peak areas were determined by weighing cut-out portions of Xerox copies of the traces. The procedures for transfer of aliquots and stripping of the solvent were essentially as described previously.22~2sThe carrier gas was freed of methanol vapor by passage through silica gel before entering the column.

Data Product Yields a5 a Function of Acid Concentration. The stability of methanolic sulfuric acid was shown by observing that the electrical conductivity of a degassed 0.28 M solution was constant for 4 days. This agrees with earlier measurements24 in the concentration range 2 X lou3 to 6 x 10-5 M which also establish essentially complete monoprotonic dissociation of the acid. The equilibrium concentration of CHaOH2+ in neutral methanol is taken as approximately 5 X 10-QM on the basis of the reported25 autoprotolysis constant of the solvent, 2.2 X lo-’’. The Journal of Physical Chemistry

K.

NORMAN N. LICHTIN

SURYANABAYANAN AND

‘P 470 I

e

7

8

1

I

1

4

-1OQ

I

a

I

I

0

I

(HeSO,,

Figure 1. U(H2)as a function of molar concentration of H2804: experiments of group A, 9; experiments of group B, solid circles with vertical lines. Ranges of standard deviations of the mean of replicate measurements are represented by vertical lines.

Values of G(H,) are plotted in Figure 1. Each point obtained by method A represents the results of between 4 and 11 independent experiments while each point based on measurements by method B summarizes 2 to 4 independent experiments. Doses were (4.958.25) X lo1*eV ml-l in experiments of group A and (0.f344.73) X lo1*eV ml-1 in experiments of type B. No dose effects were discerned over the latter range. The two sets of data are in reasonable agreement. G(H2) is increased by acid from a value of 4.91 in neutral methanol to 5.8. Most of this increase occurs at acid concentrations between lob6 and loM8M . This profile of dependence of G(H2) on concentration of sulfuric acid is in good agreement with the more limited recent data of Shermang but not with those of Baxendale and Mellows,’ of Seki and Imamura,* or of Jha and Freeman.12b The last workers report similar increases in G(H2) but a t lo-fold’ or 100-fold8Jzb greater concentrations of acid. Yields of methane were measured only by method A. Three to five independent experiments were performed with each of the solutions, 3 X 10-1 M , 10-1 M , M, and M HzS04, and in neutral methanol. The corresponding mean values of G(CH4) and their standard deviations are 0.49 f 0.014, 0.52 f 0.023, 0.51 f 0.033, 0.50 f 0.027, and 0.50 f 0.005, respectively. Thus acid does not appear to affect G(CH4). The absence of such an effect is indicated by earlier although agreement between these (21) Analyses were performed by J. K. Rowley: cf. I. M . Kolthoff and R. Belcher, “Volumetric Analysis,” Vol. 111, Interscience Publishers, New York, N. Y., 1957, p 490,for the analytical procedure. (22) J. W. Swinnerton, V. J. Linnenbom, and C. H. Cheek, Anal. Chem., 34, 483, 1509 (1962). (23) W.A. Seddon and A. 0. Allen, J. Phys. Chem., 71, 1914 (1967). (24) E. W. Kanning. E. G. Bobalek, and J. B . Bryne, J. Amer. Chem. Soc., 6 5 , 1111 (1943). (25) R. W. Gurney, “Ionic Processes in Solution,” McGraw-Hill Book Co., Inc., New York, N. Y., 1953, p 233. ’

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RADIOLYSIS OF METHANOL AND METHANOLIC SOLUTIONS reports and the present one as to the absolute value of G(CH4) is not good. Ethylene glycol was determined in experiments of group A by a methodla involving periodate oxidation followed by chromotropic acid estimation of formaldehyde, The reliability of these determinations was, however, not sufficient to define the small change in G(glyco1). Only results obtained by method B are therefore presented in Figure 2. Each point represents the average of independent analyses of multiple samples from a single irradiation. Each analysis involved the injection into the gas chromatography apparatus of several aliquots. The average standard deviation of a single analysis from such a set was 3%. Thus G(glyco1) increases from a value of 2.66 f 0.02 in neutral methanol to a plateau value of about 3.4. Increase in G(glyco1) reaches half its maximum value at about 3 X lo-* M H2S04. The observed effect of acid is in disagreement with the apparent lack of dependence of G(glyco1) on acid reported7 by Baxendale and Mellows. An increase of G(glyco1) with acid is a t least suggested by other data which are, however, limited to 10-2 MI3 and 2 X 10-1 21P acid. The effect of acid on formaldehyde yields is also summarized in Figure 2. Each data point of group A represents the average of two to five independent experiments. The product of each irradiation was usually determined in duplicate. Each data point of group B was obtained by duplicate determination of the product from a single irradiation. The two sets of data agree well in defining G(CH20) in neutral methanol as 2.12 f 0.03. Agreement in acidified solutions is less satisfactory but leaves no doubt that G(CH20) is increased by acid and that the effect is discernible M acid. The limiting yield a t with less than higher concentrations of H2S04is approximately 2.45. A similar degree of enhancement of G(CH20) by acid has been observed previously7Ja with M acid. Another papers reports no enhancement with 3.6

I

I

I

1

I

0 '

I

2 X 10-3 M acid and an increase by about 1.2 molecules/100 eV with 2 X lo-* M acid. No attempt was made to determine the acid dependence of G(C0) because of its small magnitude (about 0.1) in neutral solution. A negligible effect by N H2S04 has been r e p ~ r t e d . ~Material balance in the measured limiting enhancements of yields is within experimental error: AG (H2) lim AG (CH4) lim = 0.9 while AG(CH20)iim AG(glycol)iim= 1.0. G(H2) as a Monitor of Competition Kinetics with Acid and/or O2 as Scavengers. The kinetic analysis which is presented below is predicated on two assumptions. One is that all processes relevant to the competition between acid and oxygen are represented by eq 1 4 . The possibility that this assumption is an

+

kz

e,-

2.4

+ CH30H2+--+H + CH30H

(2)

k4

H

+ CHBOH --+Ha + CH20I-I

(4) oversimplification is considered in the Discussion section and the consequences of this eventuality are inspected. The other assumption is that consumption of O2 can be treated as negligible in kinetic analysis. The largest ratio of dose to initial ( 0 2 )employed , in an experiment involving 3.76 X M acid, was 2.54 X lOI8 eV rnl-'/1.79 X M 02. If G(-OZ) is taken as 9, a value which can be extrapolated from data reported4 for radiolysis of neutral methanol at various dose rates, 21% of the oxygen was consumed in this experiment. Acid may reduce consumption of O2 because methanol competes more efficiently with oxygen for hydrogen atoms than for electrons. However, it is not known whether acid intervenes in other ways to affect the consumption of 02. For all other competition experiments reduction in the concentration of 0 2 , calculated in this way, did not exceed 14%. No dependence on dose is apparent in the kinetic analysis which is given below. Table I summarizes relevant data. Equation 5 is readily derived from eq 14. If kl/k2 is taken to be

+ +~(O~)/~(CHIOH))] XCGH + Ges-/(l + ki(Oz)/kz(CH3OH2+))1 (5)

G(H2) P

+

-

;I

+

5

b

4

A

1

-log fH&O,)

Figure 2. G(ethy1ene glycol), 0 ;G(CH20), solid circleswith vertical lines (group A) and (group B). Vertical lines represent range of standard deviation of the mean.

i,

=

G H ~ [1/(1

equal to 0.5,12 then [ ~ I ( O ~ ) / ~ ~ ( C H ~ OIH0.11 ~+)] for the data of Table I with (acid) = 3.76 X 10-3 M . ( I t should be noted that some of these data were obtained using HClOl rather than HZSO,.) Neglecting this term and assuming that Go(H2) = Gs2 GH Ge,-, where GO(Hz) is the hydrogen yield in the absence of 0 2 , yields eq 6, where AG = Go(H2) - G(H2).

+ +

Volume 78, Number 6 May 1969

1388

K. SURYANARAYANAN AND NORMAN N. LICHTIN

Table I: Dependence of G(€12) on 108

x

(02)-

M

0 1.016

1.79 2.82 4.33 8.55 0.95 1.79 4.33 8.55 8.55 8.55 8.55 1.0 1.0 1.0 1.0 1.0

x

(02)

and (Acid)

Table 11: Quantities Derived from Kinetic Analysis

(acid)P M

G(Hz), molecules/ 100 evi

nk

37.6 37.6 37.6 37.6 37.6 37.6 None None None None None None None 0.0752 0.132 0.188 0.752 3.76

5.77 f 0.01b 5.03 f 0.06d 4.64 f 0.08b 4.18 f 0. 15d 3.85 fO . l O b 3.13 f 0. 14b 3.38 f 0.046 3.19 f0.04' 2.82 & 0.05e 2.42 f 0.06e 2.54g 2 . 46h 2.51i 3.66 f 0.028 3.82 f 0.056 3.77 i0.05" 4.25 f 0.Oge 4.55 f 0.12e

3 2 8 3 4 8 2 4 6 3 1 1 1 2 3 2 3 4

loa

Quantity ks/k4

+

GH GenGH2b kslkr

Gab kllh @es-

Magnitudea

(5.55 f0.56) X loac 3.96 f0.276 1.81 f 0.276 (6.01 f 0.90) X 108 2.02 iO . B d 0.36 f 0.116 1.45 f 0.306

* Uncertainties are calculated from standard deviations of the slopes and intercepts of the appropriate linear relationships. Molecules/100 eV. From data for 3.76 X 10-8 M acid and variable (02).d From data for neutral solution and variable (02). From data for 10-8 M ( 0 2 ) and variable acid.

obtain eq 8. Least-squares analysis of the data of

1/[G(H2)

Concn of added HzSO4 unless otherwise stated. Dose = (25.5 f 0.1) X 1017 eV/ml. c Concn of added HC104. Dose = 8.7 X 10'7 eV/ml. Dose = 8.8 X 101' eV/ml. f Dose = 12.7 X l O I 7 eV/ml. g Dose = 7.6 x lO17eV/ml. hDose = 11.4 X 1017eV/ml. Dose = 6.4 X 1017 eV/ml. f Uncertainties are standard deviations of the results of a single irradiation, k Number of independent irradiations.

=

- 3.451 [1.23/Ge8-][1

0

+ k1(02)/k2(CH30Hz+)]

(8)

Table I for individual experiments with 1 X M 0 2 and variable (acid) in the form of eq 8 provided the constants presented in Table 11. The absence of systematic deviations from linear relationships was shown by plotting the appropriate data in the form of eq 6, 7, and 8. Two values of GeB- are available from the data. Least-squares analyses of the data for individual exOne of these, 1.94 f 0.45, is obtained by subtracting l/AG = [ ~ / ( G H + GeB-)][l J C ~ ( C H ~ O H ) / ~ ~ ((6) O Z ) ] GH from G H Ges-, Since our determination of G H employs the value of G H a based on data for 3.76 X periments with (acid) = 3.76 X 10-3 14 and variable low3M acid solutions, this value of Ge8- depends on (02)in the form of eq 6 by means of an IBM 360 knowledge of three derived quantities from two indeModel 40 digital computer gave the values of GH pendent sets of data. The other value of G,,-, 1.45 f Ge8- and k4( CH30H)/ k g( 02) presented in Table 11. 0.30, is also determined with the aid of three derived Subtraction of the former from Go(H2) gives G H = ~ quantities, two of which, G H and G H ~are , also involved 1.81. in calculation of the first value. It is deduced from M solvated I n neutral methanol with ( 0 2 ) 2 three independent sets of data. Under these cirelectrons should not contribute to the yield of H2. cumstances, agreement between the two values of This follows from the fact that kz(CH30H2+),estiGe8can be regarded as satisfactory. The fundamatedl2Sz6to be 2 X lo2 sec-l, and the measured firstorder decay rate of the solvated electron, 2 X lo6 mental validity of eq 8 is, however, examined in the Discussion section. sec-I (cf. below) , are both small compared to kt2(0,) 2 Previously reported values of G,,-, GH, and G H Z from 1.9 X lo7 sec-I. Under these conditions, eq 7 is aplow LET radiolysis of liquid methanol are assembled in plicable. Least-squares analysis of the data for inTable 111. These values are compared with the dividual experiments involving neutral methanol in data of Table I1 in the Discussion section. The present 1/[G(H2) - GHJ = (1/G~)[1 k3(0z)/~r(CHsOH)I value of kl/kz, 0.36 f 0.11, agrees to well within experimental error with the value 0.49 f 0.22, available from (7) pulse data,12 which was employed in arriving a t eq 5 . the form of eq 7, taking G H = ~ 1.81, provided the The ratio k3/k4 does not appear to have been measured values of G H and k3/k4 presented in Table 11. previously in methanol. Its value in aqueous solution, Equation 5 , taken in conjunction with hydrogen 2.4 X lo4, can, however, be estimated from available yields obtained in the presence of M 0 2 and data.26j27The data of Table I for neutral methanol do varying low concentrations of acid was used to evaluate not differ greatly from limited information provided by Ge8- and kl/kz. For this computation the previously one previous report5 but show considerably larger values ~ 1.81, GH = 2.02, andkdk4 = determined values, G H = 5.78 x lo3 (average of values based on data for 3.76 X (26) E. Hayon and M. Moreau, J. Ghim. Phys., 6 2 , 391 (1965). 10-3 M acid and for neutral solution), were used to (27) H.Fricke and J. K. Thomas, Radiat. Res., S u p p l . , 4, 35 (1964).

+

+

+

+

The Journal of Physical Chemistry

RADIOLYSIS OF METHANOL AND METHANOLIC SOLUTIONS Table 111: Literature Values of Primary Yields Scavenger

G H ~

GH

1.7

FeCh or benzoquinone FeCls or benzoquinone in acid solution FeCls or benzoquinone Various arenes

1.8 1.6

2.5

1.9

Biphenyla NiClz Ne0 J1.5 1.9 1.4 1.7

I\

FeCls I1

Pyridine CCL, CsHaC1, or p-CsHrCh

1.3

N2 1.8 1.6

3.1b 1.1 1.05 1.05 1.05 2.8b

Nz0

6 7 7 3

f C

d d d d

1 1.9-2.1 2.0 1.1 2.0

NzO

Ref

6

1.7

0 2

0 2

Gee-

e

10 8 9 12b

1.8 NzO, acid OFrom pulse data. All other yields are derived from steady radiolyses. & G.,-. c E. Hayon and M. Moreau, J . Phys. Chem., 69,4063 (1965). J. Teply and A. Habersbergerova, Second Tihany Symposium on Radiation Chemistry, 1966, pp 239-245. e Z . Schweiner, I. Janovsky, and J. Bednar, Collect. Czech. Chem. Commun.,31,43 (1966). M. C. Sauer, S. Arai, and L. M. Dorfman, J . Chem. Phys., 42, 708 (1965).

+

of G (Hz) a t corresponding concentrations of oxygen than do two others,as4 although onea of these agrees closely with present work as to G(H2) in the absence of solutes. Pulse Data. Dose per pulse was approximately 1.3 X lo1' eV/ml in most cases. Unless otherwise specified, absorption was monitored a t 545 nm, 85 nm from the reported12 A,, of the solvated electron in methanol. Decay was clearly first order for 3-4 half-lives in all cases. Repeated pulsing of the same sample of deaerated methanol produced little if any change in the decay rate, up to a total dose of approximately lozo eV/ml (830 pulses), sufficient to generatea 3 X lowaM formaldehyde and 5 X M ethylene glycol. The observed decay constants were 2.8 X lo6, 2.2 X lo6, 2.3 X lo6, and 2.1 X lo6 sec-l after 6, 230, 530, and 830 pulses, respectively. Essentially the same decay rate was observed in methanol that had been treated by preirradiating a deaerated sample with 1.5 X lOl9 eV/ml of 6oCoy rays and then rectifying it on a 30-theoretical-plate column, protected from atmospheric moisture, in the presence of ' of Mg (OCH,) before pulsing approximately 0.1 wt % samples of the middle third. E.g., after 150 pulses the decay constant was 2.2 X lo6 sec-1. Twofold variation in the dose rate had no effect on the decay rate. No absorption was detected between 250 and 370 nm in a scan a t 20-nm intervals although absorption a t 290 nm, ascribed to the CH20H radical, has been reported.28 The average value of the half-life of the solvated

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electron observed in the present work, 3.2 X sec, is significantly greater than published values based on pulse data, namely, 1.512&and 1.928psec. The former value12&was obtained with a dose rate of the order of ten times that used in the present work, and is based on a decay which was not purely first order. The conditions under which the latter value28was obtained are not reported but decay is described as cleanly first order.

Discussion The Efect of Acid on Product Yields. The present work confirms previous limited observations that G(H2) and other yields from methanol are increased by acid. I n spite of disagreement of the present results with most, but not all, previous reports as to the acid concentration range in which product enhancement occurs, the present finding that it is discernible a t concentrations of the order of 10" and fully developed with 10-3 M acid, or less, is assumed for purposes of discussion. The sigmoid dependence of yields on acid concentration indicates intervention of acid in one or more processes by which, in the absence of acid, transient intermediates are converted back to methanol. Because of the material balance observed in acid enhancement, the sum of such processes must act equally on precursors of reduced and oxidized products. The high reactivity of acid toward electrons and the apparent absence of alternatives suggests that acid acts by intercepting electrons which otherwise ultimately disappear without formation of products. If these electrons are of the freely diffusing solvated variety, then acid must act by simple competition with some species the concentration of which must, in view of the insensitivity of the decay rate of the electron to repeated pulsing, remain essentially constant during irradiation. Such a role could be played by one or more adventitious solutes which react irreversibly with electrons and which also prevent formation of an equivalent amount of oxidation products or their precursors. Initial total concentration would have to be greater than 5 X 10" oxidation equivalents per liter to be consistent with constancy of the electron decay rate up to a dose of lozoeV/ml and AG(Hz) = -1. It is possible, although it does not seem probable, that the methanol employed in this work was contaminated to this extent. According to this argument, the widely recorded2g variation of G ( H2) from "pure" neutral methanol is due to variation in the impurity level. If this were indeed true, yields of Hz and other products should be most sensitive to dose in those samples which in neutral solution give yields (28) F. 8. Dainton. J. P. Keene, J. T . Kemp, G . A. Salmon, and J. Teply, Proc. Chem. SOC.,265 (1964). (29) See M. Imamura, S. U. Choi, and N. N. Lichtin, J. Amer. Chem. SOC.,85, 3565 (1963),for a summary of many of these values.

Volume 73, NumbeT 6 Mag 1060

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K. SURYANARAYANAN AND NORMAN N. LICHTIN

approaching their limiting values in the presence of k e . - + ~ and + the half-maximum value of (CH30H2+),to acid most closely, but such a phenomenon does not be unequivocal. seem to have been observed in careful relevant work.7JZb A fundamentally very different possibility is that From this argument it also follows that the seven acid intercepts species other than the freely diffusing values of G(H2) between 4.9 and 5.1 which have been solvated electrons. This viewpoint brings more radical reported3 +-IO , 3 0 4 2 since 1963 with methanol of widely proposals under consideration. One is that acid interdiffering history in eight laboratories in four countries cepts electrons which normally are recaptured by were obtained with samples of methanol which all had parent ion radicals in the spur without net decompothe same effective impurity level. sition occurring. It is difficult to envisage such a Alternatively, acid might be competing with adprocess unless the electrons and ions are solvated. ventitious solute(s) which act by catalyzing recombinThe concentration of acid which gives half the full ation of radiolytic transients back to methanol without value of AG(Hz) corresponds to an average distance of net consumption of such solute(s). There is no spurs from CHaOH2+ ion of the order of 200 A. This requirement of relatively high concentration of such model also requires that CH8OH.f ions can be stabilized solute(s) but the difficult condition must still be met by solvation, a questionable requirement in view of the that the effective concentration must have been the fact that the specific rate of reaction of CHIOH.+ same in many samples of widely varying hiswith methanol in the gas phase to give CH80H2+is tory.8,&10,30-32 greater than the relaxation rate of liquid methanol.36 Another possibility is that one or more freely difAnother possibility in this general category is an fusing radiolytic products, either stable molecule or excited methanol molecule3 in which an electron is transient species, subvert solvated electrons in such a promoted t o an antibonding orbital from which it can way as to reduce G(H2) and G(0x) by about 1 be detached by a proton or other reagent. Otherwise, molecule/100 el7. The absence of dependence of it decays to its ground state. The major obstacle to yields or electron decay rate on dose shows that the this concept is that no long-lived excited state of active species cannot be known stable products.@ methanol appears to be known. A related proposal is The data limit the possibility of intervention by stable that acid intercepts an excited anion -CH*OH* or products to as yet unidentified compounds which CH@* which otherwise reverts to stable CHaO-. quickly reach a small constant concentration. A Oxidation of an excited anion by CHsOHz+ to give simple steady-state analysis shows that if the primary CHzO 2H is roughly consistent with the data time rate of formation of such products is linear in dose although it predicts too large a ratio of AG(CH20) to AG (glycol) , This hypothesis also suff ers from lack of a rate, G(Hp) and radiolytic yields of other products derived from solvated electrons will be independent of precedent. It thus appears that none of these many proposals dose rate, in agreement with observed behavior. for explanation of the effect of acid on product yields Alternatively, freely diffusing radicals may normally stands on firm ground, On the other hand, few if any intercept solvated electrons. With CH20H or CHsO, are completely discredited. The observedae increase for instance, the result would be regeneration of niethin G(H2) from y radiolysis of ethanol by 0.8 moleanol. Accommodation of this possibility with the cule/lOO eV with 5 X M or more concentrated first-order decay of solvated electrons observed in the HC1 encourages the view that something more fundapresent work requires the assumption that the conmental than impurity effects is involved, however. centration of the involved radicals does not change The significance of the data Competition Kinetics. substantially over 3 half-lives of the electron, e.g., of Tables 1 1 1 1 must be viewed in the light of the that k C H z o H + C H K m > ability of acid to evoke additional decomposition of G(e,-), More detailed evaluation, by kinetic analysis, methanol. The data for 3.76 X M acid sumof the possibility that acid competes for solvated marized in Table I and the value of GH+~.-derived electrons with stable or transient products does not provide a firm answer. If one employs the r e p ~ r t e d ’ ~ J ~ 4 x 1010 M-1 sec-1, in conjunction value of (30) L, M. Theard and M. Burton, J. P h y s . Chem., 67, 59 (1963). with the data of Figure 1 and equates ~,,-+H+(CHB- (31) V. I. Vladimirova, G . M. Zhabarova, B. M. Kadenatskii, and G . B. Pariiskii, Dokl. A k a d . Nauk SSSR, 164, 361 (1905). OH2+)(es-) with ICR (R) ( es-) , where R represents (32) This work. transient and/or stable radiolytic products which (33) The failure of OH20 to serve as an electron scavenger in consume solvated electrons, at the value of (CHaOHz+) methanol solution is consistent with the almost complete conversion of formaldehyde to methoxymethanol and related species in this where AG(H2) = $AG(Hz),.., then it appears that solvent. See J. F. Walker, “Formaldehyde,” 2nd ed, Reinhold is Although this value ICR(R) is a t least lo6 sec-l. Publishing Corp., New York, N . Y . , 1963,p 61. five times greater than the rate of decay of solvated (34) The speciflc rate of CHaOIEat + 8.- in methanol was determined12 in 4 X 10-6 to 1 x 10-4 M methanolic HCI. Salt effect electrons measured in the present work, the difference would cause the rate to be slightly higher in 1 X 10-8 M electrolyte. is not large enough, when viewed in the light of other (35) P.Wilmenius and E. Lindholm, A r k . Kemi, 20, 255 (1963). va1ues12J8of the decay constant and uncertainties in (36) J. C.Russell and G . R. Freeman, J.Phys. Chem., 71,755 (1967).

+

l’hs Journal of Physical Chemistry

1391

RADIOLYSIS OF METHASOL AND METHANOLIC SOLUTIONS from them with the aid of eq 6 refer to essentially complete development of the additional yield. Thus = 3.96 f 0.27 is really GET+^.-+&, where Q may be that part of the yield of freely diffusing solvated electrons which is obscured by reaction with impurities or radiolytic products or it may represent one or more of the alternative species which are suggested above or some combination of all of these. Perhaps all four values of G H + e s - or GH Gea- in Table I11 which fall short of the present value by about 1 molecule/100 eV do so because they do not include GQ. Evaluation of G H Z is not complicated by the problem of Q and, not surprisingly, eight of the ten values fall within 0.2 molecule/100 eTr of the present result and their mean differs from the present value by only 0.1 molecule/100 eV. Determination of G H by means of eq 7 and the data for neutral methanol is also not complicated by the problem of Q. Subtraction of this value from 3.96 gives Ge,-+o = 1.94. Presumably, most the values of Gee.-fall short of this by about 0.9 molecule/100 eV because they were determined by methods which did not evoke GQ. This is particularly significant for the direct pulse measurement using the biphenylide anion as monitor3' since it suggests that biphenyl does not scavenge Q. On this basis, Q would not include part of the total yield of freely diffusing solvated electrons but would have to be ascribed to alternative possibilities. Neither determination of k3/k4is complicated by the problem of Q. The agreement of the two values within experimental error is therefore unexceptional. If in fact Q is not identical with es-, analysis of the M 0%and variable data for solutions containing acid in terms of eq 8 is not correct. Equation 11 results when, in addition to eq 1-4, eq 9 and 10 are

+

G(Hz)

-GH~

=

+

+

[GH Gee-/(l ki(Oz)/~z(CH30Hz+)} GQ/ { 1 b(02)/ho(CH3OH2+) }]/[1 k3(02)/k4(CH30H)I (11)

+

+

+

taken into account. Evaluation of constants or their ratios from this unwieldy relationship is not possible with the data a t hand. If the simplifying assumption is made that kl/kz = k9/klo, then eq 11 reduces to a form differing from eq 8 only in that Ges- GQ replaces Gea-., If this assumption is not valid, then the value assigned to GeB-on the basis of eq 8, 1.45, would represent a quantity intermediate between the true values of Gee- and Ge,-+Q. This interpretation is consistent with the values of Ge,-+a based on the data for neutral solution and 3.76 X 10-8 M acid, 1.94, and around 1. of the numerous published values of Gem.Similarly, the value of kl/kz presented in Table I1 is inaccurate to the extent that the evaluation is perturbed by reactions 9 and 10. This problem presumably does not arise in the determination of kl and kz separately by pulse experiments.12 Pulse Experiments. The significance of these is discussed extensively above. The possibility exists that the discrepancy between the present result and an earlier value12 is due to a significant contribution from second-order decay in the latter work. A precedent exists in the well-known behavior of the hydrated electron.as If this is indeed the case, the present value may also be greater than the true first-order decay rate of the electron in methanol. Such an eventuality would in no way affect any of the speculations offered above.

+

Acknowledgment. The authors wish to acknowledge the valuable advice and criticism provided by Drs. A. 0. Allen and H. A. Schwarz and their hospitality to K. S. during a year in residence at Brookhaven National Laboratory. (37) M. 0. Sauer, S. Arai, and L. M. Dorfman, J. Chem. Phus., 42, 708 (1965).

(38) J. P. Keene, Radiat. Res., 22, 1 (1964).

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