iodide crystal well counter and RCL-256 analyzer. A number of organic product samples, prepared by use of the inhibited sulfuric acid catalyst, were also analyzed for sodium and arsenic. Under the conditions used (10-gram samples, irradiated 2 hours a t 5 x 107 flux), the arsenic concentrations found ranged from 0 to 20 p.p.m., with a sensitivity of about 5 p.p.m. and reproducibility of about 1 5 p.p.m. The sodium concentrations found ranged from 0 to 50 p.p.m., with a sensitivity of about 10 p.p.m. and a reproducibility of about f10 p.p.m.
LITERATURE CITED
(1) Atchison, G. J., Beamer, W. H.,
ANAL.CHEW28,237 (1956). (2) Bell, P. R., Chap. V in Siegbahn, K;: "Beta- and Gamma-ray Spectroscopy, Interscience, New York, 1955. (3) Brooksbank, W.A., Leddicotte, G. W., Dean, J. A., AKAL. CHEM. 30, 1785 (1958). (4) De, A. K., Meinke, W.W., Ibid., 30, 1474 (19581. (5) Iredale, P,, Atomic Energy Research Estab. EL/M 96 (1957). (6) Jenkins, E. N., Smales, A. A., Quart. Revs. (London) 10, 83 (1956). (7) Leddicotte, G. W.,Reynolds, S. A .Yucleonics 8 , No. 3, 63 (1951).
(8) Leliaert, G., Hoste, J., Eeckhaut, J., Anal. Chim. Acta 19, 100 (1958). (9) Meinke, W. W., ANAL.CHEM.25, 778 (1953). (10) Morrison, G. H., Cosgrove, J. F., Zbid.. 28. 320 11956). (11) doses, A, J., Saldick, J., A'ucleonics 14, No. 19, 118 (1956). (12) Smales, A. A., Mapper, D., Wood, A. J., Slamon, L., Atomic Energy Research Estab. C/R 2254 (1957). RECEIVEDfor review April 6, 1959. Accepted December 4, 1959. Symposium on Properties and Applications of Radioactive Material, Nuclear Technology Subdivision, Division of Industrial and Engineering Chemistry, 135th Meeting, .4CS, Boston, hlass., hpril 1969.
Determination of Hydrocarbon Oxidation Products Reverse Isotope Dilution Analysis WILLIAM H. CLINGMAN, Jr., and HAROLD H. HAMMEN American Oil Co., Texas City, Tex.
,To determine the products from the radiation-induced oxidation of propane, reverse isotope dilution analysis has been investigated. This method was chosen because of the possible complex nature of the product and the small sample size. The analytical data demonstrated that individual members of a homologous series may b e determined without interference from each other b y using recrystallization of a suitable derivative as the purification method. If only one compound in the sample i s determined, there need b e no separation o f products prior to this final purification. The method should find application whenever the components to b e determined are particularly difficult to separate, as in analysis o f hydrocarbon oxidation products.
D
of individual compounds in hydrocarbon oxidation products normally requires separation by distillation, ion exchange, or chromatography, follon.ed by analysis of each of the fractions. I n determining propane osidation products, the separation of individual components has been simplified by using reverse isotope dilution analysis. The general principles of this technique have been recently reviewed (4,5 ) . Briefly, the method consists in using an isotopically labeled reactant, diluting the reaction product with the compound to be determined, and measuring the specific activity of the latter after purification. This isotopic method has the advantage that any ETCRMISATION
single compound of interest can be determined in a complex mixture without prior separation. S o t only is it possible to save considerable time by avoiding some of the separation steps normally necessary in the analysis, but very small amounts of the component can be determined. The method has been investigated for the analysis of the products from the radiation-induced oxidation of propane over zinc oxide. I n this reaction the total product was less than 1 mg. Thus, reverse isotope dilution analysis )\-as particularly applicable. To apply this procedure, the irradiation was carried out on propane labeled with carbon-14. Propane-2-CI4 was selected because of its availability. EXPERIMENTAL
Material. T h e propane-2-C14 was obtained from t h e New England Xuclear Corp. a n d had a specific activity of 1.9 me. per mmole. It was diluted M ith Phillips pure grade propane before use. T h e chemically pure methanol, ethanol, 2-propanol, acetone, and acetic acid were used without further purification. The pinacol was obtained as the hydrate and was used as received. The 1-propanol, propylene glycol, acetaldehyde, propionaldehyde, and propionic acid 15 ere technical grade. Each n a s distilled on a 10-tray Oldershaw column and a heart cut taken for use in the analytical procedure. Sample Preparation. The oxidation products were prepared in sealed ampoules by irradiating mixtures of air and propane-2-C14 over zinc oside. About
200 mm. of pr0psne-2-C'~, 300 mm. of air, and 3.2 grams of catalyst were used. A 200-kv. x-ray machine was the radiation source, and the samples nere exposed to intensities of about 2.5 X lo' to 2.8 X l o 4 roentgens per hour. Under these conditions, the propane conversion \\as about 15%, and the product was either in the gas phase or adsorbed on the catalyst. The specific activity of the propane-2-C14 was low enough that no significant radio selfdestruction of the reactant or products took place during a run. The propane conversion due to absorption of p-radiation from propane-2-C14 was less than 0.0002% per day. This latter figure was calculated by assuming that all of the radiation emitted by the propane2-C14 n a s reabsorbed and that less than 10 molecules of propane reacted per 100 e.v. of radiant energy absorbed. After irradiation, the ampoules were placed in a glass vessel with 15.0 to 25.0 ml. of the compound to be determined. I n some cases, 15.0 ml. of each of two or three different compounds were used, and the individual components were later separated by distillation. The glass vessel containing the liquid and ampoule was sealed and the ampoule crushed by violent shaking. Shaking was continued for half a n hour and then the solid, liquid, and gaseous phases were allowed to equilibrate for at least 16 hours. The liquid and solid were separated in a centrifuge. With acetone and 2-propanol, duplicate runs were made in which the equilibration time was increased by about 30%. Good agreement T\ as obtained between the duplicate experiments, showing that VOL. 32, NO. 3, M A R C H 1960
e
323
sufficient time had been allowed for the liquid and adsorbed product to come to equilibrium. When more than one major component was present in the liquid, distillation was used for initial purification. Once single compounds were obtained, the h a 1 purification was carried out by recrystallizing a derivative. The purified derivatives were assayed for carbon14 as described below. Preparation of Derivatives. Acetone was converted t o t h e semicarbazone by reaction of about 5 ml. with 80 ml. of an aqueous solution of 2 M sodium acetate and 1.6M semicarbazide hydrochloride. T h e derivative was recrystallized from water. Aldehydes were reacted with a saturated solution of 2,4-dinitrophenylhydrazine in 2 N hydrochloric acid to precipitate the 2,4dinitrophenylhydrazones. Recrystallization was from a mixture of ethanol and ethyl acetate. The derivatives of the carboxylic acids were the p-bromophenacyl esters. These were prepared by refluxing a n aqueous solution of the sodium salts of the acids with p-bromophenacyl bromide. Ethanol was used for recrystallization. The alcohols were converted to the phenylurethans by reaction at room temperature with a n equal volume of phenylisocyanate, and the derivatives were recrystallized from petroleum ether. With propylene glycol, however, 4 ml. of alcohol and 15 ml. of phenylisocyanate were refluxed for 1 hour to form the bisphenylurethan. KO derivative of pinacol was prepared. This solid alcohol was purified, however, by recrystallization from water. T o determine the yield of carbon dioxide, the radiation products were equilibrated with 75 ml. of water containing 10.5 grams of sodium carbonate
and 7.5 grams of sodium hydroxide. Barium carbonate was precipitated from this solution using concentrated barium hydroxide. Carbon dioxide was then liberated from the precipitate with sulfuric acid and the gas assayed for carbon-14 in an ionization chamber. Radioassay Procedure. T h e specific activity of t h e derivatives was determined by t h e method described previously (6), except t h a t t h e average sample size was about 40 mg. Samples were converted t o carbon dioxide by wet oxidation and t h e radioactivity of t h e gas was determined by measurement of ionization currents produced in suitable ionization chambers. The propane was assayed directly for carbon14 after dilution of the gas with carbon dioxide. The precision of the specific activity measurements was about j10.1 X lo-' pc. per mmole. This value nas determined from several duplicate measurements of the specific activity of typical derivatives. Determination of Propane Conversion. T h e total propane converted in a n oxidation run was determined by equilibrating t h e irradiated sample with dilute hydrochloric acid. Sufficient acid was used t o dissolve the zinc oxide catalyst. T h e solution was then shaken for 0.5 hour with about six times its volume of gaseous, inactive propane to remove a n y dissolved, unreacted propane-2-CI4. The aqueous solution was then assayed for carbon-14 in the same manner as the solid derivatives.
Gas chromatography analyses of the vapor over the catalyst after irradiation showed that unreacted propane and air were the only compounds present. Thus, the above acid solution should contain all of the carbon-14 from reacted propane molecules. Calculation of Results. T h e conversion of propane t o a n y particular product was calculated from the following equation:
% conversion to A
=
100(SAMA/SpMp)/ [1 - ( S A / S p ) ]
where S A = specific activity of derivative of A , microcuries per
millimole Sp = specific activity of propane, microcuries per millimole M A = millimoles of inactive A added to radiation product M p = millimoles of propane used in experiment DISCUSSION
Table I lists all t h e compounds for which analyses were made and t h e properties of typical derivatives. These d a t a illustrate t h a t not only can a compound which is present in the product be determined, but also the absence of any particular compound can be confirmed. This latter point is true even though homologs of the compound are present. For example, the activities of the methanol, propylene glycol, and When a sample of pr0pane-2-C~~ pinacol derivatives were not distinguishwhich had not been in contact with zinc able from background, even though oxide or been irradiated, was treated in ethanol and the C8 alcohols were presthe above manner, no carbon-14 was ent. Likewise, the acetone in the found in the aqueous solution. Thus, product did not interfere with the deterthe propane contained no radioactive mination of acetaldehyde or propionimpurities which would lead to erronealdehyde. ous results. To avoid interference from homologs that may be in the product, it is necessary to recrystallize the derivative successively to constant specific activity. Table I. Properties of Derivatives This point is illustrated in Table 11, where the specific activity of several ProDane Derivative derivatives after a different number of recrystallizations is given. Normally, Product the third and fourth recrystallizations Alcohols gave the same activity within the experi15-16 47 0.0 72 8 0.0 Methanol mental error. After only one recrystal52-53 $2 7.3 138 Ethanol 6.3 lization, however, the derivatives still 83-84 88 12 5.8 72.8 2-Propanol 5.9 ... ... ... 72.8 contained radioactive impurities. For 2.2 48-49 51 7.3 '72.8 1-Propanol example, the 2,4-dinitrophenylhydraPropylene zone of acetaldehyde in experiment 8 0.0 87.4 0.0 141-144 145 glycol still showed radioactivity after one 38 0.0 87.4 0.0 37.5-39.5 Pinacol recrystallization. This activity might Carbonyl have been due to the presence of the 72.8 0.0 148-149 147 0.0 Acetaldehyde Propionaldeacetone derivative, because acetone was 0.0 146-148 154 0.0 hyde 72.8 a product of the reaction. After an1.8 190-193 187 4.6 72.8 Acetone other recrystallization, the impurity was 1.7 186-190 187 ... 72.8 removed. The same point is illustrated 1. 5 ... ... ... 72.8 in experiment 7 with the phenylurethan Carboxylic acids of ethanol. 72.8 0.0 0.0 87-88 85 Acetic acid 0.0 62 63 0.0 Propionic acid 79.8 Recrystallization proved to be a more efficient method of removing radioactive 138 21.0 ... ... 10 Carbon dioxideb impurities than distillation. This is a hfeasured activities adjusted to common dose of 5.2 X lo4roentgens for all samples. illustrated by experiment 9 in Table 111. b Carbon dioxide from 2-carbon atom only. The product was equilibrated with equal 324
ANALYTICAL CHEMISTRY
mluines of both acetone nntl :icetic acid. This inixture was then distilled using a 2-foot concentric tube column and a 20 to 1 reflux ratio. The heart cut of acetic acid showed a significant radioactivity. Subsequent purification recrystallization of the p-broniophena ester, however, demonstrated that this activity was due to an impurity. I n experinients 10 and 11, only a single coinpound was equilibrated with the sample and then purified by distillation. I n both cases, the specific activity was higher than when recrystallization was used (experiments T and 5 ) . I t is of interest to compare esperiments 9 and 1 in Table 111. I n the latter esperiment only acetone was equilibrated with the sample, yet essentially the same specific activity was obtained as in experiment 9 where acet,ic acid was mixed with t,he acetone. Thus, this dilution of the acetone did not interfere with its subsequent determination. Likewise, in experiment 6, both methanol and 1-propanol were mixed with the product. After the alcohols had been separated by distillation and the phenylurethans recrystallized, the methanol derivative showed nil activity. Thus, even though the 1-propanol was radioactive, because it was a reaction product, it did not interfere n-ith the met'hanol determination. I n general, one sample of reaction product can be used to determine several compounds if these compounds can be separated by distillation. After this preliminary separation, final purification should be by recrystallieation of derivatives.
Table II.
Expt.
Product
1
Acctone
2
Acrtone
3
2-Propnnol 1-Propanol
4
Effect of Successive Recrystallization on Activity
No. of
Recryst:tl!izations 1 2 3 3 4 3 1 3
9.6 9.8 13.9 13.7 5.1 5.3 0.5 0.6 3.0
4
1-Propanol"
5
3
3
1-Propnnol
6
2.2 2.2 0.4
Ethanola
f.
8
Derivative Specific Activity, pc./LImole x loJ
Xi1
hIclting Point, O
c.
Found
Lit.
190-193 188-190 89-90 ... 53
187
...
...
...
... 51 51 51 ... ... 52
49-5 1 43
48-49 ... 50150.5
...
88 ...
51
... ... Xi1 ... ... Product from control experiment in which propane-air misture was not irradiated. Acetaldehyde
0.9
1
2
6
~~~
Table 111.
Expt.
~~
Purification by Distillation and Recrystallization
Boiling Point of Heart Cut, O C.
Activity of Heart Cut, pc./hlmole X lo4
Derivative Specific Activity, pc./;llmole X lo'
Product 1.7 ... Acetone 0.0 .Acetic acid iis-iis 5 1.0 1.5 1 Acetone KOdistillation 78 0.2 ... 10 Ethanoln 0.0 S o distillation 7 Ethanol= 11 1-Propanol" 95.5-96.0 1.5 ... 5 1-Propanola S o distillation 0.5 6 Methanol ... ... 0.0 1-Propanol ... ... 2.2 Product from control experiment in which propane-air mixture was not irradiated. 9
0
INFLUENCE O F ISOTOPE EFFECT ON ANALYSIS
A possible source of error in this particular application of reverse isotope dilution analysis is an isotope effect during the oxidation reaction. The rate of a reaction in which rupturing of a carbon-carbon bond is rate-determining is generally 6 to 870 slower when one of the carbon atoms is replaced by carbon-14 (2). This rate change is partly due to the lower zero point vibrational energy of the labeled molecule. The activated complex of the heavier molecule also moves across the energy barrier at a slower rate. These effects have been treated quantitatively by Bigeleisen ( I ) , and the use of Bigeleisen's equation to estimate the isotope effect has been discussed by Roginsky ( 3 ) . The maximum isotope effect would occur when the bond is completely broken in the transition state. Such a maximum was calculated for the carbon-carbon bond using average frequencies of 993 and 955 cni.-I for the C12-C1zand C12-Cl4 stretching vibrations, respectively. It was assumed that the other fundamental frequencies were unchanged on going from the reactant t o the activated complex. A maximum isotope effect of 10%
was calculated in this manner, and this value is consistent with experimental data (9, 3). A similar estimate was made for the breaking of a carbon-hydrogen bond. Average frequencies of 3050 and 3040 crn.-' were used for the C12-H1 and 04-H1 stretching vibrations, respectively. The maximum predicted effect is about 3.7%. The influence of the isotope effect on the product distribution can be estimated from the above values. The ratio of two products which involve breaking the same carbon-carbon bonds is affected only by the C14-H1 isotope effect. If, in one product, a C12-H1 bond is broken, while in another a C14-H1 bond is ruptured, the measured ratio will probably be within 2 to 3% of the value for the unlabeled reactant. For example, with propane-2-C14, the ratio of 2-propanol to 1-propanol may be 2 to 3% low. If different carbon-carbon bonds are broken in forming the two products, the error introduced by the isotope effect may be 6 to 8%. Errors of this magnitude should not alter any qualitative conclusions from
the experimental data. Also, their approximate value for any given product can be estimated as above. If the sample size is sufficient, the isotope effect can be avoided by using normal isotope dilution analysis. I n this case an unlabeled reactant is used, and the reaction product is diluted with the labeled compound that is being determined. LITERATURE CITED
(1) Bigeleisen, J., J . Chem. Phys. 17, 675 (1949). (2) Burr, J. G., Jr., "Tracer Applications for the Study of Organic Reactions," p. 3, Interscience, New York, 1957. (3) Roginsky, S. Z.,"Theoretical Principles of Isotope Methods for Investigating Chemical Reactions," pp. 41-3, Academy of Sciences USSR Presa, AEC-tr-2873 (1956). (4) Ropp, G. A., J . Chem. Educ. 34, 60 (1957). (5) Rosenblum, c.,ANAL.CHE3.I. 29, 1741 (1957). (6) Sechrist, C. N., Hammen, H. H., Znd. Eng. Chem. 5 0 , 341 (1958).
RECEIVEDfor review August 3, 1959. Accepted December 3, 1959. VOL. 32, NO. 3, M A R C H 1960
325
