such as carbon tetrachloride and for parathion and methyl parathion. In gaseous electronic detectors employing radioactive sources to provide a source of free electrons, ionization and electron capture must be competing processes (6). The kind of interaction that predominates will be determined by the types and concentration of the compounds present in the gas stream and by the geometry of the detector. Ionization as well as electron capture may be produced a t low applied voltages a t near-atmospheric pressure with electrons. Otvos and Stevenson (14) observed ion production in a number of gaseous hydrocarbons upon irradiation with electrons from carbon-14 and strontium-90 while Lovelock and Lipsky (IO) observed an increase in current for cyclohexane in an electron affiity detector when radium was used as the source. The latter authors recognized that ionization can occur a t higher applied voltages. Low results obtained by Otvos and Stevenson (14) for the apparent ionization crosssections of ammonia and Freon-12 are readily explained by postulating that the electron-capture process was competing with ionization for these com-
pounds so as to produce a lower net relative ion production. The first peak in Figures 5 and 6 at 25 nanograms of Systox, and the first peak in Figure 7 at voltages above 10 volts show maximum reversal when the amount of the thiono-systox in the detector is a maximum. This change from electron capture to ionization M the concentration of the active compound in the effluentincreases, indicates that both electron capture and ionization can take place in the same detector under the same operating conditions. Detector design is an especially critical variable for those compounds which will both caQture and ioniae. These studies
against the flow of carrier gas, minimizes the ionization phenomenon. The pulsed mode of detector operation (8) will not, however, eliminate interferences from ionization in a poorly designed detector. LITERATURE CITED
(1) Clark, s. J., 140th National Meeting, ACS, Chicago, Ill., September 1961. (2) Clark, S. J., 144th National Meetmg, ACS, Los Angeles, Calif., April 1963. (3) Coulson D. M., Pesticide Research
Bulletin, Stanford Research Institute, 2, No. 1, I (1962); Ibid., No. 2, 1. (4) Fukuto, T. R., Metcalf, R. L., J . Am. Chem. SOC.76, 5103 (1954). (5) Goodwin, E. S., Goulden, R., Reynolds, J. G., Analyst 86, 697 (1961).
( 6 ) Littl:?ood,
A. B., "Gas Chromatography, p. 286, Academic Press, Wew York, 1962. (7) Lovelock, J. E., ANAL. CHEM.33, 162 (1961).
(8) Ibid., 35, 474 (1963). (9) Lovelock, J. E., Nature 89,729 (1961). (10) Lovelock, J. E., Lipsky, S. R., J . Am. Chem. SOC.82, 431 (1960). (11) Lovelock, J. E., Simmonds, P. G., Vandenheuvel, W. J. A., Nature 197,
249 (1963). (12) Lovelock, J. E., Zlatkis, A., Becker, R. S.. Ibid.. 193. 540 (1962). (13) Mbore, A. D.', J . @con. Entomol. 55,
.-Gas Chrbmbg. I, No. 2, 23'(1963).
'
(16) Watts, J. 0.1 K l e b A. K., J . Assoc. Ofic. Agr. Chemists 45, 102 (1962).
J. E. BARXEY I1 C. W. STAXLEY C. E. COOK
Midwest Research Institute
425 Volker Blvd. Kansas City 10, Mo.
RECEIVEDfor review August 29, 1963. Accepted October 15, 1963.
Latent Heats of Vaporization from Distillation Rate Data SIR: The observation that the temperature dependence of physical rate processes follows the same relationship as the temperature dependence of chemical processes (4) evolved from a demonstration that chemical kinetic principles apply when physical reactions such as distillation, gaseous effusion, and dialysis occur (6). The following discussion shows that, for Table I.
Compound Benzene Chloroform Methanol n-Butanol Ethyl acetate Methyl ethyl ketone n-But ylamine Water
E (kcal. per mole) 18.85 18.18 19.18 21.16 19.19 18.85 18.75 21.12
Av.
Formic acid Acetic acid Ref. (6). Ref. (a).
20.86 21.38
'' Ref. @).
* Ref. (1).
2208
ANALYTICAL CHEMISTRY
distillation rate data, a relationship exists between the energy of activation as determined from the Arrhenius equation and the latent heat of vaporization. The energies of activation were calculated from the slopes of the lines in the plots of log k US. 1/T. Examples of such plots for benzene and n-butanol have been shown (4). By using these
Results Obtained
H , (kcal. per mole) C = E - H, calcd. using A (mole (kcal. av. A per min.) per mole) and C 22.31 22.04 22.02 22.25 22.08 21.90 21.61 20.77 21.87 22.47 21.98
11.40 11.34 10.69 10.73 11.51 11.21 11.11 11.39 11.18 10.39 IO. 07
H. (kcal. per mole), literature values
7.37 6.89 7.89 9.67 7.84 7.65 7.78 10.88
7.375 6.84O 8.49" 10. 45a 7 . 636 7.64" 7.67c 9.73c
9.18 10.10
10.47d 11.31d
values for the energies of activation, the Arrhenius equation, k = A e - E / R T , was solved for A. These values for A of the compounds studied were in close numerical agreement. Also, the published values for the Iatent heats of vaporization, H,, a t the boiling points differed from the calculated energies of activation by a fairly constant value, C. It appears that the constants, A and C, depend on the geometry of the distillation set up and include the work involved in moving the vapor from the surface of the liquid to the receiver. Based on distillation rate constants and on the average values obtained for A and C, heats of vaporization were recalculated using the equation
All these data are presented in Table I. Calculations of the latent heats of vaporization for formic and acetic acids presented difficulties because of their associating vapors. The values obtained are based on a volatilization forming the equilibrium vapor and depend on the degree of association under the conditions used. Because of the uncertainties in the degrees of associa-
tion for these acids, their values for A and C are not include’j in the overall averages shown in the table. The latent heats of vaporization calculated for these acids using t h ? average values obtained for A and C differ somewhat from the literature values, indicating that the degrees of assclciation reported ( 1 ) do not apply exactly to our experimental conditions.
LITERATURE CITED
(1) Armitage, J. W., Gray, P., Trans. Faraday SOC.58, 1746 (1962). (2) Dreisbach, R. R., T h y s i p 1 Properties
of Chemical Compounds, 111, No. 29 of Advances in Chemical’Series, A.C.S.,
Washineton. D. C. 11961). (3).DreisGachj ,R. R.‘, “Physi;al Properties of Organic Compounds, The Dow Chemical Co., Midland, Mich. (1953). (4) Hanna, J. G., Siggia, S., ANAL. CHEM.35,911 (1963).
(5) “International Critical Tables,” Vol. V, McGraw-Hill, New York, 1929. (6) Siggia, S., Banna, J. G., Serencha, N. M., ANAL.CHEM.35,365 (1963). J. GORDON HANNA
SIDNEY SIGGIA Olin Research Center Olin Mathieson Chemical Corp. 275 Winchester Ave. New Haven 4, Conn. for review June 18, 1963. AcRECEIVED cepted September 5, 1963. ~~~
~
Modification of Electron Probe to Detect Carbon SIR: The electron probe can analyze the atomic contents of a I-micron cube a t the surface of a solid sample ( 2 ) . This is achieved by focusing a 1micron diameter electron beam on the sample and analyzing the emitted characteristic x-rays m a vacuum x-ray crystal spect-ometer. The crystal spacing, d, of generally available crystals prevents the detection of elements with an atomic rumber less than about 11 (sodium) because of the Bragg law: nX = 2d sin 0. An Applied Reseainch Laboratory electron probe has been modified to detect carbon (Ka wavelength 44.0 A.) by making two chan,ges: A pseudo crystal with a d spacing: of -50 A. has been made of 200 monomolecular layers of barium stearate by the Langmuir-Blodgett technique (3). Monomolecular layers of barium stearate are successively picked up from an aqueous substrate. The layers go on with the barium end of the molwule alternating up and then down. This creates a layered structure consisting of planes of barium atoms separated by a distance roughly equal to twice Ihe length of the aliphatic carbon chain. The soap film crystal was laid down on an ARL Cinch radius crystal backing plate. C. L. Andren s ( 1 ) constructed a similar crystal for x-ray diffraction woi-k in 1940. The C K a pulse-height distribution from the proportional counter for a pure 300
v)
n
z 0 PO0
u
v)
?E f
100
8 0
0
10
30 VOLTS
40
40
Figure 1. Pulse height of K a line distribution
50
60
carbon
with P-10 gas (10% methane, 90% argon) a t atmospheric pressure. The counter operating voltage was -1200 volts. The central wire diameter was 0.0015 inch. The counter window was made of two superimposed layers (to avoid pin holes) of collodion (4) each 2000 A. thick. The double window was supported on a nickel mesh 250 lines per inch, 50% transmission.
IPO0
ACKNOWLEDGMENT
0
‘
1
1 I
450
Figure 2. line
50°
Se
I
I
I
I
hi
550
The help and suggestions of B. L. Henke of the Pomona College physics department are gratefully acknowledged.
,
600
Scan across carbon Ka
carbon target is shown in Figure 1 and the spectral line profile is shown in Figure 2. The peak-to-background ratio is approximately 25 to 1. The ratio of 25 to 1gives an overly optimistic estimate for the detection sensitivity of carbon in steel, for example. To give a more practical measure of carbon sensitivity in a suitation where most of the background would be coming from iron as the major component, the following experiment was done: With the x-ray spectrometer set correctly for carbon, the counting rate decreased by a factor of 12 when the target was shifted from carbon to iron. A sample of cast iron containing flakes of pure carbon was viewed in the scanning mode of operation. Figure 3 is a photograph of the cathode-ray tube which shows bright areas for carbon flakes and dark regions for iron. The snakelike regions of carbon are about 10 microns thick. The operating conditions were: accelerating voltage of electron beam 6 kv. (higher voltage causes excessive white radiation which in turn causes the soap film crystal to emit secondary x-radiation; electron beam current 0.2 pa.; and electron beam diameter -4 microns. The proportional counter was filled
Figure 3. Scan over cast iron sample with carbon K a radiation detection Width of light areas, -1
0 microns
LITERATURE CITED
(1) Andrewe, C. L., Rev. Sci. Znstr. 11, 111 (1940). (2) Birks, L. S., ANAL.CHEM.32, No. 9,
19A (1960). (3) Blodgett, K. B., Langmuir, I., Phys. Rev. 51, 964 (1937). (4) Hall, C. E., “Introduction t o Electron Microscopy,” pp. 312-16, McGrawHill, New York, 1953. JACK MERRIW C. E. MULLER W. M. SAWYER, JR. A. TELFER Shell Development Co. Emeryville 8, Calif. RECEIVEDfor review August 2, 1963. Accepted September 20, 1963. VOL. 35, NO. 13, DECEMBER 1963
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