ION-MOBILITY MEASUREMENTS OF INORGAXIC AND ORGANICPHOSPHORUS COMPOUNDS
2867
Ion-Mobility Measurements of Inorganic and Organic Phosphorus Compounds by Morton R. Kagan Department of Electrical Engineering, Catholic Univedty, Washington,I). C.
and George G. Guilbault Department of Chemistry,Louisiana State University in New Orleans, New Orleans, Louisiana (Received January 51, 1968)
This paper describes the first experiments measuring the mobilities of negative ions produced in various phosphorus-containinggases. An experimental technique is employed whereby CY particles from a polonium source produce both a reference pulse from a solid-state silicon barrier detector and thermal electrons which readily become attached to the electronegative phosphorus-containing gas. These mobility measurements were made on a number of inorganic phosphorus halides and organophosphorus compounds at various ratios of E/Po, ranging from 0.5 to 10 V/cm torr. The zero-field reduced mobilities obtained for PC&,POCla,PSCls, PSBrs, isopropyl methyl fluoridophosphonate (Sarin), and diethyl phosphorochloridothionate (DCPT) were 0.39, 0.24, 0.29, 0.41, 0.21, and 0.17 cm2/V sec, respectively. An attempt has been made to explain the nature of the ionic species in terms of the quantum-mechanical theory of ionic mobility as put forth by Dalgarno. This analysis emphasized the need for mass analysis of the ions at the end of their flight path.
Introduction Negative ions, in general, and particularly the electron, have been theorized to play a very dominant role in almost all of life processes.’ I n fact, a ‘partial explanation of the biological function of chemical carcinogens has been attributed to the electron-donating and/or -accepting capacity of a molecule.2 However, the ionization potentials of these molecular systems are poorly known, and the electron affinities are hardly known at all. An important class of compounds are the organophosphorus insecticides which inhibit basic life processes. Hence, interest developed in these laboratories in the process of electron attachment to molecules and the mobility of the subsequent ions formed. The ion mobilities of nitrogen ions in n i t r ~ g e n , ~ helium ions in h e l i ~ mcesium ,~ ions in c e s i ~ moxygen ,~ ions in He, Ne, Ar, Kr, Xe, Hz,and Nz,6nitric oxide ions,’ NO+ ions in He, Ar, Nz, and H z , Ar, ~ Nz, CH4, CzHz, GOz, and cyclopentane ions,g and the negative ions in oxygen, sulfur hexafluoride, sulfur dioxide, and hydrogen chloridelo have been reported. No measurements have been made on phosphorus-containing compounds, either organic or inorganic, however. The two selected organophosphorus compounds chosen for this study were isopropyl methyl fluoridophosphonate (Sarin) and diethyl phosphorochloridothionate (DCPT). The four inorganic phosphorus compounds chosen were PC13, Poc13, PSC13, and PSBr3. The drift velocities and reduced ionic mobilities of these compounds were studied at various ratios of E/P,, ranging from 0.5 to 10 V/cm torr.
Experimental Section Ion-Mobility Chamber. The mobility chamber used was a rectangular box made of brass plates, similar to
the one employed by NIcDaniel and Crane.6 The dimensions of the box and the proportional counter used a t the end of the flight tube were similar to those of McDaniel, except for minor thickness changes which added to the rigidity of the system. Three silicon surface barrier detectors (Oak Ridge Technical Gorp., Oak Ridge, Tenn., Model No. SB-EU-150-60) were used a t various drift distances from 7 to 14 cm to detect the primary a radiation which produces the reference pulses. These detectors have a depletion depth of 300 p and require a bias of approximately 100 V. The drift region consists of 23 stainless steel rings, mounted in the mobility chamber with their axis perpendicular to the axis of the proportional counter, C1. A very uniform potential field was observed along the flight, tube using 22 0.5-W, 0.5-Mohm carbon resistors. A brass a-particle source holder for the polonium 210 source was mounted on tracks along one side of the chamber, and a vacuum-sealed rotary shaft coupled
(1) A. S. Gyorgyi, “Introduction to Submolecular Biology,” Academic Press Inc., New York, N. Y., 1960. (2) J. A. Stockdale, G. S. Hurst, and L. G. Christophorore, Nature, 202, 459 (1964). (3) G. E. KeIler, D. W. Martin, and E. W. McDaniel, Phys. Rev., 140, A1535 (1965). (4) J. M. Madson, H. J. Oskam, and L. M. Chanin, Phys. Rev. Lett., 15, 1018 (1965). (5) L. M. Chanin and D. Steen, Phys. Rev., 132, 2554 (1965). (6) E. W. McDaniel and H . R. Crane, Rev. Sci. Instrum., 28, 684 (1957). (7) R. C. Gunton and T. M.Shaw, Phys. Rev., 140, A748 (1965). (8) R. A. Young, C. R . Gats, R. L. Sharpless, and C. M. Ablow, ibid , 138, A359 (1965). (9) T. E. Bortner, C. S. Hurst, and W. G. Stone, Rev. Sci. Instrum., 28, 103 (1957). (10) E. W. McDaniel and M. R. McDowell, Phys. Rev., 114, 1028 (1959).
Volume 78, Number 8 August 1068
MORTON R. KAGAN AND GEORGE G. GUILBAULT
2868 to a ten-turn calibrated dial was used to change accurately the position of this source from outside the box. The a source used was a 60-pCi polonium 210 source, obtained from Monsanto Research, Dayton, Ohio, on special order, plated on a brass disk covered with a thin gold-film overlay. An all-glass vacuum system was constructed, capable of pulling a pressure of 5 x lo-* mm. This system had provisions for degassing and “baking-out” impurities. All materials used were carefully purified by vacuum distillation and by a preparative gas chromatograph. These samples were introduced only after a minimum pressure of lo-’ mm was obtained in the mobility chamber. Electronics. A Power Designs HV 1556 power supply was used to furnish both the regulated negative voltage for the rings forming the drift tube and the high positive voltage required for the proportional counter. These power supplies could furnish, within 1%, 10-6000 V at 20 mA with extremely good regulation and very low ripple. Pulses which are derived from the proportional counter are fed directly into an Ortec Model 101 preamplifier and then into an Ortec 201 amplifier. The amplified pulsed signal is then fed into the y input of an oscilloscope. An Ortec Model 207 amplifier was used to supply the bias voltage to, and to amplify the output from, the solid-state detectors. Shielded coaxial cables were used for all connections, with a flat braided cable connecting the mobility chamber to a water pipe in order to eliminate troublesome ground loop problems. A Tektronix RM 35-A oscilloscope was used in all experiments to display the output signals obtained. A Polaroid oscilloscope camera system, Model No. (3-27, set on an aperture of f/4, was employed to visually record the pulse signals from the proportional counter. A picture of several hundred pulses was generally taken, and the drift time was obtained by measuring the difference between the reference trigger pulse and the maximum density of pulses produced at the proportional counter. To facilitate evaluation of the pulse distribution, the gain of the preamplifier was set such that only the strongest pulses needed to produce saturation of the pulse amplifier were used. This served to eliminate extraneous signals and contributions due to ions that markedly deviate from the normal 3Iaxwellian distribution. Principle of Operation. The principle of operation of the ion-mobility chamber has been adequately discussed by McDaniel.6 Measuring the drift time (the time between reference and ion pulses), and incorporating the drift distance, d , the total gas pressure, P, the electric field, E, and the temperature, T , one can obtain the drift velocity, Vd, and the reduced mobility, KO,which is defined by the equation
KO = ____ 273Vdp cm2/V sec 760ET
The Journal of Physical Chemistry
Results The first experiment performed with this apparatus was the measurement of the ion mobility of SFs, a highly electronegative gas. The value obtained for the extrapolated reduced mobility of SF6 was 0.58 3t 0.02 cm2/V sec. This result is within the accepted value of 0.57 f 0.01 cm2/V sec reported in the literature.6 Measurements were next made of the drift velocities and ion mobilities of several inorganic and organic phosphorus-containing compounds at their saturated vapor pressures (0.7-3.0 torr). The results obtained are illustrated in Figures 1 and 2. I n Figure 1, the variation of the drift velocity, Vd, is plotted os. various ratios of E / P o ranging from 1 to 10. At E/Po = 3.0, values of 0.37, 0.575, 0.627, 0.775, 0.90, and 1.15 cm/sec were obtained for DCPT, POcl3, Sarin, PSC13, Pc13, and PSBr8,respectively. Figure 2 shows the variation of the reduced mobility, K O ,with E/Po ratios of 0.5-10.0. The zero-field reduced mobilities obtained for the compounds DCPT, Sarin, P0Cl3, PSC13, PC13, and PSBr3 were 0.17, 0.21, 0.24, 0.29, 0.39, and 0.41 cm2/V sec, respectively. Figure 2 also shows that in all cases there exists a linear relationship between KO and E/Po for E/Po > 3, but for PSBr3, PSCl,, and Sarin, the curves are turning upward at low fields. These observations can be attributed either to an instrument effect or to a charge exchange. However, the diffuse drift time spectra we have observed for both PSC13 and PSBr3, as well as the observations made by Kiserll on these compounds, tend to support the charge-exchange interpretation. Further measurements, preferably using mass spectrometric techniques, are needed to resolve this. The composite estimated probable errors in the measurement of the various quantities involved in the determination of the mobility is about 3%, and this is the probable error for calculation of the reduced mobility,
KO
Q
Discussion The questions we wish to consider in this section relate to the identification of the negative ion(s) or ion clusters whose mobility were measured in these runs and to the nature of the mechanisms regarding their formation. It is generally agreed that much of the difficulty in explaining the measurements of K for ions in their parent gases arises from the uncertain identification of the ion concerned. The recent application of mass spectrometry to mobility measurements represents a significant, if not essential, contribution to this field. The importance of this contribution has more than amply been demonstrated by Saporoschenko, l 2Sinnott,13 (11) R. W. Kiser, 1st Quarterly Report, Contract No. DA 18-035AMC-718-4, U. 5. Army, Edgewood Arsenal, Md., 1966. (12) M. Saporoschenko, Phys. Rev.,139, A362 (1966). (13) G. Sinnott, ibid., 136, A370 (1964).
2869
ION-MOBILITY MEASUREMENTS OF INORGANIC AND ORGANIC PHOSPHORUS COMPOUNDS
I
I
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1
2
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3
I
4
I
I
I
I
I
s
6
7
8
9
I 10
E/P~V , O L T S . C M ~.TORR-'
Figure 1. Variation of drift velocity with E/PO for a number of inorganic and organic phosphorus compounds.
0
0
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1
1
2
3
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4
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EIPO, VOLTS.CM*.TORR-I
Figure 2. Variation of the reduced mobility ( K Owith ) E/Po. The arrows along the ordinate axis refer to the polarization limits for (PSBr3),-, PCls-, (lPCl&-, and (PSCl&- ions, respectively, starting with the largwt value.
and Keller, Martin, and McDaniel.8 Previous to this, owing to the limited experimentd data on molecular ions in like or unlike gases, the common procedure was to identify the ion by calculating its mass from the measured mobiiity using the constancy of KO. This procedure, however, requires justification, because the
theoretical description must be constantlly modified to include the more complicated interactions which are obviously taking place. The first rigorous, and still the most generally applicbble, mobility theory was the classical treatise published by Langevin in 1905.14 (14) P. Langevin, Ann. Chim. Php., 8, 245 (1905). Volume 78, h'umber 8 Auguat 1968
2870
MORTON R. KAGANAND GEORGE@. GUIEBAUET
This theory when applied t o the low-field region took into account the effect of inverse fifth-power attractive forces between ions and molecules, as well as rigidsphere repulsion. HasBe16and Chapman and Cowlingle incorporated some refinements in the kinetic theory and cast the Langevin expression into a form which requires very accurate values of the sum of the radii of the ion and the molecules, as well as precise values of the dielectric constant and the gas density. Although expressions are readily available for obtaining the reduced mobility anywhere between, and including, the elastic-sphere limit of the polarization, or small-ion, limit, inability to obtain accurate values of the abovementioned parameters for complicated molecular systems makes application of this theory to experimental data exceedingly difficult. If we assume that the predominant interaction of an ion with a neutral molecule is due to the Coulomb field of the ion and the dipole it induces in the neutral molecule, then the Langevin result for the reduced ion mobility at the zero temperature (polarization) limit may be expressed in the convenient form17
KO
=
35.9
-
2/01/11 cm2/V
where KO is the reduced mobility, a is the polarizability of the neutral gas measured in atomic units, and pClris the reduced mass in units of the proton mass. Using this expression, we have only the polarization, a, to contend with in order to apply it to the experimental data of our complicated molecular systems. Many of these values can easily be obtained from the tables compiled by Maryott and Buckley.l8 That this rather simple model has considerable merit has been demonstrated within an accuracy of 10% for monatomic ions in the diatomic gases of Hz,N2,and COS1' Using eq 2, a calculation was made of the mobility of the SFa- ion in its parent neutral gas. A theoretical value of 0.63 cm2/V sec was compared with the experimental value of 0.57 cm2/V sec.10,19 This apparent discrepancy (loyo)may be due to the fact that at the experimental temperature the polarization limit has not yet been reached, and the apparent difference between experiment and theory is due t o the influence of the shorter range forces.20 The success between theory and experiment for the CC14- ion in CC1,,21 on the other hand, indicates that the polarization limit has indeed been reached, indicating the validity of this equation in some cases. Similar calculations have been made for various ions produced in Pc13, POCl,, PSCla, and PSBra. The best agreement achieved was the assumption of the (PC&cluster for PCla. This calculation resulted in a discrepancy of only 3.3% (a calculated value of 0.40 compared with an observed value of 0.39). Consideration of the parent ion, Pch-, yielded a result which deviated only 20% (0.47 compared with 0.39). The more The Journal of Physical Chemistry
complicated inorganic molecules gave results which were not as good. For example, an infinite rnass had to be assumed for PSC13 and PSBr3 to give theoretical values which agree exactly with that calculated. If a cluster of two parent ions is assumed, values of 0.35 and 0.47 cm2/V sec are obtained, which are about 15% away from the experimental values of 0.294 and 0.416 cm2/V sec, respectively. The best fit for the POC13 molecule, even for an assumed-infinite effective mass ion, gave a theoretical value of 0.50 cm2/V sec, as compared with the experimental value of 0.242 cm2//V sec. These results indicate that, contrary to the polarization interactions experienced by the complt cated molecular ions produced in SFa, CCl,, and PC1%, the ions produced in some of the inorganic gases have not, reached the polarization limit. The fact t,hat the measured values are lower than those calculated is very good evidence that the mobility is being dominated by resonant charge transfer.22 Another, but less likely, possibility is that ion clusters are being formed. An explanation based on the fact that the relative temperature is so high that the "elastic-sphere limit," has been reached is extremely unlikely, owing t o the unrealistic molecular parameters required. 8inoe neither molecular radii nor collision cross-section data are available, verification of this hypothesis is difficult without additional experimental measurements. No calculations were made on the organic compounds, since none of the supporting data, such as the polariza". bilities, molecular radii, et,c., is available. I n a collaborative study of the negative tons formed at various energies, ranging from 10-70 eV in a Bendix Time-of-Flight mass spectrometer, 'Kiserl1 has showrl that a t low electron energies (10-15 eV) the predondnant negative ions obtained from ROCla are C1- and POC1POC13 --3 POC12POClzPOC1-
---j
POCI-
--j
4-C1"
+ CI
c1- + PO
A study of mee,hyIthiophosphonic dick&ride [(Cb&) PSClz],which is similar to the PSC13 used in our s'cady,
I
(15) E[. R.Has'e, Phil. Mag., 1, 139 (1926). (16) S.Chapman and T. G. Cowling, "The Mathematical Theory of Non-Uniform Gases," 2nd cd, Cambridge University Press, London, 1952. (17) A. Dalgarno, LM. R. C. McDowell, and A Williema, Phil. Tranrr. Roy. Soc., A Z O , 411 (1958). (18) A. A. Maryott and F. Buckley, "TabIe of Dielectric @onstant* and Electric Dipole Moments of Substance in $he Gaseous Statti,'* National Bureau of Standards Circular 537, U. 5. Government Printing Office, Washington, D. C., 1953. (19) K.B. McAfee and D. Edelson, Proc. Phys. SOC.,81,382 (1963). (20) E. A. Mason and H,W. Sohamp, Ann. Phys. (N. Y.), 4. 283 (1958). (21) G. G. Guilbault and M. Kagan, to be submitted for publication, (22) A. Dalgarno, Phil. Tram. ROY.SOC.,A Z O , 426 (6958). .
I
287 1
STUDIES OF MEMBRANE PHENOMENA revealed that only C1- was obtained a t 20 eV. Attempts are being made to study the negative ions produced from these and other phosphorus compounds under thermal energies from electron attachment (0-10 eV) using the mass spectrometer, and the results of this study will be forthcoming. The drift-time spectra obtained for the organophosphorus compounds, DCPT and Sarin, and for PCh are characterized by a sharp leading edge with little tailing, generally spread, over 0.1-0.2 msec (1-msec/cm sweep). This is considerably better than SFs, indicating that even for relatively high values of E / P othere exists only
Studies of Membrane Phenomena.
one negative ion produced and that this ion is probably the parent ion formed by collision with an electron
+ e- +PCl3Sarin + e- -+ SarinDCPT + e- +DCPTPC13
The drift-time spectra obtained for the other compounds, PSC13, PSBr3, and P0Cl3, on the other hand, are more diffuse (0.5-1 msec), indicating the probable existence of several ions. This is in line with observations of Kiser.ll
VII. Effective Charge
Densities of Membrane by M. Yuasa, Y. Kobatake, and H. Fujita Department of Polymer Science, Osaka University, Toyonaka, J a p a n
(Received Februarg 1, 1968)
Measurements were made of the emf which arose between aqueous solutions of KC1 of different concentrations (C, and C,) separated by a layer of aqueous solution of potassium polystyrenesulfonate (KPSS) as a function of C1 a t various equivalent concentrations, X,of KPSS, keeping the ratio of CZto C1 a t several given values. The data obtained were analyzed in terms of the theory developed in part IV of this series after it had been modified t o suit the present experimental condition that the stagnant layers of liquid outside the polyelectrolyte region were fairly thick, and a value of about 0.4 was obtained for the parameter 6'. Here 4' Characterizes the hydrodynamically effective charge density of KPSS in aqueous KC1. A simple equation was derived for use in the determination of 6 from the measurement of the Donnan equilibrium potential, where 6 is the parameter which characterizes the thermodynamically effective charge density of a polyion in salt solution. Necessary data for the Donnan potential were obtained over a wide range of C and for several values of X, where C is the concentration of KC1 in the phase containing no KPSS. It was found that the resulting values of qi varied with log ( C / X ) following a sigmoid-shaped curve, approaching about 0.4 and 0.2, respectively, in the upper and lower limits of C / X . Thus the ratio of 6 to 6' tends to approach unity a t high concentrations of KCI. Finally, an approximate method was proposed for the calculation of membrane potentials under conditions where 4 depends on electrolyte concentration, and it was demonstrated that a better fit to observed data is obtained by this method than by our previous treatment which did not take the concentration dependence of qi into account.
Introduction In preceding p a r t ~ l -of~ this series we have shown that various physicochemical features of a system which consists of bulk solutions of a 1:1 electrolyte separated by an ionizable membrane can be well explained by postulating eq 1 and 2 for the mobilities and activity coefficients of mobile ions in the membrane phase u+ = U+"C-)(C-
+ +'X)/(C- + X ) ; u- = U-O(C-)
Y+ = r+"C->(C-
(1)
+ +X>/(C- + X I ; y- =
r-O(C-)
(2)
Here it is assumed that the membrane and the added electrolyte have the same cation in common. In the above equations, Uio(C-) and rio((C-) (i = or -) are the mobility and activity coefficient of ion species i in free solution of the added electrolyte when its molar concentration is C-, X is the stoichiometric molar concentration of electric charges fixed on the membrane skele-
+
(1) Y . Toyoshima, M. Yuasa, Y. Kobatake, and H. Fujita, Trans, Faraday SOC.,63,2803 (1967). (2) Y. Toyoahima, Y. Kobatake, and H. Fujita, {bid., 63, 2814 (1967). (3) Y. Toyoshima, Y . Kobatake, and H. Fujita, ibid., 63, 2828 (1967).
Volume Yd, Number 8 August 1968
