It was shown by activation analysis that the concentration of silver in the membrane was about one third of that which was found in membranes equilibrated with silver solutions containing no albumin. This indicates some blocking of the membrane sites by the albumin, but sufficient sensitivity remains for trace determination of many metal ions by activation and anodic stripping analysis by taking advantage of the preconcentration of the Ag+ ion in the cation exchange membrane. Thus, it appears that ion exchange membrane electrodes could possibly be used in biological and natural solutions without a significant loss of sensitivity and response resulting from adsorption of organic molecules. Also, the neutron activation electrodeposition technique ( 2 ) of trace analysis is feasible for application in biological fluids, etc. Work is presently being carried out on biological fluids, and will be reported in a future detailed paper. The effects of complex formation between bioorganic molecules and metal ions of interest is also being investigated.
ions in solutions containing organic surface active agents which deactivate the electrode and, possibly, in untreated biological and natural solutions. The membrane simply acts as a barrier to large organic molecules but not to hydrated metal ions or even some coordination compounds. The same membrane-electrode assembly configuration can be employed in three separate methods for the analysis of the trace metal ions. By employing selective complexation, solvent or medium choice, and choice of ion exchange type in the membrane, selectivity could possibly be achieved for one desired ion(s) in a mixture with several others.
CONCLUSIONS
RECEIVED for review April 10, 1967. Accepted July 27, 1967. Research supported in part by Grants from the National Science Foundation (NSF GP 4620 and GP 6425) and from the U. S. Army Research Office-Durham (DA 31124 ARO-D 284).
The membrane barrier electrode might allow one to extend the methods of anodic stripping voltammetry and neutron activation electrodeposition to the analysis of trace metal
ACKNOWLEDGMENT
The authors thank American Machine and Foundry Co., Springdale, Conn., for donating the ion exchange membrane materials used in this research, and the Michigan Memorial Phoenix Project for supplying laboratory space and reactor time.
Adsorption Isotherms of Xylene Isomers on Zinc Oxide by Gas Chromatography Herbert Malamud, Ralph Geisman, and Seymour Lowell Department of Chemistrjq, C. W . Post College of Long Islund Uninersity, Brookcille, L. I., N . Y STOCK(1) succeeded in testing the theory developed by Glueckauf (2-6), that elution of a volatile adsorbed substance from a packed column by an inert gas could be detected by a katharometer and that the resulting chromatogram could be used to develop the adsorption isotherm. Gregg (7) outlines this method, giving several appropriate examples and an explanation of the theory. This method of frontal analysis has been used and discussed by Cremer and Huber (8). In the present study, the authors were concerned with the effective molecular areas of the three xylene isomers on zinc oxide and the relationships betweeen the effective molecular areas, the dipole moments, and the steric requirements of the isomers on the zinc oxide surface. METHODS
Consistent with the experimental method of Stock, the authors have developed the apparatus shown in Figure 1. This design allows the column, packed with zinc oxide adsorbent, to be charged with xylene adsorbate to any desired (1) R. Stock, Ph.D. Thesis, London University, 1955. (2) E. Glueckauf, Proc. Roy. SOC.,(London) 186A, 35 (1946). (3) E. Glueckauf, J . Cheni. SOC.,1947, 1308. 1315, 1327. (4) Zbid., p. 1302. (5) E. Glueckauf, Nature. 156, 748 (1945). (6) Zbid., 160, 301 (1947). (7) S. J. Gregg, “The Surface Chemistry of Solids,” Reinhold, New York, 1961. (8) E. E. Cremer and H. F. Huber, “Gas Chromatography,” N. Brenner, J. E. Callen, and M. D. Weiss, Eds., Academic Press, New York, 1962, p. 169. 1468
ANALYTICAL CHEMISTRY
I
FLOW’
M E T E RS
,
TEMP
I
Ff 7‘
ITEMP. BATH r[
Figure 1. Diagram of gas flow apparatus Valve ( W ) 5 balances pressure drop due to bubblers. Valve 6 balances the drop due to the ZnO sample and the B detector. Valves 1-4 direct flow to load or unload ZnO sample with xylene vapor. (Load, open 1 and 4, shut 2 and 3; reverse for unload.) Valves 7 and 8 bypass sample and 9 balances pressure drop due to sample
equilibrium relative vapor pressure up to unity and provides for a rapid and efficient means of switching the gas flow between pure carrier gas and carrier gas containing adsorbate vapor. The isotherms were measured at 50” C. This temperature is over 60 times greater than the critical temperature of the helium carrier gas; thus, the possibility of adsorption of this gas is precluded. Flow rates of about 40 cc per minute were used and measured with a soap film flow meter (9). Flow rates of two and one half times this value did not alter the (9) A. T. James, and A. J. P. Martin, Biochern. J., 50, 679 (1952).
0.9
1
I
I
I
I
I
1
-.-I
/i
~
,y’
0.71
I
Table I. Isotherms for Xylene Isomers (Values of x/rn in mg per gram of adsorbent cs. relative vapor pressure)
PIP0 0 _1;
,%m
0.01 0.02 0.025 0.03 0.05 0.1 0.15 0.2 0.25 0.3 0.33
,
p-Xy lene 0.185 0.262 0.308 0.371 0.453 0.511 0.560 0.640 0,656
0.5
0.6 0.7 0.8
0.9 0.916 0.94 0.953 0.96 0.977 0.I Figure 2.
P/Po
0.2
0.3
BET plot
The slope is equal to ( C - l ) - l ( . ~ / ~ ~ ) ~ ~and -lC intercept -l is equal to ( ~ / m ) . , ~ - l C - 1where , (s/rn).t~is the monolayer quantity of adsorbate per gram of adsorbent
0.790 1,000 1.33 2.04 2.59
o-Xylene
nz-Xylene
0.185
0.185
0.264 0.341 0.393 0.434 0.468 0,503 0.523 0.646 0.794 0.972 1.27 1.90
0.264 0.348 0.403 0.446 0.484 0.515 0.530 0.644 0.792 0.960 1.26 1.84
2.54 3.24 4.23
3.13 5.48
a Lower portions of the isotherms were constructed from the chromatographic discharge curve after charging the colum t3 (PIP,) = 0.33. Upper portions of the isotherms were constructed from the chromatographic uptake curves formed by charging the column with xylene-saturated carrier gas.
where shape of the chromatogram curve, indicating that column equilibrium existed during each run. The zinc oxide sample weighed 79.4 grams and had a surface area of 1.35 sq. meter per gram, as measured by nitrogen adsorption. The zinc oxide sample was contained in a cylindrical column 50 cm long constructed from 0.5-inch soft copper tubing. After introduction into the system, the sample was heated to 100 O C for 24 hours and exposed during this time t o a flow of helium “Zero” gas. After each run, the sample surface was necessarily uncontaminated by xylene. The adsorption process was completely reversible as evidenced by the equality of the xylene uptake and discharge signal areas as recorded on a 10-mV chart recorder. Normalization of the signal was accomplished by measuring the signal height produced by saturated vapor of known vapor pressure ( I O ) in the carrier gas, bypassing the sample. The area produced on the recorder for a given flow rate over a known time interval at saturation generates all the parameters required for calibration. The xylene vapor conof the tained in the column dead space contributed 1 to total signal area, for which all the data given have been corrected. The xylene isomers used were distilled a t their normal boiling points under nitrogen. All temperatures were maintained to 1 0 . 0 5 ” C during the absorption and desorption processes. In reducing the chromatograms to isotherms using the method outlined by Gregg (7), one can relate the number of grams adsorbed per gram of adsorbent a t any equilibrium relative vapor pressure to the chromatogram signal area by Equation 1. = PFMA SCimRPa
Zz
(IO) “Physical Properties of Chemical Compounds,” Adcarz. Chem. Ser.., 15, Am. Chem. SOC.,1955.
x = grams adsorbed m = grams of adsorbent
M = molecular weight of adsorbate A = Signal area (sq. inches) F = flow rate (cc minute-l) P = vapor pressure of adsorbate (mm Hg) s = signal height when PiPo = 1 (inches) c = chart speed (inches minute-]) T = absolute temperature R = gas constant, 82.1 ml atm deg-l K mole-] Pa = standard pressure, 760 mm Hg RESULTS AND DISCUSSION
Quantities in grams of xylene adsorbed per gram of adsorbent, q‘in, and the corresponding values of PiPo are listed in Table I. A BET plot (11) is shown in Figure 2 from which the monolayer values of x / m were calculated. The values of xim at the monolayer for ortho-, meta- and paraxylenes are 0.39, 0.40, and 0.48 mg/gram, respectively: with corresponding effective areas of 61, 60, and 50 square A. The root-meansquare error in the values of the areas given is approximately 4z, as calculated from the probable errors in the parameters of Equation 1. Because the ortho- meta-isomers have permanent dipole moments of 0.52 D (liquid), 0.62 D (gas), and 0.31 D (liquid), 0.37D (gas), respectively ( I 2 ) , it appears that these molecules are constrained to a n orientation that requires the methyl groups of adjacent molecules to face in the same direction, (11) S. Brunauer, P. H. Emmett, and E. Teller, J . A m . Chem. Soc., 60, 309 (1938). (12) A. J. Petro and C. P. Smytli, J . Am. Clzem. Soc., 80, 73 (1958). VOL. 39, NO. 12, OCTOBER 1967
1469
thus prohibiting the more efficient packing that results when this restriction is absent-i.e., in the case of the para-isomer. The variation in the effective areas implies a certain degree of mobility on the surface in order for orientation of the dipoles to occur. An alternative explanation for the greater adsorption of the para-isomer could be the possibility of a microporous surface preferentially permitting entry of the p-xylene with its smaller cross section. This explanation does not allow for
stabilization of the adsorbed state (ortho and meta) and is consistent with the observed enhancement of the para adsorption. ACKNOWLEDGMENT The authors express their thanks to the New Jersey Zinc Co. for supplying and measuring by nitrogen adsorption the zinc oxide specific area used as adsorbent in this work. RECEIVED for review April 13, 1967. Accepted June 30, 1967.
lodometric Determination of Butadiene Polyperoxide Bernard Braithwaite and George E. Penketh Imperial Chemical Industries Limited, Heavy Organic Chemicals Division, Research Department, Billingham, County Durham, England BUTADIENE forms two polymeric peroxides by the 1,2- and 1,Caddition of oxygen. The mixed polymer, in which the 1,4-form usually predominates, is a light yellow, viscous oil of average chain length 8 to 10 units, and corresponds approximately to the empirical formula (CaHeOs)n. It is relatively unreactive chemically, but is both impact-sensitive and thermally unstable, and has a low solubility in butadiene. Slow deposition over a period of time can lead to very hazardous conditions in butadiene plants, and it is of paramount importance that methods for the determination of the peroxide content of butadiene should be capable of determining low concentrations of polyperoxide. The methods in common use, however, are applications of well known procedures for reactive peroxides and have been found entirely unsuitable for the less reactive polyperoxide. Thus, for example, the ASTM method ( I ) , based on the oxidation of ferrous iron to ferric, and widely regarded as the official method, gives recoveries on polyperoxide of less than 10% of the true value. Methods involving stannous chloride (2), triphenyl phosphine (3), and various iodometric procedures (4-6) also failed to give quantitative results; although two of the procedures described by Mair and Graupner (7), involving reflux with sodium iodide in acetic acid, give better recoveries (up to 90 %), high and variable blanks precluded their use in the determination of trace quantities. Preliminary work indicated that the most promising approach was the in situ generation of hydrogen iodide using a high boiling alcohol as solvent. In order to maintain a single phase system with high iodide concentration, lithium iodide was preferred to the more commonly used sodium iodide. Phosphoric acid was selected as the most suitable mineral acid. (1) ASTM Method D1022-64, American Society for Testing and
Materials, Philadelphia, Pa. (2) D. Barnard and K. R. Hargrave, Ana/. Chim. Acra, 5, 476 (1951). (3) L. Dulog and K. H. Burg, 2.Anal. Chem., 203, 184 (1964). (4) W. E. Vaughan and F. F. Rust, U. S. Patent 2,403,771 (July 9, 1946). (5) C. D. Wagner, R. H. Smith, and E. D. Peters, IND. ENG.CHEM., ANAL.ED., 19,976 (1947). (6) J. P. Wibaut, H. B. van Leeuwen, and B. van der Wal, Rec. Tral;. Chim., 73, 1033 (1954). (7) R. D. Mair and A. J. Graupner, ANAL.CHEM., 36,194 (1964).
1470
ANALYTICAL CHEMISTRY
Butadiene polyperoxide prepared by the method of Handy and Rothrock (8) was found to be relatively impure, and their method was modified by incorporating azo-bis-isobutyronitrile as a reaction initiator, permitting a much lower reaction temperature to be used. Use of a glass reaction vessel also greatly reduced by-product formation. EXPERIMENTAL Preparation of Butadiene Polyperoxide. APPARATUS. Butadiene polyperoxide is a very hazardous chemical and in its preparation attention to detail is vital for safety reasons. All experiments involving peroxidation of butadiene were carried out in a concrete explosion bay, from which all personnel were excluded during the reaction period. The reactor was a standard QVF glass U-tube 1-inch i.d., fitted at each end with a 1-inch i.d. to 0.25-inch 0.d. capillary (2-mm i.d.) adapter. A small needle valve was attached to one adapter by means of a 2-foot length of stainless-steel capillary. The other adapter was closed by a removable blank end. The total volume of the reactor was approximately 250 ml. The reactor was suspended in a water bath maintained at 50" i 1 O C, and was gently agitated by means of an electric motor, reduction gear, and eccentric. The bath and reactor were screened by a 0.375-inch thick perspex safety screen, with the needle valve clipped on to one corner to permit pressure in the reactor to be let down with the operator behind the screen. The water bath was provided with a thermal cut out so that in the event of a temperature runaway or water failure, the heaters would be switched off. A 0.075-gram sample of azoMETHODOF PREPARATION. bis-isobutyronitrile was dissolved in 10 ml of benzene and transferred to the reactor by means of a syringe fitted with a 6-inch needle. The reactor was weighed, then cooled to about -60" C in a methanol/solid carbon dioxide bath, and 7-9 grams of uninhibited butadiene were condensed into it. Oxygen, at atmospheric pressure, was passed through the reactor for not less than 10 minutes to flush out all nitrogen, then the reactor was sealed by fitting the blank end and closing the needle valve. The vessel plus contents were weighed and transferred to the water bath, which was at ambient temperature at this stage, The shaker was switched on so that the reactor was gently agitated in a longitudinal direction and (8) C. T. Handy and H. S . Rothrock, J. Am. Chem. SOC.,80, 5306 (1958).
