reaction vessel are switched on. After equilibrating the system for 30 min, the stirrer is switched off again and the substrate is introduced into the reaction vessel with a syringe. The pressure is adjusted by valve 2, and the reaction is initiated by starting the stirrer. The hydrogen uptake is then recorded automatically against time.
gas cylinder and proved to be very useful for small-scale hydrogenations and kinetic measurements. Note. The apparatus can accommodate gas cylinders with a volume up to 1000 ml and is also useful for other gasliquid phase reactions. Full details and design of the apparatus are available from the authors.
RESULTS
The apparatus and procedure described have, for instance, been used for the hydrogenation of benzene, using a 5 x platinum-on-carbon catalyst in acetic acid at 30.0 O C . The reproducibility of the rate of hydrogen uptake of six successive runs was 1.5x. The apparatus was equipped with a 70-ml
ACKNOWLEDGMENT
The authors thank the general service department for building the apparatus. RECEIVED for review December 2, 1969. Accepted February 26, 1970.
A “Do-It-Yourself” Electron Paramagnetic Resonance Cell J. F. Ambrose and D. Dillard Department of Chemistry, Kansas University, Lawrence, Kan. 66044
A. K. Carpenter and R. F. Nelson Department of Chemistry, Sacramento State College, Sacramento, Ca1.f. 95819
AMONGTHE PROBLEMS one enounters in practical EPR spectrometry are the cost and inflexibility of commercial quartz EPR cells. Different types of cells are available for a variety of functions-e.g., aqueous and non-aqueous work, tissue samples, etc.-but the cost in acquiring and replacing these cells is quite high. We have found that a suitable type of EPR cell for some kinds of work can be fashioned from heatshrinkable transparent Teflon (Du Pont) tubing and ordinary glass. Herein, a particular type of cell is described, one that is of use to us, but it appears that other cells could easily be constructed to suit anyone’s individual needs. At present, commercial EPR cells are constructed of quartz rather than borosilicate glass because of the presence of paramagnetic species in the latter. Teflon, unless irradiated, is also free of paramagnetic impurities, so it appears to be a suitable and far less expensive replacement for quartz. This “do-it-yourself”-type cell consists of a short piece of transparent, heat-shrinkable, thin-walled Teflon tubing (a brochure including tubing dimensions and prices is available from Kaufman Glass Co., Plastics Division, 1209-21 French St., Wilmington, Del. ; nominal i.d. tubing dimensions varying from 0.023 to 1.000-inch are available) that is flattened by simply pressing and creasing, leaving a small portion of the tubing at each end still rounded. Application of heat distorted the tubing uncontrollably, so the flattening should be done mechanically. Any kind of glass can be fitted into the tubing; in this case a quartz NMR tube was used, but borosilicate glass would also be perfectly usable since the glass portions are not in the cavity of the spectrometer. The glass is fitted snugly into the Teflon tubing and then sealed in by gentle heating, making sure that the flattened portion is not heated. If the seal is not tight, an epoxy glue can be smeared around the outside to effect a more solvent-tight junction. Studies involving the chemical and electrochemical generation of radical species are of primary interest to us; we have tested this type of cell under these conditions and have found 814
ANALYTICAL CHEMISTRY, VOL. 42, NO. 7, JUNE 1970
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GAUSS
,
Figure 1. EPR spectrum of cation radical of p-ethoxyN,N-dimethyl-aniline generated in propylene carbonate-0.1F iodine it to be very satisfactory. Spectra obtained by chemical and electrochemical generation of radical species are shown in Figures 1 and 2, respectively. These spectra are at least as good as those obtained using the commercial quartz electrolytic cell supplied with the Varian E-3 spectrometer. The signal-to-noise ratio is about the same with both cells (quartz and Teflon) as is the resolution. Note that these studies were conducted in non-aqueous media. This particular cell is not suitable for aqueous work, since the Teflon portion is fairly thick and hence contains a relatively large amount of solution. Due to the high dielectric nature of water, the cell cannot be tuned because of excessive power loss. However, it should be no great problem to produce a cell with a flatter, thinner Teflon portion. If reproducibility of cell dimensions is a necessity, this may also be somewhat of a problem with these cells. Although we did not test the reproducibility in constructing these cells, it would seem to be rather difficult. In addition, because of the nature of the Teflon, cell dimensions may fluctuate somewhat from day to day, albeit slightly. Admittedly, just a “bare-bones”-type cell is presented; the
‘ I
, IO
GAUSS
the only one that might be touchy is UV irradiation work, since irradiation of Teflon gives rise to detectable EPR signals. The Teflon tubing itself was tested for paramagnetic resonance signals by tuning the empty cell and scanning from 2400 to 4400 gauss at 9.502 GHz (employing a Varian E-3 spectrometer) at high sensitivity. No signal was observed above the normal noise level. The advantages of this cell, then, include ease of fabrication, low cost, and flexibility in that one can construct different cells for various applications at a minimal expense. Clearly these cells cannot be used for UV irradiation work or at elevated temperatures (they are, however, usable for low temperature studies). Other limitations that could probably be overcome with little effort are the reproduction and constancy of cell dimensions and the unsuitability for aqueous studies. With more frequent applications of EPR spectroscopy in analytical work beginning to appear, this versatile, inexpensive cell may be of considerable utility; in addition, it seems ideally suited for routine non-aqueous EPR studies.
,
Figure 2. EPR spectrum of p-nitrotoluene anion radical electrolytically generated in N,N-dimethylformamide-O.1F tetraethylammonium perchlorate at platinum
ACKNOWLEDGMENT
idea is simply that heat-shrinkable Teflon can be shaped to serve in whatever manner desired. The configuration can readily be adapted to a particular need, since the upper and lower glass portions can be constructed in any way desired before they are inserted into the Teflon tubing. It should be usable for many types of EPR applications; about
Special thanks are due to R. N. Adams and D. E. Smith for their support and encouragement. RECEIVED for review January 14, 1970. Accepted April 9, 1970. Work supported by NSF Grant No. GP-8941.
A Syringe Pycnometer for the Accurate Weighing of Milligram Quantities of Aqueous Solutions G . C. Lowenthal and Valdis Page Research Establishment, Australian Atomic Energy Commission, Lucas Heights, N.S. W. 2232, Australia THEACCURATE WEIGHING of open drops of an aqueous solution is complicated by evaporation effects. These effects are commonly allowed for by reading the drop mass at regular intervals after the ejection of the drop; the results being extrapolated to the time of ejection. However, the loss of heat from freshly formed drops is nonlinear for the first 5 to 10 seconds because of rapid evaporative cooling immediately after deposition ( I ) . The resulting temperature changes depend among others on the surface to volume ratio and are the greater for smaller drops. For a 10-mg drop the error from a linear extrapolation could reach 1%. The errors in small drop masses can be reduced substantially if a pycnometer is used for the weighings (1, 2 ) or if the temperature of the ejected aliquots is controlled so as to ensure the validity of a linear extrapolation ( 3 , 4 ) . The currently preferred method for the accurate weighing of milligram drops of aqueous solution is to use plastic ampoule pycnometers of about 8 ml air displacement with their opening (1) P. J. Campion, F. W. G. Dale, and A. Williams, Nucl. Znstrum. Methods, 31, 253 (1964). (2) J . S. Merritt and J. G. V. Taylor, A. E. C. L. Report, Chalk
River, Ontario, CRGP-1256 (1967). (3) A. E. Oakley and G. C. Lowenthal, Proc. I.A.E.A. Symp. Vienna STIIPUBI139 SM 79/42, p 519 (1967). (4) W. van der Eijk and H. Morel, Proc. 1.A:E.A. Symp. Vienna STI/PUB/139SM 79/43, p 529 (1967).
pulled out to a 5 to 8 cm long finely drawn tube (2, 5). However, it is then difficult to vary drop sizes except by varying the diameter of the outlet tube and the upturning of the ampoule during delivery disturbs the equilibrium conditions sufficiently to require a delay of 5 to 10 minutes before reproducible balance readings (within =k 2 pg on a microbalance) are reestablished. DESCRIPTION
To provide for increased flexibility and speed without loss of accuracy we have replaced the ampoule by a syringe pycnometer mounted as shown in Figure 1. The 2-ml capacity high quality disposable syringe is made from polypropylene and is supplied by the Boots Pure Drug Company of Sydney. The piston moves smoothly and the amount of vapor escaping between piston and wall is generally less than about 1 pg h-1; only about 10% of the syringes had leak rates past the piston in excess of 5 pg h-l. The syringe outlet is either the usual steel needle or, if acid solutions are used either 4-mm bore polyethylene tubing drawn at one end to about 0.3 mm i.d. or preferably glass needles. The outlet is heat-sealed to the conical end of the syringe to ensure a vapor tight fit. Figure 1 shows a glass needle of about 0.5-mm bore and 4 to 5 cm long; it was made as described elsewhere (3) and treated inside and outside with silicone fluid to ensure a clean break of the drop formed (5) A. P. Baerg, Metrologiu, 2, 23 (1967). ANALYTICAL CHEMISTRY, VOL. 42, NO. 7, JUNE 1970
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