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Air Buoyancy Errors and the Optical Scale of a Constant-Load Balance Sir. T h e question of buoyancy corrections for the portion of a sample weight measured by the optical-scale deflection on constant-load balances was recently examined ( 1 ) . Buoyancy errors as large as a few tenths of a milligram were calculated for maximum optical-scale deflections. We wish to point out t h a t in normal calibration and use of a balance the error described does not occur. Buoyancy corrections must be applied to t h e portion of the weight obtained from the optical scale in exactly t h e same way as t o t h a t portion obtained directly from use of t h e weights. Thus, t h e buoyant effect of air on t h e object is equal to the apparent weight multiplied by No special buoyancy correction for t h e optical-scale portion of the weight is necessary when the optical scale is calibrated by a small standard weight in air, where the buoyant effect is operative on the calibrating weight. After the optical scale has been calibrated, as it must be, then the same buoyancy corrections apply to all weights observed, whether removed from the beam by dialing or read from the optical scale. In the recent treatment ( I ) the implied assumption is that t h e optical scale was calibrated by operations performed in vacuo. This is unnecessary in practice. For optical-scale calibrations performed in air, Equation 18 should be modified by multiplying Moptby the term (1 - dalr/d6.J. With this modification, Equation 19 becomes identical t o Equation 1. Our conclusion was confirmed experimentally by weighing a n object of density 1.0 and apparent weight 1.0004 g. Three Mettler H32 analytical balances (optical scale 1 g, readable to 0.1 mg) were used. Ten independent weighings were made by each of us, first by reading the total weight from the optical scale at full deflection, and then by dialing 1.0 g and reading the optical scale near its zero position. For the system tested, t h e difference in weight readings predicted ( I ) by Equation 21 is 0.2 mg. We observed no difference (average difference 0.03 mg, standard deviation 0.05 mg). Therefore, as expected from the above discussion, t h e error predicted by Equation 21 is not substantiated. Although use of a n optical-scale beam-deflection system to obtain t h e last fraction of weight of a sample is not subject
t o special buoyancy error, optical scales can nevertheless be a significant source of error if they have gone out of calibration through changes in air density, balance leveling, or position of the sensitivity-adjustment nut. Many users do not recognize how readily the optical scale can be checked without a special Class M or similar standard weight. A procedure we have recommended (2) t h a t does not require a standard external weight, t h a t provides internal consistency of optical-scale readings with the other weights in the balance, and also is rapid and convenient, is briefly as follows (for a balance with a maximum optical scale of 1 g). Calibration Procedure. Add an object (any rough weight or object can be used) of about 1 g to the pan. Dial 1 g on t h e balance and set the optical scale t o read zero. Dial 0 g on t h e balance and read the optical scale. If the reading is not 1.0000 g, adjust the sensitivity nut. Repeat the above procedure until the optical scale reads 1.0000 g. Since the optical-scale adjustment can thus be tested in a matter of seconds, daily checking is recommended. Recent instruction manuals by some balance manufacturers specify this procedure. LITERATURE CITED (1) M. R. Winward, E. M. Woolley, and E. A. Butler, Anal. Chem., 49, 2126 (1977). (2) W. E. Harris and B. Kratochvil, "Chemical Separations and Measurements.", W. B. Saunders Co., Philadelphia, Pa., 1974, p l 1
F. F. Cantwell* B. Kratochvil W. E. Harris Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
RECEIVED for review January 23, 1978. Accepted March 27, 1978.
AIDS FOR ANALYTICAL CHEMISTS Vacuum Electrochemical Cell Applied to the Oxidation of Bis( cyclopentadieny1)chromium John D. L. Holloway, Fred C. Senftleber, and William E. Geiger, Jr." Department of Chemistry, University of Vermont, Burlington, Vermont 0540 1
Designs of highly functional vacuum electrochemical cells have been published (1-3). However, our recent work has required construction of a cell which allows for (a) introduction of highly air- or water-sensitive solid starting compounds, and (b) minimal variation of potential across the surface of the large mercury pool electrode used for bulk electrolysis. The former point requires little elaboration. Many species of
Advantages arising from the use of vacuum line procedures in electrochemical studies are well established ( 1 , 2). Proper use of vacuum procedures can assure minimal contamination of solvent/supporting electrolyte systems by H 2 0 and 02, which if present will often affect electrochemical redox pathways by reacting with either the electroactive starting compound or with the products of its reduction or oxidation. 0003-2700/78/0350-1010$01 OO/O
C
1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7 , JUNE 1978
possible electrochemical interest are highly air-sensitive, being particularly common among organometallic compounds. Previously published cell designs do not provide for a way to get samples of such solids into the cell without contamination. The latter point, concerning variation of the control potential across the working electrode, deserves more comment. Harrar and co-worker have published theoretical and experimental evaluations ( 4 , 5 ) of the effect of relative positioning of the working, auxiliary, and reference electrodes on the potential distribution across t h e surface of the working electrode. It was shown t h a t the traditional “H-cell” arrangement of working and auxiliary electrodes gave rise to large potential gradients, due t o variations in ir drop across t h e working electrode surface. These gradients could be particularly damaging if one were trying to electrolyze a t a mercury pool a t t h e potential of the first of two closely spaced electrochemical waves, since improper placement of the reference electrode could cause part of t h e electrode surface to be a t t h e potential of the second reduction. Difficulties are more pronounced in nonaqueous electrolytes, in which ir loss is generally greater. An electrode arrangement in which the working and auxiliary electrodes are parallel, and of similar size and shape, was shown t o minimize potential gradient effects. T h e cell reported in this paper has a n improved electrode arrangement and allows for facile introduction of air-sensitive starting materials. In addition, it retains the versatility of our previously described cell (3). Thus we can perform experiments a t a dropping mercury electrode, a large mercury pool, stationary mercury drop, or a platinum bead, all on the same solution, if desired. Some representative experiments involving air-sensitive reactants and products are reported to demonstrate the utility of the cell.
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-1 RESERVO IR
EXPERIMENTAL Instrumentation. All experiments were performed with a Princeton Applied Research (PAR) model 173 potentiostat equipped with a PAR 179 digital coulometer. A Hewlett-Packard (HP) model 3300A function generator was used to generate triangular waveforms for cyclic voltammetry. An HP model 7001A X-Y recorder and a Tektronix model 564B storage oscilloscope were employed in recording current-voltage curves. Operations necessitating nitrogen atmosphere were carried out with a Vacuum Atmospheres dry box equipped with a recirculating Dry-Train. Solvents and Supporting Electrolyte. Tetrabutylammonium hexafluorophosphate (Bu4NPF6)was used in all experiments as the supporting electrolyte, at a concentration of 0.1 M. It was prepared by methathesis of Bu4NI with NH,PF, in acetone/HzO and recrystallized from ethanol prior to vacuum drying. Acetonitrile was Eastman Spectrograde. It was stirred over calcium hydride. Tetrahydrofuran (THF) (Fisher Spectroquality) was stirred over LiAlH,, vacuum degassed, and then distilled bulb-to-bulb onto sodium and benzophenone. It was stored under vacuum in the presence of the benzophenone ketyl until needed. Cell Description. A schematic representation of the cell is given in Figure 1. The main body of the cell consists of two halves sealed via a ground glass flange. The top half of the cell provides the mounting joints for most of the electrodes and the solvent receiving vessel. The bottom half of the cell contains the auxiliary electrode and mercury pool electrode as well as the evacuation port and the sample-loading device. The two-piece construction allows for easy assembly and cleaning of the cell. Ground joints of the standard taper variety were generally 24/40, except for the P t bead inlet (10/30) and the inlet for the quasi-reference electrode (14/35). Ball and socket joints were of 28/12 size. Other joints are as shown in individual diagrams. All ground joints were periodically reground with its mate in order to assure the most leak-free connections. The auxiliary electrode consisted of a platinum spiral silver-soldered to a tungsten lead inserted through the side of the cell bottom. It was separated from the working electrode compartment by a 30-mm sintered glass frit located concentrically above the mercury pool. The
Figure 1 . Assembled vacuum electrolysis cell
reference electrode frit (vide infra) may be aligned between the auxiliary frit and the mercury pool in an arrangement which minimizes errors in potential control. A 20-gauge piece of platinum wire served as the reference electrode. This electrode, more properly termed a quasi-reference electrode, was found to be convenient because it did not require introduction of foreign ions (e.g.,Aq+) to the cell, and had a fairly stable potential in these nonaqueous media. Species examined in this cell and in other cells which allow use of the aqueous saturated calomel electrode (SCE) have established that the potential of the quasi-reference is generally within 100 mV of the SCE. In one experiment. both chromocene and Ni[S2C2(CN)z]2were added to the solution in order to check the potential of the chromocene waves vs. a well-known standard (3),which then allows reporting of the potential versus the aqueous SCE. The Pt quasi-reference electrode was housed in a “J”-shaped compartment which terminated in a narrow (ca. 4 mm) fine porosity glass frit. The electrode compartment was filled with the electrolyte solution through a port on its side opening to the main cell body above the solution fill level. The “J” arrangement allowed the electrode to be rotated (by its mounting joint) near any of the working electrodes during an experiment in order t o minimize solution resistance effects. Several different electrodes are present which can be employed during an experiment as working electrodes. These include the dropping mercury electrode (DME), a stationary mercury drop, a small platinum bead, or a large mercury pool. The DME has a pressure equalizing arm such as was previously described ( 3 ) . For cyclic voltammetry measurements at a stationary mercury drop, a small cup was employed which could be swung underneath the DME to catch one or two drops. This was found to be much easier than trying to hang a drop after the cell was assembled. Cyclic voltammetry measurements at platinum were accomplished with a Pt bead sealed into soft glass. Finally, connection to the mercury pool (for bulk electrolyses) at the bottom of the cell was
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
f i 14/55
24/40
200 ML FLbSK
b-w-a r - n - ’I
TEFLON PLUG
Flgure 3. Solvent receiver flask. The 1 4 / 3 5 joints allow for rotation
of the flask into a position suitable for pouring of liquid through the spout. The 24/40 joint seals the vessel into the cell and the flange around the joint allows one to rotate the entire assembly so that the position of the pour spout can be controlled
I
? -2.0 -0.5 VOLTS
VI
SCE
DC polarogram of 7.4 X M Cp,Cr in the CH&N containing 0.1 M Bu4NPF, supporting electrolyte Figure 4.
Flgure 2. Device for loading air-sensitive samples into electroiysis cell. The function of the various parts is described in the text
accomplished with a tungsten lead. Air-sensitive compounds were introduced to the solution using the loading device shown in Figure 2. The most important part of this device is a 3-mm Teflon needle valve which has had the glass section shortened and sealed and the Teflon plug cut near the end of the threaded section. This allowed us to load a sample into the valve and keep it protected from the air. The valve assembly was then lowered into a piece of square Pyrex tubing attached to the cell via a 24/25 joint, which was then plugged by a 24/40 joint as shown in Figure 2. The bottom of the 24/40 joint was cut out so the notch fit around the handle of the Teflon plug, which was squared off to fit into the notch. By rotating the 24/40 joint, the Teflon plug turned, dislodging the ampule containing the sample into the cell solution. Cell Operation. To perform an experiment the cell is assembled as in Figure 1. Supporting electrolyte is weighed out and placed in the sample receiving flask. The compound to be studied is loaded under nitrogen into the preweighed loading ampule, weighed, and put in position in the sample-loading device as noted above. The cell is evacuated to a pressure of about lo4 mm Hg (usually this takes several hours) and then the solvent is distilled into the cooled (liquid nitrogen) receiver flask (Figure 3) by bulb-to-bulb transfer. The solvent can be submitted t o further freeze-pump-thaw cycles at this point if desired. Finally, the electrolyte solution is thawed and poured into the compartments housing the reference, auxiliary, and working electrodes. Background scans can be run at this time, after which the sample is introduced as noted above and the desired electrochemical measurements are made. S Y S T E M EVALUATION Background C u r r e n t s . As expected, electrolyte solutions made under these conditions are free of measurable amounts of electroactive impurities. The electrochemical “window”
of an electrolyte system is of course arbitrarily defined depending on what one considers an unacceptably high background current. The “window” for THF/O.l M Bu4NPF6was from +0.6 V to -3.1 V using the rather stringent requirement that background currents in dc polarograms must be less than 0.2 PA. Other electrolyte systems gave similar behavior. No evidence of the oxygen reduction wave was observed, even a t very high current sensitivities. Oxidation of Chromocene. Bis(a-cyclopentadieny1)chromium, Cp,Cr, is a highly air-sensitive metallocene which has been known for over 20 years (6). Metallocenes are generally electroactive and most of the more air-stable members of this series have been extensively studied by electrochemical methods (7). T o our knowledge, no voltammetric data on chromocene has appeared, in spite of the fact that Fischer and Ulm reported details of the chemical oxidation of Cp,Cr to cation salts such as Cp,CrI many years ago (8). Since Cp,Cr is so highly air sensitive (solid samples are pyrophoric), it seemed to constitute a good system on which to check out our cell and procedures. No problem of decomposition of chromocene was experienced in this study. M Cp,Cr in CH,CN/O.l M A dc polarogram of 7.4 X Bu4NPF, shows both an oxidation and reduction wave (Figure = 4 . 6 7 V corresponds to the formation 4). The wave of of CpzCr+via a one-electron reversible oxidation. A plot of the potential vs. log [i/(id - i)] was consistent with this, being linear with a slope of -54 mV (theory -59 mV). The reversibility of the oxidation was confirmed by cyclic voltammetry (Figure 5) measurements a t a mercury drop a t slow sweep rates. the iJia ratio was unity and the potential separation between the cathodic and anodic peaks was about 70 mV. Since no positive feedback was employed in these experiments, it is most likely that the increase in the peak separation over the theoretical 60-mV value is due mainly to uncompensated solution resistance effects. Another indication t h a t the oxidation was a simple, diffusion-controlled process uncom-
ANALYTICAL CHEMISTRY, VOL. 50,
F W
n n
U 3
-0 3
-0 9
V O L T S n SCE
Figure 5. Cyclic voltammogram of solution of Figure 4 at stationary mercury drop electrode at a scan rate of 100 m V / s
1978
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of the redox couple (Cp,Cr in our case), V is the volume of the solution, p is a constant dependent on the cell geometry and stirring rate, and i and F have their usual significance (9). A plot of log i vs. t for the chromocene oxidation gave a straight line with a value of p = 2.8 x (calculated from the slope) and a value of n = 1.18 (calculated from t h e intercept). The linearity of the log i vs. t relationship is again consistent with a n uncomplicated electron transfer in the oxidation. This was further proved by reversal coulometry. In this experiment, after electrolysis at 0 V for time t , t h e potential is set sufficiently negative to reduce any CpzCrf which may be present in solution. A potential of -1.0 V was employed. If the initial oxidation product is stable and not subject to follow-up reactions, t h e number of coulombs consumed in the oxidation and reductions steps, Qox and QM, respectively, are related by QRed -
-
--1
I
NO. 7, JUNE
1 - exp(-pt)
where t is the same for both electrolyses (1000 s in the present case). A value of 0.92 was predicted by Equation 2, using the value of p given above, and a value of 0.95 f .05 was measured by integration of the current-time data. A polarogram of the solution after the initial oxidation showed the 2 reduction waves of Cp2Crt very clearly (Figure 6). Cp&r can also be reduced. A diffusion-controlled wave with a half-wave potential of about -2.3 7\ involves formation of Cp@, followed by a reaction to give another electroactive product(s). Details of t h e reduction will be published in a subsequent paper concerned with metallocene reductions.
xc
-2 0
-0.5 VOLTS
YS
SCE
Flgure 6. DC polarogram of Cp,Cr+ solution produced by electrolysis of Cp,Cr at 0 V at mercury pool electrode
plicated by preceding or following reactions was obtained by our observation t h a t t h e anodic current function (ipa/t,' ' z , where iPl is the anodic peak current and u is the scan rate) was independent of scan rate over the range u = 50 to 500 mV/s. T h e long-term stability of t h e chromocene cation was investigated by bulk oxidation of t h e neutral complex a t t h e mercury pool anode, a t 0 V. For an uncomplicated anodic electron-transfer reaction, t h e current-time relationship is given by Equation 1,
ACKNOWLEDGMENT We thank Roy Clark for his glassblowing efforts, and we gratefully acknowledge the support of t h e National Science Foundation in this work. LITERATURE CITED (1) A. J. Bard, Pure Appl. Chem., 25, 379 (1971). (2) J. L. Mills, R. Nelson, S. G. Shore, and L.. B. Anderson, Anal. Chem., 43, 157 (1971). (3) W. E. Geiger, Jr., T. E. Mines, and F. C. Senftleber, Inorg. Chem., 14, 2141 (1975). (4) J. E. Harrar and I . Shain, Anal. Chem., 38, 1148 (1966). (5) J. Newman and J. E. Harrar, J . Electrochem. SOC.,120, 1041 (1973). (6) E. 0. Fischer and W. Hafner, 2. Naturforsch., 6 .8, 444 (1953); G. Wilkinson, J . Am. Chem. Soc., 7 8 , 209 (1954). (7) For leading references, see W. E. Geiger, Jr. J . Am. Chem. Soc., 9 6 , 2632 (1974). (8) E. 0. Fischer and K . Ulm, Chem. Ber., 9 5 , 692 (1962). (9) A. J. Bard and K. S.V. Santhananrn, Necfroanal. Chem., 4, 215 (1970); see especially pp 237-239.
Pt log i = -+ log nFVC, P (1) 2.3 where t is the electrolysis time, n is the number of electrons
RECEIWDfor review November 21, 1977. Accepted February
transferred, CR,is the bulk concentration of the reduced form
6 : 1978.
Liquid Sample Loop Geometry for Trace Analysis by High Performance Liquid Chromatography with Large Sample Volumes James M. Zehner" and Richard A. Simonaitis Stored-Product Insects Research and Development Laboratory, Science and Education Administration, USDA, Savannah, Georgia 3 1403
High performance liquid chromatography (HPLC) has many advantages compared with gas chromatography. However, in the case of trace analyses, the HPLC detectors sometimes lack sensitivity. This disadvantage can be overThis paper not subject to U S Copyright
come by injecting large-volume samples (1mL or larger). But when t h a t is done, it is necessary to dissolve the sample in a solvent which causes the components t u accumulate a t the front of the column and be eluted by rapidly changing the Published 1978 by the American Chemical
iii~iaiy
