The intervals when precolumn effluent was vented are indicated by arrows on the TC trace. Terminating venting a t a TC signal closer to the base line would have minimized tailing into the analytical column. Chromatographic conditions were the same as in Table I except that the oven temperature was programmed from 70 to 230 "C at 8 "C/min. In addition to GC/MS and general packed column chromatography, pneumatic switching offers a unique oppor-
Spinning Dropping Mercury Electrode-A
tunity for trace analysis using capillary columns. By venting solvent and/or major components, large injections can be used without the sample loss encountered with conventional inlet splitting. Thus, both high resolution and high sensitivity can be achieved. Received for review November 30, 1972. Accepted April 2,1973.
Practical Analytical Tool
Henry J. Mortko and Richard E. Cover D e p a r t m e n t of Chemistry, St. John's University, Jamaica,
N.Y.
11439
Many efforts have been made to adapt the dropping mercury electrode (DME) to the continuous analysis of stirred or flowing systems. A variety of cells (1) for use with the DME have been constructed to minimize the erratic effects arising from convection. A successful approach to this problem which permits direct immersion of the electrode in the agitated solution involves the controlled detachment of the mercury drops by the vibration or periodic mechanical shock of the capillary (2). This vibrating dropping mercury electrode (VDME) has been demonstrated to be superior to the DME in many respects, both as an analytical detector and for theoretical purposes when drop times are sufficiently small (2-7). As an analytical device, however, the VDME has some serious disadvantages. To obtain millisecond drop times, large mercury pressures are required and the electrode vibrator must be mechanically complex (8). At such short drop times, the charging current increases to the microampere range decreasing VDME sensitivity to below DME levels ( 5 ) .Finally, the vibration of the capillary precludes direct sealing of the electrode into flowing streams. The work reported here demonstrates that it is possible to retain all the analytical virtues of the VDME and eliminate its disadvantages by the simple expedient of spinning the capillary. The spinning dropping mercury electrode (SDME) discussed here differs mechanically from that of Kolthoff and coworkers (9) in two ways. Kolthoffs capillaries were U shaped with the orifice pointing upward while ours are straight barometer-tubing capillaries pointing downward. In addition, Kolthoffs capillaries were of relatively large bore (10) compared with the commercial capillaries employed here. The practical effects of the simple differences are striking. The geometry of Kolthoffs electrode prevents its use in small-diameter flowing streams. The large bore of his capillaries with the attendant large mercury flow rate results in omnipresent maxima of the second kind; the ad(1) (2) (3) (4) (5) (6) (7) (8)
(9) (10)
2. P. Zagorski, Progr. Polarogr., 1962, 549. R. E. Cover, Rev. Anal. Chem., I,141 (1972). R. E. Cover and J. G. Connery, Anal. Chem., 41, 918 (1969). J. G. Connery and R. E. Cover, Anal. Chem., 41,1191 (1969). R. E. Cover and J. G . Connery. Anal. Chem., 41, 1797 (1969). R. E. Cover and J. T. Folliard. J. Electroanal. .Chem., 30, 143 (1971). J. T. Foiliard and R. E. Cover, J. Electroanal. Chem., 33, 463 (1971). J. G. Connery, Ph.D. Thesis, St. John's University, New York. N . Y . , 1970. I. M . Kolthoff and Y. Okinaka, Progr. Polarogr., 1962, 357. Y. Okinaka and I . M . Kolthoff, J. Arner. Chem. Soc., 79, 3326 (1957).
dition of surface-active materials to suppress these maxima is necessary if reliable analytical data are to be obtained (11). In addition, apparently because of the large bore, treatment of the capillary with water-repellent materials was found to be essential (12). EXPERIMENTAL All capillaries were obtained from Sargent-Welch Scientific Co., (S-29419)and cut to a length of 15 cm. A variable-speed motor with tachometer feedback was used to power a pulley system with an output/input ratio of 2 : l . The capillary was rotated in a ball-bearing housing and constrained to rotate around its axis. The speed of rotation was continuously variable and precisely controllable over the range 0-7000 rpm. The rotating mercury reservoir (0.d. 7 mm, 40 cm long) was supported on ball bearings and connected directly to the capillary with Tygon tubing (see Figure 1). Reproducible, trouble-free operation with a single electrode was found over three months of intensive work. Vibration-free mounting and sufficiently high mercury pressures are essential to such performance. All other apparatus and reagents have been previously described ( 3 , 4 ) .
RESULTS AND DISCUSSION The responses of the SDME to various phenomena were studied under a variety of conditions at rotational speeds in the range 0-7000 rpm with mercury pressures as high as 51 cm. The systems studied and the results obtained are very similar to those previously reported for the VDME (3, 4 ) ; increasing rotational speed of the SDME results in behaviors parallel to those found with increasing frequency a t the VDME. Optimal analytical response is obtained with both electrodes at the highest agitational rates. Polarograms obtained with the SDME are essentially identical in appearance with those obtained with the VDME (3, 4 ) . The short drop life at both the SDME and the VDME makes it possible to record polarograms a t high speeds. This property may, therefore, make these electrodes useful with techniques other than DC polarography. Mass-Transfer Controlled Currents. The limiting current-concentration response of the SDME at 7000 rpm and 50 cm of pressure to cadmium in unstirred 0.1M KN03 was found to be linear over the range 0.01-20.0mM. The lower limits of detection and the magnitude of the residual currents were essentially identical a t the SDME and the DME. (11) I. M. Kolthoff, Y. Okinaka, and T. Fujinaga, Anal. Chim. Acta., 18, 295 (1958). (12) I. M . Kolthoff and Y. Okinaka. Anal. Chim. Acta, 18, 83 (1958).
ANALYTICAL CHEMISTRY, VOL. 45,
NO. 11,
SEPTEMBER 1973
1983
Figure 1. The spinning dropping mercury electrode The SDME like the VDME is relatively insensitive to convection permitting the direct analysis of agitated solutions; limiting currents of 5.04mM Cd in 0.1M KN03 were measured at the SDME under stirring conditions ranging from quiescence to extreme turbulence. For all data obtained, the average response found was 9.96 f 0.46 FA (*4.8%). Since the volume of the mercury reservoir of our SDME is relatively small, the effect of decreasing mercury pressure on electrode response to 1.02mM Cd in 0.1M KN03 was studied. Over a 3-hr period, the mercury column height decreased from 25 to 21 cm at 5000 rpm. The Cd limiting current increased from 2.10 to 2.31 FA. For all 16 polarograms run during this period, the mean response was 2.20 f 0.09pA (f4.1%).
1984
The limiting current of Cd in 0.1M KNOJ at the SDME was found to decrease continuously with increasing rotational speed; the ratio of SDME to DME limiting currents becomes constant at about 0.31 in the range of 5000-7000 rpm . Maxima. It was previously shown ( 3 ) that maxima of both the first and second kinds can be eliminated a t the VDME a t sufficiently high frequencies without the addition of surface-active materials. Similar behavior is found a t the SDME a t high rotational speeds. The pronounced maximum of the first kind observed on the Cr(II1-Cr(I1) wave in 0.1M NaC104, pH 3.10 ( 3 ) , is obliterated completely a t the SDME in the range 60007000 rpm. Similarly, in the same range, the large maximum of the second kind seen with the DME on the Cd wave in saturated KC1 ( 3 ) is eliminated at the SDME. The contrast with Kolthoffs RDME is striking (11). Catalytic and Kinetic Waves. The SDME responses to processes of these types are similar to those of the VDME ( 3 ) ; increasing rotational speed decreases observed limiting currents. The ratio of SDME to DME limiting currents in the 6000-7000 rpm range is 0.22 for the first catalytic wave of quinine (7) in 5.0mM HC1-1.OM NaCl and 0.055 for the kinetic wave of formaldehyde in 0.104M NaOH (3). Adsorption Inhibition. The drop time of the SDME is about 10-2 sec and its rate of area formation is five times greater than that of the DME at 7000 rpm. These factors should both tend to minimize the inhibition of electrode processes by electroinactive adsorbates; this effect is indeed seen a t the SDME. At the DME, no wave is found for Cd in 0.2M HC1 in the presence of l.OmM tribenzylamine but the SDME at 7000 rpm yields a well-defined wave a t E I , ~= 0.60 V us. SCE. Similar behavior was found at the VDME ( 4 ) . Work is now in progress on establishing a theoretical basis for SDME response to various processes as well as its incorporation into automated analysis systems of the flow type. Received for review February 15, 1973. Accepted April 11, 1973.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
