the original gold surface. The hydrogen overpotential of this electrode is about the same as that of a mercury electrode. This electrode was used for the reduction of Cd(I1) using 0.2F potassium chloride as supporting electrolyte. The sample solution was comprised of Cd(1;0&.8 HzO in 95% ethanol. The spots were applied at a temperature of about 60’. The shape of the spot was intermediate between Fe(II1) and hydroquinone-Le., most of the material was in a ring but some was in the center. The initial potential was -1.1 volt us. S.C.E. and the blank correction was 7.5 pa. Corrected currents a t t = 3 seconds as a function of the quantity of Cd(I1) are presented in Figure 5. I n this case the potential had to be adjusted to -0.1 volt us. S.C.E. after each trial in order to reoxidize the cadmium from the amalgam before the electrode was removed and a new spot was applied. T h e roughness of the electrode slowly increased with time and the currents obtained with rough electrodes were greater than with smoother electrodes. General Characteristics of the Method. T h e physical processes involved in this method of electrolysis are quite complex. When t h e violent manner in which the sample is brought into contact with the supporting electrolyte is considered, t h e fact t h a t reproducible results can be obtained is surprising. The thickness of the sample spot is very small. For example, 20 nanomoles cm.9 of hydroquinone (1.6 X distributed evenly in a circular spot of 3-mm. diameter would be 0.2 micron in thickness, 1 nanomole would be 0.01
micron in thickness. Because the electrode surface is not smooth, the average thickness is even lower -Le., the sample thickness can be as small as a few molecular layers. With the electroactive material in such close physical proximity to the electrode surface, a large fraction of the material reacts. When the supporting electrolyte contacts the sample spot, dissolution and electrolysis begin simultaneously. The sample must dissolve before it can be reduced or at least solvent molecules and ions of the supporting electrolyte must be in the vicinity to provide electrical conduction. If dissolution is the slow step in the process, the current obtained may depend upon the physical character of the spot-e.g., crystal size. Thus reproducible results would be more di6cult to obtain. With hydroquinone a change from aqueous to 5001, ethanol supporting electrolyte was necessary to obtain reproducible results. When the reductions of p-nitrobenzoic acid, ethyl pnitrobenzoate, and p-nitrophenol were attempted a t the amalgam electrode, no reproducible results were obtained in any of several ethanol, ethanol-water, and methanol supporting electrolytes. These phenomena may be caused by slow and erratic dissolution. Increasing the temperature to 40” C. did not improve the reproducibility. At still higher temperatures the alcohol tended to condense on the cool electrode and the spot was partially rinsed away before the electrolysis began. Spot electrolysis offers the advantage of high sensitivity. Less than 1 nanoequivalent of electroactive material
(usually less than 0.1 pg.) is detectable and 5 to 20 nanoequivalents can be determined with a n accuracy of a few per cent. The instrumentation required is very simple. Analyses require very little time, only about three minutes being necessary to prepare and electrolyze a sample spot. The major savings in time over conventional electroanalytical methods accrue from the fact that deaeration of each sample i s unnecessary. LITERATURE CITED
(1) Blaedel, W. J., Olson, C. L., Sharma,
L. R., ANAL.CHEM.35,2100 (1963). (2) Breyer, B., Bauer, H. H., “Alternating
Current Polarography and Tensammetry,” Interscience, New York, 1963. (3) Galus, Z., Olson, C., Lee, H. Y., Adams, R. N., ANAL. CHEY. 34, 166 (1962). (4) “Hydbook of Analytical Chemistry, L. hleites, ed., pp. 5-9, RlcGraw Hill, New York, 1963. (5) Kelley, M. T., Jones, H. C., Fisher, D. J.. ANAL.CHEM.31. 1475 (1959). stry,” 2nd ed., p. 245, Interscience, New York, 1958. (8) Perone, S. P., hlueller, T. R., ANAL. CHEM.37, 2 (1965). (9) Peters, L). G., Lingane, J. J., J . Electround. Chem. 2, l(1961). (10) Shain, I., “Treatise on Analytical
Chemistry,” I. M. Kolthoff and P. J. Elving, eds., Part I, Sec. D-2, Chap. 50, Interscience, New York, 1963. (11) Underkofler, W. L., Shain, I., ANAL. CHEM.35, 1778 (1963). RECEIVEDfor review June 1, 1965. Accepted September 1, 1965. Division of Analytical Chemistry, 150th Meeting ACS, Atlantic City, N.J., September 1965
Test of the Theory of Glass Bead Columns in Gas Liquid Chromatography S. J. HAWKES,’ C. P. RUSSELL, and J. C. GlDDlNGS Departmenf of Chemistry, University of Utah, Salt lake City, Utah The plate height theory of glass bead columns in gas liquid chromatography has been tested over a wide practical range, including bead diameters from 0.004 to 0.023 cm. and liquid loadings from 0.1 to 3.9%. Theoretical values are calculated completely independent without reference to plate height data from any chromatographic system. The agreement is very good (usually within 10% for the largest beads). The conformity deteriorates for the smallest beads, the theory predicting plate height terms too small by factors of from 2 to 4 at liquid loadings less than 1%. Possible reasons for the divergence
are discussed. The agreement is also very poor for height liquid loadings as expected from theory and observation.
T
HE glass bead column used in gas liquid chromatography is perhaps the best present-day system for comparing chromatographic theory and experiment. The “diatomaceous earth” columns have a complex liquid distribution which cannot yet be defined in detail (7). It is reasonable t o believe that the capillary column, despite its apparent simplicity, suffers the same disadvantage (3). (If the inside column
wall is “rough,” the liquid configuration will again be complex; while if the wall is smooth, the liquid film will be unstable and probably not maintain a uniform character for a long time.) The glass bead surface is very smooth and well defined geometrically. There is no instability problem, since most of the liquid accumulates around the contact points and is not expected to adhere t o the bead surface as a uniform layer (I). I n addition, the CI term is relatively large and easily discernable. This Present address, Department of Chemistry, Brigham Young University, Provo, Utah. VOL. 37,
NO. 12, NOVEMBER 1965
1523
swamps out the troublesome C, term, which is complicated by coupling and other complexities less subject to exact theory. This paper examines the applicability of Giddings’ theory of C I to smaller beads, With this it is hoped t,o move closer to a general understanding of glass bead column, so that optimum conditions can be more easily found. EXPERIMENTAL
Several packing methods were tried using a solution of a blue dye in isoactyl alcohol as stationary phase. Sucking the solution into a column of dry beads and then eluting either by gravity drainage or by sucking or blowing the liquid out gave a grossly nonuniform liquid distribution, except when the liquid was sucked into the column while almost horizontal and blown out a t the same angle. No nonuniformity was visible when the beads were packed in the usual way, with the liquid already on them. -4 number of columns with various loads of stationary phase on 2 5 / 3 5 and 120/170-mesh beads were made
Table 1.
dp, cm. 0.0040 (325400
mesh)
Theoretical and Experimental C IValues for Various Glass Bead Columns and Solutes
70 0.23 0.46 0.93 2.0 3.9
0.0095 (140170
mesh)
0.24 0.48 1.05 1.8 3.7
0.023 (60-70
mesh)
0.09 0.29 0.47 1.05
1524
by the latter method and stood upright for 2 months to observe drainage. The beads used were obtained from the English Glass Co., Leicester, Great Britain. These were much more regularly spherical than beads from several other manufacturers. Sieving the beads was difficult, since they clogged the pores of the sieve firmly and often permanently. Accordingly, the bottoms of the sieve needed to be brushed frequently and machine sieving was ineffective. The mean bead size was determined with a traveling microscope. The glass beads were coated with trio-tolyl phosphate and packed into 300cm. by l/d-inch-o.d. columns, except the 325/400-mesh beads, for which 150-cm. columns were used. Since the liquid adhered to the walls of the beaker in which the beads were coated, the original loading of the beads was not the same as that in the column. The liquid and beads remaining in the beaker were therefore separated and weighed and the loading was corrected. The columns were mounted in a water bath at 39’ =k 0.5’ C., closed with wire gauze, and connected to an inlet system and a Gow Mac microkatha-
(Liquid phase, TOTP. Temperature, 39” C.) C I (exDtl.), C I(calcd.), C I (exptl.) . . e Solute R see. sec. C I(calcd.) 0.42 Pentane 0.81 0.0087 0.0026 3.4 Hexane 0.61 0,014 0,0041 3.5 Heptane 0.54 0.013 0.0044 3.0 Pentane 0.43 0.72 0.0086 0.0047 1.9 Hexane 0.48 0.016 0.0060 2.7 Heptane 0.27 0.016 0,0049 3.3 0.44 Pentane 0.58 0.033 0.0082 4.0 Hexane 0.34 0.034 0.0077 4.4 0.024 Heptane 0.17 4.8 0.0050 0.41 0.44 Pentane 0.063 0.012 5.2 Hexane 0.21 0,056 0.0084 6.7 0.09 Heptane 0.040 0.0043 9.3 Pentane 0.25 0.086 0.42 0.013 6.6 Hexane 0.11 0.117 0.0070 15.9 Heptane 0.04 0.060 0.0028 21.4 Pentane 0.38 0.77 0.024 0.018 1.3 0.56 0.037 Hexane 0,026 1.4 0.29 0.033 Heptane 0.022 1.5 Pentane 0.66 0.033 0.38 0.033 1.0 0.42 0.041 Hexane 0.036 1.1 0.21 0,032 Heptane 0,026 1.2 0.48 0,081 0.38 Pentane 0,054 1.5 0.26 0.071 Hexane 0,043 1.4 0.12 0.040 Heptane 0.023 1.7 0.34 Pentane 0.276 0.063 0.38 4.4 Hexane 0.16 0.195 0.039 5.0 0.07 Heptane 0.105 0.019 5.5 1.114 0.19 Pentane 0.39 0,062 18.0 0.08 Hexane 0.612 0.031 19.8 0.03 Heptane 0.295 0.013 22.6 Pentane 0.39 0.92 0.029 0.027 1.1 0.79 Hexane 0.039 0.6 0.063 0.59 Heptane 0.075 0.095 0.8 0.78 0.110 0.40 0.114 Pentane 1.0 Hexane 0.57 0.175 1.0 0.167 0.33 0.165 Heptane 1.0 0.156 0.40 0.66 Pentane 0.179 1.0 0.190 Hexane 1.1 0.45 0.238 0.215 0.31 0.197 Heptane 1.0 0.192 0.38 Pentane 0.50 1.1 0.340 0.317 0.34 Hexane 0.260 0.291 0.9 Heptane 0.15 0.180 0.171 1.0
ANALYTICAL CHEMISTRY
rometer Type 470 with capillaries of negligible dead volume (- 0.06 11.). The katharometer was sealed in DowCorning silicone rubber RTV 502. A Leeds & Northrup recorder was used for most of the work, but a Sanborn recorder was used for very fast peaks. Plate height was calculated from the retention time and the width at half the peak height. CALCULATIONS
The values of C I were calculated starting from the equation
Putting 0 have
=
v o j and rearranging, we
where I? is the apparent (measured) plate height, the time-averaged gas velocity obtained by dividing the column length by the elution time of air, Go‘ the resistance to mass transfer in the gas phase a t unit pressure, p , the outlet pressure, B’ the longitudinal diffusion coefficient a t unit pressure, and C I the resistance to mass transfer in the liquid phase, and j and f are given by
C,’ was approximated by the equation
(D C,‘
=
(0.63
- 0.2R)dP2/D,’
(5)
where R is the ratio of zone to gas velocity, d, the bead diameter, and D,’ the diffusivity of the sample in the carrier gas at unit pressure. Little reliability is claimed for Equation 5 , but the calculated C, in all the columns used is so small (
