A Simple Electronic Polarographic Voltage Compensator Raymond Annino and Karl J. Hagler, Department of Chemistry, Canisius College, Buffalo,
of inve..tigators have deapparatus for the coni;wn,ut,ioii of the i R droli encouiitered in !io~iaqueou~ polarogralihy (I--,?). Tlie riistrumcnt tleicritwi liere may hc iirni)ly att:ic.hed to a commercial rc8 .ording polalogral)h (5:trg'nt ;\Iodcl XV : r i thi. ri+c.tiri.h) and has the advantage lit' R 1argc.r \-oltape compensatioii range :tt I(1n-t zt.5 niv. over a 100-volt range) tli:in the indtrunicnt p r o p o d b y .Irthiir and Vanderkani ( 1 ) ( + 5 niv. (i\.(>r:i 20-volt range:, It is siniilar to t h a t of Kellcy, Jone.:, and Fishpr ( 3 ) , twcy't that the currlmt aniplificr the>i i d tias h e n eliminatcd. A -ond s 0.003 pa.;'nini. are not provided n-ith tho standard ~Ioclol XI- Polarograph, although smallcr current- can he rwordcd with this in-trument (0.0001 pa.:/nin~.)after conilicnsation of capacitancc currentTi-ith an auxiliary attachment. m i n i senhitivity of the conil)lrtc~ may he limited 1,- tht. aniouiit of 60 c,p,s. iiickup betn-een the 11.1I.E. and rc>fcrence electrode Thi3 voltage appcm. to be ;ma11 ice it \vas not obscrvsbl(. on an osc oicope with a sensitivit!- of 10 niv. ' c m . l'olarogranis of Ctl-' in 0.lJI KC1 with external reGtances added in series uith the counter electrode ( 2 2 megohms at limiting currents of 4 pa.) yiclclcd linear lilots of log i;i,,-i with a slope of 0.030 volts. Figure 3 contains the polarographic wave of Cd-? in 1-butanol, 0.1.11 lithium
in1 erted, and applied to the counter electrode, forcing the potential of the polarized electrode, n i t h respect to the reference electrode, to be equal to the applied voltage. The circuit ihon n in Figure 1 ha. an ad\ aiitage in that it can be coiineeted directly to the polarograph n ith no internal modifications. Hon el cr, since the current mea-wing re-i-tori are iii the drop1)ing mercury electrode (I) 11 E) lrad, the iR drop encountered herc i i not compcn-ated. .In alternxtc. circuit shon n in Figure 2 alleLiate- thii difficult) sincr the current mea-urinp rt+tora are non in the amplifier output lead. Thii modihcatioii require3 cutting one \?ire in the polarograph (points .Y and I-, Figure 2 ) to make a plug-in arrangenieiit on the side of the initrumeat for the output to the L l 11 E and the output of the amplifier Tlie ori-inal D.1I.E. lead lion Iiecomes the oiitput lead of the amplifier and i- ?onnected to the counter elwtrode Tno iincompen-ated resistance path. remain through nhich the cell current may flon-: that of thc mercury thread in the capillary and that of the potentiometer supplying the polarizing Ioltage. Since theqe are u-ually quite -mall
Externally connected compensator
Counter r,lectrode Polarized electrode Reference electrode 0.001 pi. 330 K
N. Y.
i o rrcordrr
r POLAROGRAPH
4 Figure 2. Internally connected compensator vs
w S C E
Figure 3. Polarograms of cadmium ion in 1 -butanol, 0.1M LiCl Curve A, compensated polarogram (solution resistance, 0.2 megohm) Curve B, uncompensated polarogram (solution resislonce, 0.2 megohm)
VOL. 35, NO. 10, XPTEMBER 1963
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chloride. I n this case, cell resistance was 0.2 megohm. ACKNOWLEDGMENT
The authors gratefully acknowledge T. the helpful suggestions of -vyron Kelley and D* J. Fisher Of Oak Ridge Sational Laboratories.
LITERATURE CITED
(1) Arthur, P., Vanderkam, R. H., ANAL. CHEM.33, 765 (1961). ( 2 ) Jackson, W., Jr., Elving, P. J., Ibid., 28, 378 (1956). ( 3 ) Kelley, hl. T., Jones, H. C., Fisher, D. J., Ibid., 31, 1475 (1959). (4) Xcholson, M. M., Ibid., 27, 1364 (1955).
(5) Oka, S.,Ibid., 30, 1635 (1955) Presented at the 14th Pittsburgh Conference on Snalytical Chemistry and Applied Spectroscopy, March, 196%. Financial assktance of the National Science Foundation by an “Undergraduate Seience Research Participation Grant” to one of the authors (KJH) is gratefully acknowledged.
Infrared Differential Technique Employing Membrane Filters Howard J. Sloane, Beckman Instruments, Inc., Fullerton, Calif.
c o m m of routine operation Iis occasionally of a spectral laboratory, the analyst confronted with the need N THE
to obtain the infrared spectra of small amounts of materials which have been trapped on a filtering medium. Sometimes it is possible to scrape off or dissolve the sample with a suitable solvent. More frequently, especially for microgram quantities, this is impossible to do without introducing considerable and indeterminable amounts of contaminant from the filter or from subsequent sample handling. At best, such a procedure generally is inconvenient. The purpose of the study described in this note was to find a filtering material with an infrared spectrum sufficiently weak at a useful thickness to permit direct examination of the sample by means of the infrared differential technique. K i t h such a filter, one could expect to obtain the sample’s spectrum, differentially compensating out the filter’s absorption spectrum by placing an equivalent thickness of filter in the reference beam of the double beam instrument. The initial portion of this work, then, involved examination of spectra for a wide variety of cellulosic and noncellulosic filters. Among the commercial materials examined were A, a standard cellulose filter paper (0.15 mm thick-
ness); B , a glass fiber filter (0.2 nim.); C, a Teflon-impregnated glass fiber filter (0.05 mm.) ; D, a cellulose acetate foil (0.15mm.); E, an acrylic fiber sheet (0.11 mm.); F , a poly(viny1 chloride) filter (0.225 mm.); and G, a cellulose nitrate-acetate membrane filter (0.025 mm.). Of these materials, all except G gave spectra far too intense for the differential work. However, all these filterj, especially D, E, and F, showed considerably improved transmission of infrared radiation when they were “wetted” with mineral oil to decrease scattering losses. These losses are, therefore, deemed to be a major contributor to the high degree of opacity observed. The spectrum of cellulose nitrateacetate membrane filter is shorn in Figure 1. This filter is available conimerically from the Millipore Filter Corp., Bedford, Mass., in a xariety of pore sizes, diameters, and thicknesses. It has been used extensively in bacteriological work and air pollution studies and to remove contaminants in fuels and hydraulic fluids. The spectrum shown in Figure 1 is that of the thinnest membrane filter available, Millipore type TH, 0.025 mm. and is the type used in all of the Lvork described below. With this thickness. it is
unnecessary to add oil to reduce beattering losses. The infrared spectra shown here ivere recorded on a Beckman IR-9 spectrophotometer equipped with Automatic Slit Control. The function and utility of this latter device have been demibed previously (3, 5 ) ; however, its importance in the present work n arrants home additional remarks. Briefly, the purpose of Automatic Slit Control is to provide a constant energy background for differential Jvork by automatically opening and closing the instrument slits to the degree necessary to compensate for energy being absorbed in the reference beam. The result 13 a uniformly high energy background (except in regions of total absorption) tliroughout the entire spectral r s n q being scanned. -2lthough ,iutomatic Slit Control is highly beneficial for such work. nieaningful results can still be obtained on the smaller, less versatile instruments not so equipped. I n this case, honever, i t should be remembered that bpurious bands or “dead” regions will occur in those portions of the spectrum nhere the filter is heavily absorbing. T o obtain a reasonably flat base line n hen filters are scanned differentially, it is imperative to match the thicknesses of sample and reference materials as
WAVELFNGTH MCRONS
WAVENUMBER CM
Figure 1.
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Spectrum of cellulose nitraie-acetate membrane filter, Millipore, type TH, 0.025 mm. nominal thickness
ANALYTICAL CHEMISTRY
