Polarographic Drop-Time Measurement Using an Alternating Current Technique John W. Hayes,' Donald E. LeydenI2 and Charles N. Reilley, Department of Chemistry, University of North Carolina, Chapel Hill, N. C. 2751 5
been numerous reports T in literature describing devices for the measurement of the drop time of a HERE HAVE
dropping mercury electrode (D.hT.E.). hfany of these use the interruption of a light beam by the falling drop to generate an electrical pulse which operates a timer ( 1 , 5, 9). Meites and Sturtevant ('7) used the polarographic residual current to provide a signal to trigger an electrical counter. Corbusier and Gierst (5) point out that this method is unreliable because the current is zero near the potential of the electrocapillary maximum. Barker and Jenkins (2) used an 18-Mc. signal to provide a trigger pulse in square-wave polarography. Propst and Goosey (8) used the change in impedance of the D.M.E. a t drop fall to cause a burst of oscillation in a series-tuned 250-kc. oscillator and thus provide a trigger signal. This report describes a simple way to trigger an electronic counter to measure the drop time of a D.hT.E. using conventional operational-amplifier techniques. The signal from the current amplifier in a conventional operationalamplifier ax. polarograph (6, 10) is filtered to remove the d.c. polarographic current, rectified, and applied to the counter input (Figure 1). The current
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0.2
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1.0 S.C.E.
8 Figure 1 . ratus
Circuit diagram of appa-
I, currrent amplifier of a x . polarograph (Phiibrick P45A)
R , rectifier (Philbrick USA-3) D, germanium diode (Sylvania Type 1 N55A) Counter, Hewlett-Packard electronic counter, Type 5233L Resistor values in ohms, capacitor values in pf.
is of sufficient magnitude over the whole potential range of the D.M.E., even in the absence of depolarizer, to provide sufficient voltage for positive triggering of the counter. Because the counter receives a pulse each time the drop falls, the drop time of every other drop is registered. Figure 2 shows electrocapillary curves determined with the described apparatus for 1M KNOI and 1 M KN03 containing camphor. The reproducibility of the drop time was within the expected limits (*O.Ol sec.). Because the counter is triggered by a decrease in the current signal, difficulty
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may be experienced when depolarizers mhich exhibit highly distorted alternating current-time relationships are present. Biegler (3) has q h o m that the alternating current-time curve for riboflavin is highly distorted in a narrow potential region in the vicinity of the half-step potential. Figure 3 shows rectified alternating current-time curves obtained with the preient apparatus for By riboflavin a t -0.80 volt us. S.C.E. careful choice of the triggering level of the counter, it was poqsible to prevent triggering of the counter except at drop fall. Thus, even in this eytreme case, drop time measurement is possible with this apparatus. The higher the frequency, the larger is the current signal, but an upper frequency limit is imposed by the frequency response characteristic of the operational amplifiers (6, 10). Experiments reported in this paper were performed by using a frequency of 1 kc. A larger signal could be obtained by increasing the amplitude of the applied alternating voltage, but caution is 1 Present address, School of Chemistry, University of Sydney, Sydney, New South Wales, Australia. * Present address, Department of Chemistry, University of Georgia, Athens, Ga.
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Figure 2. Typical electrocapillary curves obtained with apparatus 0 1M KNO, 0 1M KNOI 3 X 1 0 3 4 camphor Applied alternating voltage: 1.0 mv. a t 1 kc. average of a t least three individual drops
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Figure 3. Rectified alternating current-time curves for 1.0 X 1 O-4M riboflavin in 0.1M H C I 0 4at -0.80 volt vs. S.C.E. Each point is
Applied alternating voltage, 1.0 mv. at 1 kc.
necessary since large amplitudes can cause highly distorted current-time relationships (3) and can also affect the drop time (4). The advantage of the present apparatus over those described previously is that no special cell design is required, as is the case with lighbbeam triggering. The instrument is a simple extension of an operational amplifier a x . polarograph and provides a means for observing alternating current-time curves while measuring the drop time. ~h~~ can be important in studies involving adsorPtion (S).
Aiutomatic recording of electrocapillary curves can be performed using a digital-to-analog converter and a recorder, as reported b y Bard and Herman ( I ) . LITERATURE CITED
(1) Bard, A. J.7 Herman, H+ Be, ANAL. cHEM. 37, 317 (1965). ( 2 ) Barker, G. C., Jenkins, I. L., Analyst 77, 685 (1952). (3) Biegler, T., Australian J. Chem. 15, 34 (1962).
(4)Buchanan, G. S., Australian J. Science 17, 103 (1954).
( 5 ) Corbusier, P., Gieret, L., Anal. Chim. Acta 15, 254 (1956). (6) Hayes, J. W., Reilley, C. N., ANAL. cHEM. 37, 1322 (1965). ( 7 ) Meites, L., Sturtevant, J., Ibid., 24,
1183 (1952).
(8) Propst, R. C., Goosey, M. H., Ibid., 36, 2382 (1964). (9) Ri’ha, J., “Advances in Polarography,” I. S. Longmuir, ed., p. 210, Pergamon, New York, 1960. (10) Smith, D. E., ANAL.CHEM.35, 1811
(1963). WORK supported by the Directorate of Chemical Sciences, Air Force Office of Scientific Research Grant No. AF-AFOSR584-64.
Variable Angle Attachment for Internal Reflection Spectroscopy N. J. Harrick, Philips Laboratories, Briarcliff Manor, N. Y.
ansen (1) has described a versatile variable angle double prism attachment for Internal Reflection Spectroscopy which takes advantage of the well known retroaction of a 90’ prism and thus simplifies hhe optical tracking required for a variable angle internal reflection element. He suggests placing this optical assembly directly in the sampling space of a spectrometer where, if the beam is sufficiently well collimated, defocusing of the source image at the entrance slit of the spectrometer caused by the increased path length is not serious. As pointed out by Hansen, the entire assembly can be moved parallel or normal to the light beam without displacing or further defocusing the light beam at the spectrometer slit. His device is designed for a single reflection. The angle of incidence is changed by rotating prism XOX’ of Figure 1 which also displaces the light beam at the monochromator slit. This displacement rRaTAi“ON.,
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Figure 1 . Hansen’s double prism variable angle attachment for Internal Reflection Spectroscopy XOX’ is silvered 90’ mirror and ACA‘ is 90’ prism. Angle of incidence on prism surface i s changed b y rotating XOX’ about the pivot point 0. Displacement of light beam a t monochromator slit due to rotation of XOX’ is eliminated b y translating ACA‘ relative to XOX’
is eliminated by translating horizontally XOX’ relative to prism ACA’. T h e purpose of this note is to draw attention to variations of this double prism assembly which we find advantageous in our applications. To change the angle of incidence, by rotating prism 4CA’ of Figure 1 about the vertex C, instead of rotating the silvered prism XOX’ about 0, there is no change in the distance d, hence no displacement of the light beam at the entrance slit of the spectrometer. Furthermore, for wide ranges of angles of incidence-e.g., 15’ to 75O-there is only a small change in optical path lengths, thus any defocusing of the source image a t the entrance slit is negligible. Any movement of the light beam relative to the sample can be corrected for by a vertical displacement of the entire optical assembly XOX’ and ACA’. No motion of prism XOX’ relative to prism ACA’ is required. By employing the necessary transfer optics and focusing the beam near the surface of the arm AC, this assembly can be used for variable angle either single or multiple internal reflection elements as shown in Figure 2, with no refraction at the entrance and exit surfaces. The prism ACA’ is replaced b y a variable angle reflection element along the surface AC, and a plane mirror along the surface CA’, which is normal to AC. When the angle of incidence is changed b y rotating ACA’ about the vertex C, the light beam will intercept the arm ilC at a different location and would thus miss the entrance aperture of a mutiple reflection element fixed on the arm AC. This difficulty can be overcome in one of two ways. The reflection element either can be mechanically constrained to move in a vertical path in line with the light beam as the assembly ACA’ is rotated or, if the reflection element is rigidly fixed on the
Figure 2. Modified double prism assembly employing variable angle either single or multiple internal reflection element After desired angle of incidence i s selected b y rotating ACA‘ about vertex C, realignment and refocusing of light beam relative to reflection element is achieved by small vertical and horizontal translations, respectively, of entire assembly. No motion of XOX’ relative to ACA‘ is required. Settings for two angles of incidence (0 = 15’ and 65’) shown
arm AC, the entire assembly can be moved vertically until the light again strikes the entrance aperture. Any defocusing of the light beam near the entrance aperture of t h e reflection element in either of these two cases can be corrected by moving the entire assembly horizontally. Thus, in the case that the element is fixed on the arm AC, the only adjustments required after the desired angle of incidence is selected are two translations, each not more than inch, for the design shown in Figure 2. These translations could, of course, be coupled to the rotation. As pointed out earlier, these motions neither displace nor defocus the light beam a t the entrance slit of the spectrometer. LITERATURE CITED
(1) Hansen, W. N., ANAL.CHEM.37, 1142
(1965).
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