Wave Spreader for Precision Determination of Half-Wave Potential with the Sargent Model XI1 Polarograph. David N. Hume and Theodore IT. Gilbert, Massachusetts Institute of Technology, Cambridge 39, Mass.
fact that the half-v-ave potential of an irreversible wave is not applicable in the usual polarographic equations, should be recognized by everyone working with such systems. A series of seven determinations of the half-wave potential of cadmium in 2 potassium nitrate at 30” C. using a 0.2-volt span gave a maximum deviation of 1.8 mv. and an average deviation of 1.1 mv. by this method. The mean value, -0.5786, compares favorably with two measurements, -0.57G5 and -0.5767, made on the same system with the manual apparatus of Lingnne and Kolthoff [J. Am. Chem. SOC.,61, 825 (1939)J. The major source of error in the automatic recording apparatus appears to be in the calibration and measurement of the photographic record, The highest reproducibility which might reasonably be expected for polarograms recorded on the 0 to 2 volt scale is A10 mv. il great increase in both accuracy and convenience is obtained if the wave spreader is used when an nnalysis of the wave is to be made. With a 0.2-volt span, eight or ten points may he taken on z)] the rising part of the wave for plotting values of log[i/(zd against E. Analysis of the cadmium waves in 2 ‘TI potassium nitrate gave uniformly straight lines with very little scatter of points and slopes within 5% of the theoretical. Although the wave spreader was designed for use with the Sargent Model XI1 polarograph, it is equally applicable to any other type of automatic recording polarographic apparatus, the span voltage of which can be varied between 0 and 0.4 volt.
HE precision with which half-mave potentials may be measured Twith automatically recording polarographic apparatus is often limited by the amount that the wave can be “spread out” on the recording paper. The Sargent Model SI1 is provided with an adjustment by nhich the voltage drop across the slide-wire may be varied between 3 volts and less than 100 mv. If the wave may be recorded a i t h a low span voltage-e.g., 0.2 volt-it becomes sufficiently spread out to allow the half-wave potential to be determined with considerable precision. The authors have found the simple wive-spreading device shovn in the figure to be of great value with this instrument, in that it allows polarogranis to be taken nith a low span voltage, even though the voltage range of interest may be in the region of 1 to 2 volts.
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Thermostated H-Cell for Polarography. Joseph C. Komyathy, Francis Malloy, and Philip J. Elving, The Pennsylvania State College, State College, Pa. I
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HILE the H-type polarographic cell first suggested by Lingane w a n d Laitinen [Lingane, J. J., and Laitinen, H. A., IXD. ENG. CHEM.,ANAL.ED.,11, 504 (1939)l has found wide application due to its simplicity and versatility, no provision was made for maintaining the sample solution a t constant temperature, as is usually necessary, except by immersion of the cell in a constant temperature bath, which is somewhat inconvenient. For simplicity of operation, a water jacket was sealed around the sample leg of the cell, permitting water a t a constant temperature to be circulated around the sample [Elving, P. J., and Teitelbaum, C., J . Am. Chem. SOC.,71, 3916 (1949)l. Subsequently, it was felt that provision should also be made for controlling the temperature of the calomel electrode in the other leg of the H-cell. Because of the variation in temperature in the laboratory throughout the day and night, the potential of the saturated calomel reference electrode was not constant. Any variation in the electrode potential of the saturated calomel electrode would cause an error in the measurement of the half-wave potential. I n addition to thermostating the entire H-cell, it was desired to modify the cell so a8 to avoid certain difficulties encountered in the use of an li-cell for polarography. The cell finally designed is shown in Figure 1.
The device consists essentially of two wire-wound potential dividers of the type used in radio receiver construction, and a source of voltage such as a 1.5-volt dry cell. By adjustment of RI (coarse) and R2 (fine), a voltage is introduced into the circuit just great enough to bring the potential of the dropping electrode to a point about 0.1 volt before the beginning of the wave to be examined. The span voltage of the polarograph is then set a t a convenient v a l u e e . g . , 0.2 to 0.4 volt. An auxiliary potentiometer is used to measure accurately the potential of the dropping electrode against the working anode a t the beginning and a t the end of the polarogram, and the calibration points marked on the photographic record by opening the shutter briefly a t each point. If desired, RI and Ra may be adjusted to set the potential of the dropping electrode a t some exact starting potential, as measured with the potentiometer-e.g., - 1.5000 volts. Using a “students”’ type potentiometer and a small lamp and scale galvanometer, the initial and final potentials of the polarogram can be measured to h O . 1 mv. without difficulty, although the resultant standardixation of the voltage wale on the photographic record is probably not better than & 2 mv. The polarogram is recorded, the residual and diffusion currents are determined, and the apparent half-Tvave potential is read by an interpolation between the calibration points. If there is apprecinlile iR drop through the cell, this fihould be determined and applied as a correction to give the true half-n-ave potential. Fur rcversible reactions, the same values of half-wave potential are obtained regardless of whether average or maximum galv:tnonieter swings are used; the latter are considerably more convcnicnt if the galvanometer swing is appreciable. With the unsymmetrical waves often obtained in irreversible electrode reactions, half-wave potentials obtained by different methods of inenswenlent may not coincide exactly. This, together with the
The H-cell is sealed into a large glass tube, BO that the cross arm of the €-Iis a t a slight angle to the horizontal; the lower end of the cross arm is joined to the sample leg of the H-cell, thus facilitating drainage of solution from the cross arm when the cell is emptied. The ends of the large glass tube are closed and fitted with connections for circulating water. A thermometer well, E, is sealed into the water jacket to allow ready monitoring of the temperature of the cell. The reference electrode leg of the H-cell W M modified by necking down the bottom of the leg in order to prevent the calomel used in the cell from coming in contact with the platinum wire 431
