pairs are separated, but lJ2-dibromoethane and 1,2-dibromopropane are A mixture containing equal not. amounts of these two packings gave poorer separations than either individually, but a 90% silicone oil-lOyo Tween 60 mixture was satisfactory. Using this ratio, three columns were prepared : a n intimate mixture of the two packings, a packing made by mixing the two liquids before adsorbing them on the firebrick, and a two-stage column. All three columns gave identical separations within small experimental variations. Use of mixed solvents has been employed frequently to modify the adsorptive properties (3, 4) or to decrease the volatility (5) of the stationary liquid. This work demonstrates that the properties of a chromatographic column prepared from a mixture of two stationary liquids can be equivalent to those of a two-stage column prepared in the same ratio. This can be expected for any combination of solvents that do not interact chemically. The order of elution with respect to boiling point for several mono- and dibromoalkanes is shown in Figure 4 for silicone oil and in Figure 5 for Tween 60. The upward curvature is a result of increasing the temperature during the determination. K i t h silicone oil, the mono- and dibromoalkanes behave as three classes of compounds. At a given boiling
point level, the monobromoalkanes are retained longer and their differences in chemical structure have no significant effects. The gem- and uic-dibromoalkanes are grouped together, while the remaining dibromoalkanes (Figure 5 ) constitute the third class. With the Tween 60 substrate, which is more polar, the mono- and dibromoalkanes constitute several distinct classes depending upon the carbonbromine skeletal structure. At a given boiling point level, the monobromoalkanes are retained considerably less than the dibromo compounds. Because the bromine atom is partly shielded by the surrounding methyl groups, 2bromopropane and 2-bromo-2 methylpropane are retained less than would be predicted from the retention time of other members of the series. This effect is slightly apparent with 2bromobutane and 1-bromo-2-methylpropane, but with 2-bromopentane it is not observed. The importance of structure is more apparent with the dibromoalkanes. Using Tn-een 60, 1,ldibromoethane is retained less than the lower boiling dibromomethane (boiling point difference = 11" C.) and 1,2dibromoethane is eluted simultaneously with lJ2-dibromopropane (boiling point difference = 12" C.). In general, with lower members of the dibromoalkane series, the effect of haying the same carbon-bromine skeletal structure will compensate for a 10" C. difference
in boiling points. This causes the dibromo compounds to behave as several classes, each with the general formulation 1,l-dibromo C,, lJ2-dibromo Cn+l, l,&dibromo Cn+l, etc., but which converge in the region of the 1,4-dibromoalkanes. This is illustrated in Figure 5 for the series starting with dibromomethane and 1,l-dibromoethane. The retention value for 1,2-dibrornobutane indicates that it may be the second member of the next series. This phenomenon may be attributed partly to the importance of similar carbon-bromine structures and partly to the shielding effect of the additional methyl group. The relatively low retention times observed for 1,2-dibromo-2-methylpropane and 2,5-dibromohexane may also be attributed to shielding of the bromine atoms by the methyl groups. LITERATURE CITED
(I) Evans, J. B., Willard, J. E., J. Chem. Soc. 78, 2908 (1956). (2) Fredericks, E. M., Brooks, F. ANAL.CHEM.28, 297 (1956). (3) James, A. T., Martin, A. J. Biochem. J . 50, 679 (1952). (4) James, A. T., Martin, A. J. Howard Smith, G., Ibid., 238 (1952). ( 5 ) Keuleman, A. I. M., Kwantes, Zaal, P Anal. Chim. Acta
357 (1955).
RECEIVEDfor review July 29, 1957. Accepted December 2, 1957.
Polarograph with Direct Recording of Electrode PotentiaI DONALD T. SAWYER Division of Physical Sciences, University o f California, Riverside, Calif. ROBERT L. PECSOK and KARL K. JENSEN Department of Chemistry, University of California, los Angeles 24, Calif.
,A new polarograph has been developed which utilizes an X-Y recorder for direct measurement of the electrode potential or applied voltage. Use of a third electrode provides automatic correction for the lR drop in the cell. The instrument is compact and versatile. Data are provided in a convenient and easily cataloged farm. The performance of the instrument indicates an accuracy within A2 mv.
T
HE accurate recording of polarograms from which precise values of half-wave potentials can be read
directly is an important problem in many applications of the polarograph. The accuracy of some commercial polarographs and an improvement for the electronic control of the span voltage have been recently discussed (6). Three sources of error are inherent in the design of these instruments: The chart drive must be synchronized with the span drive with respect to both speed and starting time; the chart records time along the X-axis, which can be interpreted as voltage applied to the cell only if the span voltage is accurately known and precisely linear; and, a t best, it is the cell voltage that is read
from the chart rather than the potential of the polarized electrode. A new polarograph utilizes a modified X-Y recorder for the direct recording of current us. either the cell voltage or (with the use of a third electrode) the potential of the polarized electrode. Thus, the sources of instrumental errors have been greatly reduced, and the accuracy for the direct reading of potentials is limited only by the accuracy of the recorder, itself. The instrument offers many advantages. It can be used conventionally with two electrodes, or with a third reference electrode; when a third electrode is used, the VOL. 30, NO. 4, APRIL 1958
481
* 1.0
TEST
- 10.
0 t o - 2.0"
I O
Io
- 1 0Y
- 2.5
Y
OHMS
1
1
-
AC 117"
60
SENSITIVITY
Figure 1.
Applied voltage circuit
Mallory battery RM42R Ea, B4. Mallory battery RMl2R Helipot. 1 0 0 0 ohms, 1 0-turn Helipot, 0.5% linearity Motor. Motor drive, 0.5 r.p.m.; clockwise drive, with two-way No. 1 friction clutch, Haydon, No. 1600 Relay. Advance relay, No. MF/3C/l 15 VA SI,SP, Sa, Sa. 4-position switch, Carling, No. 2 5 1 5 S5. Double pole double throw switch for third electrode pa/in. Set of 0.1% precision resistors, Daven Co., No. P-250 El,
82.
potential of the nonpolarizable Forking electrode need not be known nor kept constant. Errors caused by large cell currents and/or cell resistances are automatically and accurately compensated. The potentiometer, current supply, and drive for the span need not be of high quality, because they have no effect on the accuracy. Polarograms are recorded on standard graph paper, 6 1 / 2 X 11 inches, or vellum for ready and economical filing or reproduction. The versatile X-Y recorder is available for other experiments a t the flick of a switch, and it is compact, light, and relatively modest in cost. DESCRIPTION OF CIRCUIT
A lIoseley Model 3 X-Y recorder has been modified to make the X-axis a 1-volt, zero-current, potentiometric system. \Then used conventionally with two electrodes, the input leads of the X-axis are attached across the polarographic cell. Thus, the actual potential applied to the cell is recorded and no synchronization is involved. The current in the polarographic circuit passes through one of a set of standard resistors across which the leads of the Y-axis are connected. The Y-axis, set on the 5-mv. range, is also a potentiometric circuit n-ith zero current drain by the recorder. The motor-driven voltage source is shonn in Figure 1. Because the recorder plots whatever voltage is applied to the cell, regardless of speed or linearity, an inexpensive 10-turn potentiometer and a 0.5 r.p.m. synchronous motor-drive suffice for the voltage divider. Two mercury cells (B1 and Bz, Figure 1) are used in series with an adjusting rheostat to give a 2.5-volt 482
ANALYTICAL CHEMISTRY
SELECTOR
Figure 2. Sensitivity selector circuit and circuits provided by range selector switch
source of potential. This is spanned by the motor-driven potentiometer at a rate of 128 mv. per minute. Alternate scanning speeds and a reversible motor can be substituted when desired. The motor-drive on the potentiometer is fitted with a friction clutch to permit manual scanning of the polarographic wave. With the pen in the lifted position, a fast preliminary scan, obtained by turning the potentiometer dial manually, will indicate the nature of the wave and whether the optimum current sensitivity and proper voltage range have been selected. The 18 standard resistors which are incorporated in this section of the instrument are indicated by palin. in Figure 1, and are shown in detail a t the bottom of Figure 2. B sensitivity range from 0.1 to 50 pa. per inch is provided. A range selection sn-itch (SI, S2,S a, and S4 ganged on a common shaft) permits recording of any 1-volt span nithin the limits from +1.0 to -2.5 volts. The four basic circuits involved for each position of this snitch are shown in Figure 2. In the test position, the S-axis is short-circuited to provide a means of positioning the pen a t zero potential. Position 1 ( + l to -1 volt) inserts an accuratrly standardized potential of +l.OOO volt in the circuit by means of battery B3. The voltage is standardized ivith a Leeds & Korthrup Type K-2 potentiometer. This voltage is applied to the polarographic cell and, when used in conjunction with the span voltage and the zeroing control of the recorder, provides for recording any 1-volt span within the limits from +1 to -1 volt. In position 2 (0 to -2 1-olts), B3 is inserted and for position 3 (-1 to -2.5 \rolts), B3 plus Bg are inserted in the measuring circuit connected to the X-axis. B4 is also an accurately standardized potential of +1.000 volt. The function of these
two positions is to oppose the measured applied potential in such a way that the 1-volt span can be selected from any portion of the complete range. This function is accomplished as indicated in Figures 1 and 2. Full scale on the X-axis represents a span of 1 volt, but this can be any 1-volt portion of a 2-volt range, because the recorder has a full-scale offset adjustment. It should be emphasized that B3 and B4have the same stability of standardization as the X-Y recorder. ,411 three use llallory RlI-12 mercury batteries ivhich drift a t the rate of approximately 0.25% every six months. For systems with appreciable internal resistance ( I R ) potential drops, the use of a third electrode permits direct recording of the potential of the polarized electrode. The third electrode is a conventional Beckman calomel electrode such as provided with pH meters. It is placed in the polarographic cell directly next to the dropping mercury electrode, and is connected into the polarographic circuit as s h o m in Figure 1. Switch S 5 provides for recording either the conventional cell voltage (with two electrodes) or the potential of the polarized electrode (with three electrodes). As current does not flow between the third electrode and the dropping mercury electrode, there is no I R drop and the true electrode potential is recorded. In Figure 3 the effect of using a. third electrode is shown for a lOmM solution of cadmium chloride in 1 M potassium chloride. The cell resistance is 1600 ohms. Curve A is a conventional polarogram of current us. cell voltage; curve B is a polarogram of current us. electrode potential of the dropping mercury electrode. For the latter curve, the shape of the current-potential recording for each individual drop results from the variation of the I R drop across the cell resistance and the finite lag of the recorder. Because only the maxima of the oscilIations are employed, the accuracy of the current-potential
Figure 3.
Polarogram
1 OmM cadmium chloride in 1 M potasii~mchloride
A. 8.
Current VI. cell voltago Current vs. electrode potential
measurement is not affected. Comparison of the half-wave potentials for the two curves indicates an I R drop of 80 mv. for curve A . The half-wave potential for curve B is -0.641 volt us. S.C.E., which is in good agreement with the reported value of -0.642 (a). The use of a third electrode is particularly advantageous for organic solvents because of the high cell res& ances commonly encountered. Arthur, Lewis, and Lloyd ( 1 ) have also noted the value of a third electrode for high resistance systems. T o confirm these apparent advantages, polarograms have been recorded with the X-Y polarograph for carbon tetrachloride in 80% ethyl alcohol containing 0.01M tetraethylammonium bromide as the supporting electrolyte. The half-wave p ~ tential is -0.84 volt us. S.C.E. For cell resistances up to a t least 22,000 ohms, satisfactory waves are obtained without I R drop. Direct recording of electrode potentials should permit more accurate determination of the half-wave potentials for organic compounds. The X-Y recordrr was modified by the manufacturer (F. L. Mosrley Co., Pasadena, Calif.) to give a 1-volt potentiometer circuit for the X-axis. Figure 4 indicates the changes made to provide the voltage measuring circuit at zero-current. All of these changes are incorporated in a single switch which permits the recorder to he used either for polarography in its modified form or as a conventional X-Y recorder in ita normal form. Even if this instrument had few real advantages over conventional polarographs, the versatility of the X-Y recorder for other applications n,ould justify its higher cost. The direct recording features of this instrument permittrd the costs of labor and parts for construction of the
Circuit of X-Y recorder modifications
Figure 4. Mod. Nor.
Switch position for polarographic use Switch position for normal X-Y recorder use
voltage source to he significantly reduced.
half-wave potentials of copper(I1). cadmium(II), nickel(II), and zinc(I1) in various supporting electrolytes gave values in agreement with those reported in the literature (4). In determining the hslf-wave potentials, the diffusion currents were measured using the tops of the oscillations and have been corrected for residual currents hy extrapolation. Previous work (5) has shown that recorders with full-scale response times of less than 15 seconds record the instantaneous current when the drop times are greater than 2 seconds. The Moseley X-Y recorder has a full-scale response time of less than 1 second.
Figure 5. Polarograh,h The high-speed response o'f the recorder and its direct recorduig of the cell voltage suggest the posriibility of recording rapid scans of applie(1 voltage; these studies are now in progri365. It is anticipated that a I-volt span can be recorded in less than 4 seoonds. This would permit studies that h ave been possible in the past only b y oscillcVOL. 30, NO. 4. APRiL 1958
483
graphic techniques. To carry out these studies, a voltage sweep of 4 seconds or less will be substituted for the motordriven potentiometer in the present circuit. For extremely precise measurements of half-wave potentials, it would be possible to modify the X-axis to record a few tenths Of a instead Of volt as in the instrument described. The
cost of the polarograph, excluding the recorder and labor, was $103. LITERATURE CITED
(1) Arthur, P., Lewis, P. A,, Lloyd, N. A,, ANAL.CHEM.26, 1853 (1954). (2) Kolthoff, I. M., Lingane, J. J., “Polarography,” 2nd ed., p. 504,
Interscience, New York, 1952. (3) Lee, T. S., J. Am. Chem. SOC.74, 5001 (1952).
(4) Meites, L.,
“Polarographic Techniques,” Interscience, New York, 1955. (5) Pecsok, R. L., Farmer, R. W., ANAL. CHEM.28. 985 (1956). . ,
RECEIVEDfor review April 8, 1957. Accepted November 22, 1957. Division of Analytical Chemistry, 132nd meeting, ACS, New York, N. Y., September 1957. Work eu ported by Research Corp. Grant-in-lid.
Polarographic Determination of Tin in Zirconium AIIoys JOHN T. PORTER 11’ Knolls Atomic Power laboratory, General Elecfric Co., Schenectady,
b A method has been developed for determining tin in zirconium alloys polarographically. The procedure requires no separations from zirconium or the normally occurring alloy constituents.
T
application of zirconium to nuclear reactors has given rise to the Zircaloys, which contain tin, iron, nickel, and chromium in various combinations (6). Bryson and others ( 1 ) have described a titration procedure for tin, when greater than 0.5%. Because the general range of interest has been extended to about 0.25%, a more sensitive method was desired. In developing the polarographic method described, a further aim was to avoid separations. Because hydrofluoric acid is generally used in dissolving zirconium materials in amounts that are deleterious to the dropping mercury electrode, any excess must be removed prior to running the polarogram. This may be done by fuming with sulfuric acid, but the procedure is simplified if the use of hydrofluoric acid is avoided or if the fluoride is complexed prior t o running the polarogram. HE
EQUIPMENT AND REAGENTS
Equipment. A Sargent-Heyrovskf polarograph was modified t o fit a Brown recorder and a Leeds & Northrup Electrochemograph. An H-type cell was used with a saturated calomel electrode (S.C.E.) reference isolated by a sintered-disk agar bridge. Half-wave potential measurements were made with a large saturated calomel electrode and a Rubicon potentiometer. Reagents. Reagent grade chemicals were used throughout. The zirconium metal used in preparing synthetic samples was crystal bar zirco1 Present address, Research and Development Division, Corning Glass Works, Corning, N. Y .
484
ANALYTICAL CHEMISTRY
N. Y.
nium. Spectrographic analysis of this material shoiTed no significant impurities with the possible exception of iron (850 p.p.m.). PROCEDURE
Reduction by Solution Under Nitrogen. Weigh a sample of about 0.25 gram in a 25-m1. volumetric flask. Add 5 ml. of water, 3 ml. of hydrochloric acid, 1 ml. of sulfuric acid, and 1 ml. of fluoboric acid (48 to 50%). Place on a steam bath or hot plate, directing a stream of nitrogen into the flask. If evaporation requires the addition of further water, use deaerated water. In the course of solution a white crystalline solid may appear, which will usually go back into solution on dilution but will not interfere in any case. When solution is complete, cool (continuing the nitrogen stream), add 1 ml. of 0.125% peptone as a maximum suppressor, and dilute to volume with deaerated water. Transfer to the polarographic cell. Because the solution has been heated in preparation, this should be a thermostated cell. The solution may be conveniently stirred with nitrogen to speed thermal equilibration, but the purge is not required to remove oxygen, as the solution is thoroughly deaerated. Record the polarogram from -0.2 to -0.7 volt ZJS. S.C.E. In the work reported, the residual current correction was by baseline extrapolation. Reduction by Powdered Iron. Weigh a sample of about 0.25 gram into a 50-ml. flask. Add 5 ml. of water, 1 ml. of sulfuric acid, and 1 ml. of fluoboric acid (48 to 5Oye). Place on a steam bath or hot plate. Add 3 ml. of hydrochloric acid and continue heating to complete solution of the tin. If evaporation has reduced the volume substantially, add more water. Cool and add 0.5 gram of iron powder. Place a stream of nitrogen on the surface of the solution. When the reaction subsides, heat gently and finally boil to complete solution of the iron. Cool to room temperature, transfer to a 25-ml. volumetric flask containing
1 nil. of 0.125% peptone as a maximum suppressor, and dilute to volume with deaerated water. Continue as in nitrogen reduction. DISCUSSION
The use of hydrofluoric acid was avoided by dissolving the samples with a mixture of sulfuric, fluoboric, and hydrochloric acids. Despite prolonged use in this medium, the capillary behavior remains constant, as indicated by the diffusion current of the cadmium solution; dissolution was more moderate than with hydrofluoric acid. The total amount of fluoboric acid can be added initially. The presence of iron, which would interfere if present as ferric ion, combined with the more desirable behavior of stannous tin indicates that the analysis should be carried out with the metals present in the lower oxidation state. In the developmental work, the solution was reduced with iron powder and protected with a nitrogen atmosphere prior to polarographic analysis. The same result is obtained if solution of the sample is carried out under a stream of nitrogen. Under the conditions of measurement, stannous tin gives a well defined ware with a half-wave potential of -0.48 volt us. S.C.E. The average value of the reciprocal of the slope of the line obtained by plotting log ( i ) / ( i d i) z’s. potential for five measurements was 0.031, with 90% confidence limits of &0.002. The theoretical value for a reversible two-electron reduction is 0.030. In order to determine the lower limit of the procedure and whether the current was proportional to the tin concentration, synthetic solutions were made by adding tin to the zirconium solution prior to reduction with iron powder. The percentages in Table I are based on the amount of tin added
