appreciable concentration-cg., 0.01 water by weight corresponds to a 4.3 .iMsolution. 'The overall reaciion scheme for the three waves would be the sequence represented by Equations 13,14, and 15. MECHANISM OF BROMIDE OXIDATION
The single reproducible anodic bromide wave produced in aqueous solution at the rotating PGE has a constant El,* between pH 0 and 6 ; the current density increases with pH from that for the first iodide anodic wave in acidic solution to approximately the siim of the first two anodic iodide waves in neutral solution. Such a pattern indicates the occurrence of two processes in bromide oxidation with the second process being suppressed by increasing hydrogen ion concentrationi.e., probable involvement in neutral solution of a mechanism analogous to that isostulated for iodide:
+ 2e 2Br0- + 4H+ + 2e
2Br- F? Br2
Brz
+ 2H20
--t
(17)
It is unlikely that a mechanism analogous to that proposed by Toren and Driscoll ( 2 ) for iodide at platinum is operative because of the very low stability of the tribromide ion. The data suggest that, although bromide is much more easily
oxidized to the hypohalite than iodide, the kinetics of the process are such that at high hydrogen ion concentration the reaction is completely suppressed. Triangular sweep voltammetry indicates the redox behavior of bromide to be less reversible than that of iodide at the PGE, which is in good agreement with the observations (32, 3 3 , based on impedance measurements at the platinum electrode, that the reversibility of the halogen-halide systems increases in the order: C12/C1- < Bn/Br- < Iz/I-.
It was noted (32) that the adsorption of reactants in the Brz/Br- system is probably not involved in the kinetics of the electrode process, which again confirms the cyclic voltammetric behavior of bromide as compared to iodide. In fact, it has been stated (40) that the tenacity of adsorption on platinum electrodes increases with the covalent bonding ability of the halide ion-i.e., I- > Br- > C1-; the present study qualitatively confirms that the same order is observed at graphite. RECEIVED for review August 6, 1966. Accepted February 16, 1967. Work supported by the U. S. Atomic Energy Commission and the Horace H. Rackham Graduate School of The University of Michigan. (40) P. V. Popat and H. Hackerman, J. Phys. Chern., 62, 1198 (1958).
Continuous Monitoring of Ion-Exchange Column Effluents with Squaire-Wave Polarography E. B. Buchanan, Jr., and J. R. Bacon Department of Chemistry, University of Iowa, Iowa City, Iowa
The application of square-wave polarography to the continuous determination of reducible materials in the effluent stream from an ion-exchange column is demonstrated. The specific features to recommend this detecting system are high sensitivity and specific ion response. I n addition, there i s no need to deaerate the solution. "oldup and mixing problems are minimized by maintaining a small volume beyond the column. Once ithe parameters of the determination are evaluated, the equipment functions unattended for long periods of time.
THE MANY ADVANTAGES which accrue to the continuous analysis of column effluents, as contrasted to batch analysis, are described by I3laedel and Todd (I). The ideal detector for continuous analysis should have a rapid response, high sensitivity, and excellent specificity. If quantitative measurements are desired, then the detector must display accurately a linear response with respect to concentration. The instrument should produce this response reliably over long periods of operation. It I S also desirable that the detector function with very little prccessing of the solution and be nondestructive in character. Many schemes have been suggested which fulfill a portion of these requirements. The measurements of refractive index ( 2 ) , absorbance (3), and conductivity (4) are Blaedel arid J. W. Todd, ANAL. CHEM., 30, 1821 (1958). (2) R.D.Coulson, iPeu. Sci. Insrr., 34, 1418 (1963). (3) W. C.Kenyon, 1. E. McCarley, E. G. Boucher, A. E. Robinson, and A. K. Wiebe, ANAL.CHEM., 27.1888 (1955). (4)R. Wickhold, 2. Anal. Chem., 132, 401 (1951).
(1) W. J.
52240
but a few of the schemes that have been applied to detect a change of either concentration or species in the effluent stream. Under appropriate conditions a polarograph can be utilized with great advantage as a detector in a chromatographic scheme of analysis. However, certain features of the d.c. polarograph provide serious limitations to its use as a detector. The system is capable of high sensitivity only if oxygen is removed. Another limitation is revealed in Figure 1 which shows a typical d.c. polarogram together with a square-wave polarogram obtained from the same solution of copper, lead, zinc, and ccdmium ions. Both systems achieve their specificity from the appropriate values of the applied potential. In d.c. polarography the measured diffusion current is added to the diffusion current of any previously reduced ion. True specificity is thus limited to the copper step in Figure 1 if a common base line is adopted, as the current measured at any other potential could be caused by the reduction of any more easily-reduced substance. I n order to achieve specificity, it is necessary to ensure that only one reducible substance is present in the cell at any time during the analysis. Therefore, the technique is limited to those cases where the ions involved are completely separated by the ion-exchange column. In this particular instance (Figure 1) an additional limitation is imposed by the discharge of hydrogen which masks the zinc step. The peak type response of the square-wave polarograph does not suffer from these disadvantages. The a.c. diffusion VOL 39, NO. 6, MAY 1967
615
1 Figure 1. Comparison of a.c. and d.c. polarograms of solution containing Cu +*, Pb +*, Cd +*,and Zn &'
Supporting electrolyte is 0.1M HCI Reference electrode is a SCE
.?
8
-0.4
C
-0.8
D.C. Potentiol
-1.2
are applied to the cell in a cyciic patterc. The four voltages chosen in this work are the summit potentials o i copper, cadmium, lead, and zinc. .-? square-wave repetition f r e quency of 500 cps with ar. amplitude of 10 mV was used I < X all of the work. The gate position which selects the portion of the square-wave signal employed was established by instrumental gate setting of 9-9. The sensitivity was set at of maximum value by the instrumental setting of sensitivir, A Sargent Model SK recorder was converted to a form of L point plotter. One point cline! is plotted during the la?rt:r portion of the period that each of the four voltages is appl1c-i to the cell. As the recorder response is proportionai IO the a.c. Faradaic current flowing through the cell, each point. IS B measure of the concentratim of the particular ion in the eel' a t that time. Switch. Figure 2 is a diagram of the circuitry which a,.]justs and connects any of four selected voltages to the poiaro-. graph. The value of any of these four voltages is adjust by means of a 15-turn 2K potentiometer (resistors 6 through located in each branch of the circuit. The use of the 6.8-0nr.
current is measured from a constant base line. Therefore, the square-wave polarograph should serve as a highly selective detector for a chromatographic column. The fact that oxygen does not produce a signal eliminates the need for deaeratim except in those cases where oxygen would react with the electricaily-generated species. This paper describes the equipment and procedures for the continuous square-wave polarographic measurement of a n ion-exchange column effiuent . APPARATUS AND OPERATIOK
-.I ne square-wave polarograph employed in this work has been described earlier ( 5 ) . Slight modification is necessary to adapt the instrument to the continuous analysis of a multicomponent system. The ramp voltage normally employed is not used, and in its place four sequentially selected voltages ( 5 ) E. B. Buchanan, Jr., and J. B. McCarten, ANAL.CHEW,37, 29 (1965;.
+ 300 V
>>
h
WA
a:
s
t o marker relay
R IO t o summing point working amplifier
C(
'yII1
I
t
I
SIC
F
p p s I2
Figure 2. Circuit diagram of input potential switch 616
ANALYTICAL
CHEMISTRY
i
$3
switch S - 5 on voltoge selector
to
from recorder accessory plug Figure 3. Circuit diagram of relay power supply
series resistors (Resistors 2 through 5) limits the span of the potentiometer to 1.5 V, thus increasing the sensitivity of the control. A stable source of 9.1 V is produced by the combination of a 56K resistor (Rl) and the Zener diode 10M9.IZP2 (DI) (Motorola Inc.). Switches 1 through 5 are snapaction microswitches that are actuated by a motor-driven cam which completes one revolution in 80 seconds. Switch 1 is closed during the first quadrant of each revolution; Switch 2, during the second; Switch 3, during the third; and Switch 4 during the fourth quadrant of each revolution. Thus, during every revolution of the cam each of the lour selected voltages is applied t o the polarograph for one fourth of the time. Switch 5, which is also driven by the cam, is closed during the last 22" 01each quadrant and open at other times. This switch actuates the marker pen on the recorder. Switches 6 through 13 represent a four-station push-button switch. Depression of any of the push-button stations disconnects the motor-driven switches from the circuit and connects the desired potential source t o the circuit. Remrder Modification. The fact that four independent signals are t o he recorded on a single chart necessitates a form of a point-plotting recorder. The usual point-plotting recorder will not serve in this application because i t i s designed to stamp a single point on the chart each time i t i s actuated, whereas the output signal o f the polarograph requires that a short line be produced. I t is essential that the timing o f the marking mechanism be synchronized with the input signal to the polarograph and not necessarily with the chart drive mechanism. The timing mechanism 01the usual point-plotting recorder is mechanically coupled to the chart drive mechanism and i t is difficult t o adapt mechanical coupling to the type o f controJ desired. Synchronization is erected in this application by making the pen-actuation mechanism indepcndent o f the chart-drive mechanism and controlling i t with the switch illustrated in Figure 2. A Sargent Model SR drag pen recorder was modified by replacing the pen carriage with a light aluminum one which
was fastened t o both of the parallel slide bars on the recorder frame. A small relay from which the electrical contacts were removed was mounted on this carriage. A pen attached to the swinger plate of the relay contacts and marks the paper only when the relay is energized. Figure 3 is a diagram of the energizing circuit. With the switches in the open position the 250-pf capacitor i s charged to 110 V and will provide sufficient current t o actuate the relay when the switch i s closed. The 3.9K resistor limits the current through the diode to a value which will just maintain the relay in the energized state. Thus, the relay can be maintained in this condition indefinitely and not overheat. A SPDT (center off) switch allows the operator t o select either continuous or intermittent operation of the relay. Figure 4 i s a photograph of the modified recorder. Cell Design. The square-wave polarograph used in this work was of the controlled-potential type and, therefore, employs a three-electrode cell. The two-electrode cells previously described in the literature are not suitable ( I ) . However, the cell designed for this system takes advantage of and incorporates the desirable features of the two-electrode cells. Figure 5 i s a cross-sectional diagram of the cell. I t has a small volume, approximately 1.5 ml, and i s demountable for easy cleaning. The main body of the cell i s transparent so that the electrodes can be observed. The close proximity of the SCE and D M E is assured by mounting both in the top of the cell. This is accomplished by using a SCE of the fiber type with only the fiber extending through the cap of the cell. The fiber was made from an &ply cotton string and sealed in place with silicone rubber. Because the spacc
EME-
Rubber
Sample
Figure 4 .
Photograph of modified recorder
Relay power supply is locatcd on the middle of the back panel of recorder
\Sompie
Figure 5.
ond M e r c u r y
Outle'
Crw-sectional diagram of the d l VOL 39. NO. 6, MAY 1967
617
required for the fiber within the cell is minimal, there is ample space available for the entrance port on the top cap. The cell is arranged so the mercury drops fall directly through the cell and cut the bottom and thus have little tendency to bounce and stir the solution. The platinum electrode is coiled within the lower portion of the borosilicate glass tube and is protected at the top by a fluorocarbon washer. Figure 6 is a photograph of the cell. -RMANCE
OF APPARATLS
CaWmiiom. The linearity of the current us. concentration relationship was establiskd for the ions, Cu+*, Pb+*,Zn+*, and Cd+*. The range of concentration over which such linearity is applicable was found to be 1 X 1W'M to 1 X 1WV.f. These calibration studies were made with a supporting electrolyte which was 0.1M hydrochloric acid and with a nonbwingor static system. cbmedhl
c3rleds I)oe to Flaring sohiim. The
polarographic current for a particular solution has bee0 shown t o be a function of the rate of flow of the solution
pisue 7. Elntion amve for
past the electrode ( I , 6, 7,s). This effect was i n m i g a t e d for the square-wave polamgraph in the following manner. The ion-exchange column normally connected t o the cell was replaced by a column packed with glass wool to provide a back pressure equivalent to that obtained from the ionexchange column. The system was then flushed with a solution which was 1 X IW'M in lead ion and 0.1M in hydrochloric acid. The polarographic currents for solutions flowing at rates behueen l mllminute and 10 ml/minute were compared to those of a static system. At 1 mllminute there is approximately a 1 increase in current. This value rises to 4% at 3 ml/minute and to 9% at 10 ml/minute. These figures represent the contribution of convection currents and are equivalent to the values obtained in d.c. polarography. Response Time. The cell and connecting tubing were filled with 0.1M hydrochloric acid. With the potential set to determine Ph+', a base line was drawn by the recorder. The glass wool column previously described was thoroughly Bushed with a solution which was 1 X IO-'M in lead ion and 0.1M in hydrochloric acid. The flow rate through the column was adjusted to 1 ml/minute. The time interval was measured from the instant the connection was completed between the column and the cell tubing. No response was recorded for a period of 1 minute which indicated a dead volume of 1 ml. After 4.8 minutes 95% response was obtained. One hundred per cent or steady state response was achieved after 5.8 minutes. The dead time (no response) is due t o the holdup volume between the cell and the column and is the time required for the solution to enter the cell after leaving the column. The buildup time is the time required for the solution to flush the contents of the cell. These times would be lengthened if a n unfavorable density gradient were involved. I n order to keep the response time t o a minimum the cell was coupled direaly t o the bottom of the column with smalldiameter tubing. This coupling does not provide for the removal of air bubbles before the solution enters the cell. As these bubbles will have a detrimental effect upon the operation of the detector, one should exercise care to ensure the absence ofair in the column or insert a bubble trap in the connecting tubing. Separation of Copper, Lead, Zinc, and Cadmium Ions on an Ani00 Erdmge Collnu~ A 3.0-ml sample containing 1.5 x 10-1 mmoles of zinc, 3.0 x lo-> mmoles of lead and
(6)C.K. Mann, ANAL CHEM.,29, 1385 (1957). (7) 0. H. MIAIR, 3. Am. C k m . Soe.. 69,2992 (1947). (8) L. D. Wilson and R. J. Smith, ANAL. CHEM., 25,218,334(1953).
a+'Pb+i , and a+' wilh 0.1M H a
. F s
I
I
Volume
50
in Milliliters
-
1
I
25
0
75
Figure 8. Elution curve for Cu+*, Pb+*, and In+*with 0.05M HCI
1.2 X mmoles of copper was introduced onto a 15.5-cm column of Dowex, L X 8x 30&400 mesh. The column had been zquilibrated with a 0.1M hydrochloric acid solution. The elution was begun with a 0.1M hydrochloric acid solution. The Row rate was adjusted at 1 ml/minute. The square-wave polarographic current was followed when the DME had a potential of -0.17 V, -0.44 V, -0.65 V,and -1.08 V, VS. SCE. These potentials represent the summit potentials for the reduction of the copper, lead, cadmium, and zinc ions to the metal. A low sensitivity was used to keep all values on the IO-inch chart. Figure 7 is the chart record for this particuiar determination and gives an indication of the appearance of the data as it is actually obtained. The calibration points were added later t o show the measure of sensitivity toward the corresponding ion. While the separation is not complete (it could be made so at higher hydrochloric acid concentrations), the chart shows the capabilities of the detecting systcm. The overlap of the zinc and lead curves is easily detected, and the concentration of each ion is readily determinable. The elution curves for these same ions were obtained under less favorable conditions t o further
verify the applicability of the detector. A column was equilibrated with 0.05M hydrochloric acid. A 1.00-ml sample containing 4 X lo-' rnmoles of copper, 1 X lo-' mmoies of lead, and 5 X rnmoles of zinc was introduced. Elution was accomplished with 0.05M hydrochloric acid. Figure 8 is a curve drawn by connecting the maximum current at each measurement for each particular ion. The data were presented in this fashion because of the confusion resulting when the chart is reduced to a size suitable for journal publication. Figure 9 was drawn in the same manner from an elution curve obtained when the eluate was 0.02M hydrochloric acid. A 1.00-ml sample containing 4 x lo-' mmoles of copper, 1 X 10-Jrnmoies of lead, 5 X 10-3 mmoles of zinc, and 5 >( mmoles of cadmium was introduced. The curve r e p resents the worst possible case so far as the elution is concerned, and places the greatest demand upon the selectivity of the measuring device. The cadmium was included in this elution t o show the long-term stability of the apparatus. It should be noted in Figure 9 that the base line of copper stands well above the base lines of lead, cadmium, and zinc. This higher base line is cau-ed by an imperfect separation of the
n
L -
O
i15
50
480
600 . .
. .........
720
840
960
A_
1080
Figure 9. Elution curve for CU+=, Pb+=, In+=,and Cd+' with 0.02M HCL VOL 39, NO. 6, M A Y 1967
619
capacitative and faradaic currents at low d.c potentials. The imperfect separation results from an increased cell resistance which is in turn due to the low concentration of s u p porting electrolyte. The shift in the position of the base line is a constant a t any particular concentration of supporting electrolyte and therefore can be displaced back to the bottom of the chart by a base line compensation circuit, should it become desirable t o d o so. CONCLUSIONS
of high sensitivity, and therefore can detect the presence of a particular ion well out on the extremities of the peak. The selectivity is excellent as is shown by Figure 9 where two different ions were detected even though they were both eluted at the same time. The fact that oxygen is not reduced reversibly eliminates the necessity for treating the effluent to remove dissolved oxygen before the solution enters the cell. The 15-hour run for the determination of cadmium illustrates the long-term stability or reliability of this system.
The use of square-wave, and by extrapolation, sine-wave polarography provides a n excellent means of monitoring the effluent from an ion-exchange column. The scheme is capable
RECEIVED for review October 20, 1966. Accepted February 24,1967.
Particle Size Distribution Measurement in the 200 to 1200 A Range Ted Lee and C. W. Weber TechnicaI Division, Oak Ridge Gaseous Diffusion Plant, Union Carbide Corp., Nuclear Diaision, Oak Ridge, Tenn.
A relatively inexpensive method for partitle size distribution measurements in the 200 to 1200 A range has been developed for a variety of powders, including clays. The method utilizes 20-kc ultrasonics to deagglomerate and disperse the powder sample in an aqueous medium; a continuous flow centrifuge applying forces up to 60,000 x G to divide the slurry into size fractions, based on Stokes’ limiting diameters: and a gravimetric procedure (or refractive index measurements) to determine the solid content of each fraction. The simple Stairmand calculation i s applied to correct for simultaneous settling of fine particles. The method has been applied to Linde type B alumina polishing powder, and the results have been compared with electron microscopy measurements. Alumina, a major constituent of clays, permitted development with a pure compound of known density. Within the limitations of the equipment, experimental factors which influence accuracy are optimized. Deviations from basic centrifugation theory are discussed.
THISPAPER describes the development of a practical and versatile technique for general application t o particle size distribution measurement. A relatively inexpensive method for measuring the particle size distribution of a variety of powders, .;ncluding clays, was needed for the difficult size range from 200 to 1200 A. Alumina, a major constituent of clays, permitted development with a pure compound of known density. A review of the factors influencing the choice of a method for particle size analysis has been given by Scarlett (1). An exceilent review and bibliography of methods for determining particle size was prepared by a subcommittee of the Society for Analytical Chemistry ( 2 ) . Probably the two best known methods for particle size distribution measurements in the size range of interest are the analytical ultracentrifuge and ;he electron microscope. The analytical ultracentrifuge is expensive, and its use is limited to samples in which the concentration can be determined by optical means. To use the electron microscope, also expensive, requires counting large -- -
(1) B. Scarlett, Chem. Process Eng., 46, 197 (1965). (2) E. Q . Laws, et ui.,Anuiyst. 88, 156 (1963).
620
ANALYTICAL CHEMISTRY
numbers of particles for each sample; this is tedious and impractical for routine applications. A third method for size distribution measurement, also based on centrifugation, is the application of a Sharples supercentrifuge. The supercentrifuge is relatively inexpensive, provides centrifugal force sufficiently strong (up to 60,000 X G) t o settle the small particles of interest, and permits feeding the sample and collecting the size fractions while the machine is operating at sedimentation speeds. The method developed in this report would also be applicable to more refined equipment operating on the same principles, such as the preparative ultracentrifuge with a continuous flow rotor. The supercentrifuge was first applied to particle size distribution measurements by Hauser et of., (3-5). Their method is based on determining the sediment deposited in the centrifuge bowl, and calculating the size distribution from the centrifuge parameters and a calibration factor (6, 7). TO simplify the calculations for routine applications, alignment charts (3, 5, 8) and nomographs (9) were developed and applied. Using the supercentrifuge and the equations developed by Hauser et ui. (3-9, these workers and others (7, 8, 10) obtained usable quantities of sized samples. Sedimentation constants (7) obtained with the supercentrifuge showed good agreement with the more elaborate analytical ultracentrifuge. The size distribution measurement described in this report is based on determining the soiids content in etRuent fractions, rather than in the aeposits. The limiting diameter for each size fraction is calculated by the Svedberg-Nichols equation (3) E. A. Hauser and J. E. Lynn, ind. Eng. Chem.,32,659 (19401. 14) E. A. Hauser and C . E. Reed, J . Phys. Chem.. 40,1169 (1936). (5) E. A. Hauser and H. K. Schachman, Ibid., 44, 584 ( 1940).
(6) R . R. Irani and F. C. Clayton, “Particle Size Measurement, Interpretation and Application,” pp. 85-88. Wiley, New York, 1963. (7) H. K. Schachman, J. Phys. Coiloid Chem., 52, 1034 (1948). (8) S. C. Oliphant: C. R. Houssiere, Jr., and G: H. Fancher, Am. Inst. Mining Mer. Engrs.. Tech. Pub. No. 1530 (1942). (9) E. Saunders, ANAL.CHEM., 20, 379 (i948). (10) F. €3. Norton and S . Speii, J. Am. Cerom. Soc., 21, 367 (1938).
