ANALYTICAL CHEMISTRY
218 in Figure 1. This error was eliminated in the analyses by measuring the diffusion current a t the same voltage each time. The limitation of this type of polarography, when extended t o more dilute solutions, lies in the increasing influence of the residual current in working with more dilute solutions, the difficulty in removing the last traces of oxygen, and the increase in electrolysis current on increasing the electrode area.
ACKNOWLEDGMENT
The author gratefully acknowledges the help and encouragement of the research staff of the Leeds and Northrup Co. in the initial stages of this investigation. The author also wishes to thank T. S. Lee of the University of Chicago for furnishing him with prepublication copies of the articles on the rotating mercury electrode.
PROCEDURE
LITERATURE CITED
Five to 10 ml. of the unkn0v.m solution in the appropriate supporting electrolyte was placed in the cell and nitrogen passed through the sintered-glass disk for a t least 2 minutes. This initial removal of most of the dissolved oxygen is necessary before the electrode is introduced into the solution to prevent introduction of mercury or silver ions by reaction with dissolved oxygen. If sodium sulfite was used, about 0.1 mg. was then added and the cell was placed in the apparatus. .4further deaeration of 5 minutes was allowed before the polarogram was begun. The nitrogen was then by-passed over the solution and any gas bubbles adhering to the electrode were removed by sharply rapping the shaft. The polarogram was then run in the normal fashion. In the case of the cadmium polarograms, the deposited metal was removed from the electrode by anodic stripping. This was done by shorting the electrode with the reference electrode or reversing the normal voltage scanning direction. This procedure was not necessary with Go"' + Co" reductions.
(1) Friend, J. N., "Textbook of Inorganic Chemistry," Vol. X,
p. 138,Philadelphia, J. B.Lippincott Co., 1928. (2) Kolthoff, I. M.,and Jordan, Joseph, J . Am. Chem. Soc., 74, 382 (1952). (3) Laitinen, H. A., and Burdett, L. W,, ASAL. CHEM.,22,833 (1950). (4) Laitinen, H. A., and Kolthoff, I. AI., J . Phus. Chem., 45, 1079 (19411. ( 5 ) Lee, T. S., J . A m . Chem. Soc., in press. (6) Lydersen, Dagfin, Acta Chem. Scund., 3, 269 (1949). (7) Rogers, L. B., et al., U. S. Atomir Energy Commission, Doc. AECD 1876 (1948). (8) Sidgwick, N. V., "Chemical Elements and Their Compounds," p. 289, New York, Oxford University Press, 1950. (9) Skobets, E. lI.,Berenblyum, L. S., and -itamanenko, S . N.. Zacodskayu Lab., 14, 131 (1948). RECEIVED for review June 23, 1952.
Accepted October 17, 1952.
Polarographic Analysis Using Flowing Samples Efect of Cell Design and Flow Rate o n Difusion Current LYNN D. WILSON'
AND
R . J . SMITH
George M . Moflett Research Laboratories, Corn Products Refining Co., .Argo, 111. Automatic process control necessitates continuous instrumental analysis. Quantitative analysis by the polarographic method using flowing samples is shown to be feasible. Value of the diffusion current for a given flow geometry is dependent on the linear flow rate up to that point at which major irregularities in drop formation occur, resulting in greatly distorted diffusion steps. In general, for a given sampling rate, cell design should be such that the linear flow rate does not exceed that value at which distortion of the diffusion current first begins.
C
LBSSICAL polarography requires the use of static samples because of the effect of stirring on the diffusion current. As a consequence, possible application to continuous indication or recording of concentration variations in flowing samples has been overlooked except for a few scattered studies. Continuously recorded measurements have been made of the oxygen content of various media including lake water ( 6 ) , activated sludge (S, 41, and water used in metabolic studies ( 7 ) . Special adaptations have been devised for the continuous determination of oxygen in gases (1, 5 , 8). For the most part, these studies have been empirical in nature and are limited to the applications described. This article embraces a more fundamental study of the effects of flowing samples on diffusion current and on characteristics of the dropping mercury electrode; simultaneously, the effect of electrolysis cell geometry is considered. EXPERIMENTAL
A solution of 0.004 &' cadmium nitrate in an indifferent electrolyte of 0.05 M potassium nitrate was used in this study, as a maximum suppressant was not required. Preliminary observations indicated that sufficient information could be obtained by using three different electrolysis cells, two providing for horizontal flow at two different linear velocities and the third for Present address, R'ilson Industrial Hygiene and Research Laboratories. 330 South Wells St., Chicago 6 . Ill. 1
vertical flow in both upm ard and downward directions. Further exploratory trials showed that a flow rate range up to 1000 ml. per minute would suffice t o cover the apparent useful range regardless of cell geometry. Because the diffusion current is dependent on the drop time and the mass of mercury flowing per second as defined by the IlkoviE equation (Z), provision was included for studying the effect of flow rate on these factors. Figure 1 shows the schematic arrangement of the entire assembly, which, with the exception of the constant-level device and the rotameter, was thermostated at 25 ' i 0.05' C. Connections between the several parts were made with rubber tubing which had been boiled in 50% sodium hydroxide. The constantlevel device was designed with the inlet above the feed tube t o prevent entrained gas bubbles from entering the electrolysis cell, where they might have disturbed the uniformity of fl0.w. Figure 2 shows the vertical cell and Figure 3 shows the horizontal cell assembly, which was tilted downward slightly t o ensure that the mercury drops rolled into the trap. To prevent diffusion of the sample into the calomel electrode, a reservoir (not shown) of saturated potassium chloride-mercurous chloride solution was arranged to provide a slight hydrostatic head over that of the rotameter. The dropping mercury electrode was firmly supported in a male ground joint with its face at the exact center of the flowing stream-namely, the position of maximum rate of flow of sample with respect t o walls of the cell. Polarograms were obtained with a Sargent-Heyrovsk? Model X X I recording polarograph. The complete study utilized four separate units covering measurements under two different conditions of horizontal flow and two of vertical flow, one upward and one downward.
V O L U M E 25, NO. 2, F E B R U A R Y 1 9 5 3
219
the time required for 25 drops to fall into the trap was measured. As soon as the 25th drop had fallen into the trap, the stopcock was opened to discharge the collected mercury into an evaporating dish where it was washed with distilled water and acetone, and allowed to air-dry for 5 minutes before weighing to determine the mass of mercury flowing per second. RESULTS
E
Figure 1.
S c h e m a t i c .4rrangement of Assembly A. B. C. D. E. F.
5-gallon reservoir 5-gallon receiver Gas dispersion tube Gas inlet tube Release cock Constant level G . Calomel cell H . Electrolysis cell I . Rotameter J . Entrapping device
v
Polarograms obtained as outlined in the preceding section are not reproduced, because of space limitations. Following the preparation of each polarogram, irregularities in the diffusion current form were determined for an applied constant e.m.f. of 0.9 volt (os. S.C.E.). The effects of volume and linear flow rates on diffusion current magnitude and distortion for the four conditions of horizontal and vertical flow are shown in Figures 4 and 5, respectively. With the large horizontal cell (25-mm. bore), flow rates up to 100 ml. per minute (average linear flow rate of 0.34 em. per second) have substantially no effect on the diffusion current. Above this rate, distortion becomes apparent and the increase in diffusion current over that obtained with static samples becomes more pronounced. With the small horizontal cell (12.3-mm. bore), diffusion current distortion begins at about the same volume flow rate, but a t a much higher linear flow rate (2.07 cm. per second). More striking, hoffever, is the relatively greater increase in the diffusion current rclative
I
--1
'Y
.J
Figure 2.
)
Figure 3.
Typical Horizontal Electrolysis Cell Assembly
Typical Vertical Electrolysis Cell
Each experiment was preceded by a 12-hour period of thermostating a t 25" C., during which dissolved oxygen was removed by displacement with hydrogen introduced through the gas dispersion tube in the %gallon reservoir (see Figure 1). Polarograms were run a t flow rates of 0, 50, 100, 150, 200, 250, 500, 750, and 1000 ml. per minute for each cell, the sample being forced through the system by displacement with hydrogen. During each of the runs the bath stirring motor was shut off to eliminate extraneous vibration. Constancy of flow was achieved by means of the constant-level device. The several rates of flow were metered by rotameters connected in the system a t the downstream end of the cell. Immediately after each polarogram had been recorded, the applied voltage was set t o a point corresponding to the middle of the diffusion current region (about 0.9 volt L S . S.C.E.) in order to relate current fluctuations to possible variations in drop time and mercury mass. During this period
I
I 100
250 FLOW RATE
-
500 MILLILITERS
I PER
750 MlNUTE
Figure 4. Effect of Volume Flow Rate on Diffusion Current A . Large horizontal cell B. Small horizontal cell C. Vertical cell, upward flow D . Vertical cell, downward flow Normal diffusion current Distorted diffusion current
-----
I 1000
ANALYTICAL CHEMISTRY
220 to increasing flow rates. In other v-ords, the smaller the diameter of the cell or the greater the average linear flow rate, the greater is the increase in diffusion current. Similar observations were made with a vertical cell (22-mm. bore), using both upxard and dowmvard flow. With upward flow, diffusion current distortion again occurs a t approximately the same volume flow rate and a t an intermediate linear flow rate (0.66 em. per second). With downward flow in the same cell, distortion occurs a t a volume flow rate of about 200 ml. per minute (0.88 em. per second), a somewhat higher volume flow rate than for the three previous conditions of flow. Downward flow would be expected to cause less interference with drop formation, as the mercury drops grow in the direction of the flowing stream rather than against it, as is the case n-ith upward flouI t is evident also (Figures 4 and 5 ) that upward flox in the vertical cell produces a substantially greater diffusion current than downward flow at similar volume or linear veloc4ities.
Table I.
0 50 100 150 200 250 500 750 1000 0 50 100 I50 200 250 500 750 1000 0 50 100 150 200 250 500 750 1000
0 50 100 150 200 260 500 750 1000
2
Rate - t Drop, m, AIg,1/3 Cin./sec. Sec. 1lg./Sec. Sec.l/s
id(o.9 Volt), pa.
Stirring
El,:, Current, Volt
Fa.
...
0.00 0.17 0.34 0.61 0.68 0.85 1.70 2.55 3.40
Horizontal Cell, 25-Mm. Bore 2 21 2 03 34 0 2.96 2 15 2 00 3.03 34 2 2 15 3.04 2 00 34 6 2 15 3.04 2 00 34 8 2.13 3.07 1 99 36 2 2 12 3.07 1.99 36 4 2 12 3.08 1 99 38 4 2.12 39 6 3.06 1 99 2.12 42 2 3.06 1 99
0 59 0 59 0 59 0 59 0 59 0 59 0 59 0 69 0 60
0.00 0.69 1.38 2.07 2.76 3.45 6.90 10.35 13.80
Horizontal Cell, 12.3-hIm. Bore 2.23 33.8 2.92 2 04 2 17 38.2 2 01 3.02 2.17 41.2 3.02 2 01 2.16 43.4 3.02 2 01 2.15 2 01 3.05 14.6 2 15 3.05 47 1 2 01 2 14 55,5 3.05 2 00 2.14 1 99 2.97 70.8 2.13 2.63 98.4 1 94
0 59 0 60 0.60 0 60 0.61 0 61 0 62 0 61 0 59
Vertical Cell, 22-Afm. Bore, Upward Flow 2.22 2.04 33.8 0.00 2.94 0.22 3.02 2 16 2.01 33.4 3.02 2.15 2.00 34 8 0.44 3.03 2.15 2.01 34.8 0.66 1.99 3.06 2.13 36.2 0.88 3 07 2.13 2.00 40.2 1.10 3.07 2.12 1.99 57.3 2.20 3.06 2.12 1.99 67.2 3.30 2.99 2.12 1.98 75.2 4.40
0 2 0 6
0 8 2 2 2.4 4.4 .6
I
I
4
6
FLOW RATE-CENTIMETERS
I PER
8 SECOND
I
I
10
I2
Figure 5 . Effect of Linear Flow Rate on Diffusion Current A . Large horizontal cell B. Small horizontal cell C. Vertical cell, upward flow D. Vertical cell, downward flow Normal diffusion current Distorted diffusion current
----
Effect of Flow Rate on Diffusion Current, HalfWave Potential, and Capillarl- Constants m2/3tl:(,
lll.,'niin,
I
0
flov ing sample produces irregular dropping and disturbs the diifusion layer. A s a result, the current increases because of ronvertive transfer to the electrode surface. The increase in current may be appropriately labeled the "stirring current" (terminology proposed by J. J. Lingane). The relationships brtween diffusion currents and flow rates are shown in Figures 4 and 5 . I t may be seen that vertically downward flow results in the least dwiation of the diffusion cuirent from the static value.
8 2
CONCLUSION s
0.59 0.59 0.59 0.60 0.60 0.60 0.60 0 61 0 61
Vertical Cell, 22-1Im. Bore, Downward Flow 2 22 2.04 33 8 0 0.00 2.94 2 17 2 01 32 8 0 3.01 0.22 2 16 3.03 2 01 32 6 0 0.44 2 15 0 33 0 3.04 2 00 0.66 2 14 0 33 6 3.02 2 01 0.88 2 14 34 2 0 3.05 1.10 2 00 2 14 0 36 8 3.05 2.20 2 00 2 13 37 8 0 3.02 3.30 1 99 2.13 45 9 0 3.02 4.40 1.99
59 59 59 59 59 59 60 60 60
. .
4.4 7.4 9 6 10 8 13.3 21.7 37.0 64 6
...
4
-0
1 0
1 0 2 4 6.4 23 5 33.4 41,4
... -1 -1
0 2
-0 8 -0 2 0.4
3.0 4.0 12.1
Table I summarizes results on the stud>-of effect of inereasin? flow rates on diffusion current, half-wave potential, and capillai I. constants m and t. The half-wave potential remains constant within the limits of experimental measurement. There seems to be no theoretical reason why there should be any shift in deposition voltage over than an apparent shift resulting from distortion of 13-ave form. I n like manner, t , the time per drop, nz, the mass of mercury flowing per second, and the product, m 2I3f1 '6, also remain substantially constant, though at higher flow rates the constancy of t as an average for 25 drops is somewhat misleading, as evidenced by the erratic current oscillations on the polarograms. The last column of the table lists the numerical measure of the effect of increasing flow rates on the diffusion current. It seems likely that the stirring action caused by the
Experiments have established the feasibility of concentration measurements by the polarographic method using flowing samples. Examination of the polarograms has shown that the flow rate need only be held to some reasonable degree of uniformity below that rate a t which appreciable distortion of the diffusion current wave occurs. For a given sampling rate, knowing the required cell diameter for that rate, it is only necessary to refer to a seiies of polarograms covering the reduction over a suitable range of flow rates and from these to select the upper flow rate a t whirh a diffusion current wave with a relativrly smooth envelope is still obtained. The desired rate is maintained by the outlet orifice of the cell, the feed to the cell being controlled by a constant-level device. The use of downward flow precludes the necessity for perfect filtration of industrial liquors, as only large particles which might clog the cell orifice need be removed. A commercial polarograph is not required for routine measuienient, as a suitable polarizing unit allowing selection of the appropriate fixed potential can be constructed readily from apparatus usually available in a physical chemistry laboratory. The only expensive portion of the equipment is the recorder. ACKNOWLEDGMENT
The assistance of H. E. Gornian and T. G. Meilleur in this work is gratefully acknowledged. LITERATURE CITED
(1) Beckman, P., Chemistry and I n d u s t r y , 1945, 791. (2) IlkoviE, D., Collection Czechoslou. Chem. Communs., 6,498 (1934). . 14, 256( 1942). (3) Ingols, R. S.,IKD.EXG.CHEY.,A s 4 ~ ED., (4) Ingols, R. S., Sewage W o d x J., 13, 1097 (1941). (5) Laitinen, H. A., Higuchi, I., and Cauha, M.,J . Am. Chem. Soc., 70. 561 - - - 11448). ' (6) Manning, TI'. hl., Ecology, 21, 509 (1940). (7) Spoor, W.A , , Science, 108, 421 (1948). (8) Wise, W, S.,Chemistry and Industry, 1948, 37. I
\ - - -
RECEIVED for review March 5, 1952. hccepted October 27, 1952.
