sistently low, usually by several per cent or more. The presence of 15% by volume ethanol, used by Wanninen in the determination of sulfate to decrease the solubility of barium sulfate, did not seem to effect significantly the accuracy of titrations with 0.01X DTPA containing 0.005X Mg-DPTA. Titration of Strontium. Table I11 summarizes the results of the titration of four different concentrations of strontium with a solution 0.01M in D T P A and 0.015M in Mg-DTPA. Each result represents the average of a t least four titrations, with relative standard deviations of 0.3% or less. With strontium, the 0.0134 DTPA could contain as lorn as 0.005X hIgDTPA t o obtain results comparable to those of Table 111, with the error increasing as the concentration of hfgDTPA was decreased below 0.005M.
Because the Sr-DTPA chelate is appreciably more stable than the Mgit was to be expected DTPA chelate (4, that the concentration of Mg-DTPA would be less critical in the titration of strontium than in the titration of barium. Titration of Magnesium. The titration of magnesium with 0.01~34 DTPA, with or without Mg-DTPA present, was accurate to 0.2 t o 0.3%, with relative standard deviations of 0.3y0 or less. Warming to 40’ C., used by Wanninen ( 4 ) ,did not significantly change the titration results. ACKNOWLEDGMENT
The authors thank Robert B. Thurman for performing some of the check titrations and the Geigy Chemical Corp. for furnishing DTPA reagent.
LITERATURE CITED
(1) Olsen, E. D., ANAL.CHEM.36, 2461 (1964). (2) Schwarzenbach, G., “Complexometric
Titrations,” Methuen and Co., Ltd., London, and Interscience, New York,
1957. (3) Sijderius, R.. Anal. Chim. Acta 10, 517 (1954). (4) Wanninen, E., Acta Acad. Aboensis, Math. et Phys. 21, No. 17, 1 (1960). (5) Wanninen, E., Suomen Kemistilehti 29B, 184 (1956). (6) Wanninen, E., Talanta 8 , 355 (1961).
EUQENE D. OLSEN ROBERTJ. NOVAK Department of Chemistry University of South Florida Tampa, Fla. WORKsupported by the National Science Foundation (NSF-GP-3482).
A Critical Evaluation of Practical Rotated Disk Electrodes SIR: Considerable data have been accumulated a t rotated disk electrodes (RDE’s) which are cylinder shaped. These electrodes have the electroactive disk surface encased in an insulating rod of glass, Teflon, or other inert shroud. Diffusion coefficients determined with cylindrical RDE’s compared well with those obtained by other electrochemical methods and there was no reason to suspect serious deviations in limiting currents (iL) a t these electrodes (3-5). However, Riddiford suggested fluid flow mixing between layers above and below the disk plane could lead to incorrect values of iL and that cylindrical electrodes were prone to this type of mixing ( 7 ) . Azim and Riddiford recommended cone or bell-shaped electrode designs (1). Most recently Blurton and Riddiford have examined the RDE shape problem in detail (2). The present investigation substantiates the finding of Blurton and Riddiford and extends the evaluation to include the effects of immersion depth and solution volume on iL. EXPERIMENTAL
The rotated disk electrodes used in this study are shown in Figure 1. The shafts were stainless steel rod ea. 0.9 cm. in diameter. The shielding material was Quikmount self-setting resin obtained from Fulton hfetallurgical Products Corp. The liquid resin was molded around the shaft and, after it had hardened, it was machined on a precision lathe to the desired shape and size. The electroactive area was produced by drilling a ‘/*-inch diameter hole through the shielding to the steel shaft. The drilled space was filled with carbon paste. The disk radius (electroactive) was ca. 0.16 em. and
calomel electrode. The working electrode was rotated by a motor with an integral tachometer generator. The motor shaft and electrode shaft were connected via a precision chuck attached to the motor shaft. All carbon paste techniques were as previously reported (6).
SHIELDINQ
All chemicals used were reagent grade and the water was distilled and then deaerated with tank nitrogen.
CARBON PASTE (A)
(8)
ELECTRODE SHAPE STUDY
u
Ie r .
Figure 1 . A. B.
Carbon paste RDE’s Cylindrical (CEL) Bell-shaped (BEL)
the electroactive area ea. 0.079 sq. em. Reynolds numbers calculated a t the outer edge of the electroactive surfaceLe., for r = 0.16 em.-varied from about 16 to 640 over the range of rotation rates used (1-40 rev./sec.). The total radius of the disk plus insulating shroud was ca. 1 cm. A controlled potential polarograph utilizing a three-electrode system was used, and in all cases the voltage scan rate was 0.5 volt/min. The auxiliary electrode was a platinum wire and the reference electrode was a saturated
Table 1.
40.0 20.0 10.0 5.0 2.0 1.0 Av .
A stock solution ea. 1 X 10-3M &Fe(CN)G and 211.1 in KC1 was prepared. A portion of this freshly prepared solution was run a t a cylindrical RDE 0.41 em. in total diameter as a control measure. Then limiting currents were obtained a t the cylindrical electrode (CEL) shown in Figure l A , a t a number of rotation rates (R) ranging from 1.0 to 40.0 rev./sec. At each rotation rate a fresh portion of the stock solution and three new electrode faces were used. The CEL was then machined down to the bell-shaped electrode (BEL) , Figure IB. In so doing, neither the electroactive area nor the total surface area of the disk was affected. The same experi-
Effect of Electrode Shape upon Limiting Currents
70.1 f 0 . 2 49.3 f 0 . 2 35.0 f 0 . 1 25.7 =t 0 . 2 16.3 f 0.2 11.9 f 0 . 1
11.1 f 0.1 11.0 f 0 . 1 11.1 f 0.1 11.5 f 0 . 1 11.6 f 0 . 1 11.9 f 0 . 1 11.4 f 0.3
70.1 =t 0 . 5 49.8 f 0 . 1 35.8 f 0 . 4 25.8 f 0 . 1 1 6 . 1 =t 0.1 11.4 f 0.2
11.1 f 0 . 1 11.1 f 0 . 1 1 1 . 3 f 0.1 11.5 f 0 . 1 11.4 f 0 . 1 11.4 f 0 . 2 11.3 f 0.1
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153
mental procedure as above was then followed for the BEL. Finally, another portion of the stock solution was run a t the control RDE. The limiting current obtained differed by 0.5% from that obtained when the solution was freshly prepared indicating that no decomposition of the solution had occurred during the course of the extended experiment. The results of this experiment are shown in Table I and Figure 2. They indicate that, over the entire range of rotation rates studied, the differences in limiting currents obtained a t the two electrodes are slight. The ratios of iL/R"2 indicate that a t very low rotation rates, convection may be more important a t the CEL than a t the BEL. However, these effects are small, and in the range of rotation rates of interest (R 2 5 rev./sec.) these differences are negligible. One distinct difference between the two electrodes was observed. At a rotation rate of 40 rev./sec., the CEL frothed the solution while the BEL did not. Nonetheless, the limiting currents obtained were identical (as shown in Table I) and were in good agreement with values a t rotation rates where frothing did not occur. At smaller electrodes (ca. 0.5 cm. total diameter) frothing is not observed below ca. 80 rev./sec. Thus, it would seem that for rotated disk voltammetry a t all but very slow
Table II. Effect of Immersion Depth upon Limiting Currents
Im: mersion depth, cm.
R = 5.0 rev./sec.
1.85 1.13 0.62
25.5 25.7 25.8 25.8
0.00
Av.
i~ (pa.) R = 40.0 rev./sec.
f0.1 i: 0 . 1 zt 0 . 1 zt 0 . 1
25.7 f 0 . 1
Table 111.
Solution volume, ml. 9000 4000 2000 1000 600 250 100
70.2 69.9 69.7 69.4
69.8 f 0 . 2
2.0
Figure 2.
cm.
25 18 14 11 9
25 25 19 14 12
7
8
5
6
Plot of
iL
5.0
0.0
7.0
vs. R ' I e for both CEL and BEL
rotation rates the cylindrical and bellshaped electrodes produce identical results. Furthermore, for most purposes, the difficulty of fabricating the bell-shaped electrode probably outweighs its advantages. IMMERSION DEPTH STUDY
In this study, the experimental conditions were as in the previous study except that only the BEL was used. A preliminary study was made to ensure that no error would be introduced by using the same electrode surface for three consecutive polarograms. This was found to be the case as three consecutive limiting currents run on the same electrode surface a t a rotation rate of 5.0 rev./sec. were identical. Using a fresh portion of the stock solution, four polarograms were run as before, except that after each run a portion of the solution was removed from the cell. This, in effect, decreased the depth to which the electrode was immersed. The final polarogram was
i~ (pa.) 94.0 92.4 93.2 93.0 94.2 93.4 92.0
f0.4 i:0 . 4
f0.8 i:0 . 8 f 0.6
f 0.8 f 0.4
93.2 i: 0 . 6
ANALYTICAL CHEMISTRY
4.0
0 BEL CEL
T'essel size Diameter, Depth, cm.
3.0
R "'I R EV!'SBEC~
Effect of Solution Volume and Vessel Size upon Limiting Current
Av.
154
f0.2 f0.2 f 0.2 f0.2
1.0
0
54.3 54.1 54.4 54.4 54.8 54.6 54.0
zt 0 . 2 f0.2 i:0 . 2 f0.2 f0.2 f0.4 i: 0 . 2
5 4 . 4 i: 0 . 2
run with the electrode face just touching the surface of the solution. Finally, with a new portion of solution, the above experiment was repeated a t a rotation rate of 40.0 rev./sec. The results of these experiments are shown in Table 11. Theory states that the distance from the electrode surface to the solution-air boundary must be infinite as compared with the diffusion layer thickness. Riddiford indicates that the electrode surface should be at least 0.5 cm. beneath the solution-air boundary ( 7 ) . However, the results shown in Table I1 indicate that identical results may be obtained whether the electrode is immersed to a depth of 1.85 cm. or barely touching the solution. SOLUTION VOLUME STUDY
In this experiment, 9 liters of a solution ca. 1 x 10-3Jf KaFe(CS)6and ca. 1.5M KC1 were prepared. Polarograms were run in this volume of solution (using a 9-liter lab bucket for a cell) a t two different rotation rates. A cylindrical carbon paste RDE 0.41 cm. in diameter with an electrode area of 0.132 sq. cm. was used, Three polarograms were run at each rotation rate and for each polarogram a new electrode surface was made. Then portions of this solution were run in the same manner in vessels ranging down to a standard 100-ml. beaker. The results of this experiment are shown in Table 111. They indicate that limiting current is independent of solution volume and vessel size over a wide range and that in particular solution volumes as small as 100 ml. and cell diameters of the order of 5 cm. yield no erroneous results.
SUMMARY
The present study shows that, if the electroactive area of a cylindrical R D E is surrounded by a sufficiently thick insulating surface, the limiting currents appear (within ordinary experimental error) to be identical with those of bellshaped RDE’s. These results are consistent with the very recent study of Blurton and Riddiford. It should be observed that the present studies employed generally higher Reynolds numbers where the shape influence is likely to be less important. In addition it was found that vessel volume and size have negligible effects on i~ a t
cylindrical RDE’s. Although the effect of immersion depth was only checked with a bell-shaped electrode, it, too, appeared to have negligible effect. LITERATURE CITED
(1) Azim, S., Riddiford, A. C., ANAL. CHEM.34, 1623 (1962). (2) Blurton, K. F., Riddiford, A. C., J . Electroanal. Chem., in ress. (3) Galus, Z., Olson, Lee, H. Y., Adams, R. N., Zbid., 34, 164 (1962). (4) Galus, Z., Adams, R. N., J . Phys. Chem. 67, 866 (1963). (5) Miller, T. A., Lamb, B., Prater, K., Lee, J. K., Adams, R. N., AXAL. CHEW.36, 418 (1964). (6) Olson, C., Adams, R. N., Anal. Chim. Acta 22, 582 (1960).
E,
(7) Riddiford, A. C., in “Advances in Electrochemistry and Electrochemical Engineering,” Vol. 5, Delahay and Tobias, eds., Interscience, New York, in press. KEITHB. PRATER^ RALPHN. ADAMS
Department of Chemistry University of Kansas Lawrence, Kan. 66045 RESEARCH supported in part by the Petroleum Research Fund administered by the ACS and also by the National Science Foundation in the form of an undergraduate research fellowship t o K. Prater. Present address, Department of Chemistry, University of Texas, Austin, Texas.
Fluorometric Determination of Microgram Quantities of Sulfate SIR: Little on fluorometric methods for sulfate has been reported. GBto (3)described a spot-test detection of sulfate based on the quenching of the leadmorin fluorescence. A method for the determination of sulfate based on the release of the fluorescent salicylfluorone from its nonfluorescent thorium complex in the presence of sulfate has also been reported ( 5 ) . The reaction of thorium with morin to form a yellow complex which gives a bright green fluorescence under ultraviolet or near-ultraviolet radiation is well known. It has been used for the determination of thorium ( 1 , 2, 4 , 6 ) , and for the titrimetric determination of fluoride with thorium ( 7 ) . The thoriummorin system in dilute acid medium has been thoroughly studied by Fletcher and Milkey ( I , 4). They noted the serious interference of sulfate in the formation of the thorium-morin complex caused by the formation of sulfate complexes with thorium. Based on these interactions, a fluorometric determination of sulfate has been developed in the 0- to 40-pg. range. The method is more sensitive than most existing methods. In addition, it is rapid, manipulatively simple, and readily adaptable to the determination of large numbers of samples. EXPERIMENTAL
Apparatus. Fluorescence measurements were made with the Coleman Model 12B photofluorometer, using a Corning 5113 primary filter and a Corning 3486 secondary filter. A Beckman zeromatic p H meter was used for p H measurements. Reagents. All reagents used were reagent grade. The morin (3,5,7,2’,4’pentahydroxyflavone) was obtained from British Drug Houses, Ltd., and used without further purification. The water used in the preparation of solutions was double distilled or deionized.
Thorium. A stock solution of thorium containing 1.0 mg. of T h per ml. was prepared. Morin. Solutions of morin in 95Y0 ethanol, 0.002070 and 0.0027%, were used in the procedure. Sulfate. Standard sulfate solutions were prepared from reagent grade sodium sulfate Recommended Procedure. For amounts of sulfate up to 40 pg., the thorium solution containing 20 pg. per ml. and the 0.002770 morin solution should be used. For sulfate in amounts known to be 10 pg. or less, better results are obtained by using the thorium solution containing 10 pg. per ml. and the 0.002070 morin solution. Up to 7 ml. of the sulfate sample are added to 3.0 ml. of the acidified thorium solution in 50-ml. volumetric flasks. The samples should be approximately neutral and free from interfering ions. Several sulfate standards in the desired concentration range are prepared a t the same time. Twenty-five milliliters of 95% ethanol and 15 ml. of the morin solution are added. The solutions are diluted t o volume and mixed. After 20 minutes, the fluorescence measurements are made and the amount of sulfate determined from the decrease in fluorescence. RESULTS A N D DISCUSSION
The optimum pH for the determination of sulfate was 2.35. At this pH, either 30 pg. of T h and 300 pg. of morin, or 60 pg. of T h and 400 pg. of liorin were satisfactory for the determination. With the smaller amount, greater sensitivity to sulfate under 10 pg. was obtained. The larger amount gave a more linear curve which did not level as rapidly, thus giving higher sensitivity in the 10 to 40 pg. of sulfate range. .4n 80% ethanolic medium gave the greatest sensitivity to sulfate. The fluorescent intensity stabilized after 20 minutes and remained constant for a period of several hours. There was no
change in sensitivity of the system when the order of addition of reagents was altered; however, better reproducibility was obtained when the sulfate was added to the T h before the addition of the morin. The fluorescence intensity of the thorium-morin complex decreases with increasing temperature. Unless the temperature was carefully controlled it was necessary to prepare and measure standards simultaneously with the unknowns. The effect of various diverse ions on the determination of 10 pg. of sulfate was studied. Ions which did not interfere a t the 1.0-mg. level include Na+, K+, NH4+, CO+’ Pb+’, C1-, Eos-, Br-, I-, C03-2, c103-, c104-,and SCN-. Silicate, Bi+3, Ba+2, and La+3 can be tolerated up to 100. Fluoride, P04-3, Wad-', Mo04-2, citrate, As+5, Se04-*, VOa-, Fe+3,ZrO+*, and Al+3 interfere seriously a t the level of a few micrograms. LITERATURE CITED
(1) Fletcher, M. H., Milkey, R. G., ANAL. CHEM.28, 1402 (1956). (2) GBto, H., Tohoku Imp. Univ. Sci. Repts., 1st Ser., p. 287 (1940-41). (3) Ibid., p. 469. (4) Milkey, R. G., Fletcher, M. H., J . Am. Chem. SOC.79,5425 (1957). (5) Nazarenko, V. A., Shustova, M. B., Zavodsk. Lab. 24, 1344 (1958); C . A . 54,13985b (1960). (6) Sill, C. W., Willis, C. P., ANAL.CHEM. 34, 954 (1962). (7) Willard, H. H., Horton, C. A., Ibid., 22, 1190 (1950).
JOHN C. GUYON E. J. LORAH
Department of Chemistry University of Missouri Columbia, Mo. PRESENTED at the First Midwest Regional Meeting, Kansas City, Mo., November 1965. Work supported by the National Science Foundation, research grant No. GE-6443.
VOL. 38, NO. 1, JANUARY 1966
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