EFFECT O F DROP TIME
On the &ole, the diffusion curreIlt coefficient becomes smaller as the drop time approaches 0.2 second per drop. Table II,A, sholl.s that these values increase as drop time decreases, but a t 0.24 second they begin to become constant. The diffusion current coefficients in Table II,B, are reasonably constant until a drop time of 0.207 second is reached, where they begin to decrease. Even though small drop times tend t o produce higher currents, an opposite effect seems to be produced if drop time falls too low. In subsequent work using drop times much shorter than 0.2 second per drop, (Produced by scratching the electrode), it mas shown that the current decreases as the drop time decreases. Presumably, this is because the rapid drop produces a vortex action which brings some of the depleted solution to the vicinity of the capillary orifice where the
new drop is being formed, If the drop eventually grows large (as it mill with larger drop this effect is negligible~but when the drop remains this depleted solution can markedly affect the over-a11 current. ACKNOWLEDGMENT
The author wishes to thank Carl P. Tyler and kxarjorie T. Walker for their excellent technical assistance and the Humble Oil & Refining Co. for permission to publish this paper. LITERATURE CITED
(1) American Public Health Association, New Yo&, “Standard Methods for Examination of Water, Sewage, and p. 254, Industrial Wastes,” 10th 1955. (2) Buckley, F., Taylor, J. K., J . Research Natl. Bur. Stondards 34, 97 (3) Kolthoff, I. M., Lingsne, J. J., “Polarography,” 2nd ed., p. 72, Interscience, New York, 1952.
(4) Ibid., p. 80. (5) Kolthoff~1. Lf., MilIer, S.1 J . A m Chem. SOC.63, 1013 (1941). (6) Kryulrova, T. A., J . Phys. Chem. (U.S.S.R.) 21, 365-75 (1947). (7) Kryukova, T. A., Z a u o d s k ~ aLab. 14, 511-17 (1941;). (8) Kryukova, T. A., Kabanov, B. N., J . Gen. Chcm. (U.S.S.R.) 1 5 , 294-302 (1945) (English summary). (9)Am. Lingane, J. Loveridge, B* J. Chem. SOC.66, 1425 (1944). (10) Ibid., 68, 395 (1946). (11) Meites, Louis, Ibid., 73,3724 (1951). (12) Ibid., 75, 3809 (1953). (13) Milner, G. V(. C., “Principles and Applications 0. Polarography,” p. 78, Longmans, Green, London, 1957. (14) Nicholson, A[. M., J. Am. Chem. SOC.79, 7 (1957). (15) Orlemann, E. F.J Icolthoff~1. Ibid., 64, 833 (1942). (16) Taylor, J. K , Smith, R. F., J . Research Natl. Bur. Standards 48, 172 (1952). J*
RECEIVEDfor review June 9, 1958. Accepted Decemher 1, 1958. Regional Meeting, ACS, Sen Antonio, Tex., December 1958.
Polarogra phic Behavior of Iodinuted X- Ray Contrast Agents in Buffered Media PETER KABASAKALIAN and JAMES McGLOlTEN Chemical Research and Development Division, Schering Corp., Bloomfield, N. J .
b Ten x-ray contrast agents have been studied polarographically in 50% ethyl alcohol-water solutions at various pH values between 1.3 and 10.5. The effects of pH, temperature, and sample concentration on wave form and height were examined and the analytical possibilities, implied by the results, are discussed.
X
contrast agents containing iodine are usually assayed by liberating the iodine by either oxidative decomposition (9, 11) or reductive stripping (10) techniques and then determining the inorganic iodine by a volumetric, gravimetric, or electrometric procedure. The literature (1, 8, 6) concerning the polarographic fission of the carbon-iodine bond in iodophenyl compounds indicated that the polarographic determination of purity of iodinated x-ray contrast agents should be feasible. Free, Page, and Woollett (4) reported the assay of 3,bdiiodoa-phenylphloretic acid in a 20% 2propanol solution containing 0.5N sodium carbonate and 0 . W tetramethylammonium bromide. They obtained a single wave. Page (8) has reported the polarography of 3,Sdiiodo-1-meth-RAP
ylcheladamic and 3,5-diiodo-4oxo-lpyridineacetic acids in this same system. I n view of the ease and speed of the polarographic assay technique compared to the more common iodine procedures, the polarographic behavior of a number of iodinated x-ray contrast agents, in pH-controlled solutions, was studied. Compounds examined are presented in Table I. EXPERIMENTAL
Apparatus and Reagents. The buffer components were the same as in earlier work (7) with the addition of a 0.1N hydrochloric acid buffer for p H 1.3 and a 0.1M trimethylamine-0.075M hydrochloric acid buffer for p H 8.6. The polarograph, polarographic cells, and diffusion cells were also the same. The electrode capilIary delivered 1.681 mg. of mercury per second a t a column height of 45.3 em. The capillary constant, m 2 W 6 was , 1.801 mga2r3set.-"* determined a t an open circuit with the capillary immersed in a 0.1N potassium chloride solution. All p H measurements were made with the Beckman Model G p H meter. Procedure. A solvent system of 50% ethyl alcohol-water was used. Electrolysis solutions were prepared and the polarograms were recorded (7).
Current-concenti~ationratios, &/C, used in the quantitative work were determined directly jrom the polarographic curve by the method of intersecting lines and the point method. I n this latter case the constants were determined by obtaining current measurements a t a poteitial on the foot of the curve and on t l e plateau for both the sampIe solution and a blank solution, The sample c u i ~ e n tminus the blank current was used as the diffusion current for evaluation of these &/C values. Maxima suppression was effected by the use of 0.002% Triton X-100 obtained from Rohm & Hms. The diffusion coefficients were determined by the d aphragm cell technique of Stokes (1.2). RESULTS A N D DISCUSSION
Dependence of the Half-Wave Potentials on pH. The effect of p H on the half-wave rlotentials of these compounds is shown in Figure 1. There is a marked similarity in the Ellz us. p H relationshp of the compounds which have c:irboxyl and/or amido groups attach( d to the ring nucleus (compounds 111, IV, V, VI, and IX). The waves fo. these compounds all move rapidly to more negative potentials as the pE: increases from 1 to 7, VOL 31, NO. 4, APRIL 1959
513
then reverse this trend above p H 7. Compounds I and I1 differ from the above compounds in the behavior of their first wave. Compound I, which has a phenolic nucleus, gives a first wave independent of p H from p H 1 to 5, but moves to more negative potentials as the p H increases above 5. I n contrast, the first wave of comTable 1. Compound
pound 11, which has an aniline nucleus, moves to :nore negative potentials between p H I and 5, then becomes substantially cowtant at p H above 5. The revewd in the direction of movement o I the half-wave potentials with p H for most of the compounds when changing from the malonic acid buffer (pH '7) to the trimethylamine
Compounds Examined
Structure OH
-
@,2
Volts -E& 1.12 1.43
I. a-Ethy1-3-hydro~y-2,4~6triiodohydrocinnamic acid
11. a-Ethyl-3-amino-2,4,6-
1.01 1.47
triiodohydrocinnamic acid
COOH
111. 5-Acetamido-2,4,6-tri-
1.19* 1.48
IV. 3,5-Diacetamido-2,4,6triiodobenzoic acid
0.90 1.22*
V. 3,5-Dipropionamido-2,4,6triiodobenzoic acid
0.88 1 . 2 1 b
VI. 3-Acetamido-2,4,6triiodobenzoic acid
1.16b 1.52
VII. 3,5-Diiodo-cr-phenylphloretic acid
1.36 1.56
iodoisophthalic acid
OH VIII. a-Ethyl-3-hydroxy-4,6diiodohydrocinnamicacid
1.45 1.69
0
IX. 3,5-Diiodo-l-methylcheladamic acid
1.31 1.45
0
X. 3,5-Diiodo-4-oxo-l-pyridineacetic acid
1.21 1.38
IK 'f i l
I
li
CHZCOOH Half-wave potentials obtained in phosphate buffer pH 8.9 ( \ . o h us. N.C.E.). Fission of two carbon-iodine bonds.
514
ANALYTICAL CHEMISTRY
-E;z Poorly defined
1.70
buffer (pH 8.6) is apparently due to some type of buffer interaction. Increasing the buffer concentration tenfold for the acetic acid, malonic acid, and trimethylamine buffers (pH 5.5, 7.0, and 8.5, respectively) effected no change in half-wave potentials except in the case of the amine buffer. Potentials were shifted to more positive values as the amine buffer concentration was increased. The first wave for compound 111, for example, had an E,,z of -1.09, -1.07, and -1.02 volts in solutions 0.045, 0.09, and 0.27N in amine salt, respectively (enough amine base was added to maintain constant pH). There is no wave splitting here, as was found by Elving (3) in his work on chlorinated acids; however, the shifts in Ellz could still be explained by a similar complex formation. In this case the half-wave potentials of the complexed and uncomplexed species are not sufficiently different to cause a complete split in the wave, but are different enough to cause shifts in the half-wave potential depending on the ratio of complexed to uncomplexed material present in solution. The compounds were all subsequently run in phosphate buffers of p H approximating those covered by the trimethylamine buffers. No reversal in the magnitude of the half-wave potentials with p H was found in these buffers (Figure 1). Dependence of Diffusion Currents on pH. The diffusion currents obtained with these compounds are all relatively pH-independent, in that the removal of one iodine atom from any particular compound has a fairly constant diffusion current equivalent throughout the p H range studied. The only large changes in wave height with p H were observed when a complete change in reduction mechanism was caused by a change in pH-i.e., when two waves merge to form one larger wave or vice versa. The merging and splitting of waves are shown in Figure 1 by two lines converging to or diverging from a single point. Except for compounds I V and V, the wave merging and splitting involve two 2-electron (fission of one carbon-iodine bond) waves. In the case of compounds IV and V, a 6-electron nave splits into a 4-electron wave and a 2-electron wave. Dependence of Diffusion Currents on Temperature. The temperature coefficients for the currents observed with compounds I, 111, VII, and IX between 8' and 50" C. mere essentially constant a t 2.0% per degree (25' (3.)Le, of the magnitude associated with diffusion-controlled processes. Dependence of Half-Wave Potentials and Diffusion Currents on Concentration. Diffusion current-concentration relationships were observed
for conipouiids I, 111, VII, and IX from 0.1 to 5.0mH a t p H 5.2, 9.5, 5.2, and 7.0, respectively. Because of shifts in Exlawith concentration (compounds I and VI1 shifted to more positive potentials as the concentration increased) the i,/C values were obtained directly from the curve as well as by the faster point method. The id/C values all yielded standard deviations between 1 and 2% with the exception of compound VII, where the values obtained by the point method gave a standard deviation of 3%. The shifts in El,zwith concentration of electroreducible material occurred regardless of the concentration of buffer components. Diffusion Coefficients. Diffusion coefficients were obtained for these compounds in ethyl alcohol-water solutions buffered a t p H 5.5. The values in terms of IO+ sq. cm. per second were 2.2, 2.2, 2.2, and 2.1 for compounds I, 111, VII, and IX, respectively.
0.6
Compound I (a-ethyl-3-hydroxy2,4,6-triiodohydrocinnamic acid) is reduced stepwise, one iodine atom a t a time. Taking into consideration the ortho or proximity effect (1, 6), one would expect that the first reduction wave would be due to the fission of the carbon-iodine bond in the 2 position, as i t is ortho to both the hydroxyl and the methylene groups. That this position is attacked first has been demonstrated in this laboratory by the preparation of compound VI11 (a-ethyl-3 -hydroxy-4,6-diiodohydrocinnamic acid) by controlled potential electrolysis of compound I a t a mercury pool cathode. By the same reasoning, the second wave of compound I should be due to fission of the carbon-iodine bond at the 4 position, ortho to the hydroxyl group. A comparison of the half-wave potentials for the final carbon-iodine bond in compounds I and VI1 (compound VI1 has both iodine atoms ortho to the hydroxyl group) seems to substantiate this reasoning. The half-wave potentials are - 1.59 volts (us. N.C.E.) for compound I and -1.45 volts for compound VI1 a t p H 9.5. The introduction of carboxyl or amido groups on the phenyl ring greatly facilitates the ease of reduction above that due to the proximity effect. In the very acid solutions (pH 1,3), the triiodo compounds having only an amino or hydroxyl group on the phenyl ring in addition to the halogens give one 2-electron wave, whereas the compounds with two carboxyl groups and an amido group or two amido groups and a carboxyl group are completely reduced prior to the beginning of the
.
.
I COMPOUND
COMPOUND
I COY
.
COMPOUND . Vlll
0.6
DISCUSSION
.
COMPOUND
COMPOUND IX
-/:*
tI..........
0
2
4
6
IO
E
2
1
4
6
8
1 0 0
..'. . . 2
4
,
.
.
6
. 8
,
. 10
'
Pn
Figure 1.
Effect of pH on half-wave potentials
- - - Main buffer systems - - A - - A - - Supplementary phosphaie buffer *
hydrogen wave. I n fact, functional groups that are capable of accepting electrons activate the phenylring toward carbon-iodine bond fission. The halogen atoms themselves are electronegative in nature and additional halogen on the phenyl ring enhances reactivity. The carboxyl group is a well known electron-accepting group and also promotes carbon-halogen bond fission. Though having electron-donating properties in chemical electrophilic substitutions, the amido group's behavior here is analogous to that of the carboxyl group. The hydroxyl and amino groups are electron donating and in this work show no activating properties toward carbon-iodine bond fission except through the proximity effect mentioned. Even the half-wave potential us. pH relationships observed with compounds I and V are in keeping with the relative electron-accepting abilities of the phenolate and anilinium ions as compared with the phenol and aniline molecules. Because the iodine atoms in most of the carboxyl- or amido-substituted compounds are ortho to two substituents, the ease of reduction of these compounds might be attributed to a n enhanced proximity effect. That the effect of these groups is something more than a proximity effect is illustrated by compound VI (3-acetamido-2,4,6triiodobenzoic acid). Two of the three iodine atoms in this molecule are ortho
to only onc: group, yet in solutions of p H 1.3 thii; compound gives one full~7 developed wave representing the reduction of two iodine atoms and a partially completed wave for the third iodine atom before the appearance of the hydrogen wave. AWALYTICAL IMPLICATIONS
All c o r pounds examined gave well defined ar d reproducible polarographic reduction waves a t some pH in the buffered :IO% ethyl alcohol-water system used The diffusion currents obtained were linear with respect to concentratior. over a fairly wide concentration rang;e. This indicates that the polarograph can be used for routine analytica work involving the assay of iodinated x-ray contrast agents, a t a considerable saving of time over the usual or6;anic iodine assay procedures. This technique should in many cases be useful for determining the actual concentration of the totally iodinated conipound o interest, uninfluenced by the incompk tely iodinated material. For example the first wave of compound I (triiodo compound) precedes the first wave 0 ' compound VIII, its diiodo analog, 0.5 volt; with this amount of wave separation, the triiodo compound can easily be determined in the presence of the diiodo component. Because of the pronounced effects of substituents a
YO!. 3 1 , NO. 4, APRIL 1959
515
(9) Peel, E. W., Clark, R. H., Wagner E. C., IND.ENG. CHEAI.,ANAL. ED 15,149 (1943). (10) Schwenk, Erwin, Papa, Domenick, Ginsberg, Helen, Ibid., 15, 576 (1943). (11) Shahrokh. B. I
