other pyrimidines present, e.g,, if pyrimidine is to be determined in the presence of 2-hydroxypyrimidine, wave I would not be selected because of its proximity to the single 2-hydroxypyrimidine wave. Pyrimidine can be determined in the presence of 2-aminoand 2-amino-4-methylpyrimidine at ca. p H 13, where the latter compounds do not show a wave, or it can be determined from its wave IV. It is apparent from the data that purine can be determined in the presence of all of the other purines mentioned. Since purine does not give a wave in alkaline solution, purine and pyrimidine can each be determined in the presence of the other. ACKNOWLEDGMENT
The authors thank the University of Michigan Cancer Research Committee
house, D. L., Arch. Bzochem. Biophys. .~.__. 47,272 (1953). (9) Heath, J. C., LVutu.re158,23 (1946). (10) Komyathy, J. C., Malloy, F., Elving, P. J., ANAL.CHEY.24, 431 (1952). i11) Leyko. W.,Gross. S.,Polish -4cad. ' Sei., "V.' Symposium oj Biochemists,
and the U. S. Atomic Energy Commission, which helped support the work described. LITERATURE CITED
W a r s a w (1954). (12) Leyko, W., Panusz, H., BULL. soc. sei. et lettres Lodz, Class 111, 5 , S o . 1, 14 (1954); Polski Tygodnik Ledaiski 9, l(1954). (13) Luthy, N. G., Lamb, B., J . Phurm. cind Phnrmacol. 8, 410 (1956). (14) NcGinn, F. A, Brown, G. B., J . Ani. Cherri. SOC.82, 3193 (1960). ( lc5) Palecek, E., ;~nticrwissenschuften45, 1% (1958): Collection Czechoslov. Chetrl. C ' o r ~ m u n25, . 2283 (1960). (16) Smith, D. L., Elving, P. J., J . . i i r i . C'hem. Soc. 84, 1412 (1962). (1;) Ihid., accepted for publication. (18) Sugino, X., Shirai, IC., Sekine, T., -%do. K., J . Electrochem. SOC.104, 067 (1957). RECEIVEDfor review January 2, 1962. Accepted April 23, 1962.
(1) Asahi. Y . , Yakuaaku Zasshi 80, 1222 \
,
(1960). ' (2) Brezina, M., Zuman, P.,"Polarography in Medicine, Biochemistry and Pharmacy," pp. 337-41, Interscience, KeTv York, 1958. (3) Cavalieri, J. F., Lowy, B. H., Arch. Baochem. Biophys. 35,83 (1952). (4) Elving, P. J., Markomitz, J. XI., Rosenthal, I., ASAL. CHEX 28, 1179 il95A\. I - - - - ,
(5) Elving, P. J., Smith, D. L., Ibzd., 32, 1849 (1960). (6) Filipowicz, B., Leyko, \T.j Bull. soc. sei. letters Lo&, Class 111, 4, S o . 5, 1 (1953). ( 7 ) Golewski, S.,Panusz, H., Polish Acud. Sci., V . Symposium of Bzochenzists, W a r s a w (1954). (8) Hamer, D., Waldron, D. &I.,Wood-
EfFects of Dodecyltrimethylammonium Chloride on Half Wave Potentials of Aromatic Nitro Compounds DONALD J. PIETRZYK' and L. B. ROGERS' Department of Chemistry and laboratory for Nuclear Science, Massachusetts lnsfitufe o f Technology, Cambridge, Mass.
b Effects of the surface-active compound, dodecyltrimethylammonium chloride, on the polarographic reductions of aromatic nitro compounds have been investigated. In acidic solution the two waves for each compound were shifted in a more cathodic direction, the second being shifted 600 mv. or more. In basic solution, large shifts toward less cathodic potentials were found in the second wave for several nitro compounds. The application of these anodic and cathodic shifts to analyses of mixtures i s discussed.
wave and a more irreversible cathodic wave. We have observed some interesting effects of the surface-active compound, dodecyltrimethylammonium chloride (DTAC), on the polarographic reductions of aromatic nitro compounds. Although wave-height depression is
adsorption affects the extent and rate of film formation and influences the mechanism of the electrode process. Gelatin and Triton X-100 were used by these authors in their polarographic study of several metal ion complexes. I n many cases, the surface-active agent caused a depression of the limiting current of the
log DTAC Concentrotton 5x10'' ~ x I O - ~
5 x 10-3 1
I
T
HE use of capillary-active materials t o suppress polarographic maxima is well known. Along with this suppression, other effects have also been observed, such as shifts in half wave potential, decrease in limiting current, wave splitting, and wave elimination. The influence of surface-active material on electrode reactions is probably the result of its adsorption on the electrode surface (1-3, 10). Schmid and Reilley (9) have considered how this
Present address, State University of Iowa, Department of Chemistry, Iowa City, Iowa. 2 Present address, Department of Chemistry, Purdue University, Lafayette, Ind. 1
936
ANALYTICAL CHEMISTRY
I
I
-0 4
1
1
-06
1
I
- 0.8
l
1
1-0220
-I 0
Voltage vs S C E
Figure 1 . Effect of DTAC on the first (I) and second (11) waves of nitrobenzene Conditions: 10% methanol, 0.1 M sodium chloride, and 0.01 M hydrochloric acid A. 0.005% gelatin a d d e d 6. 5 X 10-3M DTAC a d d e d C. 5 X 10-4M DTAC a d d e d D. 5 X 10-3M DTAC a d d e d
0
0
0 V o l t a g s vs S.G.E.
Figure 2. Effect of DTAC and gelatin on reduction waves of nitrobenzene (A, E, C), p-chloronitrobenzene ( D , E ) , and methyl p-nitrobenzoate ( F , G). Conditions: 10% methanol, 0.1M sodium chloride, and 0.01M hydrochloric acid A, D, F. no surface-active substance 6. 0.005% gelatin added C, E, G. 0.005M DTAC added
fairly small (about 10 to 15%), shifts t o more cathodic potentials as well as small shifts t o leas cathodic potentials occur. Fusions of two waves into one also take place. I n some cases, these alterations simplify the selection of a limiting current of a polarographic wave and allow the analysis of mixtures which, otherwise. would be impossible in the absence of DTAC.
half wave potentials of the first wave and the poorly defined second wave are -0.225 volt and approximately -0.7 volt, respectively. Gelatin, O . O l ~ o , is usually used t o suppress the maximum on the first wave, but it shifts the half wave potential and decreases the diffusion current (6).
Upon adding DTAC to a nitrobenzene solution, changes in the limiting current of the tsTo naves were slight, but both shifted to more cathodic potentials. The second wave shifted beyond the hydrogen wave a t high concentrations of DTAC (Figure 1). Shifts in the first wave were not nearly so large, being approximately 70 mv. Figure 2 shows complete polarograms for nitrobenzene. A shift in the second wave was also found when progressively larger amounts of gelatin mere used. Thus, the shift observed for 0.1% gelatin was comparable to that for 5 x 10-4x DTAC. The second wave was also affected b y the concentration of inert salt used in the background electrolyte. I n 0.lM sodium chloride or lithium chloride, the second wave was the same as that in Figure 1. I n lill solution, the wave was shifted t o more cathodic potentials such that i t became a shoulder on the hydrogen wave. A similar shift occurred as the alcohol concentration increased. Many other derivatives of nitrobenzene exhibit a second reduction wave in this p H range ( 5 ) . Polarograms, with added DTAC (0.005X is sufficient), of p-chloronitrobenzene, oand p-nitroanisole, o-nitrophenol, methyl p-nitrobenzoate, and p-nitrobenzaldehyde all showed comparable shifts of the second wave. This shift resulted in a flattening of the limiting
EXPERIMENTAL
Chemicals. The nitro compounds (Eastnian Kodak Co.) were purified b y either crystallization or distillation. IITAC, obtained from K 6- K Laboratories, Inc., was used without further purification. All other chemicals were analytical reagent grade. Polarogrnnis. The polarograph (4) and cell construction ( 8 ) have been described previously. An ll-em.. 0.03mm. bore capillary with a mercury height of 100 em. and a drop time of approximately 5 seconds (m2/3t1/6 = 1,477 r n ~ . ? ' ~ a t -0.5 volt, 1Ji LiC1, 80% methanol, 0.0111i NaOH, 0 . 0 0 5 ~ ogelatin) m s used. Temperature was held constant a t 25 i. 0.1" C. Fifty-milliliter aliquots of sample containing 10-4_lI of the nitro compound were used. Due t o the foaming properties of DTAC, slow deaeration was employed a t low alcohol and high IITAC concentrations. As the alcohol concentration 17 as increased, foaming became less bothersome, and the usual deaeration speed could be used. RESULTS
Sitrobenzene has two polarographic waves in the pH range of 1 t o 4. A t pH 2 (0.01M hydrochloric acid, 0.lN sodium chloride, 10% methanol), the
Voltage vs S.G.E. Reduction waves of mixture of nitrobenzene 10-4M)and cadmium (2 X 10-4M)
Figure 3.
(1 X
A 40 0 methanol, 0.01 M hydrochloric acid, 0.1 M sodium chloride, and 0:0057 0 gelatin 8.
0.001M DTAC added to A
VOL 34, NO. 8, JULY 1962
937
0.005
M
0.0001M
1
-08
-1 0
Voltage
-0.3
I
I -1 YS
2
40% methanol, 0.1 M sodium chloride, and radium hydroxide-dibasic sodium phosphate buffer (apparent pH 10.5)
current of the first wive and subsequently alloaed greater accuracy in analysis. Figure 2 illustrates the wave flattening of several compounds. Figure 3 contains polarograms of a cadmium-nitrobenzene mixture in the presence and absence of DTAC. I n the absence of DTAC, there n-as roundness in the cadmium plateau (curve A ) due t o the second wave of nitrobenzene. I n curve B, with DTAC present, the nitrobenzene wave was removed with subsequent flattening of the cadmium limiting current. The height and half wave potential of the cadmium wave were essentially unchanged. Figures 2 and 3 illustrate improved polarograms for analytical use. The limiting currents nere easier to evaluate in the presence of DTAC. Similarly, this potential range becomes usable for further analysis by shifting the second wave beyond the hydrogen wave, as illustrated by the cadmium-nitrobenzene mixture. A lower concentration of DTAC might have been used to remove the second wave of nitrobenzene so as to shift the first wave less and thereby result in a better plateau for the nitrobenzene. DTAC also produced anodic shifts. Figure 4 shons that, as the concentration of DTAC increased, the double wave for p-nitiophenol fused into one large wave, so that at 0.001JI DTAC, the existence of tv-o n ayes was barely perceptible. The height and separation of the two waves \vas a function of the p H of the sample. As the pH went above 9, the separation increased and the height of the second n aye increased a t the expense of the first. At p H values higher than 10.5, the first wive shifted slightly to more cathodic potentials while the second shifted in the opposite direction ANALYTICAL CHEMISTRY
I -l,l
I 4.2
I -13
I -14
I -15
1 -1.6
Voltoge vs S.C E.
-1 4
SCE
Figure 4. Effect of added DTAC on reduction waves of p-nitrophenol
938
I -10
Figure
5. Wave shifts for p-nitrophenol in presence of
DTAC and/or calcium Conditions: 4070 methanal, 0.01 M sodium hydroxide, and 0.1 M sodium chloride A. 0.2M calcium a d d e d 8. 0.005M DTAC a d d e d C. 0.2M calcium and 0.005M DTAC a d d e d D. N o calcium or DTAC present
(compare curves B and D in Figure 5 ) . At 0.002X DTAC, the cathodic shift was the order of 30 mv. I n a background electrolyte of 0.01X sodium hydroxide and 0.111 sodium chloride in 40% methanol, the presence of 0.005V DTAC produced large shifts of the second ~ a v to e less cathodic potentials for p-nitrobenzene phosphoric acid, lJ3-dirnethyl-2-nitrobenzene, p-nitrobenzene arsonic acid, and p-nitrobenzene-sulfonic acid. The shifts were usually greater than 0.20 volt for anionic species and about 0.15 volt for uncharged species. Analytical use of this behavior n as demonstrated by the folloning example. One of the more common preparations of p-nitrobenzene sulfonic acid employs sulfonation of p-nitrophenol. I n the absence of DTAC, polarographic analysis was inipossible because the two waves of each compound overlapped. I n the presence of DTAC, analytical limiting currents mere easily selected. Calcium has also been found to fuse two waves into one (8). I t mas of interest to see if a competing effwt betneen the surface-active DTdC and surface-inactive calcium ion would occur, since it is believed that the calcium-shift to less cathodic potentials may be due to an ion-pair or ion-dipole interaction. Figure 5 reveals that DTAC eliminates most, but not all, of the calcium shift. Shifts of 30 and 15 mv. due to calcium were still observed in the first and second ~w.1e, respectively. DISCUSSION
Schmid and Reilley (9) have discussed how a surface-active material can affect a polarographic w v e by inter-
ference n i t h a prior chemical reaction. The behavior of DTAC appears to be unusual because the second reduction step for a compound can be shifted almost independently of the first. I n the case of nitrobenzene a t p H 2 , the limiting currents of the first and second waves are virtually unaffected. The half wave potential for the first rvave shifted only about 70 mv. (0.005X DTBC) in the cathodic direction, whereas the second reduction shifted about 600 mv., falling a t or beyond the hydrogen wave. The presence of the first n-ave indicates that nitrobenzene must be penetrating the DTAC layer to be reduced. However, the layer must he interfering rsith an interniediate chemical step which prevents the second reduction from occurring before hydrogen evolution. The fact that several nitrobenzene derivatives behave in the same fashion indicates that such behavior is not unique. Gelatin has been shown to produce a similar effect. It is clear, therefore, that the type of surface-active agent present and its concentration can gravely affect the position of a second reduction step or even eliminate it entirely. For example, the reduction of p-nitroanisole in acidic solution has been reported to give t n o waves ( 7 ) . In the absence of gelatin, 11-e found a single wave. Upon addition of DTAC, the polarogram again had t n o waves, the second being very close to the hydrogen n a\ e. This general beha\ ior is in sharp contrast to that reported by Schmid and Reilley (9) in which the limiting current gradually decreased as the result of blockage of the electrode surface. The presence of D T l C c m also facilitate electron transfer to produce an
anodic shift. At the present time, one cannot say whether or not surface activity is a necessary requirement for the observed shift. Calcium ions, which are not surface-active, have been shown t o produce similar but larger shifts (8) (see also Figure 5 ) , while tetramethylammonium ions, which are less surfaceactive than DTAC, produce a smaller shift than calcium ( 4 ) . The large shift in the half wave for p-nitrophenol shown b y calcium ion in Figure 5 n-as almost entirely eliminated by the presence of DTAC. This confirms the suggestion (8) that prior interaction of calcium with nitrobenzene does not take place; instead, ion-pairing n-ith a reduction intermediate must occur. The effects of surface-active agents
on polarographic waves of nitro compounds can be useful for analytical purposes. For certain compounds, the shifting of waves to more cathodic potentials in acidic media or opposite shifts in basic media may yield better limiting currents. This should allow greater accuracy in determining wave heights. I n addition, reducible mixtures which would normally have overlapping waves may be resolved since the potential range required for the nitro reduction should be shortened. LITERATURE CITED
S. L., Aussen, B. S.,Rec. trur. cham. 73. 455 11954). (2) Dratovsky, hI., Ebert. M.>Chem. lzsty 48, 498 (1954). ( 3 ) Heyrovsky, J., Sorni, F., Foreijt, J., (1) Bonting,
Collectton Czechoslov. Chem. Comniuns. 12, 11 (1947).
(4) Hummelstedt, L. E. I., Rogers, L. B., J . Electrochem. SOC.106, 248 (1959). (5) Nolthoff, I. M., Lingane, J. J., "Polarography," 2nd Ed., Vol. 2 , Chap. 28, Interscience, New York, 1952. (6) Korshunov, I. -I.,Kirillova, A. S., J . Gen. Chem., I'.S,S.R. 18, 785 (1948). ( 7 ) Page, J. E., Smith, J. W., Waller, J. G., J . P h y s . & Colloid Chem. 53, 545 11949). (8j Pietrzyk, D. J., Breese, R. F., Rogers, L. R., unpublished data. (9) Schniid, R. IT..Reilley, C. K., J . Am. Chem. Soc. 80, 2088 (1958). (10) Tomamushi, R., Yamanaka, T., Bull. Chem. SOC.J a p a n 2 8 , 673 (1955). RECEIVED for review October 23, 1961. Accepted April 25, 1962. Work supported in part by the U.S. -4tomic Energy Commission under Contract hT(30-1)-905.
Nitromethane as a Solvent for Solid Electrode Voltammetry J. D. VOORHIES and E. J. SCHURDAK Organic Chemicals Division, American Cyanamid Co., Bound Brook,
b Nitromethane has been used as a solvent for constant current voltammetry with the platinum electrode. Cathodic reductions and anodic oxidations of both organic and inorganic species are described. The performance of the platinum electrode in dry nitromethane solutions o f tetraalkyl ammonium salts and anhydrous M g (C104)2 has been studied. The maximum electromotive range i s obtained with 0.1M Mg(C104)?,the anodic and cathodic backgrounds occurring a t +2.4 and -2.4 volts vs. normal hydrogen electrode respectively, a t a current density o f 0.3 ma. cm.-* The effects o f water and oxidative pretreatments on the platinum electrode are also discussed. The anodic oxidation of iodide to iodine in nitromethane occurs in two discrete steps. The chronopotentiometric phenomena associated with these two steps are analogous to those reported in the literature for the polarography of the iodine, iodide redox system in acetonitrile. Some useful information on the reactivity and apparent reduction potentials of several positive chlorine compounds dissolved in nitromethane, 0.1 M in tributylethyl ammonium nitrate, i s presented.
I
s A ~ T U D I of the reactivity of organic compounds containing the nitrogen-cliloyine bond, nitromethane was a useful solvent for electrochemical
N. J .
measurements. The so-called positive chlorine compounds are not stable in many solvents because of general solvolysis, chlorination, or oxidation reactions(5,li). Common ionizing solvents such as water, alcohols, acetone, and acetonitrile react rapidly with some or all compounds containing the S-C1 bond with the result that meaningful electrochemical measurements cannot be made. -4lthough nitromethane is not completely inert to S-chloro compounds, several chloramides XT-hich react rapidly with u-at'er, acetone, and other solvents !\-ere dissolved in nit'roniethanesupporting electrolyte solutions and subjected to cathodic chronopotentionietry (4, 1 5 ) a t the platinum electrode with good results. Sitromethane vias then examined as a general solvent, for electrochemical osidation-reduction ytudies. The useful properties of this solvent for such studies are a wide polarization range coupled n i t h a low concentrat'ion of labi!e protons. Other properties contributing to its utility for elect'rochemistry are high dielectric constant [ea. 40 a t 20" C. (6'b)], a reasonnble lioiling point (101' C.)) and low viscosity [0.63 centipoise a t 25" C.(o'c)]. -1 niajor disadvantage of nitromethane as a solvent for voltammetry is t'he poor solubility of most' common supporting elect'rolytes. I n this study, t'he tetraalkyl ammonium salts, anhydrous magnesium and lithium perchlorates, and certain Lewis acid halides
such as ferric chloride and zinc chloride were the only salts soluble a t the 0.1111 level. Selson and I m m o t o ( 7 , 10) have cited the solubility of other metal perchlorates in a n electrochemical study of metal ion reductions in nitromethane. A more extensive search for soluble salts would be appropriate before further studies with this solvent are attempted. The general electroniotive range of the platinum electrode in nitromethanesalt solutions is limited on the cathodic side by the presence of small amounts of water TT hich may be asqociated n i t h the nitromethane solvent molecules (16) This limitation is very much dependent on the supporting electrolyte used, and a large number of organic compounds TT ith reducible functions can be reduced cathodically 11ell before the bachgr ound reduction. Oxidations of organic compounds such as hydroquinones anti phenols lire also possible. Inorganic redox ystenis studied TT ere fenic reduction and iodide oxidation. EXPERIMENTAL
2;
Reagents. Most organic compounds described were laboratory samples of high purity prepared a t t h e Bound Brook Laboratories, =Imerican Cyanamid Co.. Bound Brook, K. J. All salts used were of reagent grade except for the tetraalkyl ainrnonium salts (Eastman). Tetrachloroglycouril was obtained from Diamond Alkali Co , VOL. 34, NO. 8, JULY 1962
939
