electrode surface. The recording of values of r,/tl between ‘/s and 1 is, in this case, due t o the absorption of bromine by the carbon paste electrode. Difficulties were encountered in attempts to measure the extent of this absorption due to the fact that bromine from previous experiments tended t o remain in the electrode. Care mas taken to remove this bromine by cathodic scans. The electrode was considered “clean” when no observable cathodic transition time was measured for two scans five minutes apart. Under these conditions the ratio of rJt, was found t o approach 0.33 as t, became large (60 seconds), but when tl was small (7 seconds) values as high as 0.6 were recorded. Both concentration of bromide ion and current were varied b u t only relatively minor changes in r,/tf were observed. Apparently most of the bromine first produced is adsorbed on the electrode surface and diffuses slowly into the interior of the electrode, where it is held (unlike, say, the case of cadmium in mercury) by relatively strong absorptive forces. Subqequently the bromine formed remains in qolution
rather than attaching itself t o the electrode because the attachment sites are mostly taken up by previously produced bromine. Thus when t, is qhort a larger proportion of the bromine is held at the electrode surface and the value of rp/tl is consequently closer to one. The iodide-iodine couple n-as found to behave more normally at carbon paste electrodes in that r,/tt equal. close to 1/3 for the oxidation-reduction reversal. Osteryoung (9) has shown that iodine is absorbed on platinum and that rJt, ratios greater than one third are found. I n our work the concentration level was generally higher (10 milf as compared to 1 mM) which would make the adsorption effect harder to observe. No adsorption of bromine on platinum was found (9) which confirm< the data given here. On the other hand, current reversal studies of the chloridechlorine couple showed this couple to be quite irreversible. The values of rJt, recorded were much leqs than 1 3 indicating that chlorine probably diffuses into the interior of the electrodes and reacts with the electrode n-mterinl.
LITERATURE CITED
(1) Bard, A. J., ASAL. CHEX 33, 11
(1961).
( 2 ) Davis, D. G., Ibid., 35, 764 (1963). (3) Davis, D. G., Ibid., 33, 1839 (1961). (.4,) Davis. D. G.. Anal. Chim. Acta. 27. 26 11965). ( 5 ) Delahay, P., Berzins, T., J . Am. Chem. SOC.75,2486 (1953). (6) Laitinen, H. A , , Enke, C. G., J . Electrochem. Soc. 107, 773 (1960). (7) Llopis, J., Yaz uez, AI., Electrochinz. Scta. 6, 177 (1968. (8) Olson. C.. ildams. R. K., Anal. Chinz.
(9) Osterioung,‘ R. A., ANAL.CHEM.35,
1100 (1963). (10) Parsons, R., “HayIbook of Electrochemical Constants, p. 79, Butterworths, London, 1959. (11) Reinmuth, IT. H., h . % I . . CHEhf. 32, 1514 (1960). (12) Sand. H. J. S., Phil. M a g . 1, 45 (1901).
(13) T-etter, K . J., “Elektrochemisrhe Kinetik,” p. 369. Springer-Trerlag, Berlin. 1961. RECEITED for review July 5, 1963. b c cepted October 28, 1963. Division of Analytical Chemistry, 145th Meeting, ACS, New York, S. Y., September 1963. This work was supported by the Xational Science Foundation.
Polarography of 2-Aminoethanethiosulfuric Acid a nd 2 -Aminoe t ha neseIe nosu If uric Acid WALTER STRICKS and R. G. MUELLER Department o f Chemistry, Marquette Universify, Milwaukee, Wis. 53233
b The polarography of 2-aminoethanethiosulfuric acid (RSSOJH) and 2 aminoethaneselenosulfuric acid (RSeS03H) has been studied under various conditions of pH, buffer composition, ionic strength, and temperature. RSSOsH gives a single wave while RSeS03Hi s reduced in two steps, all waves being irreversible at the dropping mercury electrode. The zero current potential of RSS03H i s about 0.6 volt more negative than that of RSeS03H. At alkaline and neutral pH the height of the waves of RSS02H and RSeS03H i s independent of pH. In acid solutions the heights of the RSS03Hwave and of the second step of RSeS03H increase with decreasing pH. This i s explained by the reduction in acid medium of sulfite which i s one of the products of the reductions of RSS03Hand RSeS03H at the electrode. Mechanisms for the electroreduction of RSS03H and RSeS03H are given. At the rotated dropping mercury electrode the second wave of RSeS03H can be eliminated by the addition of surface active agents such as gelatin
-
40
ANALYTICAL CHEMISTRY
or polyacrylamide. This i s accounted for by the formation of dithionate which i s inactive at the mercury electrode.
A
STUDY of the t n o antiradiation drugs P-aminoethanethiosulfuric acid (HzX-CHzCH2--S---S03H), denoted as RSS03H, and 2-aminoethaneselenosulfuric acid (K2H-CH2-CH2-Se-S03H), denoted as RSeS03H, is reported in this paper. The results obtained n-ith the two compounds are compared. POLAROGRAPHIC
EXPERIMENTAL
Materials. RSS03H and RSeS03H were obtained from t h e Walter Reed Army Institute of Research, Department of Radiology. A11 other chemicals were commercial analytical grade products. All solutions were prepared with double distilled water. The stock solutions used for the preparation of the mixtures to be electrolyzed were 5M ammonia, 2-11 ammonium chloride, 2M potassium chlo-
ride, 1.11 hydrochloric acid, 1M acetic acid, 0.2V disodium phosphate, 0.2-If monosodium phosphate, 0.5% gelatin, 0.57, polyacrylamide, 10-2M RSSOaH, and 10-2AlfRSeS02H. Stock solutions of RSS03H, RSeS03H, and of gelatin were kept refrigerated if not in use. The gelatin solution contained one drop of toluene in 100 ml. of solution to protect it from bacterial attack. Methods. Current voltage curves were measured a t 25’ = 0.1’ C. with t h e manual apparatus and circuit described by Lingane and Kolthoff (6) and automatically with a Sargent Model XXI automatic recording polarograph. All potentials were measured against a saturated calomel electrode (SCE). Oxygen was removed from the solution in the cell Fvith a stream of Linde nitrogen (99.996y0 pure). The nitrogen was passed through three bottles containing solutions of the same composition as that of the buffers used for the electrolysis mixture. During a n experiment an atmosphere of nitrogen was maintained over the solution. Corrections \\-ere made for the residual current. The characteristics of the conventional dropping mercury electrode
-0.5 VOLT vs.
-1.9
S.C.E.
Figure 1 , Electroccipillary curves of RSS03H and RSeS0:;H Supporting electrolyte: (A) 0.05M Na2HP04, 0.005M NaH2P04, 0.85hl KCI, pH 7.42 (6) with 1 O-3M RSSOaH (C)with 1 0 - 3 M RSeS03H
(DME) were: m = 1.98 mg. per second, t = 4.07 second (open circuit, 0.1-If KCl, H = 67.8 em.). The rotated dropping mercury electrode (RDLIE) W , ~ S of the type described previously (9). The speed of rotation used for the present work was 225 r.p.m. In 0.1JI KCl, the value of m was 15.28 mg. per second and the drop time was 3.9 seconds a t open circuit. The height of the mercury column was 52 em. The pH was measired with a Beckman Zeromatic p H meter \\-ith a glass electrode of the 412eO type, usable for the entire pH range. RESULTS
RSS03H. Polarograms R ere taken with solutions of v irious concentrations in RSS03H iii buffers from 1" 1 to 11. A411RSS0:H colutions gave a well defined cathodic wave. I n strongly alkaline medium (0.05n.l S a O H , 0.95M KCI) RSSO3H decomposes slowly in air free solutions. After 10-3Jf RSSO:H stood in this solution for 1 hour the polarogram was unchanged. However, after 11 hours the height of the wave was reduced from 7.2 to 4.9 pa. and the wave (E1 2 = - 1.16 volt) mas preceled by a small prewave (E1,*= -0.72 volt) of a height of 0.86 pa. I n anodil; wave \vas not detected in the partially decomposed solution, indicating that mercaptoethylamine (RSH) is not formed in the course of the dec3mposition (10). RSS03H has hardly m y effect on the surface tension of mei-cury as indicated by the electrocapillzry curve (drop us. potential) obtaineil in a phosphate buffer a t p H 7 (Figuw 1). The diffusion currents and half-n-ave potentials of 10-3Llf IISS03H solutions a t various p H values are listed in Table I. Two pH regions can be distinguished. Between p H 11 and 7 the diffusion current is przctically constant and the half-n ave potential becomes more negative with increasing pH. Inspection of the data shows that halfnave potential and diffusion current are little affected by the compoqition of the
buffer. I n the acid pH region the diffusion current increases markedly n i t h decreasing pH and the half-wave potential is unaffected by a change of p H in this region. The presence of sulfite (0.2 to 0.02X) in alkaline medium has a blight suppressing effect on the diffusion current of RSS03H hut no effect on the half-wave potential. Experiments n i t h a 10-3.1f RSS03H solution in a phosphate buffer at pH 5.4 were performed n i t h varying heights of the mercury column (48 to 101 cm.). The ratio i/dH obtained after correction for back pressure was conitant. RSeS03H. From curve C of Figure 1 it is seen t h a t RSeSOsH, in contrast with RSSO,H, is capillary active a t thc mercury solution interface. Current voltage curves of RSeS03H were taken with solutions a t p H ranging from 1.0 to 12.4 and ionic strength froin 0.1 t o 1.0. Examples of current 1-oltage curves of RSeS03H xrC illustrated in Figure 2 TT hich also gives a c-v curve of RSS03H. I n contrast with the single iva1.e of KSS03H, RSeS03H gives polarograms \T hich con4st of two n-aves in solutions at pH lower than 5.4 and higher than 9.3. In the pH range between 5.5 and 9.3 the c-v curve exhibits a high maximum at a potential of about - 1.6 volt. In the presence of O . O O l ~ o gelatin the maximum is suppressed and a third well defined wave is observed at thiq potential. RSeS03H (Figure 2) iq reduced a t a considerably more positive potential than RSS03H. Thus, in a phosphate buffer a t pH i . 4 , the zero current potential of RSeSO3H is about -0.2 volt while that of RSS03H is -0.8 volt. Half-wave potentials and nave heights of RSeS03H are listed in Table 11. At pH values lower than 3 the plateau of the first wave is poorly defined and its height is larger than that observed a t less acid pH. Over the p H range 10.9 to 3.6 the limiting current of the first wave is practically constant a t the same ionic strength and independent of the composition of the buffer (compare the data with phosphate and acetate buffer a t p H 5.36 and 5.54,
31
2( Y
Y PT
3
0
m
u .¶ 11
.O
VOLT vr.S.C.E.
Figure 2. Polarograms of (A) 1 0 d 3 M RSS03H; (61, (C1,( D ) 1 04M RSeS03H Supporting electrolyte: (A), (6) 0.05M NazHP04, 0.005M NaHzPO4, 0.85M KCI, p H 7.42 (C) 0.005M NazHPO4, 0.05M NaHzPO1, 0.85M KCI, pH 5.38 ( D ) 0.1 M HCI, 0.9M KCI, p H 1.08
respectivel>-). The ionic strength has little effect on the height of the waves as indicated by experiments with ammonia buffers a t ionic strengths 0.1 and 1.0. The low currents obtained in solutions at pH 12.4 can be explained by the alkaline decomposition of RSeS03H which is much less stable than RSSOaH in this medium. When a solution of 10-3Jf RSeSOSH in 0.05V NaOH0.95X KC1 v a s allowed to stand, the height of the first wave decreased from 2.32 to 0.38 pa. after 4 and; 690 minutes, respectively, while the height of the second wave decreased only from 2.85 to 2.46 pa. in the same time. Apparently the second vave corresponds to the reduction of a new substance formed during alkaline decomposition of RSeSO3H. The variation ivith pH of the height of the second waye is similar to that of
Diffusion Current and Half-Wave Potential of 1 O-3M RSSOaH in Various Buffers E w , volt Buffer pH id, pa. us. SCE - 1,198 6.80 10.94 lilf NH3, 0 451 KCl, 0 . 2 M Na2S03 - 1.197 6.85 10.94 13f SH3,0 7111 KCl, 0 1M Na2SO3 - 1.176 7.08 10.84 1M NH3, 0 9M KC1, 0 02M NaeSOa -1.176 7.72 10.86 1Jf KH3, 1 O M KC1 -1.139 7.18 9.20 1M XHa, 0 1M NH4Cl -1.120 7.11 9.10 5 M Na2B407, 0 8 5 M KC1 - 1.065 7.92 7.32 05M NaZPO4, 0 005hl NaH2P04, 0 85-71 KCl - 1.070 10.02 5.38 005M Na2HP04, 0 05M NaHzPOa, 0 85M KCl - 1,065 14.60 2.90 1M CHgCOOH. 1 O M KC1 -1.06 17.00 1.08 1M HCi, 0,9M' KCI
Table I.
0 0 0 0 0 0 0 0 0 0
VOL. 36, NO. 1, JANUARY 1964
41
Table 11.
W a v e Heights il, i?, and i 3 and Half-Wave Potentials, l € l ~ ??El,*, , 3€1.? of 10d3MRSeS03H in Various Buffers at 25" C.
All solutions contain 0.00170gelatin and have ionic strength of one if not indicated otherwise. Current in pa., half-wave potentials in volt us. SCE First wave Second wave Third wave__ Buffer pH ii 1Eit2 22 :Bi,? z3 3El2 0 . Id1 HCl, 0 9;M ICC1 0 . 1 J I CHaCOOH, 1-11KC1 0 131 CHqCOOH. 0 OlJ1 CH3COi)Sa, 1 ' O M KC1 0.1JI CHaCOOH, 0 1 X CH3COO?u'a,0 9M KC1 0.005-11 Sa2HP04, 0 0531 iYaHZP04, 0 85M KCl 0.01J1 CHqCOOH. 0 1Jf CHCOO"Xa, 0 9M KC1 0.1W CH3COOYHa, 0 9-11 KCl
0.0531 Sa-BhO-. 0 85'11 IiCl 0 . l h S H , 0.1hf NH4CIc
0 , l M SHZ, 0 , l M NH4C1, 0,931 KC1 0.1M "3, 0 . 1 M KCld 0.1M XHaj 1J1 KC1 0.0jA11SaOH, 0 . 9 5 N KCP a
1 04 5 91 2 86 5 05
-0 29 -0 33
11 03 10 .50
-0 3s -0 31
3 60 4 00 -0 36
9 87
-0 81
4 56 3 70 -0 35
9 50
-0 56
6 36 3 80
-0 36
9 40
-0 ,i6 155 0
5.54 3.65
-0.36
8.95 -0.66
35.0
-1.45
7.12 4.00 -0.35
4.04 -0.58
22,2
-1.63
7.40 3.65 -0.39
3.36 -0.57
18.0
-1.59
7.76 4.39 -0.36 9.00 3.36 -0.41 9 . 2 8 3.90 -0.41
2.85 -0.57 3.23 -0.62 3.39 -0.58
18.9 -1.56 4 . 3 -1.74 7.35 -1.69
9.32 10.80 10.88 12.40
3.75 -0.42 4.15 -0.48 3.80 -0.48 2.28 -0.47
3.38 3.15 3.34 2.66
-0.59 -0.65 -0.65 -0.64
2.45 -1.69 . . . . . ..
...
... . .
i3is the height of a peak at -1.9 volt, hence no half-wave potential is given.
* Ionic strength is 0.35.
c
d 0
Ionic strength is 0 10. Ionic strength is 0 10. Polarogram taken 5 minutes after addition of RSeS03H to the buffer.
strongly affect the value of 2E12 . h similar although weaker effect is observed with boras and amtnonia buffers of t'he same pH. The halfwave potential of the third wave is little affected by the p H but an inspertion of the data of Table I1 shows that the compo-ition of the buffer may ha1.e an effect on the values of 3/
