Effect of Structure on the Stereochemistry of Electrode Reactions Monobromo
C 4
Dibasic Acids and Esters
PHILIP J. ELVING, ISADORE ROSENTHAL,' JOHN R. HAYES, AND AARON J. MARTIN2 The University o f Michigan, Ann Arbor, Mich., and The Pennsylvania Sfafe University, University Park, Pa. Monobromomaleic acid (MBMA), monobromofumaric acid (MBFA), and their diethyl esters have been polarographically and coulometrically reduced between pH 0.5 and 10.5; the reduction products were identified polarographically and, in the macro scale electrolyses, b y isolation and physical and chemical examination. The esters are quantitatively reduced in a 2-electron process to diethyl fumarate. MBFA undergoes a 2e reduction to fumaric acid. MBMA, however, i s reduced to maleic and fumaric acids and a dimer, 1,2,3,4butadienetetracarboxylic acid; the relative amounts of the three products formed are dependent on pH, but the maleic-fumaric ratio a t any pH is the same as that found when cr,cr'-dibromosuccinic acid i s reduced. This represents the first instance of dimer identification in electrochemical carbonhalogen fission and i s positive proof for a free radical intermediate. Reduction mechanisms are proposed to explain the products observed. Originally, one electron enters the molecule to release the halogen as an ion and to form a free radical. The latter can dimerize or can a d d another electron and a proton to form the unsaturated acid. The cis acid i s formed if the species i s in the singly ionized state where a hydrogen bond between the two carboxyl groups stabilizes the cis configuration; otherwise, the trans acid i s formed. Dimerization occurs only with MBMA, likely because of an interaction between mercury and the cis acid, which might sufficiently stabilize the free radical that enough can accumulate to enhance a second-order dimerization.
T
HE AUTHORS have investigated over a number of years the electrochemical reduction of the Cd dibasic acids and their esters, especially from the
Present address, Rohm & Haas Co., Philadelphia, Pa. 2 Present address, F & 31 Scientific Corp., New Castle, Del. 1
330
ANALYTICAL CHEMISTRY
viewpoints of the effect of experimental conditions on the nature of the products obtained and the applicability of such information to developing methods of analysis from the compounds, individually and in mixtures (2-5, 9, 11-13). Previous studies ( 4 , 12) on oicinal dibromo C4 acids and esters showed that such compounds could undergo 2electron reduction t o form the corresponding olefinic compounds, and that, under some conditions, one geometrical isomer \vas preferentially produced. The present study continues this study with the monohalogen compounds, for which polarographic and macro scale reduction data have not been previously reported. The compounds investigated include monobromofumaric acid, monobromomaleic acid, their diethyl esters, and monobromosuccinic acid. EXPERIMENTAL
Chemicals. Y B X 1 was prepared b y heating a n aqueous solution of meso-a,a'-dibromosuccinic acid (6) ; ether extraction and recrystallization from water yielded a polarographically pure material, n hich melted a t 140" C. LIBFA\ nas prepared by heating a n aqueous mixture of LIBMA and hydrobromic acid (IO); recrystallization produced a polarographically pure material, which melted a t 183" C. MBSA was meoared as described bv Holmberg: (8). The diethyl esters " of M B X < (DEE-MBLIA), and of MBFA (DEEMBFA) were made b y reacting the respective acids in ethanol which had been previously saturated with dry HC1 gas (10). Buffer solutions were prepared from reagent grade chemicals as previously described ( 4 ) ; HC1-KC1, p H 0.4 to 2.0; Sa2HPO4-citric acid, p H 2.0 to 7.8; nTH3-XH4C1, p H 8.1 to 9.7; KanHP04-KaOH, p H 10.5. Apparatus. T h e apparatus and techniques have been described (4). The dropping mercury electrode had a capillary constant, m2'3t1'6,of 1.26 mg.2/3sec.-1/2 a t open circuit ( h = 40 em.) a t 25" C. into distilled water, and was used with a Sargent Model XXI Polarograph. Procedures. T h e polarographic and coulometric procedures mere t h e same ils those reported earlier (4). The I
I
acids were studied a t 25.0' i 0.1' C.; the esters a t 2.0 i 0.5' C. The latter test solutions had 4 volume % of ethanol to ensure solubility of the esters; this concentration of alcohol had no effect on the results. POLAROGRAPHIC BEHAVIOR
Table I summarizes the polarographic behavior of the five compounds studied over the p H range of 0.5 t o 10.5. RIBhIA (cf. Table I footnote for definition of symbols) gives two cathodic waves, except in the p H region where the familiar wave-splitting phenomena are observed (S). At lorn p H the first ~v3ve represents C-Br bond fission; the second wave represents reduction of either or both AIA and FA; a t these p H values, the olefinic acids cannot be distinguished. The relative wave heights indicate that products other than IIX and FA\ are also formed. A third wave appears a t p H 5,3. I n the pH region 6.8 to 10.5, two cathodic \vaves again appear; wave I represents C-Br bond fission and possibly MA reduction; wave I1 represents FI reduction. DEE-i\.IB?\IX shows two cathodic waves over the pH range; both represent 2e processes. The more negat i r e wave corresponds to the one wave given by DEE-FA. Below p H 2, XBFA gives two cathodic waves; the second is characteristic of 11-4 or FA, singly or in mixture. Above p H 2.7, only one wave is observed (except in the ware-splitting pH region), which a a v e closely corresponds to that of FA. The diffusion current constant, I , indicates 4e reduction. DEE-hIBFX gives two cathodic waves over the entire p H range; the second wave corresponds to that for the reduction of DEE-FA. hlBSA is reduced in a single 2e step over the entire p H range, except in the region of wave splitting. COULOMETRIC BEHAVIOR
Table I1 summarizes the data obtained from electrolyses at a massive mercury cathode.
Table I.
Compound* MBMA
Wave
I I1
I11
DEE-MBMA
I I1
I
DEE-MBF A
I1
MBSh
I
Effect of pH on Polarographic Properties of Monobromo C4 Dibasic Acids and Esters"
pH (Actual Experimental Value Unusually within f O . l pH Unit) 0.5 1.o 2.0 2.7 4.1 5.3 6.8 0 . 9 7 1.29 0.80 0 . 6 1 0 . 5 7 0 . 4 7 0 . 4 4 Eli2 1.5 5.2 I 5.3 5.2 4.7 4.2 3.7 1.07 1.51 0.73 0.90 0.68 0.58 0.54 Em I 2.9 3.3 4.1 3.8 3.2 0.4 1.7 1.17 IE'n 4.4 0.83 0.64 0.74 0.55 E112 0.45 2.9 3.0 3.3 3.0 I 3.3 1.00 0.75 0.88 0.68 El12 0.54 2.7 2.9 3.3 2.8 I 3.2
ValueC
Em I E112 I
0.48 3.1 0 55 3 0
IE"2
0 25 3 5
0.58 3.2 0 67 2 8 0 29 3 6
0.660 3.00 0 740 290
0.70 3.1 0 87 2 9 0 50 3 4
0 78
1 9 1.15 1.2
0.75 2.9 1 01 2 9 1 18 2 8
8.2 1.38 3.9 1.56 3.2
9.7 1.41 3.5 1.60 3.0
10.5 1.4' 3.3 1.76 3.3
0.f4
1.p
0.78' c
0 97 c
1 09 3.1
1 llh 3 I*
Temperature was 25' C, for acids and 2" C. for esters; concentrations vere 0.1 to 0.5mM. Data for a number of intermediate pH values have not been included. * bIBMA, monobromomaleic acid; MBFA, monobromofumaric acid; MBSA, monobromosuccinic acid; DEE-, diethyl ester of; RIA, maleic acid; FA, fumaric acid. Eliz is in negative volts; I is the diffusion current constant in customary units. d Raaid hvdrolvsis in ba-ic media renders current values relativelv meaninaless. Q
@pHAwas>.85." pH was 5.00. pH was 2.45. * pH was 9.40.
DEE-MBMA is coulometrically reduced to DEE-FA. The recoveries were less than 100% because of hydrolysis during the electrolysis. No evidence was seen for the production of any DEE-LLi. A t low pH LIBFA is quantitatively reduced to FA; above p H 3, the yield decreases because the C-Br bond fission requires very nearly the same energy as the double bond reduction. The coulometric data show beyond question t h a t C--Ur bond fission occurs before reduction of the double bond. DEE-LIBFA is reduced exclusively to DEE-F.1 n i t h loss due to hydrolysis being noted also. Yo evidence was seen for the production of any DEEMA. Bromomaleic Acid. Reduction of XIBI1-I produces b o t h bIX and FA; the amounts formed, however, do not account for t h e starting material; some other material must also be produced. Figure 1 shon s t h e relative amounts of t h e reduction products n i t h t h e unknown product or products being called "dimer" for t h e reasons advanced in t h e following discussion. During coulometric (macro scale) electrolysis, aliquots of the solution mere removed, diluted, and polarographed. With increased reduction, polarographic wave I decreased in height, wave I1
Table II.
Products of Electrolytic Reduction of Monobromo Cd Dibasic Acids and Esters at a Massive Mercury Cathode
Compounda NBMA
DEE-LTBLIA
MBFA
DEE-MBFA
PH
Applied Potential, - Volt
Found
Cis
Trans
0.6 1.9 3.2 4.3 4.8 5.8 8.2 9.7 10.5 0.5 2.4 4.2 6.8 0.6 1.1 1.9 3.3 4.2 6.0 8.2 0.5 2.4 4.1 6.7
0.48 0.62 0.73 0.84 0.97 1.05 1.45 1.50 1.55 0.48 0.60 0.80 0.93 0.45 0.52 0.62 0.81 0.95 1.50 1.53 0.50 0.68 0.75 0.90
2.1 2.3 2.5 2.4 2.3 2.3 2.1 2.0 2.0 2.0 2.1 2.0 2.1 2.1 2.0 2.0 2.1 2.0 1.8 1.9 2.0 2.1 2.1 2.1
5 11 22 37 25 13 ~0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
44 36 38 21 38 53
n
yo Reduction Productsb
so
90 100 95c 94c 970 92c 100 97 100 86 8gd 84 96c 96" 92c 90"
See Table I for meaning of compound symbols. Reduced solutions were polarographed over entire pH range to analyze for product. Values are given in mole per cent. Low values are probably due t o hydrolysis of the ester. Potential required to reduce the C-Br bond is only slightly less negative than that of the olefinic acids; consequently, it is difficult to reduce the C-Br bond without also reducing the double bond. a
VOL. 33, NO. 3, MARCH 1961
331
a D 0 c
C 0,
Y 40 0, a 20
0
"
0
20
40
60
80
IO0
0
12-0
2
332
ANALYTICAL CHEMISTRY
6
8
I2
IO
Figure 2. Variation with pH of calculated relative amounts of reduction products formed in polarographic electrolysis of monobromomaleic acid
Figure 1 . Variation with pH of relative amounts of reduction products of monobromomaleic acid produced on coulomktric (macro scale) electrolysis
decreased slightly, and wave 111 appeared betmeen m v e I1 and the hydrogen-discharge wave. Wave I11 was long and drawn-out, so that the original and final leveling-out portions of the curve mere not in evidence. On completion of reduction of the C-Br bond, the solution was diluted with buffers and investigated polarographically over the entire p H range. Waves appeared due to fumaric acid, maleic acid, and some other species. The half-wave potential of the wave given by the unknown reduction product mas slightly pH-dependent; the 15 ave slope was small, covering as large a span of potential as 0.6 volt. Xttempts to isolate this substance failed, because only lorn concentrations of the material could be obtained and it was w r y unstable. Whenever a solution containing the unknown polarographically active material was evaporated, the activity disappeared; it is suspected that the material polymerized to an insoluble substance. Since this unknown reduction product was reducible, it was believed that if its reduction product could be isolated, the structure of the original product could be determined. Using a bridge filled with p H 1 hydrochloric acid solution so that no metal ions would be present, a solution of RIBlIX acid in HC1 (pH 1.0) was electrolyzed until every reducible species was reduced. This converted any hIA or FA initially produced t o succinic acid and further reduced the unknown reduction product. The totally reduced solution was evaporated to dryness under vacuum; the residue was heated in a test tube immersed in an oil bath. -kt 1 3 5 O C., sublimation was noted; the temperature was raised slowly and sublimation fractions were collected, At 182' C., the remaining mass melted and was subsequently shown to be succinic acid.
4
PH
PH
'
Each sublimed fraction n-as e-terified in methanol saturated with d i y HCl gas. The ester of the first fraction melted a t 69' to 72' C., had a molecular weight (Rast method) of 266, and was slightly discolored probably due to decomposition products; it contained no bromine. The molecular weight of the ester indicated a dimerized structure of the original starting material; the most logical is tetramethyl-1,2,3,4-butanetetracarboxylate. The melting point reported (1) for the latter ester proved it to be the one obtained The melting point of the sublimate also corresponded to that of the pure acid; melting points of mixtures of the sublimate n i t h pure 1,2,3,4-butanetetracarboxylic acid proved the two substances to be identical. DISCUSSION
Relative
Amounts
of
Products
When a polarographic reduction produces another polarographically reducible species. t h e Z values for both waves should be the same if t h e conversion is quantitative and both waves involve the same number of electrons. T h e IlkoviE equation can be applied to the second wave if t h e diffusion coefficient of the original species is used, since the second wave current is dependent on the diffusion of the species producing the first wave. Inspection of Table I shows that for hIBA the Z value of the second wave due to MA and FA is smaller than the corresponding I value of the carbonhalogen bond-fission wave (except at p H 10.5), even though both waves represent 2e processes. The explanation is obvious; the reduction is not quantitative to PvIA and FA; another product must be formed. Since evidence was obtained in the macro scale reduction of RIBlIA that a dimer n-as formed, it is
Formed.
logical to assume that the same products are formed polarographically. Up to p H 4, the first polarographic wave is due to the reduction of (a) b I B X 4 to llIX (n = a), (b) 11IBMA to FA (n = 2), (e) 1 I B M h to the dimer (n = 1 for each unit used), and (d) the reduction of the double bonds in the dimer ( n = 2 for each AIBRIA unit used). The second wave is due to reduction of FA and RIA ( n = 2 ) . Starting with the IlkoviE equation, i t is possible to derive an expression which will allow calculatioii of the combined RIA and FA produced from the RIBRI.4 (9) :
% MB1IA reduced to M A and FA
=
where I I and I 2 are the diffusion current constants for the first and second waves, respectively. By substituting experimental I values into Equation 1, the percentage of the sum of maleic and fumaric acid produced in solution of p H below 4.15 can be calculated; unfortunately, it is impossible to distinguish between the t x o acids a t such pH. By assuming that only the three products indicated are formed, the percentage of MBMA which forms the dimer equals 100 minus the percentage calculated by Equation 1. Such calculations have been made using Table I and other polarographic data; the results are shown as squares in Figure 2. Above p H 4.4, the first wave represents the following reduction processes: (a) RIBMA to 1LIA (n = 2), (b) M A to succinic acid (n = 2), (e) RlBMA to the dimer (n = l ) , (d) dimer to its saturated form ( n = 2), and (e) RIBMA to FA ( n = 2 ) . The second wave corresponds to reduction of FA to succinic acid ( n = 2). It is impossible in this case to calculate the percentage
of any one species formed on reduction as was done in the preceding case. However, it is possible t o calculate the percentage of FA formed if we assume only one other product. Then, the percentages of FA calculated for the situations when (a) only i t and the dimer are formed and reduced, and (b) only it and T\IA are produced, may be plotted as the limits of the true valueLe., the percentage of FA actually formed will fall between these limits if all three products are formed. The perceutage of FA formed, assuming onlj- M A and FA t o be produced on reduction of lIBlIA, is given by
yo JIBMA reduced to F A
=
212 I1
x
+
100 11
(2 )
The percentage of FA formed, assuming only FA and the dimer to be produced, is given by
7,JIBMA reduced to FA
=
312 X 100 211 1 2 (3)
+
Results of such calculations using Table I and other data are given in Figure 2 (circles for Equation 2 and triangles for Equation 3). Another procedure can be used to calculate the percentage of FA formed above p H 4.3. By estimating a value for the diffusion coefficient of MBlI.4e.g., assuming i t equal to that of NBSX in each solution-the concentration of FA formed can be calculated from the IlkoviE equation. Results of such calculations largely fall within the limiting values given m Figure 2 . Differences between Polarographic a n d Coulometric Reduction Products. 'h o dissimilarities observed in t h e products formed in t h e polarographic aiid coulometric (macro scale) reductions deserve consideration. T h e maximum a m o u n t of 31-1 formed occurs a t a higher p H polarographically t h a n coulometrically. This is due to t h e relative rates of reduction and of ionic equilibrium. I n the study of wave splitting with maleic and fumaric acids (3), i t was pointed out that the ionic species could equilibrate rapidly enough to supply the dropping mercury electrode m ith sufficient of the more easily reduced undissociated molecules so t h a t the reduction went through this species up to p H 6. At a massive mercury electrode where the material is reduced much more rapidly than in polarographic electrolysis, the equilibrium shift cannot supply the more easily reduced species fast enough, so t h a t reduction proceeds through the ionic form a t this pH. One can predict that factors which increase the speed of reduction (mass per unit time) will shift the p H a t which the monovalent ion is reduced to a lower p H value,
The dimer is formed to a lesser extent in polarographic than in coulometric reduction. This is to be espected in view of the concentrations used since dimerization is a secondorder reaction. The larger currents and extensive stirring employed in the coulometer would enhance dimerization. The fact that the dimer is produced mostly in the acid region may be considered due to adsorption on the electrode surface, which would tend to give a higher concentration of intermediate species a t the interface, thus enhancing dimerization. A polarographically reducible acid gives maxima (a measure of adsorption on interface) much more readily in acid solution than in neutral or basic solution. (Because of the negative character of the cathode the undissociated species can be assumed to be the most readily adsorbed.) I n polarography belolv p H 2, the concentration of undissociated species in solution and, consequently, dimer production increase. I n macro scale reduction, the stirring and large current effects cause a greater amount of dinierization e w n a t higher pH. Coulometric n Values. T h e apparent number of electrons, n, involved in t h e coulometric reduction of MBMA is given in Table 11. As was shown, t h e reduction involves several different processes. Thus, t h e consumption of 2.5 electrons in t h e reduction of one MBbIA molecule includes the 52% of t h e molecules which are reduced t o M A or F A n i t h a n electron change of 2; plus 48% which are reduced to the dimer with a total electron change of 3 for each MBlIA niolecule-Le., for formation and subsequent reduction of 1,2,3,4-butadiene-
c b
H
m - t -C-COO*
~i
HOOC-C-C-COOH HI
H 1
Irr
Figure 3. Reaction mechanism proposed for electrochemical reduction of monobromomaleic acid and monobromofumaric acids
tetracarboxylic acid. -It low pH, this type of calculation does not produce the desired figure simply because some of the dimerized product remains unreduced as shown by the polarograms. Reduction Mechanism. I n the cou ometric reduction of racemic-cu,a'-dibromosuccinic acid, varying amounts of M A were fornied along with FA ( 4 ) . blaximum production of RIA occurred a t p H 4.2 and accounted for about 70y0 of the reduction products. This effect was esplained by hydrogen bond formation, n hich stabilized the singly ionized starting material in a relatively rigid configuration and a l l o w d the reduction to proceed to the cis product. Analogous phenomena occur in the coulometric reduction of PIIBRIA; in fact, the ratio of MA to FA formed from lIB,ZIA is, within experimental error, the same a t any p H as that formed from &,a'-dibromosuccinic acid. This is strong evidence for a common intermediate species in the two reductions, a n intermediate n hich included the hydrogen-bound singly ionized spwies. The mechanism u-hich most closely fits the available data i. given in Figure 3 (the structures drawn are not meant to define rigidly the position of the structures, but are meant to be an aid to following the discussion). PIII3RIA (I) accepts a single electron to liberate a bromide ion and form a radical species (11). This species can either dimerize to I\' or can add a second electron to form intermediate species 111, which is the species common to the dibromosuccinic acid reduction. During the formation or existence of IT or JII, rotation is able to occur to form the trans product, which is obtained from the unionized species, the doubly ionized species, and the ester. \17ith the singly ionized form, an intramolecular hydrogen bond betneen the t n o carboxyl groups maintains a rigid configuration and prevents rotation. The same general mechanism can be applied to RIBFA reduct on. Here, however, the intermediate structures are stable to rotation. One might argue that the monionized form would tend to rotate to the cis structure to form the hydrogen-bound species; this, however, would require time and energy, which the reaction makes unavailable. The single-bonded intermediates (I1and 111) exist for an extremely short time and, if the species has no really great need to rotate, the reaction will proceed and refreeze to the original structure uithout rotation. Consider the energy diagrams for species 11 and 111 with regard to orientation: the un-ionized, doubly ionized, and ester species of both acids mould show one energy minimum, that corresponding to a trans conformation; the singly ionized species would show two minima, one VOL. 33, NO. 3, MARCH 1961
0
333
corresponding to trans conformation and the other to cis (bound) configuration. An energy barrier exists between the latter forms, so that for interconversion, time and energy must be supplied. Consider the question of why reduction of the cis compound, MBMA, yields the dimer, while none of the latter is observed with the corresponding trans compound, MBF.4. As was mentioned, the intermediate structure I1 undergoes two opposing reactions, whose rates, in the case of the cis form, are nearly equal in acid solution. It has been shown (14) that the double bond in maleic acid reacts with mercury while the one in fumaric acid does not. The nature of this reaction and t h a t of a free radical with mercury (7) will be to stabilize the cis form of intermediate 11, thus slowing its reaction to I11 and resulting in a greater concentration of free radicals at the interface. These effects enhance dimerization with the cis form. The formation of a dimeric product, 1,2,3,4-butadienetetracarboxylic acid,
fission, at least in a n aliphatic compound. Since dimer production is strong, if not conclusive, evidence for the existence in the course of the reaction of a free radical intermediate, the present study offers the strongest experimental support yet obtained for electrochemical carbon-halogen bond fission proceding, a t least in part, through a free radical stage. ACKNOWLEDGMENT
The work described was supported by the U. S. Atomic Energy Commission, to which the authors express their thanks.
(51 Elving, P. J., Teitelbaum, C., Zbid., 71,3916 (1949). (6) Fittig, R., Petri, C., Ann. 195, 62 f i o?n\ (LOIYJ.
(7) Gilman, H., ed., “Organic Chemistry,” Xol.. I, 2nd ed., p. 549, Wiley, New York, 1943. (8) Holmberg, B., Ber. 60B, 2198 (1927). (9) Martin, A. J., ph.D. thesis, The Pennsylvania State University, 1953. , ~ Michael, - - - ~ A., J . pract. Chem. 52, 301 (10) (18Y
a).
(11) Rosenthal, I., Elving, P. J., J . Am. Chem. SOC.73, 1880 (1951). (12) Rosenthal, I., Hayes, J. R., Martin, A. J., Elving, P. J., Zbid., 80, 3050 I1 9.58). (13) Warshawsky, B., Elving, P. J., hlandel, J., ANAL.C H E R19, ~ ; 161 (1947). (14) Whitmore, F. C., Organic Compounds of Mercury,” p. l-50, Chemi\--__,-
cal Catalog Co., Inc., Xew York,
LITERATURE CITED
(1) Auwers, K., Jacob, A., Ber. 27, 1123 (1894).
(2) Elving, P. J., Martin, A. J., Rosenthal, I., ANAL.CHEW25, 1082 (1953).
1929.
RECEIVEDfor review August 24, 1960. Accepted December 30, 1960. Division of Analytical Chemistry, 137th Meeting, ACS, Cleveland, Ohio, April 1960.
Investigation of N-Methylacetamide as a Nonaqueous Polarographic Solvent D. E. SELLERS’
and G. W. LEONARD, Jr.*
Department o f Chemistry, Kansas S a f e University, Manhattan, Kan.
b N-Methylacetamide serves as a satisfactory nonaqueous medium for polarographic studies of many inorganic and organic compounds. Tetraethylammonium bromide serves as a satisfactory supporting electrolyte. The mercury pool, after preconditioning, is a suitable reference electrode with a reproducibility within i=0.003 volt. In concentrations less than 5%, water has no effect on either the half-wave potential or the diffusion current constant. The half-wave potential is dependent on the species being reduced and the diffusion current is proportional to the reducible species, both inorganic and organic.
N
NOKAQUEOUS solvents have been investigated for their suitability in various polarographic studies. Relatively few of these solvents are suitable for the polarographic investigation of both inorganic and organic substances (1, 6 ) . N-hIethy1acetamide possesses certain properties which were considered advantageous for its utilization as a nonaqueous solvent-Le., a high dielectric constant,
334
UMEROUS
ANALYTICAL CHEMISTRY
178.9 and 165.5 (3,4) a t 30’ and 40’ C., respectively, and a liquid range from 29” to 206’ C. An organic solvent whose dielectric constant is of this magnitude offers the distinct possibility of serving as a solvent for many polarographically reducible substances, both inorganic and organic. Using K-methylacetamide as solvent, this work investigated the utilization of the mercury pool as a satisfactory reference electrode, the effect of atmospheric moisture on the half-wave potential and diffusion current constant, and the linearity of the diffusion current us. concentration plot for both inorganic and organic compounds. Other factors which are also important in the characterization of a suitable polarographic solvent such as oxygen interference, reversibility of the electrode reduction process, formation of maxima and maximum suppressors were also studied. EXPERIMENTAL
Apparatus. The electrolysis vessel consisted of t h e upper 3 inches of a large 25 x 150 mm. test tube fitted
with a 4-holed rubber stopper which held t h e dropping mercury electrode, a glass-platinum reference pool connection, the nitrogen bubbler, and the nitrogen outlet. T h e Sargent Polarograph hlodel XXI was used t o record the current-voltage curves. Calibration points were marked with either the up or down scale indicators. The smallest voltage span obtainable, approximately 0.4 volt, was used for the determination of half-Ivave potentials except when i t was impossible to record the entire reduction process, then a span of approximately 0.6 volt was used. A Rubicon portable potentiometer was used to measure the voltage a t the beginning and end of each polarogram a t the calibration points (6). The resistance of the cell circuit, measured n-ith a n Industrial Conductil4ty Bridge Model RC, was 1300 ohms. The temperature of the sample being polarographed was maintained at 35’ =t0.2 C. with a Sargent constant temperature bath. The dropping mercury electrode 1 Present address, Department of Chemistry, Southern Illinois University, Carbondale, Ill. Present address, Code 452, Naval Ordnance Test Station, China Lake, Calif.
