Some Aliphatic Nitro Compounds and n-Butyl Nitrate in Nonaqueous Solvents Polarographic Study NATHAN RADIX A"D THOMAS DE VRIES Purdue Uniuersity, Lafayette, Ind. The polarographic method of analysis, which has been so successful for determinations in aqueous solutions, has hardly been investigated for the determination of compounds dissolved in organic solvents, especially organic compounds. The study showed that a variety of solvents would be suitable, provided the viscosit? coefficient was small. Of the solvents studied methanol was excellent, whereas benzene was unsuitable hecause it did not furnish protons required for the reduction process. It was also possible to calculate the diffusion coefficient required in the Ilkovi; equation in order to determine the number of electrons used in the reduction process and to show that hydroxylamines were the end product of the reduction of the nitro compounds at the surface of the dropping mercury electrode. Wherever an analytical problem involves organic compounds that can be oxidized or reduced at the surface of a dropping mercury electrode, an organic solvent can be used to prepare the base solution.
The current-voltage curves were obtained with a Leeds and Northrup Type E Electrochemograph and a Fishpr Elecdropode. The polarographic cell was of the H-type with a sintered-glass disk in the cross arm. One side of the cell contained a saturated calomel electrode; an agar plug was inserted in the cross arm next to the sintered-glass disk on the calomel side. The temperature of the cell was controlled a t 25' i 0.1" C. T h r resistance of the cell was measured a t maximum drop foimation with a conductance bridge and 1000-cycle oscillator, the average resistance being 4/3 times the minimum resistance a t the instant of drop fall. For two solutions it wae necessary to measure the resistance by applying a variable voltage from a B battery and using the straight-line portion of a plot of current versus voltage to calculate the resistance. Capillary constantF \%eredetermined in the base solutions a t different cathodic voltages. The same capillary was used for all the polarographic studies. .1 typical value is 2.03 mg. per second vcith the range of t from 3.48 to 4.00 seconds. Viscosity measurements were made with a Hoeppler viscometer with a precision of 0.1%. The densities of the solutions were measured with a Westphal balance. Oxygen was removed from the solutions by passing nitrogen through an acetic acid solution of chromous chloride t o ensure oxygen-free nitrogen (IO), then through a portion of the solution being examined polarographically, and finally through the solution in the cell. From separate experiments it was determined that traces of water had no effect on the magnitude of the diffusion current or the half-wave potential, and that 1% of \Tatter did not change the current by more than 3%. However, care was taken to use anhydrous solvents.
ERT little has been published on the subject of polarography in nonaqueous solvents. This is not surprising, because analytical conditione in such solvents are often not ideal and do not compare favorably with those in water. However, work in nonaqueous solvents indicates t,hat here is an analyt,icaltool for the study of compounds that are insoluble in water, and for special cases where anhydrous conditions are desired. Lewis, Quackenbush, and De Vries ( 5 ) made polarographic studies of organic peroxides in nonaqueous solutions. The solvent used was a solution of equal volumes of methanol and benzene. Lithium chloride proved t o be more effective as a supporting electrolyte than methyl hydrogen sulfate or lit,hium met,hoxide. Parks and Hansen ( 7 ) used glycol et,hers as solvents for the study of naphthalene, 1- and 2-methyl napht'halene, and tetraethyllead. Hale ( 2 ) determined mono- and dinitroxylenes in ethyl alcoholbenzene as solvent with sodium acetate as supporting electrolyte for the det,ermination of purit,y of nitroxylene feeds and xylidine products. In t.he present, invest,igat,ionpolarographic studies were made of nitroethane, 1- and 2-nitropropane, 1- and 2-nitrobutane, 1,3-dinit.ropropane, l,j-dinitropentane, 2,2-dimethyl-1,3-dinitropropane, 2,2-dinitropropane, and n-butyl nitrate. The solvents used n-ere niet,hanol, 1 to 1 methanol-benzene mixtures, 1 to 1 methanol-1,Cdioxane mixtures, methanol-glycerol mixtures of varying conc~entration~, isobut,yl alcohol, ethylene glycol, and glycerol. Lithium chloride was used as t,he supporting electrolyte in concentrations varying from 0.2 tmo0.5 M.
~-
Table I. Polarographic Studies in Alethanol Benzene (1 to 1) as Solvent, 0.3 M in Lithium Chloride Concn., Afillimolar 1.16
id El ' 9
1's. 9.C.F:.
cm2/3t1/6
n-Butyl nitrate -1.32 4.1 Nitroethane 1 20 -1.16 13.5 Nitroethanea 1 20 -i.i4 iS.2 Nitroethanea 1 20 -1.32 13.7 Xitroethanec 1 20 -1.23 13 7 1-Xitropropane -1.16 1 00 12 8 2-Sitropropane -1.31 1 00 12,s 1-Nitrobutane -1.22 11.1 1 07 2-Nitrobutane -1.31 1 03 11.0 1,3-Dinitropropane 1.02 -1.16 14.6 2,2-Dinitropropane 1.07 -0.86 5.2 1,b-Dinitropentane 1 .oo -1.17 19.1 2,2-Dimethyl-1,3-dinitropropane 0 . 9 0 -1.22 17.3 Cell resistance 1810 (with bridge) a Plus Cellosolve t o make final soluiion 0.001% in maxirnuni suppressor. b Plus ethylcellulose t o make final solution 0.027, in maximum suppressor. C Plus ethylcellulose t o make final solution O . O @ l % in maximum suppressor.
RESULTS A h D DISCUSSION
Solutions of nitroalkanes, dinitroalkanes, and n-butyl nitrate were studied in six base solutions. The polarograms in all the base solutions had single waves n-hich were drawn out over a considprable voltage range and frequently had maxima. In the cases where maxima occurred, it was found that they were not reproducible. There were no maxima with the ethylene glycol and glycerol base solutions Attempts were made t o find a maximum suppressor. Methyl red and Cellosolve v w e tried for the
EXPERIMENTAL
The materials used were purified with the usual care. All solvents were thoroughly dried or distilled until they were anhydrous. 97 1
ANALYTICAL CHEMISTRY
972
Table 11. Results for Various Compounds in Five Supporting Electrolytes ~~
Conon., Millimolar n-Butyl nitrate 1.16 Nitroethane 1.20 1-Nitropropane 1.00 2-Nitropropane 1.00 1-Nitrobutane 1.07 2-Nitrobutane 1.03 1,3-Dinitropropane 1.02 2 2-Dinitropropane 1. 0 7 l:5-Dinitropentane 1.00 2,2-Dimethyl-l,3-dinitropropane 0 . 9 0
Half-Wave Potentials u s , Saturated Calomel id/crn'/W/a, Microamp.-Millimolar -1 hfg. -21LSeo.W Electrode in Supporting Electrolytes A B C D E A B C D E -1.46 -1.23 - 1.21 -1.01 ... 3.9 4.3 3.1 1.1 KO wave -1.17 -1.12 -1.15 -1.03 -0.93 11.1 11.4 6.2 2.4 0.2 -1.11 -1.18 -1.02 -1.09 -0.98 10.6 11.0 2.2 6.0 0.2 -1.34 -1.16 -1.20 -1.08 10.3 2.1 -1.00 11.2 6.4 0.1 -1.22 -1.11 -1.17 -0.93 ... 5.5 9.3 1.7 9.3 No wave -1.32 -1.21 -1 28 -1.11 1.4 9.3 9.7 . . . 5.0 No wave -1.10 -1.14 -1.00 -0.97 11.8 13.0 4.9 2.3 N o wave No -0.78 -0.70 -0.70 -0.61 4.4 5.0 1.0 2.7 -1.15 -1.06 -1.05 -0.99 15.0 16.5 6.2 2.8 No wave -1.21 -1.10 -1.05 -1.01 13.2 14.4 2.2 5.3 ...
... ... ...
Supporting electrolyte. A. Methanol-1,4-dioxane (1 t o 1). 0.3 k? in lithium c B. Methanol. 0.5 M in lithium c a
0.001 hf nitroethane solutions in methanol-benzene base solution, but maxima were still observed. Ethyl cellulose did suppress the maxima in the nitroethane solutions, but the half-wave potentials were shifted t o more negative values. Studies were made of the variation in diffusion current with concentration over the range 0.2 to 2 millimolar in all the solvents except glycerol. The fact that the relation was a linear one makes it possible to use nonaqueous solutions for analytical purposes. The polarograms do not show waves which are as well defined and as regular aa those usually obtained in aqueous solution?. Consequently, the value for the half-wave potentials can be given only to within 20 mv. The values given in the tables were corrected for the I R drop across the cell (Tables I and 11). The half-wave potentials of the primary nitroalkanes generally became more negative with increasing molecular weight. For the secondary nitroalkane the half-wave potentials are about the same, and are more difficult to reduce than the primary compounds in all the cases studied. The dinitroalkanes reduce at about the same potentials as the nitroalkanes. I n all solutions, with the exception of methanol, the half-wave potential increases for 1,3-dinitropropane. 1,5dinitropentane, and 2,2-dimethyl-1,3-dinitropropane in that order, but the potential for 2,Zdinitropropane is less in all the base solutions than the half-nave potentials for the other compounds. In glycerol as solvent with 0.3 M lithium chloride as electrolyte, only nitroethane, I-nitropropane, and 2-nitropropane gave polarographic waves. Presumbly, because of the high viscosity of glycerol, larger molecules than nitropropane cannot diffuse to the electrode surface. The diffusion current for a given material in different solvents was inversely proportional to the square root of the viscosity coefficient of the solvent. This was also true for 1-nitrobutane in a series of methanol-glycerol solvents with 0.3 N lithium chloride as the supporting electrolyte. Typical data are given in Tables I11 and IT, and data for 0.001 Af nitroethane in a series of solvents are given in Figure 2. Such a relationship between the diffusion current and the square root of the viscosity coefficient is reasonable. According to the Ilkoviit equation, the diffusion current is proportional to the square root of the diffusion coefficient, and the Stokes-Einstein equation D = - kT
6aqr
gives a relation between the diffusion coefficient and the viscosity, provided the moving particles or molecules are spherical and larger than the molecules of the solvent medium-i.e., when the medium is continuous with reqect to the diffusing molecules, so that the retarding forces are entirely frictional in nature and proportional to the bulk viscosity of the solution.
By combining the IlkoviE equation with the Stokes-Einstein equation and grouping all the constants into one new constant, K , the following equation is readily obtained id
=
?l.K/&/
where n is the number of electrons involved in the reduction step.
00
-02
-04
-06
-08
-10
-12
-14
-16
APPLIED POTENTIAL, VOLTS
Figure 1. Polarogram of 0.0012 M Nitroethane in Benzene-Methanol (1 to 1) as Solvent, 0.3 .Win Lithium Chloride
Table 111. Viscosities of Solvents and Diffusion Current of 0.001 M Solutions of Nitroethane and n-Butyl Nitrate
Methanol Methanol-benzene Methanol-1.4-dioxane n-Propyl alcohol n-Butyl alcohol Isobutyl alcohol Ethylene glycol Glycerol
Viscosity, If illipoises 5.46 5.61 6.37 19.9 25.9 32.8 173 9450
id, Microamperes Yitroethane n-Butylnitrate 25.4 8.8 25.8 7.8 22.7 8.0 13.6 6.6 12.6 ... 12.2 6.1 4.8 2.1 0.4 ...
Table IV. Viscosities of Glycerol-Methanol Mixtures and Diffusion Current of 0.00107 M 1-Nitrobutane in These Solvents (0.3 ,N in lithium chloride) Volume % Methanol in Glycerol
Viscosity, hfillipoises
id,
Microamp.
V O L U M E 2 4 , N O . 6, J U N E 1 9 5 2
973
For a particular molecule, if a plot of the diffusion currents against the reciprocals of the square root of the viscosity coefficients of the solvents shows a linear relationship, then, as the slope, nK, is a canstant, the electron change would be the same in all the solvents. This condition was met by all the nitroalkanes and n-butyl nitrate. I
00
Figure 2.
I
20
I
4 0
I
60
1
yfi,
8 0
I
100
1
I
120
POlSES-"2
Effect of Viscosity of Solvents upon Diffusion Current of 0.001 M Nitroethane
The diffusion coefficient of nitroniethane in methanol is given as 2.53 X 10-6 cm.2 sec.-l a t 16" C. ( 3 ) . From polarogram obtained for 0.001 M solutions of nitroniethane in 0.5 f i f lithium chloride in methanol a t 16", the calculated value of the electron change, using the Ilkoviit equation, was 3.9. As the wave heights and the half-wave potentials of the nitroalkanes are of the same magnitude, the electron change would be expected to be the same for nitromethane, nitroethane, 1-nitropropane, 2-nitropropane, 1-nitrobutane, and 2-nitrobutane. The final reduction products would probably be the alkylhydroxylaniines. There is general agreement that the nitroalkanes in aqueous solution are redured polarographically xith an electron change of 4 to the hydroxylamines ( 1 , 6, 8, 9). On the assumption that IZ is equal to 4)the diffusion coefficients of the nitroalkanes in methanol were calculated froni the IlkoviE equation and were compared with the diffusion coefficients calculated with the follo\~-ingrelation :
derived from the StokesEinstein equation, Khere ,T' is the molal volume of t h e molecule. These data are given in Table 1- and a comparison of the square root of the diffusion coefficient cal-
Table
Y. Diffusion Coefficients of 0.001 M Nitroalkalies in Methanol as Solvent (0.5 J1 in lithium chloride)
( D g )
c
Diu DSE~ Cm.2 sec.-l X 10s Sitraniethane 26.4 2.69 1.43 Nitroethane 24.4 2.30 1.30 1-Nitropropane 22.7 2.00 1.21 2-Nitropropane 23.3 2.10 1.20 1.41 1-Nitrobutane 19.1 1.14 2-Nitrobutane 20.1 1.55 1.14 D I calculated from Ilkovia e uation, n = 4. b D&, calculated from Stokes-%instein equation. 16
111
0.73 0,75 0.78 0.75 0.90 0.86
culated from the Stokes-Einstein equation; that calculated from the IlkoviE equation shows that this ratio approaches 1 as the nitroalkane molecular size increases. This indicated that in methanol for molecules as large as nitrobutane, or larger, the Stokes-Einstein equation can be used to calculate diffusion coefficients for polarographic use. For n-butyl nitrate a 2-electron change was calculated for t,he reduction in all the base solutions studied. A 2-electron change has been reported for the reduction of ethyl nitrate and cyclohcsyl nitmtcl in a q u v o u ~vthmol (4). LITERATURE CITED
(1) De Vries, T., and Ivett, R. TV,, IND.ENG.CHEM.,AN.AL.ED., 13,339 (1941). (2) Hale, c. H., ANAL. CHEM.,23, 572 (1951). (3) "International Critical Tables." Vol. V, p. 72, S e w Tork, McGrsw-Hill Book Co., 1929. (4) Kaufman, F., Cook, H. J . , and Davis, S. >' "Electrolytic I., Reduction of Organic Sitrate Esters," Abstracts, 117th lfeetine. -4MERICAN C H E X I C I L SOCIETY. Detroit. Mich.,,~1950. (5) Lewis, WY'R., Quackenbush, F. IF'., and De Vries, T., ASAL. C H E Y . , 21, 762 (1949). ( 6 ) Miller, E. TV., Arnold, A. P., a n d ;Istle, 11.J., J . A m . C'hevr. Soc., 70, 3971 (1948). (7) Parks, T. D., and Hansen, K. A., ASAL. CHEM.,22, 126s (1950). (8) Petru, F., ColEection Ctechosloc. Chem. Cornmum., 12,620 (1947). (9) Stewart, P. E., and Bonnei, JV. A,, AXAL.CHEM.,22,793 (1950). (10) Stone, H. W.,and Skavinski, E. R., ISD. ENG.CHEM.,.Isa~. ED., 17,495 (1945). I ~ L C E I ~for E Dreview March 10, 1951. Accepted April 7, 1952. Presented before the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 5 , 1951. Abstracted from t h e Ph.D. thesis submitted by S a r h a n Radin t o t h e Graduate School of Purdue University, February 1951.
Reproducible Platinized Platinum Electrode for Anodic Polarography WILLIAXI 31.~IAcNEVINAND RIICK-IEL LEVITSKY' Ohio S t a t e Unicersity, Coluntblts, Ohio
K
ECEKTLY ( 2 ) it mas observed that the current obtained with the microelectrode in anodic polarograplly could be appreciably increased if the surface of the electrode were coated with a layer of electrolytically deposited platinum. Difficulty was experienced in reproducing the current, using succee sively prepared electrodes, and it is with this problem that the present paper is concerned. The electrode support used in this work was a rotating microelectrode similar t o that described by Kolthoff and Lingnne ( 1 ) .
A 3-mm. length of 20-gage platinum wire was attached to the rotating shaft. In the origina1,design the shaft was coated with 1 Present address, Grasselli Division, E. 1. d u pant de N~~~~~~ and Wilmington, Del.
c0,,
wax and the wax was removed from the end Of the platinum wire. This process usually loosened the wax surrounding the wire, and electrode surface of unknown area was exposed to the solution. This problem lvr&s early in the prasentwork by coating the platinum wire with glass. Glass tubing was drawn to the diameter of the wire. A short section was placed OVW the wire and heated with a small flame until it collapsed and attached itself to the wire. The glass over the end of the wire waa then ground off with emery paper. The esposed end of the platinum wire was polished with fine silicon carbide powder suspended on cloth. Finally the shaft was coated with ceresin was, which overlapped on the glass-covered shank of the platinum electrode. When prepared in this way, the electrode area is constant. The exposed end Of the electrode may be polished repeatedb- without changing the area of the electrode surface. The glass surrounding
