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R. BRUCE MARTINAND MARYTASHDJIAN
Vol. 60
THE POLAROGRAPHIC REDUCTION OF N-NITROSAMINES BY R. BRUCEMARTIN AND MARY 0. TASHDJIAN Department of Chemistry, American University, Beirut, Lebanon Received Sepbmber I d , 1066
The irreversible reduction of dimethyl-, piperidine- and 'diphenyl-N-nitrosamines has been studied over a range of p H and concentration in well buffered solutions. One step symmetrical curves are obtained. The diffusion current i d varies directly with the concentration. The ratio of id/C varied with no definite trend. Temperature and height experiments indicate diffusion control. El/a varied linearly with pH. Plots of Ed.e. versus log ( i d - i)/i yield straight lines with values of no1 less than one. A correlation between El/a and absorption spectra is described.
The polarographic reduction of N-nitrosamines has received little study. English' investigated the reduction of dimethyl-N-nitrosamine incidental to determining dimethylamine.2 I n general nitroso compounds are more easily reduced (ca. 0.4 volt) than the corresponding nitro compounds3and have been shown to be a reduction product of the nitro group.4 The ammonium salt of N-nitrosophenylhydroxylamine has been studied over a range of pH values.6 At pH 1 a six electron wave is observed corresponding to the reduction to phenylhydrazine. At pH 7 to 9 the reduction involves 4 electrons. Two waves occur, of which the first is pH dependent. I n this paper the polarographic reduction of an aliphatic, heterocyclic and aromatic N-nitrosamine is reported. They are, respectively, dimethyl-, piperidine- and diphenyl-N-nitrosamine. Experimental
the case of alcoholic solutions small oxygen waves were observed, but their presence did not interfere with the interpretation of the polarogram. Instruments.-A Sargent Model XI1 Polarograph was used with a Heyrovsky type cell. The mercury height was kept constant at 67 cm. The drop time ( t ) and weight in mg. of each drop ( m ) were measured at an applied potential of 1 volt (versus the saturated calomel electrode). m = 3.21 mg. and t = 2.32 sec., therefore ma/at'/s = 2.504. The cell resistance was about 200 ohms, so no correction for I R c$op was necessary. Temperature was controlled to f0.01 . All pH measurements were made on a Cambridge glass electrode pH meter. In the case of alcoholic solutions no correction was made for the change in activity coefficients. Spectra were determined on a Beckman Model DU spectrophotometer using matched one centimeter silica cells.
Results I n general, single well defined polarograms were obtained. I n all cases the diffusion current (id) varied directly with the concentration (c). The ratio i d / c is expressed in I.L ampere/mole. El/, varied slightly with concentration. AverReagents .-The N-nitrosamines were prepared according age values versus the saturated calomel electrode are to the procedures described by Hickinbottom.6 The dried reported. The deviation was less than 0.03 volt. liquid nitrosamines were distilled twice and the physic$ Plots of E d . e . versus log (id - i)/iyield straight constants determined: N-nitrosodimethylamine, b.p. 153 , lines, where E d . e . is the potential for current i. 12% 1.4373; N-nitrosopiperidine, b.p. 215-217', 12% On the assumption that the equation 1.4932; N-nitrosodiphenylamine, m.p. 66-67'. Commercial ethyl alcohol was distilled after the addition of MnOz. The middle portion of the distillate wa8 used to prepare a 50y0 by volume solution with water. Pro Analysi potassium chloride was used as a supporting electrolyte. Reagent grade hydrochloric acid, sodium acetate, acetic acid, sodium monobasic phosphate and citric acid were used in preparing buffer solutions. Hydrogen was produced in a Kipp generator from C.P. hydrochloric acid and C.P. mossy zinc passed through a 30% potassium hydroxide solution and a sample of test solution before being passed through the sample. All rubber connections were soaked in concd. potassium hydroxide, rinsed with distilled water and dried. Solutions.-Water solutions of the dimethyl and piperidine-N-nitrosamines were prepared. It was necessary to use a 50% by volume of ethanol solution for the dipheny1-Nnitrosamine. All solutions were 0.1 N in potassium chloride. All solutions were buffered. The concentration of the buffer mixture was usually 0.2 N . Hydrogen gas was passed through the prepared solutions for 15 minutes. With alcoholic solutions the time and rate of flow was set a t optimum for efficient oxygen removal, with the least change in concentration due to evaporation. In (1) F.L. English, Anal. Chsm., 28, 344 (1951). (2) A referee mentions two references which duplicate in part thia work. M. Lechercq, Me? Poudre, 85, 365 (1953). and G. Sifre, ibid., 85, 373 (1953). We do not have access to these papers or their abstrac ts. (3) H. J. Gardner and L. E. Lyons, Rev. Pure Appl. Chem., 134 (1953). (4) I. Ril. Kolthoff and J. J. Lingane, "Polarography," Interscience Publishers, Inc., New York, N. Y., 1952, p. 746. (5) Ref. 4,page 765. (6) J. W. Hickinbottom, "Reactions of Organic Compounds," Longmans, Green and Co., London, 1945,p. 286.
Ed.0. =
El/n
- 0.0590 7 2 ~ log y (id - i)/i
was applicable, na was determined. The upper limit of pH was restricted in all cases by the hydrogen wave. Higher concentrations than those reported gave irregular maxima. TABLE I N-NITROSODIMETHYLAMINE Concn. = 3.35 to 14.87 X mole/l., temp. = 30.0". PH
1.41 1.41 2.69 2.80 3.03 3.37 3.62 3.71 3.74 4.00 4.22 4.34
p
id/C,
amperes/mole
E l l r (volts)
no!
23.4 22.3 17.4 17.1 19.2 22.2 20.5 21.5 15.1 22.6 19.6 17.5
0.90 0.90 1.01 1.04 1.07 1.09 1.11 1.12 1.12 1.15 1.19 1.23
0.77 .76 .72 .71 .69 .67 .64 .60 .60 .47 .47 .43
A plot of Et/, versus pH yields a straight line to pH 4.0. The slope is 0.096 volt/pH. The temperature coefficient of El/, is +1 mv./ degree. The temperature coefficient for id, l/i X di/dT X 100 = 0.80%.
THEPOLAROGRAPHIC REDUCTION OF N-NITROSAMINES
Aug., 1956
TABLEI1 N-NITROSOPIPERIDINE Concn. = 3.55 to 8.87 X P H ~
a
mole& temp. = 25.0'. El/*
id/C
1.89 17.5 0.83 2.24 23.8 0.80 .. 0.96 3.97 4.50 18.0 1.02 4.96 17.6 1.09 Below pH 1.89 a maximum was observed.
nu
0.79 .80 .75 .58 .56
A plot of El/, versus p H yields a straight line to pH 4.0. The slope is 0.063 volt/pH. The temperature coefficient of El/, is 1 mv./ degree. The temperature coefficient for id, l/i X di/dT = 1.5% a t 25.0'. The diffusion current was found to vary as the square root of the height of the mercury. El/, was found to be independent of the height.
+
TABLE I11 N-NITROSODIPHENYLAMINE Concn. = 4.77 60 24.70 X 10-4 mole/l. in 50% by volume of 95% ethyl alcohol; temp. = 25.0'. PH
id/C
El/*
na
1.15 2.51 3,93 4.60 5.27
13.0 10.2 13.4 10.1 12.3
0.56 .69 .83 .88 .92
1.03 1.03 0.78 .92 .89
Below pH 1.15 the first wave of oxygen made the interpretation of the polarogram difficult. A study a t a higher p H than those reported was limited by interference with the second wave of oxygen. A plot of El/, versus p H yields a straight line over the entire range studied. The slope is 0.090 volt/pH. The temperature coefficient of El/,is random and of the order of 7 mv./degree. The temperature coefficient for i d , l/i di/dT X 100 = 2.3%. Since N-nitrosodiphenylamine had to be studied in the 50% by volume solution of alcohol, a run was made on the other two compounds in the same solvent at similar pH for comparison.
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diphenylamine where a mixed process may be taking place. Since the values of na in Tables I, 11, I11 and IV are less than one the process is irreversible. The curves obtained are all symmetrical and Ellzvaries linearly with pH. These facts tend to indicate that there is a direct transfer of electrons from the electrode to the organic compound, but that either the product reacts further or the final equilibrium is attained s10wly.~ Assuming the validity of the Ilkovic equation values of n were calculated assuming the following diffusion coefficients.8 N-nitrosodimethylamine D = 8.5 X 10-8 cm.2 sec.-l N-nitrosopiperidine N-nitrosodiphenylamine
D D
8.0 X 10-8 cm.2 sec.-l = 3.3 X 10-6 cm.$sec.-l
=
The values of n vary from about 3.7 t o 5.3. This would indicate a 4 or 5 electron process. A four electron process would yield the corresponding hydrazine plus a molecule of water. A five electron process would probably yield the corresponding amine plus one-half m,oleculeof hydrazine. Attempts to determine n directly proved unsuccessful. Micro-coulometric methods which depend on an external coulometer for the evaluation of the current passed require an increase in voltage.9 This is sufficient to cause the reduction of hydrogen ions and vitiate the results. Studies of the absorption of N-nitrosamines gave evidence for the mesomeric systemlo
"> R
N-N=O
R R>+
t--+
-
N=N-0
Bands due to a resonating system have relatively high molar extinction coefficients. Influences increasing the polarity facilitate the electron migration and displace the band to longer wave lengths. The molar extinction coefficient ( E ) for ethanol solutions of N-nitrosodimethylamine is 7,000 a t 231 mp and for N-nitrosopiperidine E = 8,100 a t 238 mp. lo The value for N-nitrosodiphenylamine as determined in this study is E = 5,720 a t 293 mp. The diphenyl compound has therefore by far the greatest tendency to be polarized as indicated by its maximum at 293 mp followed by the piperidine compound with a maximum a t 238 mp and finally by the dimethyl compounds with a maximum a t 231 TABLE IV mp * N-NITROSO COMPOUNDS AT 25.0' IN 50% ALCOHOL SOLVENT It was shown above that in all probability there N-NitrosoPH id/c El/, nu is a direct exchange of electrons between the 1.23 0.44 4.06 11.5 Dimethylamine electrode and the organic compound. It is likely 1.08 0.69 4.06 11.9 Dipiperidine that this exchange takes place with the polar0.83 0.78 3.93 13.4 Diphenylamine ized form of the molecule, with the practically simultaneous addition of an electron from the Discussion electrode to the plus nitrogen and the addition of a I n all cases i d varied directly as the concentra- proton t o the negative oxygen. If this is the tion. Plots of i d versus concentration gave straight case the compound most easily polarized should be lines. The irregularity in the values of id/c is dif- most easily reduced. ficult to explain. Different buffer components Therefore, a correspondence should be observed gave the same results. Solutions run immediately between ease of reduction and wave length of light after mixing and a week later gave the same value, absorbed. The most easily reduced compound ruling out a time factor. The only conclusion should have the longest wave length absorption, seems t o be that some variable(s) has not been suf- etc. ficiently isolated. (7) J. E. Page, Quart. Rev. Ckcm. Soc., VI, 262 (1952). The results of the temperature and height ex(8) Private communication from Dr. J. Heyrovsky. periments seem to indicate a diffusion controlled (9) T. De Vries and J. L. Kroon, J . Am. Chem. Soc., 7S, 2484 (1953). process with the possible exception of N-nitroso(10) R. N. Haszeldine and J. Jander, J . Chem. Soc., 691 (1964).
-
1030
SALLIEFISHER AND ROBERT KUNIN
That this is in fact the case may be seen from Table IV where the El/; values are compared under identical solvent conditions. The diphenyl compound is seen to. be much the easiest to reduce followed by the piperidine and dimethyl compounds. Thus there is a correspondence between the absorption due to the resonating system and the polarographic reduction potential. Comparing the absorption in alcohol with the
Vol. 60
reduction potential in 50% alcohol does not alter the conclusions, as the shift caused by a change of solvent would be about the same absolute value for each compound and hence the relative positions would not be affected. Acknowledgment.-The authors greatly appreciate the support of the Research Corporation and wish to thank Dr. Robert H. Linnell for aid in initiating the study.
EFFECT OF CROSS-LINKING ON THE PROPERTIES OF CARBOXYLIC POLYMERS. I. APPARENT DISSOCIATION CONSTANTS OF ACRYLIC AND METHACRYLIC ACID POLYMERS1 BY SALLIE FISHER AND ROBERT KUNIN Rohm & Haas Co., Philadelphia, Pa. Received September 913,1966
,
The apparent dissociation constants of cross-linked polyacrylic and polymethacrylic acids in contact with an aqueous phase 1 M in KC1 have been determined. Polymers cross-linked with divinylbenzene and three other vinyl compounds have been studied. The determined values of pK, increase with increasing degrees of cross-linking in both acrylate systems. The magnitude of the change is dependent on the mole percentage of acrylate in the polymer. Limiting values of pK, as the cross-linking is decreased to zero are equal to the dissociation constants of the linear polymers of the corresponding acids.
Introduction A number of studies have been made of the effect of the degree of polymerization on the apparent disaociation constant of linear polymers of acrylic and methacrylic acids.2-4 Although Katchalsky and Michaelis have proposed an equation to fit the case of the cross-linked polymers of these acids, few experimental data were published concerning these systems until recently when Gregor6 has reported that the apparent dissociation constants of these materials decrease with increasing degrees of cross-linking with divinylbenzene. I n the present work the effect of cross-linking has been studied over a wide range of cross-linking concentrations and the materials cross-linked with divinylbeneene have been compared with those prepared using other vinyl monomers. Experimental The comparison of the apparent dissociation constants of polymers of different degrees of cross-linking was made under arbitrarily chosen experiment@ conditions in which all variables except polymer composition were held constant. In particular, the system in which the hydrogen form of the polymer is neutralized with KOH has been chosen for study. Since all of the polymers included in the investigation are insoluble in water, the direct titration method wherein the pH is followed as known increments of KOH are added to the sample was not used because of the slow rate of establishment of equilibrium in such two-phase systems. This difficulty has been circumvented by weighing out a series of sampres of each olymer and adding to each sample a known fraction of the I?OH needed to neutralize it. The mixture of polymer and alkali was then allowed to stand until equilibrium, as shown by a constant p H in the aqueous phase, (1) Presented before the 128th Meeting of the American Chemical Society, Minneapolis, Minn., September 11-16, 1955. (2) R. M. Fuoss,Ann. Rev. Phys. Chem., 8 , 81 (1952). (3) A. Katchalsky and I. Michaeli, Bull. Research Council Israel, 2, No. 3 (1952). (4) A. Katchalsky and P. Spitnik, J . Polymer Sci., a, 432 (1947). (5) H. P. Gregor, M. J. Hamilton, J. Becker and F. Bernstein, THISJOURNAL 69, 874 (1955).
was reached. All measurements were made with the resin in contact with an aqueous phase 1 M in KCl to minimize the contribution to the equilibrium arising from changes in the ionic strength of the aqueous phase. This relatively high salt concentration also minimized the polymer swelling a t low degrees of cross-linking and hence reduced the error arising from the imbibition of water by the polymer with a resultant shift of the Donnan concentrations. A constant ratio between milliequivalents of functional groups in the polymer phase and the volume of the aqueous phase was also maintained. A series of methacrylic acid and acrylic acid samples copolymerized with known amounts of divinylbenzene and with other vinyl cross-linking agents were prepared for this study. All of the samples were in the form of 20-40 mesh beads. The size of the polymer particles was not reduced by grinding lest some change in functionality or structure be introduced thereby. The finished polymers were leached with a thousand-fold excess of 1 M HC1 and then rinsed free of excess acid with deionized water until the effluent was neutral to methyl orange prior to the determination of the dissociation constants. No attempt was made to define the entire titration curve of the polymers studied since several weeks are sometimes required for the establishment of equilibrium in the region of the equivalence point. Instead,. the number of milliequivalents per dry gram of each polymer was determined by equilibrating a known dry weight of it with a known excess of standardized KOH solution that was also 1 M in KCl. The milliequivalents of the hydroxide reacting with the carboxyl groups of the polymer were determined by back titration of the supernatant liquid after the slurry had been shaken for 16 hours according to the method of Fisher and Kunin,e for the determination of total cation exchange capacity of ion exchange polymers. Once the total capacity of each polymer was known, the constants in the Henderson-Hasselbalch equation were determined by measuring the pH of a series of partially neutralized samples in the following manner. Procedure.-To four dry samples of the hydrogen form of each polymer containing 10 m 2 . of carboxylic groups amounts of the standardized 0.1 M OH solution containing 2.0, 4.0, 6.0 and 8.0 meq. of hydroxide ion were added. This solution was also 1 M in KC1. The volume of the a ueous phase was adjusted to 100 ml. by the addition of 1% KC1 solution. The samples were stoppered and sealed (6) 5. Fisher and R . Kunin, Anal. Chem., 27, 1191 (1955).
