at column outlet w = column resistivity, defined by p i 2 = po2 w~o~oL
uo = carrier gas velocity
+
LITERATURE CITED
(1) Bohemen, J., Purnell, J. H., in “Gas Chromatography,” D. H. Desty, ed., pp. 12-13, Academic Press, New York, 1958. (2) Desty, D. H., Goldup, A., Third
International Symposium on Gas Chromatography, Edinburgh, 1960. (3) Gidding% J. c., S a i W , 8. L.7 Stuc& L. R., Stewart, G. H., ANAL. CHEM. 32,867 (1960). (4) Kieselbach, R., Ibid., 32,880 (1960). ( 5 ) Purnell, J. H., Nature 184, 2009 (1959). (6) Purnell, J. H., Quinn, C. P., Third International Symposium on Gas Chromatogra hy, Edinburgh, 1960. (7) Scott, %. P. W., Hazeldean, G . S. F.,
Third International Symposium on Gas Chromatography, Edinburgh, 1960. DONALD D . DEFORD
Northwestern University Evanston, 111.
BUELL0. AYERS ROBERT J. LOYD
Phillips Petroleum CO. B a r t l e s h k Okla. RECEIVEDfor review July 15, 1960. Accepted September 14, 1960.
Ultraviolet Absorption of Aliphatic Nitro Compounds in the Presence of a Base SIR:The short-chain mononitro compounds are known to react in the presence of a base (3). However, the ultraviolet absorption of these compounds has not been examined under these conditions. Initial observations indicated strong absorbance in the 300mp region. From this it was hoped that B simple means for the detection and quantitative determination of the shortchain nitro compounds could be developed. Unfortunately, such is not the case. These observations may, however, indicate a need for caution in the use of 302- to 305-mp absorbance for determining the nitrate ion (Z), if nitro compounds are present. The solutions were examined from 240 to 340 mp on a Beckman DK-2 spectrophotometer with H2 lamp and multiplier phototube, using a 1-minute scan, 1-cm. path length, sensitivity setting 12. The nitromethane (Eastman Kodak 189), nitroethane (Eastman Kodak 2095), Znitropropane (Eastman Kodak 4608), and 1-nitropropane (Commercial Solvents) were obtained commercially. The 1-nitropropene was prepared by the method of Schmidt and Rutz (4) and Bnitropropene by the method of Blomquist, Tapp, and Johnson ( 1 ) . The initial observations of the absorbance of representative aliphatic nitro compounds in the presence of
1M sodium hydroxide are shown in Table I. For comparative purposes the data have been expressed as apparent molar absorptivity, because comparison of these compounds at a single concentration was precluded by the wide range of absorbance. However, these values should not be regarded as absolute, as Beer’s law is not strictly observed and the change of absorbance with time is dependent on concentration. At the concentrations used, the observations indicate that the 1-nitro compounds have a t least 10 times more absorbance than the nitrate ion. The wavelength maximum is given as a range, as there were variations from experiment to experiment. Subsequent work suggests that these variations are a function of actual time of reading and shift to lower wave lengths with increasing absorbance. Table I1 shows that the concentrations of the nitro compound and of the base as well as the time of readings are factors in the actual values observed. The effect of using a weak base is shown in Table 111. Nitroethane absorbance has been decreased, although the concentration has been increased to almost 150 times that indicated in Table I. The lack of correlation of absorbance with concentration is seen with 1-nitropropane. This raises the
question of whether the calculation of molar extinction coefficients (molar absorptivity) is entirely justified. Table IV presents the effect of concentration, pH, and time on the absorbance in phosphate buffer, which itself shows no absorption in this region at p H 6.0, 7.0, and 8.0 compared with sodium hydroxide. The sodium hydroxide solutions showed a definite peak, while the buffered solutions did not. Hence the values given for the buffered solutions were taken a t the corresponding peaks shown in the presence of sodium hydroxide. The absence of a definite peak and the decreasing absorbance suggest that for this concentration a p H above 8 is required to show absorbance. The inherent difficulties in using this reaction as an analytical method are most apparent from Table V. The indicated concentrations of nitromethane were prepared in 1.OM sodium hydroxide and the absorbance was determined over a 48-hour period. From
Table
II. Effect of Time of Reading on Absorbance of Nitroethane
Time of Reading
Absorbance 26.6 mhf 2.66 mM in 0.1M in 0.01M NaOH NaOH
All Diluted a t 0 Time Table 1.
Absorbance of Nitro Compounds in 300-Mp Region in the Presence of Sodium Hydroxide
Wave Length of Maximum’ 296-302 296-302 302-308
Apparent Compound Concn., mM E&. Nitromethane 0.09 244.0 Nitroethane 1.2 402.0 1-Nitropropcne 0.3 169.7 2-Nitropropane 112.0 ... 2-Nitropropene 230.0 316%0 7.35 Sodium nitrate 23.0 309 16.8 Original solutions contained 0.1 ml. of nitro compound per 5 ml. of 1iM XaOH, subsequently diluted with HzO to indicated concentrations. a .4ctual maxima varied from experiment t o experiment and subsequent observations suggest this to be a function of time and concentration.
1712
ANALYTICAL CHEMISTRY
0 5 min.
10 min. 25 min.
55 min.
125 min. 18 hr.
0.000
0.013 0.013
0.000
0.000 0.000
0,022
0.000
0.051
0.000 0.000
0.027
0.201
0.000
All Diluted at Time of Reading 0.000 0.000 0.523 0,027
0 2 hr. 18 hr.
2.
+
0.161
Original solution 266 mM nitroethane in
1M KaOH, diluted with HzO to indicated
conceutrations.
Table 111.
Absorbance of Nitro Compounds in the Presence of Lithium Hydroxide
Compound Nitromethanc Nitroethane 1-Nitropropane
(Read immediately at 300 m r ) Concn., mM Absorbance
E&. 2. < 6 . 2 x 103 0.161 0.91 1.699 4.60 0.027 0.735 2-Nitropropane 0.032 0.17 Solutions prepared to indicated concentrations in 0.25M LiOH (pH 12.3). Table IV.
0.328 177.0 368.0 36.8 188.0
+
EfFect of Dilution and Time of Reading for Nitroethane in Phosphate Buffers and Sodium Hydroxide
Time of Dilution (1 : 10 H&) 0 90 min. 90 min.
Time of Reading 0 90 min. 20 hr.
NaOH
Absorbance, 298-300 Mg Phosphate Buffers PH 6 PH 7
0.022 0.301 0.409 2.000
0.244 0.229 0.168 0.168 0.102 0.097 20 hr. 20 hr. 0.137 0.181 Solutions prepared as 266 mM nitroethane in 0.15M phosphate buffers or in and subsequently diluted 1: 10 with water a t times indicated.
the data the apparent extinction coefficients were calculated and are given for selected times. Since the apparent extinction coefficient expresses the relationship between absorbance and concentration, i t appears that at zero time proport~onate~y of the ab at the lowest sorbing material is present concentration.
Table V. Effect of Concentration and Time on Absorbance of Nitromethane in 1 .OM Sodium Hydroxide
hpparent E&. X 103 Concn., mM/L.
time
0.074 0.128 0.256
0.60 0.55 0.54
0
4 hr. 0.60 0.55 0.54
24 hr. 1.03 1.07 1.48
48
hr.
1.24 1.32 2.28
would not be fruitful as a method of analysis. LITERATURE CITED
PH 8 0.222 0.131 0.086 0.097 lnii NaOH
T., Tapp, W. J.: Johnson, J. R., J . A m . Chem. SOC.67,
( 1 ) Blomquist, A.
1519 (1945). (2) Dalance, A., Healy, P. W., ANAL. CHEM.17,718-19 (1945). (3) Maron, S. H., LaMer, V. K., J . A m . Chem. SOC.60, 2588 (1938). (4) Schmidt, E., Rutz, G. G., Ber. 61, 2142 (1928).
JOSEPHH. GAST As i t has not been possible to find FRANCES L. ESTES Biochemistry Department conditions of complete or equilibrium Baylor University College of Medicine conversion, the absorbance does not Houston 25, Tex. follow B ~ law,~and ~the inJ ~ crease”of absorbance with time is not RECEIVEDfor review January 18, 1960. Accepted August 29, 1960. Work s u p a function of concentration, particularly ported by a research contract with the Air Pollution Medical Program, Public if dilution is required to permit a valid Health Service, U. S. Department of reading, it was concluded that this Health, Education and Welfare.
Spectrophotometric Determination of Sulfide End Groups and Number Average Molecular Weight in High Polymers SIP.: A number of methods have been devcloped for determining the number average molecular weight of polymers by end group analysis. A Ppectrophotometric method for the determination of number average molccular weight of polymers, with known sulfide end groups, is described. This procrdure can be applied directly for the qualitat,ive identification of the sulfide m d group in polymers if the number average molecular weight is known. The basis of this method is the complrs formrd between molecular iodine :tnd sulfidc sulfur. This complex absorbs intcnsc4y in the ultraviolet region a t 308 nip. In determining number average molwular wcight, the assumption is made that thrw is onc sulfide group per v h : i i r i : the fact that this method givcs ~ ~ c ~ ~ wiiii t e agreement with osmotic ] n o l (i~h r wcigl!ts is rvidcnce that the :isSuinption is v:ilid, ! (h]Jn!C’Ilt of this proccdure \vas I x i ~ t ~ 011 ~ 1 prc,vious iodine-sulfidc coni-
plex absorption studies concerned primarily with materials found in petroleum. The procedure as applied to polymers parallels that described by Hastings (3). EXPERIMENTAL
Identification of End Group. Before determining sulfide end groups, or number average molecular weights, i t is necessary t o “clean up” the polymer t o remove interfering contaminants. Various compounds, usually aromatic, introduced as additives in the preparation of polymeis, may also absorb strongly a t the same wave length as the complex. The rcmoval of such material IS done preferably by column chromatography. The procedure consists of dissolving 5 t o 10 grams of polymer in mcthylene dic~liloritlrand passing the solution through a column of baiic duniina (Woelm). l‘lie column is i\arhcd two or thrt,cl tinics 111thmethyl(’ne divhloride, thcx rllripnt is c.ollcc*ted,
and the solvent completely evaporated, resulting in a clean sample of polymer. For our purposes, the interfering materials were sufficiently removed by column chromatography; however, this purification may be supplemented by countercurrent extraction using nitromethane-carbon disulfide as the solvent system.
A weighed portion of clean polymer is dissolved in 100 ml. of spectrograde methylene dichloride (approximately a 4% solution for molecular weights around l00,OOO). An aliquot of this solution is diluted with methylene dichloride, if necessary, to permit a reasonable absorbance reading of the iodine-sulfide complex. The iodine-sulfide complex is prepared by adding 5 ml. of the polymer-methylene dichloride solution to 5 ml. of the 0.374 iodine reagent (Irepared by dissolving 3 grams of iodine in methylene dichloride and diluting to 1 liter). The solution is agitatcd to obtain a uniform mixture. VOL. 32, NO. 12, NOVEMBER 1960
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