Ultraviolet Spectrophotometric Determination of Polymerized Styrene in Styrenated Fatty Acids and Alkyd Resins R. C. HIRT, R. W. STAFFORD, F. T. KING’, and R. G. S C H M l l l Stamford Research Laboratories, American Cyanamid Co., Stamford, Conn.
Even though only a one-component analysis with corrections for two other absorbers is actually desired, the overlapping of the absorption requires essentially a three-component analysis with nine known absorptivities. The longest wave-length maxima of monomeric styrene, ronjugated triene acids, and polyst,yrene occur a t 291, 282, xiid 269 mp, respectively, so these wave lengths seemed logical choices for the analytical wave lengths. Purified styrene [99.75’% pure by the freezing point depression method of Witschonke ( H ) ] and polystyrene [reprecipitated, assaying less than 0.14% hy the spectrophotometric method of hIcGovern and coworkers ( 7 j] were used to determine the absorptivities for these two compounds. These values checked ne11 with those obtainable from the papers by Newell (8) and by McGovern and colvorkers ( 7 j. For the conjugated triene acid, absorptivities were taken from the work of Brice and Swain (3,41,these being values determined on samples of much higher purity than ordinarily available. The absorptivities of the reference compounds at the analytical wave lengths are shown in Table I.
A rapid method for the determination of polymerized styrene in styrenated fatty acids and alkyd resins has been made desirable by the increased use of styrene for modifying alkyd resins. A n ultraviolet spectrophotometric method has been developed, which makes corrections for the overlapping absorption of conjugated triene fatty acids and residual monomeric styrene. The method as tested on a variety of styrenated fatty acids is superior in both speed and accuracy to the chemical method based on saponification value.
T
HE increased use of styrene for modifying alkyd resins has made desirable a rapid method for determining the polymerized styrene content of either the styrenated fatty acids used in manufacture or the filtrate residue from the Kappelmeier separation of the alkyd resin ( 1 ) . Prior to 1953 the principal approach was that proposed by Kappelmeier ( 6 ) , which was based on saponification value ratios. Since ultraviolet spectrophotometry had been successfully applied to such related problems as the determination of phthalic acid (9) in the precipitate from the modified Kappelmeier procedure (I), the determination of conjugated polyene acids in fatty acids ( 2 , Sj,and the determination of monomeric Etyrene in polystyrene ( 7 , 8 ) , it seemed plausible that a spectrophotometric procedure might be developed. based on the strong absorption of polystyrene near 269 mp, if suitable corrections could be applied for other absorbing materials present. Since the completion of this spectrophotometric method, a noninstrumental method has been described by Swann (11). These methods have recently been reviewed (IO).
Table I.
Absorptivities of Reference Conipounds
.inalytical wave lengths, 11111 Polystyrene Conjumated trienr acid hlonoGeric styrene
2CO
1.61 214
8.27
282 0.0342
23 1 0.0175 8.50
166.9
7.30
5.64
BASIS OF METHOD
Spectroscopically, polystyrene may be regarded as a monosubstituted benzene. I n either the homopolymer (polystyrene) or in a copolymer the chromophoric grouping is the benzene ring attached to the polymer chain, and the absorption wave length is essentially unaffected h y the details of the composition of the polymer chain. The absorption intensity a t 269 mp thus gives a measure of the number of benzene rings present regardless of the exact composition of the polymer chain. The term “polymerized styrene” is here used to cover both true homopolymer and any copolymers or interaction products present. I n styrenated fatty acids or oils and in the Kappelmeier filtrate residue, the possible interfering materials are the unreacted monomeric styrene and the conjugated polyene fatty acids or oils. The spectra of polystyrene, monomeric styrene, and conjugated diene and triene acids in cyclohexane solution are shown together in Figure 1. I n the 260- to 300-mp region the absorption of the conjugated diene fatty acid is so low that its contribution may be ignored, even though it may be present in appreciable concentrations. The amounts of conjugated tetraene acids that are present in commercial fatty acids ( 2 , 3) and oils are so low that their contribution is negligible. Thus only conjugated triene acids and monomeric styrene need be considered as interfering materials. Although these both have considerably higher absorptivity than polystyrene, their concentrations are relatively very much lower, and consequently their net contribution to the observed absorbance is smaller but appreciable.
200
250 Figure 1.
------.-.. . .. .. -
Monomeric st>rene (S, Polvmerir 4t)rene (P, Coijugated diene acid (2, Conjugated triene acid (3,
Present address, Department of Chemistry, Kansas S t a t e College, M a n h a t t a n Kan. 1
226
350
300 Ultraviolet spectra )
.)
MI_I
V O L U M E 21, NO. 2, F E B R U A R Y 1 9 5 5
227
Using Beer's law in its general form, a t a given wave length,
Aobsd.= b Z a&c,
(1)
a
where A is the observed absorbance a t the stated wave length, b is the cell length in millimeters, a, is the absorptivity of component i, and c i is the concentration (in grams per 100 ml ) of component i. There also exists the relation that cI = c, X C, where cB is the Concentration of the sample in solution (in grams per 100 ml.) and C, is the relative concentration, or fraction, of component i. The nine absorptivity values of Table I may be fitted into three simultaneous equations for the observed absorbances a t the three selected analytical Tvave lengths where the =
r,b (0.0175 C,
.4$iqd = csb (0.0342 =
C p
c,b (1.61 C,
+ 8.50 Ct + 5.64 C,) + 166.9 Ct + 7 30 Cm) + 214 Ct + 8.27 C,)
-
- 0.310 C p ]
BY BP Kappelmeier Kappelmeier Sapon. By spectro(actual sapon. (190sauon. F a t t y d c i d Value Preparation i~liotometry value) value) Saturated 289.7 0.0 0.25 0.0 - .52. 9.5 10 I 12.3 -33. 30 4 X0.ii 30 8 -.LO .I8 3 48.0 51.5 2R.0 - 1 .ti 0.0 -6.0 Soybean 201.4 0 0 8 . $1 7 , ii 7.7 2.2 28.4 2i.o 27.0 22.6 44.0 48.(i 44.3 41.0
I.insPd
206.5
p.0 n
(i
1.0
0.0
7 3
3 8
-9.0 -4.0
16.8
(4)
This equation may be applied to data in this form, unless the analyst desires t o re-evaluate some or all of the absorptivities given in Table I on his available instruments and Kith samples of his own choosing. It may he seen in application that Equation 5 acts as a onecomponent analytical equation with one negative and one positive correction term, and that A 2 6 9 contributes the largest amount, with the 1.293A~~2 term considerably smaller correction, and the 0.207A291 term even less. Fortunately, the two correction terms virtually cancel out a constant or slight rising (toward shorter wave lengths) background. If the A291 reading becomes appreciable in respect to the A269 and AB2 readings, it generally signifies an unusually high concentration of residual monomeric styrene, and Equations 2, 3, and 4 may be solved to permit an estimate of thp monomeric styrene concentration to he made. This equation is
[ctb (19.64 A;!$d -
P w Cent Polystyrene
ny iiltraviolet
(2) (3)
subscripts p , t, and nz represent polymerized styrene, conjugated triene acids, and monomeric styrene, respectively. As it is the concentration of polymerized styrene in the original sample that is desired, and this is the same as C,, above, the equations may be solved for C,, and the numerical terms rearranged and combined, giving
C, = 0.00967
Table 11. Polymerized Styrene Determinations
(6)
,4n estimate of the conjugated triene .acid concentration, C , may be obtained by substituting values of C, and C,, obtained from Equations 5 and 6 into any of Equations 2, 3, or 4, and solving for C t . &PP&RATUS
The data on standards and test'samples were obtained by use of a Cary automatic recording spectrophotometer, Model 11, No. 67, using 1-, lo-, 20-, and 50-mm. quartz cells and micrometer Baly cells ( 5 ) which were adjustable from 5.00 to 0.02 mm. in length. Any spectrophotometer capable of measurements a t the analytical wave lengths with reasonably narrow slits may be used, and the sample concentration and cell length can be adjusted t o suit the preference of the analyst. Purified cyclohexane was used as the solvent throughout. TEST SAMPLES
In order to test the spectrophotometric method, specially prepared samples of styrenated fatty acids were made up. Care was taken to obtain g o d material balance, and escaping vapors were trapped and analyzed spectrophotometrically for monomeric styrene ( 7 ) . Fatty acid samples were selected to give a variety of commercially available acids, with saturated, random diene (soybean), conjugated diene (castor), and random triene (linseed) acids represented. These were made up to contain roughly 10, 30, and 50Yo of polymerized styrene. Styrene was polymerized in the presence of the fatty acid, using di-terf-butyl per-
oxide as the catalyst, a t 170" C. for 3 to 5 hours. At 200" C the mixture was bloan A ith nitrogen and the volatile fraction \yas collected and analyzed spectrophotometrically for monomeric styrene ( 7 ) ; this value was used as a correction to the composition by preparation. These samples were also analyzed by the method proposed by Kappelmeier (6). Here it was assumed that all the acids mere CI8acids and that the saponification value of the acids alone would he 190. Polystyrene acts as an inert diluent and may be estimated from the ratio of the saponification value of the stvrene-fatty acid mixture to that of the fatty acids alone. I n addition, to ensure optimum conditions for this approach, the saponification value determined for each acid (instead of an arbitrary 190) was also used to estimate the styrene content. These data are summarized in Table 11, where the polymerized styrene content by preparation, by ultraviolet spectrophotometry, and by the corrected and uncorrected Kappelmeier saponification value methods gre compared. Data are also included for fatty acid samples which had not had styrene polymerized in their presence. DISCUSSION
Examination of Table I1 shov s considerably better agreement between the preparational and the ultraviolet data than between the preparational and the corrected Kappelmeier saponification data; the uncorrected Kappelmeier saponification data are comparable only when the saponification values are near 190 and then only a t the higher polystyrene concentrations. Thus the spectrophotometric method may readily be applied as a direct method to samples of unknoxn qualitative composition for which the saponification value of the original fatty acid component is not known and cannot be obtained. The effectiveness of the correction term for the conjugated triene acids is sh0R-n by the agreement within 2% of the preparations and the ultraviolet spectrophotometric data on the unreacted fatty acids, where the only absorption present is due to the conjugated triene acids ( 2 , 3). However, an attempt to use the ultraviolet spectrophotometric method on styrenated tung oil acids was unsuccessful; the v m - high concentrations of conjugated triene acids encountered could not he handled b>- the correction terms of the equation. rilthough styrenated tung oil acids are not generally considered to be economically important and are unlikely to be encountered, their presence would be a t once apparent bv the appearance of the spectrum, n-hich would no longer resemble that of polystyrene. A possible means of handling samples with very high conjugated triene acid concentrations may result from the destruction of the conjugated unsaturation by halogenntion or hydrogenation of the sample prior to examination. To test the limit of effectiveness of the correction trrm for conjugated triene acids, mixtures xvere made of tung oil acid and styrenated linseed acid, and the samples were analyzed spectrophotometrically. The data 'ire shown in Table I11
228
ANALYTICAL CHEMISTRY
It appears that the conjugated triene correction term is effective up to about 6% conjugated triene acids in the presence of about 20% polymerized styrene. The error in the amount of polymerized styrene increases as the differences between the terms of Equation 5 become of the same order as the photometric error and as the contribution of the trace amounts of conjugated tetraene becomes appreciable; the absorptivity of the conjugated tetraene acid (2, 3) is lo4 greater than that of polystyrene a t 291 mp.
Table 111.
filtrate is concentrated, transferred to a separatory funnel, and extracted with ether. The ether layer, which contains polystyrene, fatty acids, and interaction products of these, is evaporated to dryness. A sample of this fraction is weighed and dissolved in cyclohexane for spectrophotometric examination. ACKNOWLEDGMENT
The authors wish to acknowledge the assistance of William G. Deichert with sample preparation and separation and of Mrs. E. S.Davis and John G. Koren with the spectrophotometric work.
Effect of Conjugated Acids
Per Cent Conjugated Sample Triene Tetraene Description acids acids Linseed 0.12 0.040 19 : 1, linseed-tung 2.3 0,05! 6:1,linseed-tung 6.2 0.0711 3:1,linseed-tunc: 1 1 . 8 0.103
Per Cent P o l y s t y r e n L BY By ultrar.iolet preparation sp~ctiopliotornetry 25.0 23. 1 21.3 23.3 2 2 . ii 19.3 20.2 13 7
APPLICATION TO ALKYD RESINS
The details of the application of this spectrophotomrtric method to the analysis of resins have been described bj- Stafford and coworkers (10) and are summarized here. The ,\ST111 method (1)is applied to the alhvd resin as received. The sample is saponified and the dibasic acids are isolated hv prwipitation a9 insoluble potassium salts, in Lvhich the phthalic acid ma! 1)c determined by the ultraviolet spectrop3otometric Shreve method (9). The filtrate is neutralized with concentrated hydrochloric acid, and the precipitated potassium rhloride is filtered oft. The
LITERATURE CITED
( I ) Ani. SOC.Testing Materials, Method D 563. (2) Bradley, T. F., and Richardson, D., Ind. Eng. Chem., 34, 237 (1942). (3) Brice, B. -4.,and Swain, AI, L., J . Opt. SOC.Amer., 35, 53% (1945). (4) Brice, B. A., and Swain. 31. L., Oil & Soap, 22, 219 (1945). (5) Hirt, R. C., and King, F. T., Ax.ir.. CHEM.,24, 1545 (1952). (6) Kappelmeier, C. P. A , and Van der Seut, J. H., Chem. Weekblad, 47, 157 (1951). (7) JIcGovern, J. J.. Grim. .J. AI., and Teach, W. C., AN.AI..CHEM., 20,312 (1948). (8) Yewell, J. E.,Ibid., 23, 445 (1951). (9) Shreve, 0. D., and Heether, AI. R., Ibid., 23, 441 (1951). (10) Stafford, R. TI-,, Hirt, R. C., and Deichert, W.G., Division of Paint, Plastics, and Printing Ink Chemistry, 126th MeetNew York, 1954. ing, .‘hERIC.AN CHEMICAL SOCIETY, (11) Swann, 31. H.. . ~ x . A L . CHEM.,25, 1735 (1953). (12) Witschonke, C. It., Ibid., 24, 350 (1952); 26, 562 (1954). R E C E I V E for D review August 25, 1951. Accepted November 9, 1954. Presented before the Pittsburgh Conference on .4nalytical Chemistry and h p plied Spectroscopy, March 5 , 1951.
Far Ultraviolet Absorption Spectra of Aromatic Hydrocarbons L.
C. JONES, JR., and
L. W. TAYLOR
Wood River Research Laboratory, Shell O i l Co., Wood River, 111. A survey of the anal) tical potentialities of absorption spectroscopy in the far ultraviolet region has been initiated. In the first phase of this program the far ultraviolet (1700 to 2300 A.) spectra of 69 pure hydrocarbons have been measured either in the vapor or solution state with a recording vacuum spectrometer, employing a 1-meter concave grating and a photoelectric detector. Experimental methods are described and spectra of representative paraffins, naphthenes, cyclic and noncyclic mono-olefins, allenes, conjugated and nonconjugated diolefins, acetylenes, benzenes, naphthalenes, and polycyclic catacondensed aromatics are presented. Useful correlations between the spectra and structure of these hydrocarbons are discussed. It is concluded that far ultraviolet absorption spectroscopy is a promising new tool for the analyst.
A
BSORPTION spectroscopy in the near ultraviolet (2100 to 4000 A,) is one of the most widely used physical methods of
analysis. Its popularity can probably be ascribed to two factors: the large number of organic compounds with characteristic absorption in this region, and the availability of well-designed, reliable, and relatively inexpensive commercial instruments. The value of the method in the analysis of petroleum fractions is enhanced by the existence of extensive compilations of accurately measured and uniformly presented spectra of pure hydrocarbons.
This’is in marked contrast to the present status of absorption spectroscopy in the far or vacuum ultraviolet region. Despite a half century of academic research in far ultraviolet spectroscopy, the technique has not, t o the authors’ knowledge, been applied by the analyst. The lack of adequate instrumentation has undoubtedly been a large factor in delaying such application. Until very recently intensity measurements in this region have depended upon photometry with special photographic emulsions of very poor reproducibility. Consepently, emphasis in the early research was largely directed toward accurate meamrement of absorption band positions and upon calculation of molecular energy level*. The intensity data obtained in these studies were not of the degree of accuracy required in analytical applications Recent advances in photographic photometry in the far ultraviolet have improved this situation considerably, but the fact remains that most of the published spectra are of limited value to the analyst. The qualitative and semiquantitative features of these spectra suggested, however, that the far ultraviolet region had potential utility in the analysis of hydrocarbons. The photographic measurements of Carr and Pickett and their collaborators at Mt. Holyoke College (1, 12, 19, 35) on the aliphatic olefins, diolefins, and cyclic unsaturates, and of Platt, Klevens, and others a t the University of Chicago on the aromatics (16, i7, 21, 23) and other unsaturated hydrocarbons (14, 18, 24, 26) are of special interest in this connection. Measurements of analytical accuracy in the far ultraviolet
