Spectrophotometric Determination of Total Phthalic Anhydride In Alkyd Resins and Other Phthalate Esters 0 . D . SHREVE AND M.R, EIEETHER Marshall Laboratory. E . I . dir Pont de N e m o u r s & Co., Znc., Philadelphia, Pa.
The standard Kappelmeier procedure for the determination of total phthalic anhydride in alkyd resins and other phthalate esters cannot be applied in the presence of esters of other poly carboxylic acids, many unsaponifiable resins, and other materials used in the formulation of modern surface coatings. The work described was undertaken with a view to fulfilling the widespread need for a more specific and generally applicable method for this type of determination. A new method based on ultra\ iolet absorption spectrophotometry and possessing a much wider range o f application than the Kappelnleier method was developed. Accurate results are rapidly obtained from a single absorption measurement at 276 nip on an acidified aqueous solution of the usual Kappelmeier precipitate. The method should prove especially useful in laboratories concerned with the analvsis of surface coatings and related materials.
THE
well known Kappelmeicr method ( 3 ) and its various niodifications have for many years represented the most mtisfactory means for estimat,ing the total phthalic acid (or anhydride) content of alkyd resins and other phthalate esters. Unfortunately, however, this method fails completely in the presence of any material that is insoluble in anhydrous alcoholic alkali or that yields a saponification product insoluble in this niedium. Because such materials include monomeric and polynieric esters of practically all other polycarboxylic acids, unwaponifiable or partially saponifiable resins, pigments, etc., this liniitatiori represents a serious hnndicap in the nnalysie of paints, varnishes, and other types of costing materials. Thcl iiecd for a more specific method for phthalic as well as generally improved techniques in the analysis of present-day coating materials has been emphasiied in several recent publications ( 2 , 6, 6). When the Kappelmeier precipitate contains only potassiuni phthalate and completely water-insoluble material, the phthalic coritt:nt can sometimes be efitimated by weighing the dried residue after aqueous extraction of the precipitate. It is also possible in sonic caws t o obtain an approximate estimate by heating the precipitate containing the phthalic as the monoalcoholate to constant weight a t elevated temperature and assuming the weight loss to be due to volatilization of the alcohol of crystallization. These procedures, however, have obvious limitations and fall far short, o f filling the need for a more specific itrid reliable method for the phthalic determination. Swanri in a recent publication (6) has described methods for drtermining phthalic, sebacic, fumaric, maleic, succinic, and adipic acids in alkyd resins. His procedure for phthalic acid is tht. only published method claimed to be directly applicable in the pn’sence of other dibasic acids. The method requires preliminary isolntion of the dibasic acids as dry solids by acidification and evaporation of the dissolved Kappelmeier precipitate; phthalic acid is then precipitated as the lead salt from glacial acetic acid. While this method is useful, it is time-consuming and suffers from the disadvantages inherent in procedures involving precipitation of lead salts. By comparison, the ultraviolet spectrophot,onietric method is much more rapid and generally satisfactory. Swann’s methods for the other dibasic acids mentioned above probably represent the best now available, although more recent studies ( 4 ) suggest the applicat,ion of partition chromatography t o this general problem.
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WAV€LENGTH-MILLIMICRONS
Figure
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1. Ultraviolet Absorption Spectrum Potassium Phthalate Monoalcoholate
of
In distilled water, phthalate ion 0.0469 gram per liter In aqueous 0.1 N hydrochloric acid, phthalic acid 0.0500 gram per liter
In the present investigation the ultraviolet spectrophotometric method has been shown t o yield excellent results in determination of the phthalic content of alkyd resins and other phthalate esters, alone or in the presence of a variety of esters of other polybasic acids, including those studied by Swam ( 6 ) . It has also been successfully applied to this type of determination in the presence of other coating materials that interfere with the Kappelrneier procedure. 44 1
ANALYTICAL CHEMISTRY
442 Table I.
Absorptivity of Potassium Acid Phthalate in 0.1 N Hydrochloric Acid at 276 mp
Concn., G./L. Phthalic Aeid 0.0246 0,0344 0,0492 0,0640 0.0836
Absorptivity
7.84 7.76 7.73 7.74 7.76
BASIS OF SPECTROPHOTOMETRIC METHOD
Phthalic acid and its esters show strong absorption in the ultraviolet region of the spectrum, this absorption arising from electronic transitions accompanying resonance in the aromatic ring present. In some types of mixtures it should be possible to determine the phthalate ester content by direct absorption measurements on a solution of the mixture in a suitable ultraviolet transparent solvent. In the case of paint vehicles and other types of coating materials, however, such a procedure would be impractical, because the organic solvents, oils, and other materials usually present would almost invariably interfere. Fortunately, however, application of the Kappelmeier saponification procedure followed by aqueous extraction of the precipitate obtained offers a simple and effective means for completely eliminating this difficulty in moat cases and greatly minimizing the probability of interference in all cases. This is true because relatively few of those materials likely to appear in the Kappelmeier precipitate possess the simultaneous properties of water solubility and strong ultraviolet absorption under the conditions selected for the phthalic acid determination. Figure 1 shows the ultraviolet absorption spectrum of aqueous potassium phthalate monoalcoholate (distilled water being used in the reference cell) and that of the same compound in 0.1 A’ hydrochloric acid, with 0.1 N hydrochloric acid in the reference cell. “Absorptivity” in Figure 1 is equivalent to specific extinction coefficient in the older nomenclature-Le., absorptivity
-
log I d 1 Concn. in grams per liter X cell length
In the case of the neutral salt, concentration of phthalate ion was used in calculating the absorptivity values; in the case of the acidified salt, concentration of phthalic acid was used. Because the inherent absorption of the free acid (acidified salt) is greater than that of the phthalate ion (neutral salt), a method based on the absorption of the acidified salt was adopted. Exact acid concentration is not critical in the analytical procedure but should be held constant a t about 0.1 Although phthalic acid shows stronger absorption a t shorter wave lengths, the wave length of the absorption maximum at 276 mp was selected for the analysis for the following reasons: Unsaturated dibasic acids such as maleic, fumaric, and itaconic show strong absorption a t shorter wave lengths. Even the saturated dibasic acids, as commercially obtained, may show some absorption at shorter wave lengths but negligible absorption in the region selected. In general, the probability of interference from small amounts of unsaturated water-soluble organic materials which might be extracted withthe phthalate salt is greater a t shorter wave lengths. The absorptivity of phthalic acid a t 276 mp (7.73 under the conditions employed) is sufficient to ensure accuracy and precision comparable with those of the Kappelmeier method, not only in the case of straight phthalate esters but in the presence of large amounts of a number of materials that interfere with the latter method. The adherence of phthalic acid (as acidified potassium acid phthalate) to Beer’s law is demonstrated by the data of Table I. MATERIALS AND APPARATUS
Spectrophotometer. All absorption measurements were made using a Beckman Model DU quartz ultraviolet spectrophotom-
eter, housed in a constant temperature room maintained a t 25 O C. Matched fused silica absorption cells 1 cm. in length were used. Calibration Standards. Baker’s C.P. potassium acid phthalate and Eastman &phthalic acid (purified by recrystallization) gave identical results in the calibration procedure described below ( a p = 7.73 under the conditions employed). Polycarboxylic Acids and Esters. The polycarbox lic acids used in preparing the synthetic acid mixtures were tge purest commercial grades obtainable. With the exception of phthalic acid, they were used without further purification. The alkyd resin used was an oil-modified glyceryl phthalate type especially prepared for this investigation. The monomeric phthalate esters and dibutyl maleate were the purest obtainable commercial grades. The adipate and sebacate polyesters were commercially available plasticizing resins. The various monomeric saturated esters were pure samples kindly supplied by H. B. Knight of the Eastern Regional Research Laboratory. The polystyrene and polyvinyl chloride-acetate samples were commercial grades and the styrenated alkyd was synthesized for the purpose. ANALYTICAL PROCEDURE
Calibration. Calibration consists of determining the absorptivity (specific extinction coefficient) of pure phthalic acid in approximately 0.1 N hydrochloric acid a t 276 mp on the spectrophotometer. Because potassium acid phthalate is obtainable in a high state of purity, the acidified solution of this salt is a convenient standard. Weigh accurately about 0.12 gram of pure potassium acid phthalate into a calibrated 1-liter volumetric flask, dissolve, add 10 ml. of concentrated hydrochloric acid, and dilute to the mark with distilled water. Measure 50 ml. of this solution from a buret into a 100-ml. volumetric flask and dilute to the mark with approximately 0.1 N hydrochloric acid (prepared by diluting 10 ml. of concentrated hydrochloric acid to 1 liter with distilled water). Measure the absorbance (optical density) a t 276 mp against 0.1 N hydrochloric mid (10 mi. of concentrated hydrochloric acid per liter) as blank, using matched 1-cm. silica or quartz absorption cells. Repeat the measurement with the hydrochloric acid solution in the cell previously used for the sample, and sample solution in the cell previously used as blank. Average the measurements to obtain the absorbance, A , and calculate the absorptivity of phthalic acid, a p ,from the formula:
where C, is concentration expressed as grams per liter of phthalic acid (not salt) present in the final diluted solution One calibration suffices for all subsequent analyses unless conditions are changed. Procedure for Analysis of Samples. Accurate1 weigh into suitable reflux flasks three samples of the paint velicle or other phthalate-containing material, using a sample size expected to yield the equivalent of about 0.1 gram of phthalic acid on saponification. Add 5 ml. of benzene and 35 ml. of 1 N absolute alcoholic potassium hydroxide to each flask, attach to a reflux condenser, and reflux 1.5 hours. From this point on follow the standard A.S.T.M. procedure ( I ) , or other approved procedure, for isolating and drying the precipitate, which will contain the phthalic as dipotassium phthalate or the monoalcoholate (depending on the drying procedure used), together with any other materials present which are insoluble in the saponification medium. Unless the salt of an interfering acid is present (see below), the precipitate need not be weighed. Evtract the soluble salts thoroughly from one of the three precipitates by passing about 200 ml. of distilled water through the crucible with suction. To the aqueous extract in the suction flask, add 10 ml. of concentrated hydrochloric acid. If sebacic acid or other slightly soluble acidic material is present, a precipitate or cloudiness may appear a t this point. If the acidified solution is perfectly clear, transfer with rinsing to a 1-liter volumetric flask and dilute to the mark mith distilled water; if, however, even a slight cloudiness is evident in the acidified extract in the suction flask, filter the entire solution with suction through No. 42 Whatman filter paper on a Buchner funnel, wash the residue with about 100 ml. of distilled water, transfer the combined filtrate and washings to the 1-liter volumetric flask, and dilute to the mark with distilled water. After thorough mixing, introduce from a buret a 50-ml. aliquot of the diluted extract into
443
V O L U M E 2 3 , NO. 3, M A R C H 1 9 5 1 a 100-ml. volumetric flask and dilute to the mark with 0.1 hydrochloric acid (10 ml. of concentrated hydrochloric acid per liter). Measure the absorbance a t 276 mp against a 0.1 N hydrochloric acid blank, using the technique specified under calibration above. For maximum accuracy, the absorbance of the final solution measured should fall in the vicinity of 0.4 absorbance unit.
If the absorbance lies b e t w e n 0.35 and 0.45, dissolve the other tJTo Kappelmeier precipitates and proceed exactly as in the case of the first. Calculate the phthalic acid concentration in grams per liter, C,, of each of the final diluted solutions from the formula:
c,
=
A UP
where A is the measured absorbance and u p is the absorptivity value for pure phthalic acid as determined in the calibration procedure above. Calculate the per cent of total phthalic anhydride in the solid portion of the original sample from the following formula: 70
P.A. =
89.16 C, X aliquot factor
wxs
(3)
where W is sample weight and S is fractional solids content of the original sample if a volatile solvent was present. Average the three values obtained. If the absorbance of the final diluted extract from the first sample lies outside the 0.35 to 0.45 range, calculate the approximate phthalic acid concentration from Equation 2. With this knowledge a t hand, extract the other two precipitates (filtering after acidification if necessary), and adjust the volume to which the evtract is diluted or the aliquot taken for final dilution or both, so as to obtain absorbance values in the desired range. In order to minimize dilution error, the volume of the original
solution in each case should be such that not more than a fourfold final dilution is required.
If a mixture is being analyzed, the per cent phthalic anhydride calculated from Formula 3 can, of course, be converted to equivalent phthalate ester content by applying the appropriate factor. Use of Alcohol for Final Dilution. When filtration of the acidified extract is necessary, small amounts of insoluble matter may pass through the filter in some cases. An apparently clear acidified estract may sometimes contain very small amounts of suspended matter not readily detected. The presence of such material will cause error in the absorbance measurement due to scattering of the ultraviolet radiation. The difficulty can usually be eliminated by using 0.1 N hydrochloric acid in absolute synthetic methanol, or a spectroscopic grade of ethyl alcohol, in the final dilution of the aqueous aliquot taken. In fact, users of the method may wish to adopt the use of alcohol a t this point as constant standard practice, in order to minimize the possibility of scattered radiation error. When alcohol is used, the potassium biphthalate solution and blank used for calibration as well as the blank used in the sample measurement must be adjusted to the solvent composition present in the sample solution. Presence of Interfering Acids. The method cannot be directly applied in the presence of acids showing strong ultraviolet absorption a t 276 mp under the conditions of the analysis. However, in the presence of one interfering acid, which exhibits only moderate absorption a t this wave length, such as maleic or fumaric, accurate results can be obtained as follows:
Determine the absorptivitiy, a,,of a pure sample of the interfering acid (suitably diluted in 0.1 N hydrochloric acid) a t 276 mp; then apply the method exactly as outlined ahove, but calculate C, from the following pair of simultaneous equation; rather than from Equation 2: Table 11. Analysis of Synthetic 4cid Mixtures ( I ) a,C, UiCi = '4 (4) Composition of Mixture Phthalic Found, Wt. yo
Phthalic, wt. yo Succinic, wt. % 1 2 3 4 5
5.74 25.82 51.47 75.91 95.60
1 2 3 4 5
5.12 27.49 50.21 75.46 94,72
1 2 3 4 5
6.43 26.14 50.56 74.92 96.47
1 2 3 4 5
5.79 25.33 52.66 75.10 95.28
1 2 3
97.79 92.73 84.86
94.26 74.18 48.53 24.09 4.40
+
Error,
AV.
%
5.80 25.83 51.55 76.10 95.38
2 5.90 25.75 51.63 76.04 95.46
5.85 25.79 51.59 76.07 95.42
+ o . 11 -0.03 +o. 12 +0.16 +o. 18
5.22 27.29 50.29 75.17 94.44
5.36 27.33 50.19 75.27 94.60
5.29 27.31 50.24 75.22 94.52
+0.17 -0.18 +0.03 -0.24 -0.20
6.65 26.40 50.27 74.65 46.28
6.53 28.22 50.35 74.69 96.12
6.59 26.31 50.31 74.67 96.20
+ O . 16 $0.17 -0.25 -0.25 -0.27
5.62 25.44 52.66 75.00 95.07
5.67 25.53 52.77 74.90 95.03
5.65 25.49 52.72 74.95 95.05
-0.14 4-0.16
97.78 92.62 84.81
97.90 92.46 84.69
97.84 92.54 84.75
4-0.05
98.40 93.56 84.79
98.24 93.74 84.79
98.32 93.65 84.79
+0.30 -0.06 -0.38
1
FpCp
Sebacic 93.57 73.86 49.44 25.08 3.53
Itaconic 94.21 74.67 47.34 24.90 4.72
The above pair of equations assumes complete solubility of the interfering acid on acidification of the dissolved potassium salts.
+0.06 -0.15 -0.23
Maleic 2.21 7.27 15.14
-0.19 -0.11
RESULTS AND DISCUSSION
Fumaric 1 2 3
98.02 93.71 85.17
1.98 6.29 14.83
Table 111.
1 2 3 4
Sample Oil-modified alkyd resin Dimethyl phthalate Dibutyl phthalate Dioctyl phthalate
Analysis of Phthalate Esters Phthalic Anhydride 7 (Kappelmeier Metgod7 1 2 Av.
35.85 75.48 53.24 38.40
35.95 75.62 53.18 38.42
35.90 75.55 53.21 38.41
Phthalic Anhydride, o/ (Spectrophotometric Metgod) 1 2 Av. 35.98 75.60 53.17 38.57
36.06 75.60 53.21 38.81
36.04 75.60 53.19 38.69
(5)
where a p t u r , and A have been previously defined; Ci is the concentration of interfering acid in the final solution measured; F , and Fi are the factors which convert phthalic acid and interfering acid, respectively, to their potassium salt equivalents as they occur in the Kappelmeier precipitate; R is the weight of the Kappelmeier precipitate; and f is the aliquot factor involved in the procedure.
Adipic 94.88 72,51 49.79 24.54 5.28
+ FiCi = B-f
E
~ o/, ,I
+0.14 f0.05 -0.02 +0.28
~
Before the method was applied t o esters, it was evaluated on synthetic mixtures of phthalic acid with various other polycarboxylic acids. All analyses were made in the pres~ence~ of ,approximately 0.1 N hydrochloric acid, the technique being identical with that described above for the aqueous extracts of the Kappelmeier precipitates. In the case of
ANALYTICAL CHEMISTRY
444
salt is obviated and the probability of error due to small P.A. Found, %, amounts of unsuspected maP.A. (Spectrophotometric) Error, terials which may appear in this Present, yoa 1 2 A v . % Sample precipitate is minimized. Furthermore, the method is free from the so-called “carbonate error,’’ characteristic of the 1 33.71 33.71 33.51 33.61 - 0 10 Oil-modified alkyd Kappelmeier procedure, be2 23.82 23.51 23.68 23.60 - 0 22 sebacate polyester t0.18 18.12 17.94 18.29 17.95 3 cause potassium carbonate does 4 7.76 7.99 7.85 7 92 +0.16 5 3.16 3.27 3.57 3 42 +0.26 not interfere. It will probably 1 68.21 68.72 68.51 68.62 4-0.41 be possible to eliminate or Dimethyl phthalate 52.42 52.42 52.20 52.31 -0.11 dimethyl succinate 2 greatly shorten the drying time 39.81 -0.05 39.86 39.76 39.86 3 4 21.32 21.30 21.04 21.17 -0.15 required for the Kappelmeier 5 5.27 5.51 5.31 5.41 +0.14 precipitate, although the dry1 6 8 . 2 6 6 7 . 9 1 68.11 6 8 . 0 1 0 . 2 5 Dimethyl phthalate 2 51.51 51.66 51.42 51.54 f0.03 ing step is included a t present dimethyl sebacete 35.86 35.72 36.79 -0.13 3 35.92 before extracting with water. -0.21 21.77 4 21.98 21.60 21.94 5 6.04 5.76 5.99 5.88 -0.16 The value of ai (absorptivity 71 69 -0.OA 71.60 71.79 1 71.74 Dimethyl phthalate of maleic acid) used in calculat 65 20 -0.21 65.07 65.33, 2 65.41 dibutyl maleate f0.27 61.24 61 00 60.94 3 60.82 ing results on the phthalate+0.09 67.33 67.24 67.15 1 67.15 maleate mixtures (Table IV) Dimethyl phthalate + 0 . 2 2 5 4 . 2 4 5 4 . 3 2 54.40 2 54.12 dimethyl sdipate was obtained by saponifying -0.21 36.24 36 33 36.42 3 36.54 +O. 18 21.08 21.00 20.91 4 20.82 dibutyl maleate under the con+0.33 5.32 5 26 5.19 5 4.93 ditions of the analytical proa Calculated from phthalic anhydride content of phthalate ester present as determined b y Kappelmeier method. cedure, dissolving and acidifying a weighed sample of the precipitated salt, and measuring the absorbance after suitable dilution. Measurement of a, mixtures containing slightly soluble acids such as sebacic, the in this manner, rather than on a solution of pure maleic acid, solutions were clarified when necessary, before making the abminimizes the possibility of error which may rise from isomerisorbance measurements. Equations 4 and 5 were used in calculatzation of some maleic to fumaric in the saponification process, ing results on the phthalic-fumaric and phthalic-maleic mixtures. Such error should be small, as the absorptivity values for maleic Results on the acid mixtures studied are given in Table 11. Table and fumaric acids at 276 mp are low and do not differ greatly. I11 shows the results obtained on an oil-modified alkyd resin and Results on the phthalic-maleic acid mixtures (Table 11)indicate three monomeric phthalate esters. Table IV gives results for no apparent decrease in accuracy up to 15% maleic. Mixtures mixtures of phthalate esters with those of other polycarboxylic containing larger proportions of maleic can be analyzed, but acacids. Tables V and VI illustrate the applicability of the method curacy will decrease with increasing maleic content. In geneml, t o samples containing styrene and vinyl resins. maleic acid as such is seldom used in large amounts in finishes; furthermore, when present it is oftmen“adducted” to drying oils, rosin, etc., in which case little if any potasaium maleate will apTable V. Analysis of a Styrenated Alkyd and ilkydpear in the Kappelmeier precipitate. Polystyrene Blends As indicated in Tables V and VI, the method is not limited to PA. P.A. Found, % present, (Spectrophotometric) ester mixtures. The Kappelmeier precipitate from a styrenated 1 2 Av. Error Sample % alkyd resin or an alkyd-polystyrene blend always contains polyOil-modified alkyd styrene; t,hat from a phthalate plasticized vinyl resin contains a + polystyrene 26.69 26.54 -0.16 1 26.70a 26.39 mass of vinyl degradation products and potassium chloride. In 29.71 29.51 -0.29 29.91 2 29.805 the case of the styrene types the phthalic can be estimated by a Styrenated alkyd 27.95 27.79 27.64 1 29.37b modification of the Kappelmeier method; such an estimate in the Caloulated from phthalic anhydride content of alkyd as determined presence of vinyl resins, however, is very difficult to obtain by by Kappelmeier method. b Synthetic value. conventional methods. In either case reliable results are rapidly obtained by the new method. Table VI. Analysis of Vinyl Resin-Phthalate Ester Blends The various acids thus far studied include most of the type% P.A. Found, % likely t,o be encountered in analysis of iinishes. In addition to p&!it, (Spectrophotometric) those for which analytical data are given, the authors have measSample % 1 2 Av. Error ured the absorption of diglycolic, oxalic, tartaric, and citric acids. Polyvinyl chloride acetate + dibutyl These measurement,s indicate that none of the* will cause seri13.44‘ 13.85 13 57 13.71 f O 27 phthalate ous int,erference. Although only three acids in the saturated diPolyvinyl chloride acetate + dioctyl basic series have been studied (those more commonly used in 9.54“ 9.16 9.33 9.24 -0.30 phthalate finishes), it is safe t’o predict that saturated dibasic acids as a 6 Calculated from phthalic anhydride content of phthalate ester present aa determined by Kappelmeier method. class will not interfere. Most aromatic polycarboxylic acids will interfere seriously, becauw they appear in the Kappelmeier precipitate and absorb strongly in the ultraviolet. Although few aromatic polycarboxylic acids other than o-phthalic are as yet It is evident from the data that accuracy and precision are widely used in finishes, such acids as terephthalic, iaophthalic, and good in all casea. While the chief advantage of the method is methylene disalicylic may occasionally be encountered. Tererealized when it is applied to mixtures that cannot be analyzed phthalic acid can be largely removed owing t o its very low water by the Kappelmeier procedure, it also offers some advantages solubility, but the low concentration remaining will still cause inover the latter procedure in the case of a straight alkyd resin or terference. Other types of interfering materials will occasionally other straight phthalate ester. The somewhat controversial be encountered and separation of these may be required. question as to the best weighing form of the precipitated phthalate Tahle IV.
+
+ +
+
+
Analysis of Synthetic Ester Mixtures (I)
V O L U M E 23, NO. 3, M A R C H 1 9 5 1 In the case of complete unknowns, the original sample and/or the aqueous extract from the Kappelmeier precipitate must, of course, be examined qualitatively before the method can be applied with confidence. In such cases the complete ultraviolet spectrum of the extract, together with various chemical tests for interfering materials likely to be present, will indicate whether the method is directly applicable or whether suitable modifications should be sought. The authors have found infrared absorption spectroscopic examination of the original sample and the precipitated salts from saponification (as a S u j o l mull) to be very useful in this connection. Stafford et al. ( 5 ) have suggested thtk use of the infrared spectra of the dibenzyl amides of the dibasic acids as a means of determining what dibasic acids are present. Although the method was primarily developed for use in the
445
analysis of surface coatings, it should have useful applications i n other fields. LITERATURE CITED
(1) Am. SOC.Testing Materials, “A.S.T.M. Standards,” Part 4, p. 237, Designation D 563 (1949). (2) Bradley, J. J., Oficial Digest Federation Paint & Varnish Production Clubs, No. 266, 162 (1947). (3) Kappelmeier, C. P. A.. Farben-Ztg., 40, 1141 (1935); 41, 161 (1946); 42, 561 (1937); Paint Oil Chem. Rm.,6. 10 (1937). (4) Marvel, C. S.. and Rands, R. D., Jr., J . Am. Chem. Soc.. 72, 2642 (1950).
(5) Stafford, H. IT., Francel, R. J., and Shay, J. F., 21, 1454 (1949). (6) Swann, M. H., Ihid., 21, 1448 (1949).
;INAL. CHEM.,
RECEIVED September 5, 1950. Presented before the Division of Analytical Chemistry at the 118th Meeting of the AMERICANCHEMICAL SOCIETY. Chicago, Ill.
Residual Monomer in Polystyrene Spectrophotometric Analysis J. E. NEWELL, United States Rubber Co., Passaic, .V. J . A rapid method for the determination of monomeric styrene in polystyrene is described. Styrene absorbs ultraviolet radiation of wave lengths from 250 to 260 mp, 40 to 100 times as intensely as polystyrene. Determination of small amounts of monomer in polymer, 0.1 to 2.09’0, with precision and accurac? suitable for routine analysis, has been accomplished by absorption spectrophotometry. The method is most accurate in the absence of impurities other than styrene monomer. The presence of significant amounts of interfering substances is made evident by poor agreement of calculations for monomer from absorption measurements at three wave lengths.
251, 282, and 291 m p , interference by other materials is likely to be much less a t 251 m y than a t the other wave lengths. In acldition, the low concentration of polymer solution necessary for absorption measurements a t 251 m p is likely to cause less error from light scattering than the more highly concentrated solutions required for absorption measurements a t longer wave lengths. This paper presents a spectrophotometric method of analysis of styrene in polystj-rene by absorption measurements in the 251 mp region. .Is in the methods of Owens ( 7 ) , and McCovern et al. (j),multiple wave length determinations show the presence of interfering substances.
T
HE pronounced effect of residual monomer on the physical properties and molding characteristics of commercial polystyrene makes the determinat,ion of residual monomer in the polymer important. On account of the tenacious retention of styrene by polystyrene, especially by large particles, the usual determination of total volat,ile matter by treatment with heat and vacuum often gives low results. In a single determination, the frozen benzene technique of Lewis and Mayo ( 4 ) removed the styrene quantitatively but left in its place a few tenths of a per cent of benzene. Because of the difficulty in removing the styrene from its polymer in u rt.ate suitable for titration or precipitation, the methods of Kolthoff and Bovey (S),or Bond ( I ) , are not easily applied t o tht. determination of monomer in polymer. Styrene absorbs ultraviolet radiation of wave lengths of 282 and 291 mp, whereas polystyrene has only general absorption in this region of the spectrum. Owens ( 7 ) made use of this difference to analyze monomer in partially polymerized styrene, using a medium quartz spectrograph. McGovern, Grim, and Tfsach (6) adapted this method to a spectrophotometer and invcsstigated variables affecting the analysis. .it a wave length of 251 mp, styrene absorbs ultraviolet. ratli,ition 15 times as intensely as it does a t 282 mp, and 25 times as intensely as a t 291 mp. At this wave length it also has 100 t iriitbs the absorption of polystyrene. Therefore conditions are I‘:ivorable for the determination of small amounts of styrene in polystyrene by absorption measurements in this spectral region. Owing to the relative intensities with which styrene absorbs a t
WAVELENGTH. MU
Figure 1. Ultraviolet Absorption Spectrum of Commercial Polystyrene Curve 1. Before purification Curve 2. After precipitation i n alcohol Curve 3. Difference between curves 1 and 2
The absorption near 250 m p of R niisture of styrene and polystyrene can be resolved by simple mathematics into the absorption of the styrene and that of polystyrene. However, t o give significant results, two conditions must be met. The optical interference of impurities other than styrene must be low enough
