PRECISION A N D ACCURACY
The data obtained indicate the pccted standard deviation between duplicate determinations is less than 1.5% of the amount of phthalic acid present i n the I and Ir and recovery data supplied by the
organic laboratory shovr- that the accuracy Of the method iS within 1 . 5 7 LITERATURE CITED
(1) Milas, S . A., Walsh, IT. L., J . Chen?.Sot. 57, 1390 (1935).
-4iit.
t
2, Shreve, 0. D., Heether, 41. R , . ~ N A L .CHEM. 23, 441 (1951). 3 1 Warshowsky, B., Elving, P. J., Ilandel, J., Ibrd., 19, 161 (1947).
RECEIVED for review Xovember 10, 1958. Iccepted March ,6, 1959. Division of dnalytical Chemistry, 134th Meeting, ACP Chicago, Ill., September 1958.
Ultraviolet Absorption Spectra of Phthalic Anhydride and Related Substances Analytical Method for Naphthalene and 1,4-Naphthoquinone HANS
PETERS
Research laboratory, Reichhold Chemie AG, Hamburg, Germany
b Ultraviolet absorption spectra of phthalic anhydride and related substances are described. Changes in materials, and, in particular, the reaction kinetics of photochemical changes of naphthoquinone have been examined. A spectrophotometric method of analysis for naphthalene and naphthoquinone has also been developed. Naphthalene can b e determined more accurately b y using methanol as a solvent and b y measuring the absorbance of the suitable absorption maximum at 31 1 mp after 17 hours. Owing to ester formation, phthalic anhydride makes practically no contribution to the absorption after this period. The naphthoquinone content can b e calculated b y measuring absorbance at 347 mp immediately after dissolving the sample to b e analyzed. To avoid serious errors in analysis caused b y photochemical changes in the naphthoquinones, the substance must be stored in the dark and work done in red light.
T
quantitative determinations of phthalic anhydride and related substances. such as maleic anhydride, benzoic acid, naphthalene, and 1,4naphthoquinonr. are difficult when only conventional methods. such as ~ 0 1 1 1 metric analysis. gravimetry, and oxidimetry, are used. 1.4-Saplithoquinone interferes in almost all methods and in some very considtrably. -4 number of publications describe polarographic ( 2 , 4, 9) and spectrophotometric (1, 3, 5 6 , 8.14, 16) determinations. Optical methods, however, usually record only the spectra of thesr bodies and are very often concerned just with the detection of (he isomeric phthalic acids in resins and polyesters. I n the determination of the compoHE
1326
ANALYTICAL CHEMISTRY
nents of crude phthalic anhj-dride iron. a nianufacturing process by optical methodc, alloaance must be made for changes taking place before and durinz analysis. -in analytical method I. described here for phthalic anhydride, naphthalene, and I ,4-naplithocliiinoiie and particular consideration 1- given to changps in these substancps. In developing the method, special importance nap attached to the accuracj- of determination of 1,4-naplithoquinone and naphthalene.
All solvents were filtered through a sintered-glass funnel (G. 3) from Schott & Genossen (pore size, 15 to 40 mw). EXPERIMENTAL
Figures 1 3 show t h e absorption spectra of phthalic anhydride, maleic anhydride, benzoic acid, naphthalene, and 1 - 4 naphthoquinone deterrninpd iinmediately after dissolving in methanol, carbon tetrachloride, and 1,4-dioxane. Brigg's logarithm of the molar absorptivity in liters per mole em. n-as APPARATUS A N D MATERIALS chosen as ordinat,e and the n-ave length in millimicrons as abscissa. The Spectrophotometer. T h e absorptransmittancies of methanolic solutions tion nieasurements were made in of both dicarboxylic acid anhydricles a Carl Zeiss quartz ultraviolet spectron-err expected to be affect'ecl by ester, photometer, Model P M Q 11. Slit widths of 0.003 t o 1.5 mm. were used. or rather half-ester, formation. The Ultraviolet Lamp. Photochemical spectra for phthalic anhydride and reactions ere carried out with a maleic anhydride in Figure 1 are ncPhilips ultraviolet lamp (HPR 125 cordingly indicated by broken lines. IT), This is a high-pressure mercury After 17 hours the sprctruni of lamp n i t h an internal reflector and phthalic anhydride had changed fundashort wive length cutoff of its radiation iiirnt,ally (Figure l, curve P-411). As of about 308 mp. Reference Materials. PHTHALIC maleic anhydride in methanol solut.ion has no interest anal!-tically, the correASHTDRIDE, technical (Bayer), rcsponding change in this spectriiiii n-as crystallized twice from carbon tetrachloride. not examined in detail. ~ I A L EASHYDRIDE, IC technical (ReichThe absorption maxima of the soluhold), recrystallized twice from carbon tions of all five substances in the three tetrachloride. sol\-ents occur a t about' the same wave RESZOIC ACID, analysis grade (E. lengt,h. The absorption range of naphMer&). thoquinone extends farthest into the PUREKAPHTHALEKE (Russian procilong w v e length portion of the ultrauct), recrystallized twice from carbon violet.. tetrachloride and sublimed once. Changes in Spectra of Naphtho~,~-S.IPHTHOQUISONE, technical (Schuchhard), distilled with steam once quinone under Short-Wave Light. and sublimed twice. Esperinients carried out in rooms of Solvents. ~~IETHAX analytical OL. differing light intensity revealed that,, and chromatographical 'grade' (E. under the influence of short-n-ave Merck). light, the absorption spectra of napliCARBOS TETRACHLORIDE, analytical thoquinone in methanol solution. carand chromatographical grade (E. bon tetrachloride, or dioxane change Mer&). considerably. E r e n solid naphtho~ , ~ - D I O X A Sfor E , ultraviolet cpectrnquinone undergoes chemical changes, photometry (E. Nerck). Ultraviolet Absorption.
to
300 350 Wuve length Imp1 Wave length [ m f l Figure 1 . Absorption spectra of substances dissolved in methanol PAI. PAII. MA. EA. N. NQ.
Figure 2. Absorption spectra of substances dissolved in carbon tetrachloride
Phthalic anhydride immediately after dissolving Phthalic anhydride after 17 hours Maleic anhydride Benzoic acid Naphthalene 1,4-Naphthoquinone
mainly a t the surface, under the influence of daylight, and the color of the solutions changes from pale yellon- t o intense yellow and even to brown. These changes, however, are not eo astonishing, as quinones react photochemically in various ways (?, 10-15). Seutral methanolic naphthoquinone solution n-as of interest in the analysi. planned and \vas examined closely for light response. Thfl 1.91 X 1 0 - 4 V solution was first warmed to 40' C. in a 100-cc. quartz-glass measuring flask and then irradiated from the side by a Philips ultraviolet lamp. The distance from head to head n.as S cni. At various times after the start of irradiation, a small saniple of solution mas r m o v e d by means of a pipet, placed in a suitable cell, and absorbance. -4.meaqured a t 311 and 347 mp. The reaction temperature of 40' C'. n-as maintained practically constant throughout the whole series of nieasurements. The ultraviolet radiation of the light sourcc of the spectrophotonieter was so small during the determination that virtually no changes occurred in the spectrum.
For reasons of reaction kinetics, absorbances measured a t 311 and 347 nip n-ere first converted to the values for 1-em. cells, and then Brigg's logarithms were plotted against times of light exposure (Figure 4). At 347 mp, the absorbance decreased linearly and rapidly to lon- values, passed through a minimum, then increased slightly for a longer period of time, and finally decreased again linearly and slowly after passing through a maximum. The curve at 311 mp followed a similar course, but, in this case, the first linear decrease was not clearly seen. K i t h a 3-em. cell, the minimum value of the absorbance a t 311 m p was 0.034 and a t 3-17 mp, 0.001. After a longer period of radiation, the absorbances finally increased again slowly. The three linear portions of the two curves shom- that three first-order reactions occur consecutively. If the qtraight lines are extrapolated to the point of intersection, the logarithnis of the absorbances of the first tn o intermediate products are obtained. The absorbances of the third product can be calculated approximately from their
niiiiinium values. The absorptivities of these products computed from their logarithms are recorded in Table I. I n a special eyperiment, the naphthoquinone solution mas evposed to ultraviolet light until the absorbances reached the maxinimn and minimum values shown in the curves in Figure 4. The irradiation as then interrupted and the solution. ere kept in the dark. I n all e m ' s . the absorbances nom remained constant, proving that none of the three reactions is either a dark reaction or light-induced chain reaction, but a direct, or possibly partly indirect, photochemical reaction. If the absorptivities of the starting material and of the end product are known, according to Lambert-Beer's lan., the existing coilcentration of the starting material, c l , can be determined after a given period of irradiation, t , from the absorbance, A, measured after this period as follon 3:
Ct
=
A b
- € 2 >( c, - € 2 (mole/l.)
(1)
nhere and e2 are the molar absorptivities of the initial and end products; co is the molar concentration of the original substance before irradiation (mole per liter), and b is the thickness of the layer in the cell (em.). If the concentrations of the starting materials are calculated in this way for all reactions and their logarithms plotted VOL. 31, NO. 8, AUGUST 1959
1327
against the time of irradiation, the yelocity constant, k , for the three reactions can be derived from the ascents of the straight lines, respectively, of constant. the equation In c = - kt From measurements a t 347 mp, where the slope of the first linear portion of the logarithmic curve is readily seen (Figure 4), velocity constants of 0.530, 0.046, and 0.0097 minute-' were found. The first very rapid reaction after irradiation begins seems to be the formation of electronically excited naphthoquinone molecules which appear as phototropic isomeric biradicals (10). This reaction can be symbolized as follom:
If there is no source of suitable Hdonors, the two biradicals can combine to form cyclobutane derivatives. It can be supposed that the second and third reactions (Figure 4) also occur before final naphthoquinone formation. T h e n naphthoquinone solutions more concentrated than those described are irradiated, the absorptivities of the reaction mixture incrcase a t 311 and 347 mp approximately up to values normally expected for naphthohydroquinone (15). I n more dilute solutions, a much longer time is required to obtain higher values.
+
ANALYSIS OF NAPHTHALENE AND 1 ,I-NAPHTHOQUINONE
Possibilities and Limits of Determination. Without physical or chemi-
cal separation, ultraviolet spectroscopy can be used t o determine with greater accuracy only naphthoquinone, naphthalene, and phthalic anhydride in a phthalic anhydride batch. Even naphthalene, however, requires the use of t h e special process described below. When maleic anhydride and benzoic acid also have to be determined by means of absorption spectra, both these products must be separated by extraction n-ith suitable solvents or by chromatography. A reliable method of determination of phthalic anhydride in lJ4-dioxane has now been developed. A wave length of 297.5 mp is suggested as cnlibration point. Photochemical changes in naphthoquinone must be considered here also. Kaphthoquinone, the most easily determined component, has a strong absorption maximum in the long-wave ultraviolet band in all thrce solutions. If light sensitivity is taken into ac-
(2)
It is well known that the phototropic isomeric biradicals of the quinones can function as dehydrogenating agents. I n the presence of suitable H-donors, such as alcohols or aldehydes, the biradicals of the quinones furnish, according to Equations 3 and 4, hydroquinone or naphthohydroquinone, respectively, or their derivatives (?', IO). O*
I
0+
CHI.CH~OH=
0
?H
I
Table I. Molar Absorptivities of the Starting Naphthoquinone and Intermediate Products
-
C,
L./Mole Cm.
311 mp
317
w
Starting naphthoquinone 1974 Intermediate 1 1840 2 2137 3 36
2339 881 1231 1.06
IOH Table
II.
Molar Absorptivities and Related Specific Absorptivities of Substances Dissolved in Methanol
347 Mp
311 M p
P-411 SQ N
1328
One to five grams of material were eighed out and dissolved in methanol in a 100-cc. measuring flask made of Jena glass. After the volume n-as adjusted and shaken, 10 cc. of this solution were transferred by means of a pipet into a second 100-cc. measuring flask and made up to 100 cc. A suitable quartz cell was then filled with this solution and the absorbance was measured a t 347 mp. TI
E,
P.11
count, the compound can be analyzed with accuracy in every case. A suitable absorption maximum for naphthalene in dioxane and methanol solutions occurs a t 311 mp; iii carbon tetrachloride, a t 313 mp. Saphthoquinone absorbs approximately eight times more strongly than naphthalene in all three solvents. At these nave lengths, the absorptivity of phthalic anhydride is smaller than that of naphthalene, but as it usually accounts for more than 90% by weight of the crude phthalic anhydride, it contributes in corresponding strength to the absorption. Errors in the determination of naphthalene n-ill accordingly be larger. Methanol was chosen as solvent. A suitable wave length for absorbance measurement in the determination of naphthoquinone n-as established a t 347 mfi * The accuracy of the naphthalene determination was improved by measuring the absorbance of the mixture, dissolved in methanol, a t 311 mp after a lapse of 17 hours. Because of the high degree of ester formation, phthalic anhydride contributes so little to the absorption that it may be ignored for the most part. Seither maleic anhydride nor benzoic acid show :my absorptions a t either 11ave length. The molar absorptivity, E, and the related specific absorptivity, K , for the three substances dissolved in methanol are recorded in Table 11. The coefficients for naphthoquinonp, n hich is very sensitive to light, were calculated from an ayerage figure derived from 30 separate determinations. The measurements nere carried out in red light immediately after dissolving the naphthoquinone. Newly sublimed crystals were used for each test. By making a large number of mpasurements and by judicious avoidance of the shortwive light source, error. 17-ere not greater than =k0.3% on the awrage. The greatest actual error amounted to 52.57,. Analytical Procedure. To show how greatly t h e photochemical changes of naphthoquinone can affect the analysis when no precautions are taken, some samples were analyzed in a room lighted normally by daylight. T h e analyses were carried out as follows:
l./mole cm. 34 0 22 i 0 008 1969 f5 250 =I= 1.6
ANALYTICAL CHEMISTRY
K,
l./g. em. 0 23 0 0015 f 0 000054 12.45 f 0 03 1.951 f 0 013
e,
l./mole em. 0 0 2339 i 6 0
K,
l./g. em.
0 0 14 79 rt 0 . 0 4 0
I n all cases, the absorbances lay as nearly as possible between 0.2 and 0.7. As is known, the accuracy of a
1 Wave length [mil
Time Of Ultraviolet lrmdiation [Hours]
Table 111.
Analyses in Room Lighted Normally by Daylight
Composition of Mixture Grams
PA SQ S
Gram
95 2 3
0.0305 0.0442
- 1
62 19 19
0.0249 0.0621
- 21
S
PA XQ S
0 8950 0 0310 0 0317
P.1 NQ
0 1234 0 0315 0 0318
66 17 17
0 7520
50
0 2276
0.0316 0.0225
15
cs = CxQ
- KgiA,, x
-4 B 11
-
-L7
b
-
12.45 X
&Q
XQ
N
- 9
+ 1.9
+ 82 -
29
0.0233 0.0610
- 26
+ 92
0.2299 0.1167
+ I
-
22
-
11
-
16
cPA (g./l.)
K 3 1 1
Error, %
Found, c/2L
0 1100 0 0330 0.0345
P.4 SQ
N
48:: - KF& x b
1 4
3
Figure 4. Change of absorbances b y ultraviolet irradiation
Figure 3. Absorption spectra of substances dissolved in 1,rt-dioxane
spectrophotometer can only be utilized fully when the values to be measured lie in this range. When values fell outside this range, solutions were diluted to an appropriate extent or more appropriate quantities n-ere weighed out. The remainder of the solutions was stored without special precautions, and, after 17 hours, absorbance measurements were made a t 311 mp on these solutions. The concentrations of lJ4-naphthoquinone and naphthalene in the sample solution are derived as follovl-s:
2
1
- 0.0015 X
0.1545 0.0465 0.0296 0.0442 0.0357
50 15
N
0.0741 0.0565
60 40
0.0746 0.0475
EQ
0.0315 0.0318
50 50
0.0208 0.0643
- 34
+102
0.0307 0.0343
47 53
0.0218 0.0683
- 29
+ 99
CPA
1.95
k A . 1 (6) The upper indices for A and K , respectively, indicate the wave lengths a t which both amounts are determined. The lower indices are characteristic of the substances for which these values apply. AI, refers to measurements carried out after 17 hours.
ZrQ
;Q
10
0.0469 0,0263
+ 0.8
14 11
+ 0.7
VOL. 31, NO. 8, AUGUST 1959
1329
Tnhle I11 shows the results of those an:ilyses in which no precautions were taken against the influence of light. The folloKi1% method is sugeestecl for carrying out an analysis in which
Table IV.
precautions are taken against the influence of light. A sample of phthalic anhydride. stored as far as possible in the dark up to the heginning of the analysis, is
Analyses with Precautions Taken against Comnosition of Rlixtnre Found, Grams Gram 2,0968 70 1 0.0341 0.0339 10 0.3127 0.3115 u 0.2811 0.3185 10 1,2415 57 7 0 0338 0.0340 0.3076 11 0.3042 0.2749 13 14 0.3065 9 0,0302 0,0305 0.2977 0.3004 91 0.0290 0 0.0292 0.3029 91 0.3032 0.0313 0.0311 9 0.3105 91 0.3083 0,0309 0.030; !I 0.3091 0.3110 91 0.0294 0.0296 18 a1 0.3027 0.3027 0.0304 2 0,0306 0.3137 21 0.3112 46 0.6927 11 0.1691 20 0.3036 0.0299 0.0302 9 0.3033 91 0.3018 0.0370 0.0368 10 0.3191 90 0.3256 0.0307 9 0.0304 0.3047 91 0.3035 0.0287 0.0288 9 0.3067 91 0.3034 0.0319 0.0322 10 0.3018 0.3018 90 0.0307 9 0.0306 0.3032 91 0.3038 1.2336 80 0.0309 2 0.0311 0.0879 5 0.0902 5 0.0752 r I 0.1076 3 0.0311 0.0309 0.0650 4 0.0632 54 0.8387 21 0.3255 19 0.2974 48 0.0386 0.0285 0.03235 ti2 0.0314 0.0287 44 0,0294 0.0365 56 0.0378 0.0314 51 0.0321 0.03215 49 0.0312 0.0324 0.0326 50 0,0341 0.0331 50 48 0.0310 0.0315 52 0.0335 0.0341 0.0304 0.0303 50 0.0273 50 0.0301 0.0329 53 0.0326 47 0.0292 0.0290 0,0301 43 0.0312 0,0393 0,0405 57
-
-
1330
ANALYTICAL CHEMISTRY
Influence
of Light
Error, c> SQ
S
$0.5
-0.4
-0.6 -1.1
-1.0
-0.9
-0.7 -0.1
$0.5 -0.7
+0.7 S0.C
-0.7 ;0.3
-0.i $0
s
-1.0
+0.5 -0.5 -2.0 +1.2
-0 4 $0.3 +1.1 -0.9
0 '0.3 -0 2
-0.6 -2.3
-0.6
+".s
+0.4 -3
0
-2.4 -:3.4
- 9- . -3 -3.1
-0.6 +3 1 -1.6
-1 8
+0.3
-9.2
-0.9
-7 0 -0.6 -3 0
weighed out under red light and dissolved in methanol. If the crystals are dirty, the solution must be filtered through a sintered-glass funnel (G. 3). After dilution to a suitable concentration, the absorbance is measured a t 347 niF. The remainder of the solution iy stored in the dark for a t least 17 hour
