Infrared Spectrophotometric Analysis of Ethy lene-Propylene Copolymers HARRY V. DRUSHEL and FRANK A. IDDINGS ESSO Research laboratories, Humble Oil & Refining Co., Baton Rouge, l a .
b
Carbon14-labeled standards were prepared b y copolymerization of propylene and tagged ethylene. Compositions were established by liquid scintillation counting. These standards were used for calibration purposes in the rocking, wagging, bending, stretching, and combination regions of the infrared spectrum. Ratios of band intensities were used throughout for better precision (as contrasted to measurements involving film thickness) and interchangeability with other infrared spectrophotometers. The band shape and intensity of the CH3 rocking band a t 968 cm.-l in relation to the CHa wagging band a t 1150 cm.-l provides a measure of the relative number of isolated propylene groups. This relationship correlated with the temperature of insipient melting ( a measure of block polymer content) and the relative intensity of the 7 4 5 cm.-' band as compared to the 720-cm.-' band (a measure of the relative number of isolated ethylene units).
S
spectrophotometric procedures making use of the C-H bending or rocking region have been reported for the determination of ethylene-propylene copolymer coniposition. Natta (16) used the 1378-cm.-I methyl symmetrical C-H bending band of the copolymer dissolved in carbon tetrachloride, but this approach is limited because of solubility problems. TVei (20) overcame the solubility problem by developing a pressed film technique making use of the ratio of band intensities at 1150 (methyl wagging) and 720 em.-' (methylene chain rocking). However, calibration was made from secondary standards previously analyzed by Xatta's (16) solution technique. Although Gossl (11) recently claimed success in using a technique similar t o that of Wei, some objections to the use of the ll5O-cm.-l and 7 2 0 - ~ m . - bands ~ on the basis of their sensitivity to crystallinity have been made (9). Natta (16) used a radiochemical method t o determine composition and Stoffer and Smith (17) published a procedure for preparing standards by liquid scintillation counting. I n the
present work, carbon14-labeledstandards were prepared by a modification of the procedure of Stoffer and Smith (17). These standards were used for calibration purposes in the C-H stretching, bending, rocking, and wagging region of the infrared spectrum and in the combination region of the spectrum. I n each case, a ratio of band intensities was used which eliminates the necessity of measuring film thickness. Not only does the ratio technique provide a greater degree of precision, as contrasted to the use of film thickness, but the derived working curves may be applied to other instruments with an acceptable degree of accuracy. Advantages and disadvantages in the use of each specific region of the spectrum are presented. Methyl and methylene rocking frequencies and intensities were interpreted and used to elucidate the degree of randomness in the addition of both ethylene and propylene to the copolymer. For interpretation, use m s made of the intensity ratio of the 115O-cm.-' and 968-cm.-' methyl group bands and the intensity ratio of the 745-cm.-' and 720-cm.-I methylene group hands.
EVERAL INFRARED
28
ANALYTICAL CHEMISTRY
EXPERIMENTAL
Analysis of C14-Labeled Standards by Liquid Scintillation Counting. All samples were prepared and counted in 20-cc., low-potassium, giass counting A Packard Tricarb Model vials. 314-C liquid scintillation spectrometer was used for counting. Counting efficiencies of 50.9 and 74.07, were obtained a t 10- t o 50- and 10- t o 100-volt discriminator settings with the phototube high voltage a t 865 volts. Background count rates for unlabeled polymer samples mere 14 and 19 counts per minute at the above instrument conditions. Four copolymer compositions and a polyethylene were prepared using C14labeled ethylene. The polyethylene was used to obtain the specific activity for reacted ethylene. By determining the specific activity of the ethylene as polymer in a fashion identical t o the determination of ethylene compositions for the copolymers, errors resulting from isotope effect, nonreactive C14-labeled impurities in the ethylene, and differences in counting techniques are either eliminated or minimized. A series of samples of from 0.05 t o 0.5 gram of each polymer FTas dissolved
in 20 cc. of a scintillator solution containing 4 grams of 2,5-diphenyloxazole (PPO) and 0.05 gram of 1,4bis-2(5pheny10xazolyl)benzene (POPOP) per liter of xylene ( 1 7 ) . Solution was obtained by warming the mixture a t 130' C. for 10 to 20 minutes. The samples were then cooled and shaken. The polyethylene samples produced milky gels, while the copolymer samples n-ere less milky. Internal standard samples using benzoic acid-CI4 Ivere prepared in the same manner with each series of polymer samples. Specific activity as counts per minute per gram of ethylene in the polymer was calculated to be 35,200 and 51,200 for the 10- t o 50- and 10- t o 100-volt windows, respectively. Slight variation in the counting efficiencies b e t w e n the polyethylene and copolymer samples was found as well as between some samples of the same polymer. Counting efficiencies m r e corrected t o the same value using a plot of the ratio of 10to 50- t o 10- to 100-volt windows against the count rate of the internal standards in each of the windows (Figure 1) (2, 6). Such a plot permitted all the copolymer samples t o be corrected to the same counting efficiency as the polyethylene. Calculation of ethylene content for 10- to 50-volt n indow is: Wt. % ethylene = icounts/min. sample - 14)(100) X (sample wt.) (35,200 counts/min./g.) efficiency figure for polyethylene efficiency figure for sample and for 10- to 100-volt 17-indow is: Wt. % ethylene = (counts/min, sample - 19) (100) X (sample wt.) (51,200 counts/niin./g.) efficiency figure for polyethylene efficiency figure for sample Ethylene contents of the four copolymer samples are shon-n in Table I. Infrared Studies. PRESSED FILM TECHNQCE.Films of the polymer were prepared using a hydraulic press equipped with both heated and water-cooled platens. The heated platens were maintained a t a temThin films perature of 200' C. (about 1 to 2 mils) were pressed between two flat mold plates. 1Iylar film n-as placed on each side of the sample to facilitate removal of t h e sample film. Aluminum foil mas placed between the Mylar film and the mold plates. Only moderate pressure (200
4.0
I
I
I
1.50
-
1.45
5:
1.40
c
51 x. 0
1.35
2
1.30
El 2 v
1.25
1.20
t
I, EFT IC IENCY F I GLTRE x 10-3)
(COLNT RATE
Figure 1. figure
Internal standard sample plot of R vs. efficiency MOLE %' PROPYLENE
pounds per square inch) was applied during the period of time the mold and sample were retained between the heated platens. Heating time was kept t o a minimum (1 t o 5 minutes) to prevent decomposition of the sample. Immediately after the heating period the mold was placed between the watercooled platens and subjected t o 10,000 pounds per square inch pressure for 5 minutes. Thicker films necessary for the near-infrared examination of the copolymer n-ere prepared from special molds having depressions providing films several tenths of a millimeter in thickness. C A ~ TFILM TECHNIQUES.Rapid snalysis of cement samples (copolymer in polymerization diluent) was made by casting a thin uniform film on a n appropriate support by evaporation of the diluent. For the bending, rocking, or wagging regions of the spectrum, plates or disks of S a C l or KBr were used. A 0.5-inch KBr disk prepared v i t h a Perkin-Elmer 0.5-inch pellet die was satisfactory. A drop of the copolymer solution mas alloned t o dry on the KBr pellet follon-ed by 10-minute treatment
Table I. Results from Carbon-14 Analysis of Ethylene-Propylene Copolymer Standards for Ethylene Content
Weight yGethylene from carbon-14 content in standard sample Yo. 1 KO.2 Yo. 3 No. 4 23 . 2 32.9 45.2 56.1
23 .7 23 . 6 23 . 8 23.0 23.6 23.8 AT. 2 3 , 5
32.8 32.6 32.8 32.4
45.4 45.2 45.4
32.7
45.3
55.2 55.8 55.3
55.6
Figure 2. Calibration curve for infrared spectrophotometric determination of EPR composition from the rocking and wagging region of the spectrum C a r b ~ n ' ~ . l a b e l e dstandards Admixture of homopolymers, for comparison
with an infrared lamp to remove traces of solvent. The pellet containing the cast film was mounted in the usual manner in the well of the Perkin-Elmer Model 221 spectrophotometer housing. Measurements in the stretching region were made with optical quartz disks (General Electric), 1 mm. in thickness. These quartz disks were transparent in the C-H stretching region and were very easily cleaned for further use. Because of the slight nonuniformity of films cast from solution, i t was necessary to use a ratio technique for the determination of composition. Appropriate bands of about the same relative intensity were chosen for calculation of ratio values at each region of the spectrum. Spectrophotometers. Spectra in t h e normal rock salt region were obtained with both a Beckman Model IR-5 spectrophotometer and a PerkinElmer Model 221 spectrophotometer with prism-grating interchange. Most spectra on t h e Model 221 were obtained with a slit program of 980. Near-infrared spectra mere scanned with a Cary Model 1411 a n d a Beckman Model DK-2 spectrophotometer. DISCUSSION OF RESULTS
Rocking and Wagging Frequencies.
A calibration curve prepared from the C'elabeled standards is presented in Figure 2. For this purpose, the pressed film technique was used. Also included
is a calibration curve prepared by thorough mixing (by remolding several times) of weighed blends of the two homopolymers, polyethylene and polypropylene. Typical spectra showing base lines and base line points are reproduced in Figure 3. It is interesting, but undoubtedly fortuitous, that the C'elabeled standards and the homopolymer admixtures produce nearly the same calibration curve. Precision. T h e analysis of large numbers of samples from development a n d production of t h e copolymer requires a knowledge of t h e precision of t h e method. Precision d a t a for b o t h t h e pressed film a n d t h e cast film (supported o n a K B r pellet) techniques a r e presented i n Table 11. .is might be expected, the pressed film
Table II. Precision Data for the Analysis of Ethylene-Propylene Copolymers Using Bands in the 1200- to 700-cm.-' Region
Sample A (solid) B (cement) Technique Pressed film Cast film Xumber of replicate analyses 10 11 Determined wt. % ethylene 41.7 45.4 (average) Confidence limits, 95% f l . 2 52.2
VOL. 35, NO. 1, JANUARY 1963
29
160
k
-
"110 V
e
5100 W
E
E
-3 z
1
-
9080
-
70
-
3 2 6o
6
50-
w
40-
30
-
20
-
10
-
0.5
0
0.6
0.7 0 . 8
0.9
1.0
1.1
1.2
1.3
1.4 1.5
1.6
,RATIO i 9 6 8 C M . ' l l ) ( u ~ ~ ~ 1150 CM.. MOL % C3) 13,23
1200
1100
1000 900 F R E Q U E N C Y (CY.
800
700
600
Figure 3. Typical infrared spectra of EPR in the rocking and wagging region 1. 2. 3. 4. 5. 6.
7. 8. 9. 10.
Base lines Base line points CH3 wagging (mixed with CHz and CH bending) CHa rocking (mixed with CHI and CH rocking) Polypropylene crystalline-sensitive bands Vinylidene group (from propylene termlnation) CH2 rocking (perhaps mixed with CHI and CH rocking) ( C H h rocking (CH2)a rocking (CHz) 2 5 rocking
technique is more reproducible. Nonuniformity of the cast film introduces some errors, even though the ratio technique was used. -4comparison of results from three laboratories (three different spectrophotometers) using the same calibration data is presented in Table 111. Spectral Characteristics and Polymer Structure. As t h e crystallinity
in solid n-paraffins or polyethylene
Table 111.
30 *
increases, the 720-cni.-' -and splits ani the intensity of the 7 3 0 - ~ m . -compo~ nent increases (16, 18). Of the two bands, the 720 cm.-'is least affected by crystallinity and, indeed, the ratio of absorbances a t 730 and 720 em.-' may be related t o the degree of crystallinity. Likewise, in polypropylene, as the crystallinity increases, numerous temperature-sensitive bands increase in intensity with respect to the tempera-
Comparison of Results from Three Different Laboratories Using the Pressed Film Technique and the Same Calibration Curve
Sample C D E F G H I
Figure 4. Relationship between the temperature of incipient melting and the ratio of -CH3 bands sensitive to the number of isolated propylene units
Lab. numbers and spectrophotometers I, Perkin2, Beckman 3, Baird Associates Model IR-5 Recording Infrared Elmer Model 221 41.0 43.7 46.5 51.5 57.5 65.0 57.0
ANALYTICAL CHEMISTRY
42.4 48.5 49.6 52.9 56.9 64.6 57.6
39.6 42.6 47.2 51.6 57.8 62.5 57.0
ture-insensitive band a t 968 cm.-l ( 1 , I d ) . The ratio of absorbances of the crystalline-sensitive to the insensitive bands provides a measure of crystallinity. Also, the isotactic content is reflected in the shape and relative intensity of the 117O-cm.-' band as compared to the temperature-insensitive band at 968 cm.-' ( 5 ) . When large blocks of the same repeating units are formed in a copolymer, i t is possible that bands will appear which correspond t o crystalline-sensitive bands of the homopolymer discussed above-e.g., see the CScrystallinity bands in the spectrum of polymer X in Figure 3. However, many of the desirable properties of an elastomer are lost when excessive block polymerization has occurred leading t o a partially crystalline product, Therefore, a technique which will permit estimation of the extent of random copolymerization is of practical value in characterizing the polymer. Examination of the spectra of many different ethylene-propylene copolymers resulted in the observation that the intensity of the 968-cm.-l methyl group band (relative t o the 1150-cm.-l band) varied depending upon the catalyst used or the physical properties of the finished elastomer. The behavior of the 968-cm.-l band with change in elasticity, presence or absence of crystallinity (as detected by infrared), etc., established that as copolymerization be-
9 0 0
0.1
0.2
(RATIO A745
A720 cy. - 1)
0.3
MOL % CQ
m)
Figure 6. Relationship between the shape of the 968 cm.-‘ methyl band and the relative intensity of the 745 cm.-’ methylene band Figure 5. Relationship between the infrared bands sensitive to the appearance of isolated ethylene and propylene units
came more random (yielding more “isolated” propylene units) the 968em.-’ band became less intense and inore diffuse in relation to the 1150mi.-’ band. The beha7 ior of the ratio of the CH3 bands, mentioned above, is quite unPxpccted, as the 968-cni.-l band rather than the l150-cm.-1 band remains e-entially unchanged in polypropylene homopolymer as one proceeds from d i d to melt or from isotactic to atactic material (I, 14). Liang, Lytton, and 13oone ( I $ ) , on the basis of frequencies, relative intensities, polarization propeities, and effects on deuteration of 1 jolypropylene, have tentatively assigned t h e 968-cm.-’ band in polyIiropylene t o the methyl rocking mode mixed with CH2 and CH rocking T ibrations. From this assignment, perhaps it is a change in the magnitude of the mixing with the CH2 and CH niodes in the case of the isolated propylene units in the copolymer which gives rise to a more diffuse absorption band at 968 c i - ~ i . - ~ I n this connection some nonstereospecific polypropylene has been prepared (13) in which the 968-cm.-l and 115O-cm.-l bands are either missing or very weak. Also, the occurrence of head-to-head arrangements of contiguous propylene units would be expected to influence the behavior of the methyl rocking mode and any mixing \T hich may be involved.
For the purpose of polymer characterization the ratio of absorbances a t 968 and 1150 cm.-l should provide a measure of the degree of randomness with respect to the introduction of propylene units into the copolymer. On t h e basis of the initial observations mentioned above, t h e ratio .AQ= Dm.-I/ O m . - ~ should decrease as t h e randomness increases. I n addition, an increase in ethylene content is expected to increase the probability of producing propylene units isolated by ethylene units. For comparison of the 968-crn.-I t o 1 1 5 0 - ~ n i . - ~absorbance ratio with other data given below, correction for the effect of composition is made by multiplying b y the ratio mole % C,/ mole yo C3, the molar ratio of ethylene t o propylene in the copolymer. Relationships between this modified absorbance ratio and other established physical or spectral properties indicative of randomness (or conversely, block polymer) are discussed below. The temperature of incipient melting (or crystallization) obtained from stresstemperature relationships has been used t o estimate the extent of “tacticity” or block copolymerization (3, 4,8). The original relationship developed by Flory (10) for copolymers, which was extended by Coleman (@, describes the relationship between the melting temperature of the copolymer (T,)and the homopolymer (Tmd)t o the probability
( p ) of the appearance of coiitiguous monomer units in the copolymer.
I/T, - 1/Tm0= - ( R / A H , ) I n p
(1)
Therefore, as the temperature of incipient melting increases, the ratio A 9 ~ o m . - ~ / A ~om.-^ 1 S 0would be expected to increase (see Figure 4). A change in the catalyst or conditions which affect the degree of randomness (or conversely the amount of block polymer) would be reflwted in the number of isolated ethylene units as well as isolated propylene units. Therefore, the absorbance ratio A868 C m Allso -I should correlate with the number of isolated ethylene groups. Very recently, T’eerkamp and Veermans (19) reported on the differential measurement of (CH?)?and (CH,), units a t 745 cm.-’ and 630 cm.-l, respectively, in ethylene-propylene copolymers. The band a t 720 cm.-’ was assigned t o units of 5 or more CH, units. Thus, the absorbance ratio A-45 cm -I/A,?o cm -1 provides a measure of the number of isolated ethylene units, and -&en corrected for the effect of composition (by multiplying by the ratio mole % C,/ mole yo C,) is related to the inverse of the ratio S 9 s 8 - I ~ ’ A C~m~-IS times ~ the ratio mole % Cnjmole Ca (see Figure 5 ) . The more random the copolgmer, as indicated by the absorbance ratio A745 cm -1/A720 cm -I for the relative number of isolated ethylene units, the -$/
VOL. 35, NO. 1, JANUARY 1963
31
logarithm is linear with mole % propylene in the copolymer. These bands are too intense to be used in examination of the usual pressed films. Therefore, this region has proven useful for the study of extremely small samples as in polymer molecular weight fractionation work. Solid state effects present a problem and for this reason such calibration must be applied with caution over wide ranges in molecular weight or polymer type. Fundamental Stretching Frequencies. T h e C-H stretching frequencies are useful for measurement of copolymer composition when spectrophotometers of sufficient resolution
-0WcT'TY
tCM.
- 1)
Figure 7. Typical infrared spectrum of an ethylene-propylene copolymer in the C-H bending region of the spectrum
FREQUENCY (CU.
Figure 9. Typical infrared spectrum of an ethylene-propylene copolymer in the fundamental C-H stretching region
Base line points shown by arrows Grating to prism changeover at 141 6 cm.-'
Base fine points shown by arrows
30
40 !,I&
more diffuse the 968-crn.-' band becomes (see Figure 6). I n summary, the appearance of contiguous us. isolated monomer units may be studied by the spectral characteristics of the methyl and methylene group rocking and wagging bands. Bending Modes. A typical spectrum showing base lines is included i n Figure 7. Figure 8 is t h e calibration curve for this region of t h e spectrum prepared from the CI4-labeled standards. Separate base lines were drawn for the symmetrical methyl bending and methylene scissoring absorption bands. Because of contribution from the asymmetrical methyl bending mode to the absorption a t the methylene scissoring frequency, neither the ratio A13,*Em. -1/A1467 Cm.-I nor its
0.6
I
I
,
GO
Ij0
70
% PIapyiens
ethylene-propylene copolymer, showing the base line, is reproduced in Figure 9. The ratio between the CH, and CH2 asymmetrical C-H stretching bands was not used because of the large difference in relative intensities. The ratio between the asymmetrical CHa and the symmetrical CH2 bands, which have nearly the same relative intensities, produced more reliable results. Resolution of the CHs from the CH? asymmetrical as well as the symmetrical bands must be satisfactory as no curvature of the calibration curve was seen. An equation for the calibration line is given below:
Figure 8. Infrared calibration curve for the determination of ethylenepropylene copolymer composition from the C-H bending region of the spectrum CI4-labeled standards
are available. Large absorptivities provide t h e sensitivity t o examine extremely small samples from fractionation studies. Use of this region is most convenient since quartz disks may be used to support the cast film. -1 typical spectrum of a cast film of 0.
0
0.
1
E
0.
4
0.
P
0.
P
8
3
w 0.
u
$
m 4
0.
0.
0. 0.
0. 0 15
I 0
10
I
I
I
I
20
30
40
50
!
60
70
dS
93
100
WEIGKT % ETHYLENE I 8 COPOLYMER 0.
2.2
2.25
2.3 2.35 WAVELENGTH (MICRORS)
2.4
Figure 10. Typical near-infrared spectrum of an ethylene-propylene copolymer
32
ANALYTICAL CHEMISTRY
Figure 1 1. Near-infrared calibration curve for ethylene-propylene copolymer composition C"-labeled standards Base point at 2.20 I./ Beckman DK-2 spectrophotometer
log10( A 2 9 6 2
om.-1/Asaa0 cm. -1)
=
0.0125 (mole % propylene)
- 0.585
(2)
If the CHI symmetrical band was not resolved from the CH2 band, a change in the slope of the curve a t high propylene contents would have been observed. T h e Combination Region of the Near-Infrared Spectrum. Relatively thick pressed films may be studied in the combination region of the spectrum where the absorptivities are much lower. A typical spectrum of the copolymer is shown in Figure 10. Assignment of the specific frequencies in the C-H stretching and bending regions which could give rise to these combination bands was not made. However, the band a t 4396 cm.-' (2.275 microns) probably corresponds to a combination of an asymmetrical methyl C-H stretching frequency and some bending frequency near 1400 cm.-l and is sensitive to the propylene content of the polymer. The bands a t 4329 and 4255 cm.-l (2.31 and 2.35 microns) are sensitive to the methylene (hence, also, the ethylene) content and are probably combinations of the asymmetrical and symmetrical methylene stretching frequencies with bending frequencies. Therefore, the ratio of the 4396 (2.275 microns) absorption to either the 4329 em.-' (2.31 microns) or the 4255 cm.-' (2.35 microns) absorption should be an
indication of the composition of the copolymer. These relationships are depicted in Figure ll. Precision is not as good as can be attained by use of the rocking and wagging frequencies. At low propylene contents, the methyl band a t 4396 cm.-' (2.275 microns) is not too well resolved. Very recently, Bucci and Simonazzi ( 7 ) reported the use of overtone bands near 5882 cm.-' (1.7 microns), but resolution of the CH, from the CH2 bands was not as good as found for t,he combination region discussed above. ACKNOWLEDGMENT
The authors thank Esso Research Laboratories, Humble Oil & Refining Co., for permission to publish this material. We also thank J. F. Ross, P. B. Lederman, and E. V. Fasce for supplying the polymers described in this study and for many helpful suggestions. Much credit also goes to J. R. Ellis and W. A. Daniel, who obtained the spectra, and to J. T. Wade who assisted in the preparation of the standards. LITERATURE CITED
(1) Abe, K., Yanagisaw.Et, K., J . Polymer Sn'. 36. - -, 536 - - - 11959). ( 2 ) Baille, L. A., Atomlight 19, 1 (1961). (3) Baldwin, F. P., Ivory, J. E., Anthony, R. L., J . A p p l . Phys. 26, 750 (1955). . - - - - I
(4) Boonstra, B. B. S. T., Ind. Eng. Chem. 43, 362 (1951). (5) Brader, J. J., Spencer Chemical Co., private communication (1960). (6) Bruno, G. A., Christian, J. E., ANAL. CHEM.33, 650. (1961). 17) . , Bucci. G.. Simonazzi. T.. Chim. Ind. (Miianj 44,' 262 (1962)'. ' (8) Coleman, B. D., J . Polymer Sci. 31, 155 (1958). (9) Corish, P. J., Small, R. M. B., Wei, P. E., ANAL.CHEM. 33, 1798 (1961). (10) Flory, P. J., Trans. Faraday SOC.51, 848 (1955). (11) Gossl. T.. Makromol. Chem. 42, ' l'(1960): ' (12) Liang, C. Y., Lytton, hl. R., Boone, C. J., J . Polymer Sci. 54, 523 (1961). (13) Liang, C. Y., Watt, W. R., Ibid., 51, 514 (1961). (14) Luongo, J. P., J . A p p l . Polymer Sci. 3, 302 (1960). (15) Natta, G., Mazzanti, G., Valvaasori, A., Pajoro, G., Chim. Ind. (Milan) 39, 733 (1957). (16) Stein, R. S., Sutherland, G. R.B. M., J . Chem. Phys. 21,370 (1953). (17) Stoffer, R. L., Smith, W. E., ANAL. CHEM.33, 1112 (1961). (18) Tobin, M. C., Carrano, &I. J., J . Polymer Sci. 24, 93 (1957). (19) Veerkamp, T. A., Veermans, A., Makromol. Chem. 50, 147 (1961). (20) Wei, P. E., ANAL. CHEM.33, 215 (1961). RECEIVEDfor review June 25, 1962. Accepted October 29, 1962. Presented at the Symposium on Spectroscopy of High Polymers, Joint with Division of Pol mer Chemistry and Division of Organic :oatings and Plastics Chemistry, 142nd National ACS Meeting, Atlantic City, N. J., September 9-14, 1962.
Spectrophotometric Study of Cobalt and Nickel Corn pI exes with 2,3-Qu inoxa I ined it hi01 GILBERT H. AYRES and ROBERT R. ANNAND' Deparfment o f Chemistry, The University o f Texas, Ausfin, Jex.
b The interaction of 2,3-quinoxalinedithiol with cobalt(l1) and nickel(l1) has been studied. The reagent and the metal ion complexes are very sparingly soluble in water; hence the reactions were carried out in 80% (volume) dimethylformamide solution, slightly acidified with formic acid. The red cobalt complex has an absorption peak a t 505 mp; the blue nickel complex has absorption peaks a t 598 and 650 mp, the former showing slightly greater absorbance. The colors form rapidly and are stable for several hours. The sensitivities, for 0.001 absorbance, are 0.00163 pg. cm." for cobalt and 0.00367for nickel. Optimum range, for measurement a t 1-cm. optical path, is about 0 . 4 to 1.4 p.p.m. for cobalt and 0.9 to 2.3 p.p.m. for nickel. Simultaneous determination of cobalt and nickel i s
made from absorbance measurements a t 505 and 650 mp. In the solvent system used, the reaction ratio of metal ion to reagent varies from 1 : 2 to 1 :2.5 for nickel and from 1 :4 to 1 :5.5 for cobalt. It appears that the absorbing complex consists of polymeric structures in which the polymer chains consist of alternating metal ion and quinoxalinedithiol links.
T
HE simultaneous spectrophotometric determination of cobalt and nickel with 2,3-quinoxalinedithiol has been reported recently by Burke and Yoe (4, who gave references t o the preparation and previous uses of the reagent. The present paper reports work done in this laboratory (1) involving the separate and the simultaneous determination of cobalt and
nickel with this reagent, and a study of the chemistry of the reagent and the complexes, with results in several respects essentially identical with those reported by Burke and Yoe. However, perhaps because of somewhat different solution conditions, some of our findings are a t variance with those of Burke and Yoe, especially in the matter of the composition of the complexes formed. EXPERIMENTAL
Apparatus. Spectral curves were recorded with a Beckman Model DK-1 spectrophotometer. RIeasurements at fixed wavelength were made with a Beckman Model DU spectrophotometer. 1 Present address, Tracor, Inc., Austin, Tex.
VOL. 35, NO. 1, JANUARY 1 9 6 3
33
