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. The 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 CornpI 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
Table I. Absorptivities and Sensitivities
Wavelength, Element Cobalt Nickel
mp
Specific absorptivity, p.p.m.-'cm.-l
505 598 650
0.155 0.0264
505
0.080
598 650
0.613
0.272 0.265
Reagents. STANDARD COBALT SOLUTION. Potassium hexacyanocobaltate(III), prepared and purified as described by Yardley (I6), was chosen a s most nearly fulfilling the criteria of a suitable standard (IS). The high molecular weight of this compound permitted direct weighing, on a microbalance, of the quantity of solid for preparing a 25.0-p.p.m. cobalt stock solution. The requisite amount of K&o(CN)G, transferred to a semimicro-Kjeldahl flask, was decomposed with concentrated sulfuric acid and evaporated over a microburner just to dryness. The residue was taken up in hot water, and finally diluted t o volume. STANDARD NICKELSOLUTION. Nickel dimethyl glyoximate was prepared by double precipitation by the usual analytical method (6). The requisite amount of solid was decomposed with concentrated nitric acid, followed by evaporation with concentrated sulfuric acid and subsequent treatment a s for the cobalt solution. 2,3 - QUINOXALINEDITHIOL (QDT). The reagent (Eastman No. 7317) was used as received, except in studies of the reaction ratio, for which it was recrystallized as directed by Morrison and Furst (9). A 0.10% solution in 4 to 1 dimethylformamide-water was used for developing the color. Solutions in this solvent mixture were suitable for use for a t least 48 hours, or even longer if protected from light. (Further observations regarding the reagent are given in the Discussion.) In the nickel experiments, some batchto-batch variations were noted, although the sample-to-sample correspondence was good within a batch of the reagent solution. N,N-DIMETHYLFORMAMIDE (DMF) This solvent (Eastman No. 5870) was used as received, except that for'some experiments on the stability of the reagent solution the solvent was freshly distilled. SOLVENT MIXTURE. As a result of preliminary experiments, the solvent system chosen consisted of a 4 to 1 (volume) mixture of D M F and water, cooled to room temperature before use. FORMIC ACIDSOLUTION. A 1 to 1000 solution of formic acid in DMF-water solvent mixture was used. The solution, stored in a brown bottle, was suitable for use for several weeks. OTHER REAGENTS. All other reagents were analytical reagent grade. I
34
ANALYTICAL CHEMISTRY
Molar absorptivity, liter mole-' cm.-'(X 10-4) 3.61 0.913 0.156 0.467 1.58 1.48
Sensitivity, pg,cm.-2 0.00163 0.00645 0.0379 0.0128 0.00367 0.00392
For interference tests, the various ions were added in the form of their readily available soluble salts. Recommended Procedure. Into a 25-ml. volumetric flask measure 1 to 5 ml. of standard solution or sample for analysis, appropriate to the range of the methods; use an identical volume of water for preparing the blank. Add 1 ml. of formic acid solution, and DMF-water solvent mixture to bring the volume to about 20 ml. Add 1 ml. of 0.10% QDT reagent, mix thoroughly, allow to stand for 5 minutes, then dilute to volume with the solvent mixture. Measure the absorbance against a corresponding blank. The red cobalt complex has maximum absorbance a t 505 mp, The spectral curve of the blue nickel complex has a major peak a t 598 mp, a peak of only slightly lower absorbance a t 650 mp, and a minor peak a t 470 mp. In the simultaneous determination of cobalt and nickel, the 650-mp, rather than the 598-mp, absorbance is used because of the lower absorbance of the cobalt complex a t the longer wavelength. Optimum concentration range for measurement in 1.00-cm. cells is about 0.9 to 2.3 p.p.m. for nickel, and about 0.4 t o 1.4 p.p.m. for cobalt. Specific and molar absorptivities and sensitivities a t the wavelengths used for the separate and the simultaneous determinations of cobalt and nickel are given in Table I. In the preliminary experiments on nickel, multiplicate samples prepared and measured on the same day with the same reagents were in excellent agreement. However, there were dayto-day and/or batch-to-batch deviations that were too large for analytical reliability. These deviations, although not completely eliminated, were satisfactorily minimized by the use of the 4 to 1 DMF-water solvent. Seven sets of triplicates (1.54 p.p.m. of nickel) were prepared, and the measured absorbances were subjected to extensive statistical treatment ( I " ) , including subset and pooled values, and the I"-ratio test. The standard deviations for a single measurement were 1.1 and 1.6% for the 650- and 598-mp measurements, respectively.
The plot of absorbance us. concentration of nickel, for both 6.50 and 598 mg, m s a straight line which, however, did not quite pass through the origin, but intersected the zero-absorbance axis a t a small value. Here also, the data were treated statistically (17) for best fit to a line and test for significance of departure of the line from the origin. For the 650-mh measurements, a line not constrained to pass through the origin did not fit the data any better than a line so constrained; hence, a zero intercept is expected. The slightly larger intercept for the 598-mp data was shown to be of some significance. At the 1.54-p.p.m. level for nickel, a standard deviation of 0.03 p.p.m. is representative. Statistical treatment of the data for the cobalt complex showed a definite, although small, drift (decreasing absorbance) with time over several days. The absorbance us. concentration curve was a straight line which passed through the origin. At the 1-p.p.m. cobalt level, the standard deviation is 0.03 p.p.m. STUDY
OF
VARIABLES
In addition to the day-to-day and/or batch-to-batch variations with the nickel complex, the presence of chloride ion caused decreased precision in the absorbance measurements of the cobalt complex. The method development work was therefore done with sulfate solutions of both nickel and cobalt, prepared as described earlier. The studies reported below were made with solutions containing a final concentration of 1.02 p.p.m. of cobalt and 1.54 p.p.m. of nickel. Effect of DimethylformamideWater Ratio. Accurately measured volumes of water were included in the make-up of solutions and blank according to the procedure described earlier. Volumes of mater from 1 to 6 nil. (including the water of the standard cobalt or nickel solution) had no significant effect on the absorbance of either the cobalt or the nickel solutions. The most pronounced effect of the larger amounts of water was to precipitate the QDT reagent, and for a given amount of water this occurred first in the blank. The time interval to the appearance of precipitate was roughly proportional to the amount of water added. Samples containing 4 ml. of (added) water did not precipitate within 6 hours; those containing 8 ml. precipitated before the absorbance could be measured. Effect of Amount of Reagent. Use of 1 nil. of 0.10% QDT solution-Le., 1 nip. of QDT-per 25 ml. of final volume corresponds t o about 8 times the amount of nickel in the optimum range, and an even larger excess for
cobalt. Yaiying the concentration of the QDT reagent solution from 0.05% to 1.0% had no effect on the absorbance, if the same amount of reagent was added to the blank; a O . O l ~ o solution, however, was insufficient for full color development. Effect of Acidity. The cobalt-QDT complex is red in either acidic or ammoniacal medium, whereas the nickel complex is blue in acidic and red in ammoniacal solution (10). Formic acid, used in the standard procedure to provide a very slight acidity, could be present in concentration up t o 10-zaTfwithout effect on the absorbance of the blue nickel complex a t either 598 or 650 mp, and 10-154 formic acid could be tolerated a t 650 mp. Equivalent concentrations of sulfuric acid, and 1Jf formic acid, slightly enhanced the absorbance; 1 N sulfuric acid produced a yellow-green color and a lower absorbance. iZbsorbances of the cobalt complex solutions containing formic acid up to 1JP &-ere constant within experimental error, and sulfuric acid concentration up to 10-*N was also without effect: higher concentrations of sulfuric acid gave decreased absorbances. Uie of formic acid-formate buffers was precluded because of the slight solubility of the alkali salts in the DMFsolvent mixture. Effect of Mixing Volume. Because of the water-insolubility of QDT and its complexes, i t was necessary to establish the minimum amount of solvent mixture required to prevent precipitation when QDT is added. This depends upon the volume of ityueous metal ion solution taken; 5 ml. mas adopted as the largest feasible volume. Varying volumes of DMF-water solvent mixture, from 5 to 15 ml., were mixed viith the metal ion solution prior to addition of the QDT reagent. Measured absorbances were within experimental wror. However. the blank solution in which only 6 ml. of solvent mixture was used precipitated after about 4 hours. The addition of 10 to 15 ml. of solvent mixture is rcconimended. Effect of Order of Addition. The solvent mixture should be added to the metal ion solution before addition of the reagent; otherwise the complex is partially precipitated. The QDT reagent must then be added without much delay, because cobalt shows a tendency toward low absorbances if allowed t o stand in contact with the mlvent. Effect of Temperature. Heating the solutions on a 60" C. water bath for as little as 5 minutes produced a pronounced decrease in absorbance of both the nickel and the cobalt complexes. Heating a t 40" C. for as long as 1 hour was without effect.
Rate of Color Formation. Samples of the nickel complex showed an initial rise in absorbance during the first 2 minutes after addition of the Q D T reagent, then constant absorbance for a t least 8 hours. Cobalt samples showed constant absorbance from the time at which the earliest measurement could be made following addition of the reagent (about 40 seconds). Measurements for longer than 8 hours were not made because of precipitation in the blanks on standing overnight. Effect of Foreign Ions. The tolerance concentrations for the various ions were determined on the basis of an absorbance difference (from t h a t of cobalt or nickel) equal t o twice the standard deviation of the absorbance measurements in the reproducibility studies. The tolerance limits are shown in Table 11. In addition, barium, calcium, cadmium, thallium(I), and tin(I1) produced precipitates a t once and strontium did so on standing; osmium precipitated when the acidity was adjusted. Table 111 shows the effect of various anions and miscellaneous reagents on separate solutions of the cobalt and nickel complex. SIMULTANEOUS DETERMINATION
For the simultaneous determination of cobalt and nickel, absorbance measurements mere made a t 505 and 650 mM. These wavelengths fulfill the criteria given by Mellon (8)-namely, wavelengths a t which the ratio of the sensitivities (absorptivities) of the two components is maximum and minimum. The solutions measured contained cobalt and nickel in varying ratios, but with each element a t a concentration within its optimum range. From the measured absorbances a t 505 and 650 mp, concentrations were calculated in the usual way by solving the two simultaneous equations of additive absorbances. The results are shown in Table IV. COMPOSITION OF COMPLEXES
By application of the method of continuous variations (7, 12) and the mole ratio method (16). the ratio of metal to QDT obtained as the average of several runs was found as follows: Continu-
ous Mole ratio method varia- Metal QDT tions constant constant Coba1t:QDT 1:5.4 1:4.8 1:3.9 Nicke1:QDT 1 : 2 . 0 1 : 2 . 4 1:l.g
No discernible difference in the combining ratios was obtained by using the QDT reagent as received, or purified by three recrystallizations by the method of Morrison and Furst (9), or purified by vacuum aspiration of an ammoniacal solution of the reagent.
Table
II. Effect of Foreign Ions (Values in p.p.m.) Cobalt deterNickel mlnadetermination tion
At At at Foreign ion 650 mp 598 mp 505 mp Arsenic(111) 70 170 145 Chromium(II1) 40 25 65 Cobalt(11) 0.3 0.09 0.9 1 0.7 Copper(11) 8 103 Iridium(IV la 4 Iron(II1) 180 465 170 IroniII)' 70 180 175 Lead(I1) 450 65 30 Magnesium(11) 360 560 115 Manganese(11) 1.1 6 2.3 0.6 Mercury(11) 45 140 Molybdenum 165 103 5 x 108 (VIP 0.3 Nickel(11) 0.2 1.4 0.2 Palladium(11) Platinum(IV) 2.7 2.2 5 150 30 Rhodium(111) 175 Ruthenium (VIII) 100 125 135 Silver(I) 3 24 6 480 Tungsten(VI)" 300 465 Vanadium(111) 2 x 1034 x 103 5 x 104 Produced decreased absorbance. '
(I
Table 111. Effect of Anions and Miscellaneous Reagents"
Effects * Precipitated reagent 0.75M produced no effect on nickel but decreased cobalt absorbance KBr Nickel and cobalt absorbances both enhanced Drastically decreased color formation KNCi 0.15M slightly enhanced both colors; cobalt slightly cloudy 0.6% slightly enhanced both colors 1.2 x lO--'M drastically decreased both absorbances Citric acid 1.2% slightly enhanced both colors Tartaric acid 1.2% slightly enhanced both colors "4HI"Z 1.270 sharply decreased nickel color, slightly decreased vobalt color Both samples cloudy 0.1% sharply decreased both absorbances NHzOH . HC1 1.270 enhanced nickel color but decreased cobalt color Tested in presence of both cobalt and nickel solutions. * Concentrations in final solution measured.
VOL. 35, NO. 1, JANUARY 1963
3$
Table IV.
Cobalt, p.p.m. Taken Found 0.614 0.819 0.861 1.024 2.458 2.458 1.536
0.644 0.864 0.864 1.040 2.379 2.467 1.561
Relative error, yo +4.9 +5.5 +0.4 +1.6
+3.2 +0.4 +1.6
Av. $ 2 . 5
DISCUSSION
The spectral curves of the cobalt and nickel complexes and the sensitivities found in the 4 to 1 DMF-water solutions were essentially the same as found by Burke and Yoe (4) for alcoholwater solutions. The small differences in the wavelength of the absorption peaks might be due to differences in wavelength calibration of the instruments used. The QDT reagent in 4 to 1 DMFwater solution used in this work was suitable for a t least 48 hours, and storage in the dark delayed the formation of yellow precipitate from a few days to several weeks. Burke and Yoe reported that their reagent, in 1 t o 1 DMF-alcohol solution, was almost nonreactive after about 12 hours. The QDT reagent is sparingly soluble in ethanol, acetone, gIacial acetic acid, 2-propanol, and tributyl phosphate, as well as in D M F and in alkali. The solutions are not very stable except those containing D M F or alkali. On exposure to daylight, solutions in the above solvents form yellow precipitates whose infrared spectra (in KBr wafers) are different from the parent QDT and from each other. The compounds which precipitate from ethanol or from ammonia are soluble in D-MF; the compound which precipitates from D M F in the dark is soluble in ethanol. On prolonged heating in air a t 150' C., QDT (yellow brown) turns bright yellow, and its infrared spectrum is very similar to that of the yellow compound precipitated from ethanol solutions exposed to light. These and other considerations may lead to the conclusion that the precipitate formed from solutions in ethanol and other solvents, except D M F in the dark, is due to a photo-oxidation reaction in which the thione tautomer of QDT forms polymers through the -S-Slinkage. It is also possible to conceive of polymers involving a linking of QDT molecules through the DMF: H
-s-c-s-
I I
N(C&)2
36
ANALYTICAL CHEMISTRY
Nickel, p.p.m. Taken Found 1.538 1.538 1.538 1.538 1.538 1.538 2.269
1.444 1.457 1.481 I . 452 1.510 1.518 2.238
reducing properties of the -SH or Recent work here has shown that cobalt(III), a t least in certain solutions, does not react with QDT, and QDT can be used t o determine cobalt(I1) in t8hepresence of cobalt(II1). In our work on the nickel complex a reaction ratio of 1 to 2 was indicated by the continuous variations method, and also by the mole ratio method using a constant amount of QDT and varying the amount of nickel. However, with nickel concentration constant and variable QDT, the reaction ratio was about 1 to 2.5. With cobalt solutions the different methods gave different reaction ratios, from about 1 to 4 t o about 1 to 5.5; in no case was a 1 to 3 ratio closely approached. StevanEeviE and Draiid (11) investigated the combining ratio of cobalt, nickel, and palladium with QDT in alkaline solution. They also determined the charge on the complexes by migration studies, and concluded that all of the complexes had the structure
=S groups of the reagent.
Simultaneous Determination of Cobalt and Nickel
Relative
error, yo -6.1 -5.3 -4.1 -4.9 -1.5 -1.3 -1.4
-3.6
Also a possibility is the acid-catalyzed hydrolysis of the quinoxaline ring followed by polymer formation of the hydrolyzed molecules. Anhydrous dimethylformamide has a very high heat of dilution with water (3, 6), and other lines of evidence also indicate hydrate formation. On mixing water and DMF, the temperature rise is maximum a t about 20% (by volume) of water, at which concentration D M F and water are present in equimolar amounts. Because of the sharp temperature rise on mixing D M F and water (or aqueous solutions) and the fading of the cobalt and/or nickel QDT complexes a t temperatures above 40" C., the QDT reagent was prepared in 4 to 1 DMF-water mixture, and dilutions in the preparation of final solutions for measurement were also made with the same solvent mixture. The slight acidity desired was provided by addition of a small amount of formic acid, chosen in preference t o other acids because it would be a product of hydrolysis of D M F if any such reaction occurred. In a general sense, the results of our interference tests were similar t o those reported by Burke and Yoe. However, some notable differences appear. Burke and Yoe found serious interference from iron(III), but little or no interference from manganese(I1). Under the conditions of our work, relatively large amounts of iron(II1) could be tolerated, but manganese(I1) constituted serious interference. In any event, adequate methods for analytical separations of the various ions have been published, so that no difficulty should be experienced in effecting such separations as are required in any particular case. A significant difference between our \T-ork and that reported by Burke and Yoe relates to the reaction ratio of the metal ion to the QDT reagent. They reported a 1 to 2 ratio for nickel and a 1 to 3 ratio for cobalt, and stated that cobalt(I1) apparently was oxidized to cobalt(III), followed by reaction with the semi-aci form of the reagent; the oxidizing agent was not specified. The supposition that cobalt(I1) is oxidized seems inconsistent with the
They failed to shoiv more than one ionizable hydrogen. In their continuous variations curves for cobalt and nickel complexes in ammoniacal solution the reaction ratio was clearly 1 to 2, whereas in 2-propanol solution the ratio for cobalt was 1 to 3. In their mole ratio plots a 1 to 2 ratio for both cobalt and nickel was clearly indicated. StevanEevi6 and DraiiE faded t o mention a 1 to 1 (in addition to a 1 to 2 ) ratio for the palladium-QDT solutions. although i t was clearly indicated by their data, The existence of both the 1 t o 1 and the 1 t o 2 complexes of palladium in acidic D M F solution has been demonstrated by Byres and Janota (@, the former complex predominating when palladium(I1) was in excess, the latter complex when QDT nas in excess. It appears, especially in the case of cobalt, that the complex(es) formed mill be dependent upon several factors, such as pH, solvent, excess of one reactant or the other, etc. Our experimental observations are most compatible with a polymeric structure of the complexes in which the polymer chains consist of alternating metal ion and QDT links. Depending upon the number of coordination positions occupied by QDT molecules and the length of the chain, any combining ratio (including nonintegral values) from 1 to 1 to 1 to 6 is possible. In addition, the conclusion that the complexes are polymeric is consistent with the following observations: Solutions of the nickel complex
exhibit a constant ratio of the absorbances for the 595-mp and 650-mp peaks over a wide range of conditions, including different acid concentration, different solvents, and different ratios of reagent and metal concentration. The wavelength of the absorbance peak of the cobalt complex is also constant. Whether or not the complexes precipitate does not depend directly on their concentration, or, within the limits described earlier, on the composition of the solvent. Precipitation is never immediate and the precipitates contain all of the metal ion from the solution-Le., none is released on precipitation of the complex. Polymeric complexes have been found with rubeanic acid complexes of cobalt, nickel, and other cations ( 1 4 ) ; the Y-C-S structure in QDT is similar t o t h a t in rubeanic acid. Molecular models allow any of the configurations necessary t o give the observed combining ratios in polymer structures, but crowding is prevalent for those
higher than 1 t o 4. On the other hand, the square planar configuration about the metal atom and the bond angles and planarity required by the hetero ring geometry in the structures postulated by StevanEeviE and Draiid and by Burke and Yoe induce a considerable strain in the chelate ring, Finally, it is difficult t o reconcile either the rapid formation of the complexes or their tendency t o precipitate compounds retaining their spectral characteristics with structures other than polymers. LITERATURE CITED
(1) Annand, R. R., Ph.D. dissertation, University of Texas, June 1961. (2) Ayres, G. H., Janota, H. F., ANAL. CHEM.31, 1985 (1959). (3) Blankenship, F., Clampett, B., Proc. Oklahoma Acad. Sci. 31, 106 (1950). (4) Burke, R. W., Yoe, J. H., ANAL. CHEM.34. 1378 (1962). (5) Hecht, K . T.,’Wodd, D. L., Proc. Roy. SOC.(London)A235, 174 (1956). (6) Hillebrand, W. F., Lundell, G. E. F., Bright, H. A., Hoffman, J. I., “Applied Inorganic Analysis,” 2nd ed., pp. 40810, Wiley, New York, 1953.
(7) Job, P., Ann. Chim. (Paris) (10) 9, 113 (1928). (8) Mellon, M . G., “Analytical Absorption Spectroscopy,” pp. 369-73, Wiley, New York, 1950. (9) Morrison, D. C., Furst, A., J . O r g . Chem. 21, 470 (1956). (10) Skoog, D. A., Lai, M., Furst, A., ANAL. CHEM.30, 365 (1958). (11) StevanEeviE, D. B., Drazit, V. ,G., Bull. Inst. Nucl. Sci. “Boris Kidrach” 9, 69 (1959). (12) Vosburgh, W. C., Copper, G. R., J. Am. Chem. SOC.63,437 (1941). (13) Williams, W. J., Talanta 1, 88 (1958). (14) Xavier, P., Ray, P., J . Indian Chem. SOC.35, 432 (1958). (15) Yardley, J. T., Analyst 28, 156 (1950). (16) Yoe, J. H., Jones, H. L., IXD.EKG. CHEM.,ANAL.ED. 16, 11 (1944). (17) Youden, W. J., “Statistical Methods for Chemists,” Chap. 5, Wiley, New York, 1951.
RECEIVEDfor review August 16, 1962. Accepted October 30, 1962. Condensed from a dissertation submitted by Robert R. Annand t o the graduate school of The University of Texas in partial fulfillment of the requirements for the doctor of philosophy degree, June 1961. Work supported by The University of Texas Research Council Projects 637 and 829.
A Study of the Near-Infrared Spectra of Some Aliphatic and Aromatic Isocyanates D. J. DAVID Mobay Chemical Co., New Martinsville, W. Va.
b The spectral absorption of both aliphatic and aromatic isocyanates has been measured between 1.60 and 2.69 microns. By means of these spectra information can be gained regarding the structure of the molecule. In particular, the presence of the H -NCO
and aromatic
H
\
/
/
\
c=c
groupings can be ascertained and information about the structure of the molecule can be obtained from the combination bands of the -CH3, >CH2, and 3 CH. Near-infrared spectra can be utilized for the general characterization of the isocyanates in mixtures b y use of the -NCO band. Information can be gained about the effects of substituent groups on aromatic isocyanates b y correlating the wavelength of a particular band with Hammett’s u values.
T
structure of many organic compounds, including isocyanates, has been studied in the infrared region HE
(1, 4). This region shows functional groupings and interatomic linkages present, while the ultraviolet spectrum shows conjugated unsaturated SJ -t ems (6). With the development of suitable detectors and good resolution instruments for the near-infrared region, more attention has been devoted to this portion of the spectrum ( 1 7 ) . The most general area of the spectrum studied is in the region from 2 t o 15 microns. Admittedly, much less specific information can be gained in the nearinfrared region (0.8 t o 3.0 microns), and sensitivity in this rpgion is poorer, since we are mainly dealing with the overtone and combination bands of -CH, -KH, and -OH stretching vibrations. Inasmuch as no information on the near-infrared spectra of isocyanates is yet available, it was felt that a study of the near-infrared spectra of representative isocyanate structures might provide a valuable tool in isocyanate chemistry. ?;ear-infrared spectra have been comprehensively reviewed by Kaye ( I d ) , Wheeler (17), and Goddu and Delker ( 7 ) . TQ
EXPERIMENTAL
A Cary Model 14 spectrophotometer R a s used for recording all near-infrared spectra. Solutions were approximately 0.5 to 1.061 or as indicated on the individual spectrum traces. The spectral region from 1.6 t o 2.69 microns was studied. For the study of isocyanates the region below 1.6 microns was surveyed and not found t o be particularly useful. ,411 data were obtained using a scan speed of 0.001 micron per second. Carbon tetrachloride was used in the reference cell for recording all spectra and nominal slit widths were 0.304 mm. at 1.6 microns, 0.442 mm. a t 2.0 microns, and 1.262 mm. at 2.5 microns. Since the cutoff of the Cary Model 14 is 2.69 microns, it established itself as the upper wavelength limit. From 2.G t o 2.69 microns the accuracy of the Cary 14 is questionable. However, the spectra appear adequate for qualitative purposes. All isocyanates were prepared in Mobay’s Research Laboratories from the corresponding amines and were distilled t o ensure maximum purity. Typical amine equivalents showed them pure. Table I lists the t o be 99% isocyanates studied, solvents used, and
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