rhodamine B in the presence of sodium perchlorate. Spectrophotometric Titrations. The results for the titrations of representative strong, intermediate, and weak bases are recorded in Table 11. Rhodamine I3, the weakest basic dye studied, was used to indicate the equivalence point in the titrations with perchloric acid. Although the binary solvent composition is not critical for mixtures richer in acetic anhydride, the highest coniposition [70% (v./v.) acetic anhydride] evaluated for the indicator bases above was used as the titration medium. The pK, values listed in Table I1 are those assembled by Streuli (1%). With the exception of the acetates of lead and cadmium, all of the bases in Table I1 were reported by Streuli ( l a ) to assay 9701, or greater in potentiometric titrations in acetic anhydride. (Unpublished titration curves on lead acetate in acetic acid suggest that this compound is among the weak bases.)
The data show that rhodamine B is a sufficiently weak base to serve as a spectrophotometric indicator in the quantitative titration of 0.1- to 1-meq. samples of strong and intermediate bases with perchloric acid. However, for the weakest bases (lead acetate, and potassium bromide, iodide, and nitrate), the dye is not a weak enough base to yield acceptable recoveries. As was observed in the use of indicator base in spectrophotometric titrations in glacial acetic acid (Q),Type I1 plot gives consistently more accurate end point values in media of low dielectric constant than does the simpler graphic method of Hummelstedt and Hume ( 5 ) . The per cent relative errors listed in Table I1 were computed by comparing the spectrophotometric results from the former method with the mea. added. The over-all mecision for the' sDectrophotometric titration of tr;plicate samples of each base was =k3p.p.t., with the exclusion of the less precise results on lead acetate. A
LITERATURE CITED
(1) Bell, R., Robinson, R., Trans. Faraciay SOC.57.965 (1961). (2) Bruckenstein, S:,J. Am. Chem. Soc. 82,307 (1960). (3) F:itz, J., Fulda, M., ANAL. CHEM. 25, 1837 (1953). (4) Higuchi, T., Rehm, C., Barnstein, C., Zbid.. 28. 1506 (1956). ( 5 ) Hummelstedi, L.,' Hume, D., Zbid., 32,576 (1960). (6) Hummelstedt, L., Hume, D., J . Am. Chem. SOC.83, 1564 (1961). ( 7 ) Kolling. 0. J . Chem. Educ. 35, 452 11958). ( 8 ) Koliing, O., Smith, M., ANAL.CHEM. 31, 1876 (1959). (9) Kolling, O., Stevens, T., Zbid., 33, 1384 (1961). (10) Mather, W., Anson, F., Zbid., 33, 132 (1961). (11) Shkodin, A., Karkuzaki, L., Zhur. Analit. Khim. 15, 676 (1960). (12) Streuli, c. A., AXAL.CHEM. 30, 997 (1958). -I
w..
RECEIVEDfor review kb' 7 , 1962. ~ ~ ~ & ~ d t~ ~$ h ~ ~ ~ $ ~ ~ ~ carp. for financial supportof this investi-
gation.
Determination of Alkyd and Monomer-Modified Alkyd Resins by Attenuated Total Reflectance Infra red S pectro met ry ROBERT L. HARRIS and GLENN R. SVOBODA Freeman Chemical Corp., Port Washington, Wis.
b The principle of attenuated total reflectance (ATR) has been utilized to develop a systematic approach to the routine analysis of alkyd resins without prior chemical treatment. Quantitative analyses of phthalic anhydride, isophthalic acid, vinyl toluene, and styrene are described.
T
PRESENT WORK was undertaken to determine a method for the determination of the aromatic constituents of alkyd resins without prior chemical treatment and to outline a procedure for routine analysis. Four methods (1, 2, 4-6) of sample handling for the quantitative analysis of polymers are suggested in the literature but each has a serious drawback. A preferred method would be to use free films, but it is extremely difficult to obtain cured films of uniform thickness. Even if this obstacle were overcome, there remains the problem of accurate measurement of film thickness of the order 0.005 to 0.010 mm. HE
Present address: Connecticut Instrument Corp., Wilton, Conn.
Solution analyses of commercial resins per se were not practical. Because one is interested primarily in the aromatic ingredients, the presence of aromatic solvents, such as xylene, in commercial resin solutions presented a problem, Polystyrene has been determined in monomer-modified oils and alkyds by plotting the ratio of the aromatic band intensity a t 14.3 microns to that of the carbonyl band intensity a t 5.8 microns us. the ratio of polystyrene to oil or alkyd (4). The latter ratio is dependent on oil identity as well as the oil to monomer ratio. Should the resin also contain an acid, as would an alkyd, the aromatic to carbonyl ratio would be further complicated. The use of an internal standard has been suggested ( 5 ) . The requirements with respect to alkyd analysis are that the proposed standard be film forming, compatible with the alkyd, and free of absorption from 13 to 15 microns. A standard which met all of these requirements could not be found. Attenuated total reflectance (ATR) techniques offered an interesting alternative, inasmuch as the absorption
intensity is independent of sample thickness (3, 8 ) . The results of such a study are presented in this paper. EXPERIMENTAL
Apparatus and Reagents. The reflectance attachment used in this study was a prototype of t h a t manufactured by the Connecticut Instrument Corp. The prism material was KRS-5. Because of the low level of energy transniission associated with ATR techniques as compared to conventional transmission techniques, the Beckman IR-5 damping control was conveniently moved t o a more accessible position since frequent changes were necessary. A more convenient method of compensating for energy losses was to equip the reference beam with a iModel BA-1 variable beam attenuator (Connecticut Instrument Corp.). The resins used in this work were either commercially available Chempol (Freeman Chemical Corp.), alkyd resins, or special laboratory preparations. Laboratory preparations using known charges were especially desirable and used in the study of monomer-modified preparations, because these could be 1 0 0 ~ oconverted on a theoretical solids VOL. 34, NO. 12, NOVEMBER 1962
1655
AROMATIC ABSORPTION OF ALKYD RESINS WAVELENGTH, MICRONS 12 13 14
CONSTITUENT
15
PHTHALIC ANHYDRIDE ISOPHTHALIC ACID TEREPHTHALIC ACID BENZOIC A'CID
I a a1
VINYL TOLUENE STYRENE WEAK
I
P
MaMEDIUM
Figure 1. Cothrup-type chart for a series aromatic constituents
basis, whereas commercial batches, for economic reasons, may be considered complete a t 98 to 100% conversion. All commercial grade materials were used without further purification. PROCEDURE
Sample preparation for use with the reflectance attachment was accomplished in the following manner. In all cases, cobalt octoate drier was added to the resin solutions. The resin was smeared across the face of a 22 X 30 mm. microscope cover glass and allowed to air cure. When it was desirable to obtain a permanent sample for filing or for some special purpose, the following procedure was used. A wet film of known thickness (0.5 to 2 mm.) was applied to a piece of Teflon F E P (E. I. du Pont de Nemours &- Co.) film by means of a draw-down bar. The film was then air-dried or baked. It was desirable to undercure slightly those resins which gave brittle films. When the film had reached the desired state of cure,
/a
IF I SxSTRONG
of alkyd resin
it was removed from the Teflon and mounted in a 35-mm. slide mount. Because of the surface properties of the Teflon, the use of this method of film preparation was limited to resins which possessed fairly high viscosities. Films prepared in this fashion could be used to determine the region of the spectrum best suited for subsequent quantitative determinations. Except for isophthalic and terephthalic alkyds, the regions best suited for analysis were those as cited in the literature (1, 2, 4-6). However, the ATR absorption of isophthalic and terephthalic acids did not correspond to that normally expected of meta or para substitution. A review of the significant absorption wavelengths for aromatic constituents found in alkyd resins is presented in the Colthrup-type chart of Figure 1. RESULTS A N D DISCUSSIONS
I
0 05 70
I
,
GO
I
55 50 45 ANGLE OF INCIDENCE. DEGREES
65
40
Figure 2. Dependence of absorbance on the angle of incidence
Picture a beam of radiation passed into a triangular prism, reflected off the second face, and passed out of the third face. It has been demonstrated (3) that a portion of the beam enepgy escapes from the reflecting face. If the ratio of the refractive index of the prism material to that of the material behind the reflecting face is properly arranged, this escaping radiation can be made to return into the prism. Absorbing material placed in intimate contact with the reflecting surface absorbs a portion of the escaping energy
The principle of attenuated total reflectance may be illustrated in the following manner:
T Table I.
Experimental ATR Parameters for a Series of Alkyd Constituents
Constituent Phthalic anhydride IsoDhthalic acid Viriyl toluene Styrene Table II.
Wavelength, P
13.5 13.7 12.3 14.3
(4
Concentration range, %
Slope EM
24-42 27-45 45-75 35-75
0.55 1.57 0.30 1.41
40 O
35 45 O 30'
AA/AC
Analysis of the Major Aromatic Constituents of Alkyd and MonomerModified Alkyd Resins
Constituent Phthalic anhydride
Absorbance
Theoretical, %
Found, 70
0.20 0.26 0.30 0.34 0.42 0.50 0.62 0.09 0.12 0.15 0.18 0.37 0.51 0.65 0.75
24.9 35.0 42.4 26.8 32.5 37.8 44.5 45.0 55.0 65.0 75.0 45.0 55.0 65.0 75.0
24.5 35.1 42.2 26.9 32.1 37.2 44.9 44.9 55.0 65.0 75.1 45.0 55.0 65.0 74.8
Isophthalic acid
Vinyl toluene
Styrene
1656
Angle of incidence
ANALYTICAL CHEMISTRY
I PERCENT CONSTITUTENT IN RESIN
Figure 3. Dependence of absorbance upon concentration of various aromatic constituents of alkyd resins 0 Styrene; 1 unit = 0.10 A., 1 unit = 10% constituent concentration A Vinyl toluene; 1 unit = 0.02 A., 1 unit = 10% constituent concentration 0 Isophthalic acid; 1 unit = 0.05 A,, 1 unit = 5 % constituent concentration 0 Phthalic anhydride; 1 unit = 0.05 A,, 1 unit = 5% constituent concentration
before it returns. Spectra obtained in this manner are independent of sample thickness, so long as the sample is a t least of minimum thickness. The minimal thickness is equal to the depth of penetration of the escaping energy and is usually 0.005 mni. or less. The depth of penetration as well as the band intensities can be controlled by varying the angle of incidence of the radiation. As the angle of incidence becomes smaller, the energy penetration becomes greater and the band intensity increases.
Figure 2 shows the dependence of the absorbance a t 13.5 microns on the angle of incidence for an alkyd that contains 247, phthalic anhydride. -4s the angle of incidence mas decreased, the amount of absorbance increased and the position of the absorbance curve was raised with respect to the ordinate. The choice of the angle of incidence was based on a compromise which gave the best balance between absorbance and spectrum orientation. Before the spectrum was run, the angle of incidence was set a t the desired value and the prism alignment was adjusted to give minimum absorbance a t 5.0 microns, an area of the spectrum which mas free of alkyd absorption. In preparation of the quantitative procedures for the analysis of phthalic anhydride, isophthalic acid, vinyl toluene, and styrene, the concentration
range was limited to that encountered in commercial resins. For example, the phthalic anhydride curve in Figure 3 extends from 24 to 42%. The experimental ATR parameters for preparation of the calibration curves shown in Figure 3 are cited in Table I. All resins were laboratory preparations with known quantities of ingredients. As can be seen in Figure 3, the curve for styrene does not obey Beer’s law a t higher concentrations. This may be due to a slight misalignment of the zero line. Because the 14.3-micron band of styrene at high concentrations has an absorbance greater than unity, a slight misalignment of the zero line introduces a large error (‘7). Calibration curves for benzoic and terephthalic acids are not included. Benzoic acid is used commercially only as a modifier and not as a major constituent. When used in conjunction with phthalic anhydride, the additive absorptions require a calibration curve based on ratios of absorbances for analysis. Terephthalic alkyds are not commercially available and hence not included. The results of the analyses of commercial resins are presented in Table 11. The results agree me11 with the theoretical concentration values. CONCLUSION
The theoretically recognized concepts of attenuated total reflectance
can be readily applied to quantitative analysis of polymeric materials. Further application of ATR techniques to quantitative analysis of polymeric materials is indicated.
LITERATURE
(1) Adams, M. L., Swann, M. H., ANAL. CHEM.30, 1328 (1958). (2) Bellamy, L. J., “The Infrared Spectra
of Complex Molecules,” Meuthuen, London, Wiley, New York, 1954. (3) Fahrenfort, J., Spectrochim. Acta. 17, 698 (1961). (4) Fraser, J. G., Pross, A. y., Ofic. Di,g.,
Federation Paint & Varnzsh Productaon Clubs 29, 75 (1957). (5) Infrared Spectroscopy Committee, Infrared Spectroscopy: Its Use as an Analytical Tool in the Field of Paints
and Coatings, Federation of Societies
for Paint Technology, Phdadelphia,
1961.
(6) Kappelmeier, C. P. A., “Chemical Analysis of Resin-Based Coating Ma-
terials.” ChaD. XIII, Interscience, New York, ’1959. (7) Robinson, D. Z., ANAL.CHEM.33,273 (1961). (8) Wilks, P., Ohio State Symposium on Molecular Structure and Spectroscopy, 1961; cf. C.I.C. Newsletter 14, Connecticut Instrument Gorp., September 1961. RECEIVEDfor review April 2, 1962. Accepted September 24, 1962. Abstracted from a paper presented at the Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1962.
Indirect Spectrophotometric Determination of Zinc and Cobalt Using Eriochrome Blue Black R D. W. ROGERS Department o f Chemisfry, Roberf Kolej, Istanbul, Turkey
b Methods of achieving selectivity in the indirect spectrophotometric determination of metal ions with Eriochrome Black T [l -(1 -hydroxy-2-naphthylazo)-6-nitro-2 naphthol-4-sulfonic acid] and Eriochrome Blue Black R [l (2 hydroxy 1 naphthylazo) 2 naphthol-4-sulfonic acid] have been discussed. The principle has been illustrated by the simultaneous determination of cobalt and zinc in synthetic binary solutions using Eriochrome Blue Black R as the chromogenic agent. The absorbance is measured a t 625 mp where the absorbances of the zinc and cobalt complexes are approximately the same, The addition of EDTA [(ethylenedinitri1o)tetraacetic acid] removes zinc, but not cobalt, from its complex with the chromogenic agent. In the concentration range of 0.05 to 0.6 p.p.m., the calibration curves (analogous to
- -
- -
-
Beer’s law plots) are linear for cobalt and nearly linear for zinc. The average error is 2 p.p.b. for cobalt and 10 p.p.b. for zinc.
T
HE USES of
Eriochrome dyes both as chromogenic agents and as metallochromic indicators have been reviewed (1, a). Until recently their use as chromogenic agents had been largely restricted to the determination of calcium magnesium ions. Within the past few years, horrever. the use of 1 - (1 - hydroxy - 2 - naphthylazo) - 6nitro-2 naphthol-4sulfonic acid (Eriochrome Black T) and 1 - ( 2 - hydroxy-lnaphthy1azo)-2 naphthol - 4 - sulfonic acid (Eriochrome Blue Black, R) in the indirect spectrophotometric determination of calcium (Q), magnesium (Q), titanium (5), and thorium (6) ions has been described. Our work has
shown that the indirect spectrophotometric determination of copper, zinc, cobaltous, cadmium, and nickelous ions, using Eriochrome Blue Black R, is possible in simple solutions containing no interfering ions. Clearly, Eriochrome Black T and Eriochrome Blue Black R show promise of becoming chromogenic agents of rather wide application. Their principal disadvantage is lack of selectivity. A number of methods of increasing the selectivity of chelate formation have been suggested in connection with the analogous problem of achieving selectivity in titrations involving (ethylenedinitri1o)tetraacetic acid (EDTA) and related chelating agents. Singly or in combination, they should be applicable to many specific problems of elimination of interference or to simultaneous multicomponent spectrophotometric analysis. VOL. 34, NO. 12, NOVEMBER 1962
1657
