from reaction mixtures. h receiver, similar to the receiver of this still but of reduced dimensions, has been used recently in this laboratory for the recovery of tiny amounts of pure components from a gas liquid chromatography apparatus (after admission of a complex mixture). I n this context, it is of interest to note that mercury was injected into the receiver to recover the material thus collected, a technique of possible relevance to the work discussed in the preceding sections. All three stills described in this paper appear to be applicable to genetic and isotopic studies in plants and to certain toxicological studies.
ACKNOWLEDGMENT
The authors thank the Director and Trustees of the Museum of A4pplied Arts and Sciences for making available facilities to carry out this work, and H. H. G. McKern for his valued advice. The assistance of J. If,Gauke, C. Debenham, and E. Ham must also be gratefully acknowledged. LITERATURE CITED
(1) Bryant, L. H., Forestry Commission of N. S. Wales Tech. A’otes, 4 (special issue), 6-10 (1950). ( 2 ) Cheronis, S. D., “Micro and Semimicro Methods,” Vol. VI of “Technique of Organic Chemistry,” 1st ed., pp. 7084, Interscience, New York, 1954.
(3) McKern, H. H. G., Smith-White, S., Australian Chem. Inst. J . d% Proc. 15, 276-8 11948). (4) McKern, H. H. G., Spies, M. C., Willis. J. L., “Researches on Essential Oils of the Australian Flora,” 3, 7-14, Museum of A~alied$rts and Sciences. Sydney, 1953.’ ( 5 ) Penfold, A. R., Morrison, F. R., McKern, H. H. G., Ibid., 1, 3-11. Museum of Applied Ar ts and Sciences, Sydney, 1948. (6) Reitsema, R. H., Cramer, F. J., Farr. S . E.. J . A a r . Food Chem. 5. 779-80 (1957). ( 7 ) Seidell, A,, “Solubilities of Organic Compounds,” Vol. 2, 3rd ed., p. 683, Van Nostrand, Sew York, 1941. A
RECEIVEDfor review April 9, 1962. Accepted August 6, 1962.
Acid-Base Indicators in the Binary Solvent, Acetic Anhydride-Acetic Acid ORLAND W. KOLLING and THOMAS L. STEVENS Department of Chemistry, Southwestern College, Winfield, Kan.
b The effect of the composition of acetic anhydride-acetic acid mixtures upon the half-neutralization number (pL1:2) was determined for the indicator bases, acridine orange, brilliant green, rhodamine B, and thioflavine T. The apparent base strength of each dye increased with increasing mole fraction of acetic anhydride, with the greatest influence occurring in the range 0 to 0.30. In acidity function measurements the logarithm of the color ratio for each dye was a linear function of pCaclo,, but the slope differed from unity. The least basic dye, rhodamine B, was demonstrated to be a suitable indicator for the spectrophotometric titration of leveled and intermediate bases with perchloric acid in the mixed nonaqueous solvent containing 7Oy0 (v./v.) acetic anhydride.
A
has been used as a nonaqueous medium for the titration of uncharged and anionic bases m-ith perchloric acid, in which the equivalence point is usually detected potentiometrically (3, 10-12). Bases that are too weak to be determined in acetic acid can be titrated in the presence of acetic anhydride ( 3 ) . However, acetic anhydride is not a usable solvent in the titration of bases capable of being acetylated. Limited information concerning the behavior of acid-base indicators in solvents containing acetic anhydride has been reported by Fritz and Fulda ( 3 ) . The present study was initiated t o extend t o this solvent system the same general experimental approach applied CETIC ANHYDRIDE
by the authors to the evaluation of indicator bases in glacial acetic acid (9). Since all other investigators have utilized binary mixtures with varying (and often unspecified) composition with respect to a second solvent component, the solvent medium in this study included a range from 0 to 70% acetic anhydride in acetic acid. The indicator properties evaluated were the halfneutralization numbers, empirical acidity function measurements, and equivalence point determination in spectrophotometric titrations. EXPERIMENTAL
Apparatus. Absorbance measurements and spectrophotometric titrations were made with a Bausch & Lomb Spectronic 20 spectrophotometer. The Beckman Model H2 p H meter, equipped with the usual glass calomel electrode pair, was used in all potentiometric titrations in which the equivalence point was determined from the second derivative of electromotive force with respect t o volume increment. Reagents and Solutions. The glacial acetic acid (Mallinckrodt X.R.) was used without additional purification; water content (by Karl Fischer titration) was within the range of 0.002 t o 0.008%. Reagent grade acetic anhydride was distilled, using a 34-cm. Raschig column, and solvent having a boiling range of 138.5” t o 139.5’ C. (corrected t o 1 atm.) was used in all acetic acid-acetic anhydride mixtures. Stock solutions in glacial acetic acid of 0.1M perchloric acid and the bases, cadmium acetate, lead acetate, N , N dimethylaniline, sodium acetate, and
sodium formate, were prepared and potentiometrically standardized as before (8, 9). Solutions of the dyes, as well as the potassium salts, in the same solvent were prepared determinately. The final concentration of the dye in the determination of the pL1I2values and in the acidity function measurements was in the range 10-6 to 10-’M. PROCEDURE
Spectrophotometric Titrations. T h e course of the spectrophotometric titration of each base mas followed by measuring the absorbance of the acid form (470 mp) of rhodamine B (8).After correction of the absorbance for the dilution occurring during the titration, the end point was determined by the two graphic procedures of Hummelstedt and Hume (6) and Higuchi et al. ( 6 ) . The 70y0 (v./v.) acetic anhydride composition of the solvent refers to the make-up of the base plus solvent mixture at the start of the titration. RESULTS AND DISCUSSION
Indicator Constants. From the 14 dyes evaluated in the previous studies 9 ) , four tertinry on glacial acetic acid (8, amines were selected to determine the effect of the binary solvent composition upon the indicator half-neutralization numbers. Perchloric acid was the reference acid. Because the solubility of potassium perchlorate changes with the composition of the solvent, the regulation of ionic strength using this solute could not be continued. However, a fixed stoichiometric concentration of 0.004M sodium perchlorate was maintained for the salt concentration to be at least ten times that of the acid and a hundred times that of the indicator VOL. 34, NO. 12, NOVEMBER 1962
0
1653
salt in all solutions. At a salt concentration below 0.005M the fraction of the salt in ion aggregates larger than ion pairs can be neglected. The curves showing pLl,t as a function of the mole fraction of acetic anhydride are plotted in Figure 1. Because of the very low solubility of sodium acetate in acetic anhydride and the formation of a yellow color in perchloric acid solutions in this solvent, the data could not be extended to pure acetic anhydride. The spectrophotometrically determined constants show clearly that the indicators are stronger bases in acetic anhydride than in acetic acid, and that these neutral bases are not leveled by the addition of acetic anhydride. The observations of Streuli (12) from half-neutralization potentials also show that aromatic amines are not leveled. The greatest change in the pL,z value occurs below a mole fraction of 0.3 or about 40% (v./v.) acetic anhydride. The precision of the measured indicator constants was poorer in the binary solvent than in glacial acetic acid. The average deviation ranged from 10.03 unit on pLlil for rhodamine B and acridine orange to h0.26 unit for brilliant green and thioflavine T. I n the more polar binary solvent mixture, water-dioxane, Bell and Robinson (1) found a linear relationship between the pK. value for weak acids and the mole fraction of dioxane for the range 0 to 80% dioxane. For the acetic acid-acetic anhydride system,
Table 1.
2 -I
n -4
0
Rhodamine B Thioflavine T
0.8
0.4
X Figure 1 . The change in indicator half-neutralization number (pLl/J with composition of the binary solvent X is the mole fraction of acetic anhydride in the acetic acid-acetic anhydride solvent. Curves are: [A) brilliant green; (6) thioflavine T; (C) acridine orange, and (D) rhodamine B
an approximately linear relationship exists between the pL1,2 of the base and the mole fraction for mixtures greater than 0.38 in anhydride. Acidity Function Measurements. Although perchloric acid reacts as a strong acid in glacial acetic acid, its pKd value is 4.87 in t h a t solvent. However, the logarithm of the indicator color ratio is a linear function of the analytical concentration of the
70% (v./v.) Acetic Anhydride
Sodium Perchlorate-Perchloric Acid PLliZ PCHClOl Range -4.20 -5.20a -3.96 -4.83
t
-3t
Acidity Function Measurements in
Dye Acridine orange
a
-5
3.75-4.30 3.20-3.60 2.95-4.10 3.50-4.10
Slope 1.36 f0 . 1 0 . 3 2 f 0.05 1.20 z!= 0 . 1 0 . 2 3 & 0.03
N,N-dimethylaniline Perchlorate-Perchloric acid -5.20 3.25-4.75 0.22 f0.03 Acridine orange Rhodamine B -3.96 3.25-4.25 0.88 iz 0 . 0 5 Determined graphically by extrapolation to the condition where log I A / I B= 0. Table II. Spectrophotometric Titrations in
70% (v./v.) Acetic Anhydride
Meq. found Relative Base PKa 7 e error ( % 1 Cadmium acetate ... 0.975 0.962 0,968 -0.7 Lead acetate ... 0.112 0.110 0.110 -1.8 N,N-dimethylaniline 5.2 0.496 0.494 0.493 -0.6 Potassium bromide - 7.9 0.100 0.0976 0.0985 -1.5 Potassium acid phthalate d 0.251 0.252 0,253 0.8 0.0970 -3.0 0,0944 -10.7 0.100 Potassium iodide 0.0975 -2.6 0.0953 - 1.3 0.100 Potassium nitrate Sodium acetate d 0.488 0.488 0.0 0.488 Sodium formate d 0.485 0.2 0.484 0.486 Based upon potentiometric standardization of solutions, except for the potassium salts which were used determinately. b Equivalence point determined by the method of Hummelstedt and Hume (6). Equivalence point obtained from the Type I1 plot (4). d These solutes are leveled in the presence of acetic anhydride ( 1 2 ) . 0
1654
ANALYTICAL CHEMISTRY
acid, thereby permitting the definition of an empirical acidity function in terms of perchloric acid (2, 7). Several indicators have been applied to the evaluation of PCHC~O, in acetic acid (8, 9). The acidity function measurements in acetic anhydride were made on solutions containing a reference base, the base perchlorate, and perchloric acid. The data obtained for 0.005M baseperchlorate salt solutions containing varying concentrations of perchloric acid, and using three dyes in the binary solvent having a mole fraction of 0.585 acetic anhydride are summarized in Table I. Two reference bases were used: sodium acetate (a strong base leveled in this solvent); and N , N dimethylaniline (an intermediate base). Although plots of the log (acid form)/ (base form) vs. log (analytical concentration of acid) gave linear graphs of acceptable precision, the slopes differed from the value of 1.00 found in pure acetic acid as the solvent. I n sodium perchlorate mixtures, acridine orange gave a difSerent slope at higher concentrations of perchloric acid than a t low, suggesting that the indicator base reacts separately with two equivalents of acid. This is consistent with the data on the same dye in N,N-dimethylaniline perchlorate (the salt of a weaker base) in which only one linear function of low slope is obtained. With a given strong or intermediate reference base, rhodamine B (the weakest base studied) can be used to determine a 1.5-unit range of PCHCIO, from empirical plots for the specific system. For plots having slopes considerably less than 1.0, the reaction is not only one between the acid and indicator base, but includes the measured net effect of the competition of two bases of similar strength (one a dye and the other colorless) for the available protons of the added perchloric acid. Acridine orange and N,N-dimethylaniline exemplify such a base pair, and thioflavine T (the strongest basic indicator studied) seems to have a base strength approaching that of sodium acetate. From studies on the reaction of indicator bases with low concentrations of HCl in aprotic solvents of low dielectric constant (but greater than 6.1), Hummelstedt and Hume (6) have proposed that a slope greater than one may result from the association of the proton acid with the indicator salt and that such a slope does not necessarily require the formation of higher ionic aggregates for the indicator salt. Because the binary solvent used in the present investigation is largely acetic anhydride, but yet contains significant amounts of acetic acid, either or both indicator salt-perchloric acid association and indicator salt association into higher ionic aggregates may be responsible for the abnormally large slopes for acridine orange and
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. w.. 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
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 a n 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
