Solubilization of Sulfonic Acids for Infrared Studies Some Analytical Applications MEYER DOLINSKY and CHARLES STEIN Division of Cosmetics, Food and Drug Administration, Department o f Health, Education and Welfare, Washington 25, D. C. Simple procedures are described for solubilizing many sulfonic acids, as well as certain other types of compounds, in the common infrared solvents, carbon disulfide and carbon tetrachloride. Solubilization i s accomplished by the use of Amberlite LA-2, a liquid anion exchange resin. Where applicable, the method eliminates the uncertainties due to crystal differences, encountered in the use of mulls, potassium bromide disks, or dispersions in other media. Some quantitative applications are described.
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HE investigation of sulfonic acids and their derivatives by infrared methods is difficult because of the insolubility of such compounds in the 1000
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WAVE LENGTH MICRONS
Figure 2.
Spectra
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A. Carbon disulfide vs. air 8. 5% Amberlite LA-2 in carbon disulfide vs. air C. Amberlite, LA-2, film
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Extraction procedure
A.
FD&C Red 4 [2-(5-sulfo-2,4-xylylazo)-lnaph!hol-4-sulfonic acid], 8 mg./ml. 8. FD&C Yellow 6 (1 -p-sulfophenylazo-2-
naphthol-6-sulfonic acid), 6 mg./ml. C. FD&C Red 2 [ 1 -(4-sulfonaphthylazo)-2naphthol-3.6-disulfonic acid], 6 mg./ml.
solvents that are suitable for use in this spectral range. hfulls, potassium bromide disks, or dispersions in other media may be used, but the spectra of solids are influenced by variations in crystal form or formation of niised crystals, and many sulfonic acids and their salts exist in several crystalline modifications. d r d and Fontaine ( 1 ) have solubilized certain carboxylic acids in carbon disulfide and carbon tetrachloride solutions by adding up to 2 . 5 7 , of triethylamine, but their method does not solubilize sulfonic acids. This paper describes a simple method of solubilizing many sulfonic acids, as well a s certain other acidic compounds, in carbon disulfide. The solubilizing agent is Amberlite LA-2, a liquid anion
exchange resin. According to the manufacturer, Amberlite LA-2 is a mixture:of secondary amines with a molecular weight of 353 to 359 and a n acidbinding capacity of 2.2 to 2.3 meq. per ml. Carbon disulfide solutions of the acidresin salts can be obtained in several nays. I n one method, a n aqueous solution of the sulfonic acid (or its salt) is acidified and the sulfonic acid is extracted with a 5% solution of LA2 in carbon disulfide (extraction procedure). Some substances, such as sulfanilic acid or N-ethyl-N-phenylsulfobenzylamine, are not completely extracted under these conditions, but can be solubilized by treating the solid material with a 5% solution of LA-2 in ethanol, VOL. 34, N O . 1, JANUARY 1 9 6 2
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and a final 5.0-ml. portion. Wash the contents of the separatory funnel twice by shaking with small portions of carbon disulfide. Evaporate the combined extracts and washings on a steam bath t o a volume of about 10 ml. Transfer quantitatively to a 25-ml. volumetric flask and make to volume with carbon disulfide. Pour the carbon disulfide solution into a glass-stoppered 125-m1. flask containing 4 to 5 grams of anhydrous sodium sulfate, shake, and allow to stand for 1 to 2 minutes. Determine the infrared spectrum of the solution, using a blank solution carried through the same procedure, in the reference beam. Ethanolic Procedure. Transfer the sample to a 100-ml. beaker. Add 25.0 ml. of Solution B. Remove the alcohol on a steam bath and continue heating for 10 t o 15 minutes after the odor of alcohol can no longer be detected. Take up the residue in about 10 ml. of carbon disulfide, warming if necessary. Transfer the
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