Determination of Alkylbenzenesulfonates by Ultraviolet Spectrophotometry SIR: I n the course of investigations on the adsorptive removal of a number of alkylbenzenesulfonates from aqueous solution we have found that precise quantitative determination of the concentrations of these materials in dilute aqueous solution can be made rapidly and conveniently by measurement of ultraviolet absorption at 225 mp. Although the method is not generally suitable for determining concentrations of these compounds in complex u-astes becausr of iiumerous interferences, it is far superior in simplicity, accuracy. and sensitivity for work n ith synthetic solutions to methods currently being used. Rcid. ,41ston, and Young (2) in 1955 proposed the use of ultraviolet spectrophotometry as a method for qualitative analyses of surface-active agents for the characterization of commercial detergent products. The analytical procedure developed b y those authors provided for the identification of the nature of the surface-active material in a detergent compound by comparison of an e~perimental ultraviolet spectrum for the compound with spectra for various reference compounds of known chemical structure. Dodecylbenzenewlfonate Table 1. Molar Absorptivities for Alkylbenzenesulfonates
Molar absorpII-ave-
Compound Benzenesulf onnte p-Toluenesulfonate Benzenesulfonates, (2hexyl)-, (2-octyl)-. (2dec;-l)-, (2-dodecyl)-, (3-dodecd)-. (6dodecy1)-, (?-tetradecl-1)
Table 11.
tivity, .If-'
length,
cm. -l
nip 21.5 220
X10-3 7 9 10 9
225
12 9
Analytical Data for Solutions of Alkylbenzenesulfonates
Solute (2-0ctJ 1)benzenesulfonnte
(2-Drcy1)benzenesulfonnte
(2-Dodecy1)benzeneeulfonate
Concn. taken, mg./'l. 0 29 0 58 1 75 2 34 1 2G 2 56 3 20 3 84 0 35 1 40 2 09 2 59
1844
was one of the surface-active agents for which Reid and coworkers measured a spectrum. A similar technique has been adopted b y the American Society for Testing and Materials for characterizing commercial detergents (1). I n the present work the use of ultraviolet spectrophotometry has been estended to the quantitative determination of alkylbenzenesulfonates in very dilute aqueous solutions. Of particular interest is the observation that all of the long-chain alkylbenzenesulfonates studied exhibit maximum absorption a t the same wavelength and have equal molar absorptivities at this wavelength within the precision of experimental measurements. Hence, the molar concentration of the homologous and isomeric alkylbenzenesulfonates commonly found in commercial detergents can be evaluated regardless of the relative amounts of individual alkylbenzenesulfonates present. It is also possible to approximate an average molecular weight b y this procedure. For example, a technical-grade alkylbenzenesulfonate specified by the manufacturer t o contain 92.9% active material consisting predominantly of dodecylbenzenesulfonate was determined b y ultraviolet spectrophotometry t o have an average molecular weight of 372 as compared with 349 for pure dodecylbenzenesulfonate. An essential feature of the method is the use of dihydrogen phosphate in the sample solution to minimize the strong tendency for alkylbenzenesulfonates to adsorb on the quartz surfaces of the absorption cells and on the surfaces of glass vessels used in preliminary steps: such adsorption tends to decrease tile accuracy and precision of the analytical procedure, particularly at low concentrations. I n the presence of 10-2A1f K H 2 P 0 4 adsorption is reduced enough to permit quantitative determinations
ANALYTICAL CHEMISTRY
Absorbanre 0 069 0 125 0 0 0 0 0
379 505 250 505 639
0.773 0 067 0 257 0 382 0.509
Concn. found, mg./l.
Difference, mg./1.
0 32 0 56 1 72 2 28 1 25 2 50 3 17 3 84 0 36 1 39 2 07 2.75
+0 03 -0 02 -0 03 -0 06 -0 03 -0 06 -0 03 f O 00 +0 01 -0 01 -0 02 -0.04
a t concentrations 3 x 10-6JP or less. The dihydrogen phosphate should be used not only in the solution for measurement, but also as a preliminary rinse for all absorption cells, pipets, and other pieces of apparatus that may come into contact with the detergent solution. PROCEDURE
Pipet a 10.0-ml. sample from the solution to be analyzed, using a pipet that has been rinsed with 10-2111 KH2P04. Introduce the sample into 10.0 ml. of 2 X 10-2X K H 2 P 0 4 in a small beaker or flask and mix thoroughly. Transfer a suitable portion of the mixed solution to a 5-cni. quartz cell that has been rinsed with 10-2L11 KH9POIand measure the absorbance a t 225- m i against a blank of 10-2J1 KH,PO+ RESULTS
Aqueous solutions of alkylbenzenesulfonates exhibit intense characteristic absorption of ultraviolet radiation, as shown in Figure 1. After the ultraviolet spectra for solutions of a number of these substances had been measured from 320 to 210 mp with a Beckman Model DK-2 spectrophotometer, the absorption maximum for each of the substances was precisely located with a Beckman Model DU spectrophotometer. Subsequent spectrophotometric determinations were made with the Model DU. For the range of concentrations studied (lo-@ to 10-4Alf) all of the aqueous solutions of these compounds exhibit excellent conformity to the Beer-Lambert law, showing good proportionality of absorbance to concentration at the warelength used. The molar absorptivities calculated from the calibration data are listed in Table I. The data in Table I1 show something of the precision and sensitivity of this method for simple solutions of alkylbenzenesulfonates. Concentrations of 0.3 to 3 mg. per liter are determined readily within 0.03 mg. per liter. DISCUSSION
Benzenesulfonate in dilute aqueous solutions begins to absorb in the ultraviolet at about 230 mp, as shown in Figure 1, the absorbance increasing with decreasing wavelength until a plateau is reached a t mvelengths less than about 215 mp. When a methyl group is substituted para t o the sulfonate group, the absorbance is increased and occurs as somejvhat longer wavelengths;
06-
[
\HIGHER
polyethoxyethanol class indicate that these substances also have a,n absorption maximum at 225 mp, but the molar absorptivities are somewhat smaller, about 9.5 X lo3. l17hile adsorption uf the nonionic detergents on the surfaces of absorption cells and other glassware does not seem to be as much of a problem a,s does that of the alkylbenzenesulfonates, the use of dihydrogen phosphnte as a rinsing and dispersion nietiium appears also to enhance the Irecision of measurements of these materials.
i
OLKYLBEI’;ZENESULFONA~~~
05 W
0
2
04
LL 0
2 a
0.3 02 01
c
210
Figure 1.
220
230
240 260 260 WAVELENGTH, rnp
LITERATURE CITED
300 320
(1) Am. Soc. Testing Mat,erinls, Phil-
adelphia, Pa., “.4ST;\.I St,andards,” Part 10, pp. 1045-8, 1961. (2) Reid, V.W., Alston, T., Young, B. IV., -4ndyst 80, 682-9 (1955).
Ultraviolet spectra of alkylbenzenesulfonates Concentration 1 .OO X 1 0-SM 5.00-cm. cell
thus ~-tolueiiesulfonatehas an absorption maximum near 220 mp. With further increase in size of the p:dkyl group the shift in wavelength and inten4tj- ot nisnimum abqorption continucs. Iioth Tvavelength and intensity have reached limiting ~ n l u e sat 225 mp and 12.9 x IO3 molar absorptivity, respectively, for alkyl groups containing
si-i or mo1e carbon atoms. It is this property that makes the technique useful not only for solutions of the pure compounds but also for common mixtures of the long-chain alkylbenzenesulfonates with strong surface-actil e and detergent properties. Preliminary experiments ivith nonionic detergents of the alkj-lphenosy-
WALTERJ. WEBER,JR. J. C A R R E L I . A I O R R I S J T E R S E R STUB131
Division of Engineering and Applied Physics Harvard University Cambridge 38, Mass. RESEARCH supported in part by the I-,d.
Fublic Health Service, Contract SAph (6295, and by N. I. H. Research Fellowship GF11,989 (147.J. Weber, Jr.).
Extension of the Use of Lanolin as a Substrate for Gas Liquid Chromatography SIR: l y e have previously reported the use of lanolin as a substrate in a gas chromatographic procedure for the determination of thymol and its isomers ( 3 ) . Khile the thymols are derivatives of 772-cresol, o-cresol on isopropylation also yields four isomers, the best known of which is carvacrol. The carvacrol isomers can also be resolved by lanolin in a similar manner as described for the thymol series.
point data or order of volatility during distillation in their studies of the isopropyl cresols. The isomeric composition of a mixture of carvacrols is of practical as well as academic use. Carvacrol, principally, is found in a variety of essential oils and used industrially ( 2 ) . EXPERIMENTAL
Apparatus. A Perkin-Elmer Model l54D Vapor Fractometer equipped
with 5000-ohm thermistors was used. Column Preparation. Lanolin, ticodorized U S P (Robinson-Wagner), 10% by n-eight was used on 60- to SO-mesh acid washed Chromosorb-K (Johns - Manville). After slurryiiiq with acetone, evaporating, drying, ant1 sieving, 16 grams of t h e support iv:i\ packed into 3 meters of 1 4-inch coplwr tubing. The isomers used were originally prepared bv Carpenter and Easter for their studies. P h i t i e s were confirmed by GLC and IR.
A 3-Isopropyl-2methylphenol
‘i
4-Isopropyl-2methylphenol
4 ISOPROWL-2~H~LRIEOL
5-Isopropyl-2methylphenol (carvacrol)
6-Isopropyl-Pmethylphenol
Carpenter and Easter ( I ) list some physical constants for carvacrol and its isomers but do not mention boiling
Figure 1.
The four carvacrols
Lanolin 10% on Chromosorb-W, 60- to 80-mesh, 3 meters, 160’ C.
VOL. 34, NO. 13, DECEMBER 1962
1845
