Determination of Sodium Dodecyl Sulfate in the Presence of Lauryl Alcohol Robert D. Vold and Kashmiri L. Mittall Department of Chemistry, University of Southern California, Los Angeles, Calif. 90007
ALTHOUGHSODIUM DODECYL SULFATE (SDS) can be satisfactorily determined by a two-phase titration method (1-4) with cationic detergents in the presence or absence of inorganic salts, no information is available on the effect of the presence of fatty alcohols on the accuracy of the method. This is an important matter since such analyses are necessary for determination of adsorption of surfactants used as emulsifiers or suspending agents, and in the study of molecular complexes between the alcohols and the surfactants. Epton ( I ) states that the cationic detergent titration method is most precise when the concentration of SDS calculated on the basis of the total aqueous phase after addition of methylene blue solution is between 0.86 to 2.29mM (10 ml of 0.003 to 0.008M SDS, 25 ml of Me B solution), all of which concentrations are below the critical micelle concentration (cmc) of solutions of pure SDS (5, 6). At higher concentrations, there was difficulty with formation of emulsions of chloroform and water which were too stable, while at lower concentrations the titer was too small. In the present work the effect of the concentration of the SDS on the applicability of the titration method was investigated both above and below the cmc, and with and without added lauryl alcohol. EXPERIMENTAL
The sodium dodecyl sulfate used was obtained from Eastman Kodak. The surface tension-concentration curve of this particular preparation was determined by the DuNuoy ring method and showed a minimum of about 1 dyne per cm in the vicinity of the cmc below the limiting constant value of 38.5 dynes/cm. By interpolation from the results of Miles and Shedlovsky (7), this corresponds to the presence of not more than 0.05 Z lauryl alcohol as an impurity, a sufficiently small amount to be inconsequential for the present work. The cetyl pyridinium chloride used (CPC) (from Matheson, Coleman and Bell) was determined to be 98.0% active by two-phase titration against a pure sample of SDS (8) which had no inorganic impurity and showed no minimum in the surface tension-concentration curve. The lauryl alcohol (Matheson, Coleman and Bell) was fractionally distilled under vacuum, and the fraction used was shown to be free of any consequential amounts of impurity by gas chromatography. Methylene blue was used as received from the National Aniline Division of Allied Chemical Company. All other chemicals were of A.R. grade. Present address, Department of Chemistry, University of Pennsylvania, Philadelphia, Pa. (1) S. R. Epton, Trans. Faraday Soc., 44, 226 (1948). (2) A. S. Weatherburn, J. Amer. Oil Chem. SOC.,28, 233 (1951). (3) R. D. Vold, and R. C. Groot, J. Phys. Chem., 66, 1969 (1962). (4) V. W. Reid, G. F. Longman, and E. Heinerth, Tenside, 9, 292 (1967). (5) J. N. Phillips and K. J. Mysels, J. Phys. Chem., 59, 325 (1955). (6) S. J. Rehfeld, ibid., 71, 738 (1967). (7) G. D.Miles and L. Shedlovsky, ibid., 48, 57 (1944). (8) R. D. Vold and A. K. Phansalkar, Rec. Trac.. Chim. Pays-Bas, T74,41(1955).
Table I. Determination of Sodium Dodecyl Sulfate in the Presence of Lauryl Alcohol SDS solution taken for Volume of analysis 60 ml M1 of Concn of CPCa CPC required, containing added in titrant, in replicate initially mg/ml LOHb mdml titrations, ml 4 4
0 1
28.90, 28.95 29.00, 29.10 0 29.10, 29.10 1 29.15 0 29.10, 29.15 1 29.20, 29.15 a Concentration of SDS in the aqueous phase before addition of n
10 10 5 5 2.5 2.5
28..80, 29.00, 29.15, 29.10, 29.20, 29.15,
the aqueous methylene blue solution. bSDS is sodium dodecyl sulfate; LOH, lauryl alcohol; CPC, cetyl pyridinium chloride.
In the titration procedure followed in this work, the volumes of the organic and aqueous phases were always kept constant, respectively, at 15 and 85 ml, since Vold and Groot (3) had found that this procedure eliminated the need for a blank correction which had been suggested by Weatherburn (2). Sixty milliliters of SDS solution (1 to 4 mg/ml) were taken in a 250-ml glass-stoppered bottle containing 25 ml of methylene blue chloride solution (prepared by adding 0.016 gram of methylene blue chloride, 25.0 grams of anhydrous sodium sulfate, and 4.4 ml of concentrated sulfuric acid to enough water to make 500 ml of aqueous solution). Then 15 ml of chloroform were added, the contents of the bottle mixed thoroughly for 1 minute, and allowed to stand for 1 minute to form a colorless top layer and a dark blue bottom layer. During titration, the CPC solution was at first added from the buret in 1-ml portions, and the bottle vigorously shaken and allowed to stand for 1 minute before further addition of the titrant. As the end point was approached, the titrant was added in 0.1-ml quantities until an equal depth of color was obtained in the aqueous and chloroform phases. In titrations in the presence of lauryl alcohol, it was added directly to the aqueous SDS solutions and between 10 and 30 minutes was allowed for attainment of solubilization equilibrium before titration with the CPC solution was started. At high concentrations of SDS and lauryl alcohol, the bottom blue layer had a whitish tint but this did not interfere with detection of the end point. In agreement with Weatherburn’s observations (2), it was found that vigorous shaking and standing for a minute after each addition of CPC solution was necessary in order to obtain sharp reproducible end points. RESULTS A N D DISCUSSION
The results obtained are shown in Table I. It is clearly apparent that in all cases identical amounts of cetyl pyridinium chloride were required for the titration of the sodium dodecyl sulfate independent of the presence or absence of lauryl alcohol in the solution. This is equally true both above and below the critical micelle concentration, since even though ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
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Table 11. Comparison of Concentration Ranges in the SDS-CPC Titration Used by Different Workers
Original concn of SDS solution, mM/ml
Authors Epton ( I ) 3.00-8.00 Weatherburn (2) 1 .74 Present work 3.47-13.88
Final volume of aqueous phase after Volume of SDS addition solution of Me B solution, used, ml ml 10 1C-40 60
35 75 85
Final concn of SDS in aqueous phase, mM/ml 0.86-2.29 0.23-0.93 2.45-9.80
Schick and Fowkes (9) found that the cmc of SDS was reduced to one third of its initial value in the presence of 22.2 lauryl alcohol, the present solutions containing initially 1 mg of SDS/ml (3.47mM/ml) would still be expected to be below the cmc (0.238 wt. (8.lmM) for pure aqueous SDS) based on the volume of the aqueous phase after addition of CPC solution. The presence of micelles in the solution apparently does not interfere with the exchange reaction, (9) M. J. Schick and F. M. Fowkes, J . Phys. Chem., 61,1062 (1957).
+
SDS.methylene blue CPC + SDS-CPC methylene blue, which provides the end point of the titration by developing a blue color in the aqueous phase. Two additional conclusions can be drawn from the data in Table I. Although variable times of standing between 10 and 30 minutes were allowed after addition of lauryl alcohol before titration with CPC, good precision among replicate runs was obtained in all cases. This suggests that the solubilization of lauryl alcohol by the SDS solution was complete in 10 minutes, although there has been no study of the kinetics of this process. The absence of a dilution effect, as reported by Epton ( I ) , in the present results may be due to use of a higher concentration range of SDS (cf. Table 11). This would be in accord with Epton’s observation that the dilution effect became smaller at higher concentrations. A comparison of the concentration ranges of SDS studied by different workers is shown in Table 11. It is evident from these results that the method is satisfactorily applicable over a wide range of concentration of SDS from 0.86 to 9.80mM, (based on the concentration in the aqueous phase after addition of methylene blue solution but before titration with CPC solution), and need not be restricted to the concentration range between 0.003 and 0.008M used by Epton (1). RECEIVED for review July 22, 1971. Accepted November 4, 1971. Support of a fellowship which made possible this work is gratefully acknowledged to the Foods Division of the Anderson Clayton Company.
Use of Metal Tungsten Bronze Electrodes in Chemical Analysis M. A. Wechter,’ H. R. Shanks, G. Carter, G. M. Ebert, R. Guglielmino, and A. F. Voigt Institute for Atomic Research and Departments of Chemistry and Physics, Iowa State University, Ames, Iowa
THEMETAL TUNGSTEN BRONZES, nonstoichiometric compounds of formula M,W03 function as indicating electrodes in several widely differing electrochemical systems. Current investigations indicate that electrodes made of the bronzes may be used to determine pH, pMetal for certain reducible species such as Ag(1) and Hg(II), and to follow potential changes involved in some oxidation-reduction systems. Even more significant perhaps, is that they appear to function as oxygen sensitive electrodes. The use of tungsten bronzes as “catalytic” electrodes in fuel cells has been reported ( I - j ) , but little has been published on applications as measuring electrodes. Recently a Russian group has published several papers (4-6) on uses of bronzes 1 Present address, Chemistry Department, Purdue UniversityCalumet, Hammond, Ind.
(1) A. Damjanovic, D. Sepa, and J. O’M. Bockris, J. Res. Inst. Catal. Hokkaido Univ., 16, 1 (1968). f2) D. Seva. A. Damianovic, and J. O’M. Bockris, Electrochim. Acta, 12,746 (1967): 13) L. W. Niedrach and H. I. Zeliger, - J. Electrochem. SOC.,116, 152 (1969). (4) A. G . Kohsharov and V. F. Ust-Kachkintsev, Uch. Zap. Perm. Gos. Univ., 111,63 (1964); Chem. Abstr., 64,16178(1966). (5) Ibid, 178, 117 (1968); Chem. Abstr., 73,6197e (1970).
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in pH and other measurements. At this time details of these experiments are not available to the authors. The metal tungsten bronze crystals used for this work were prepared by the fused salt electrolysis of the appropriate metal tungstate and WOs. The details of the single crystal growth are available elsewhere (7). The compositions of the bronze crystals were determined either directly by neutron activation analysis or from the measurements of lattice constants. The relationship between lattice constants and composition has been determined by neutron activation analysis (8). Individual pieces, single crystals, of bronze to be used for electrodes were cut from larger crystals with a diamond saw. The electrodes were prepared by attaching the crystal to one end of a glass tube with epoxy cement. The tube was partly filled with mercury which made electrical contact to the crystal. A copper wire could then be inserted in the mercury and used as an electrical lead to the electrometer used for potential measurements. Details of the electrode are shown in Figure 1. (6) A. G. Koksharov and V. F. Ust-Kachkintsev, lzv. Vyssh. Ucheb. Zaved. Khim. Khim. Tekhnol., 10, 243 (1967); Chem. Abstr., 67, 39.5126 (1967). (7) H. R. Shanks, J. Crystal Growth, in press. (8) M. A Wechter, H. R. Shanks, and A. F. Voigt, Inorg Chem., 7, 845 (1968).
