Spectrophotometric determination of anionic surfactants by conversion of the leuco-bases of triphenylmethane dyes

Analytica Chimica Acfa, 141 (1982) 419--425 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherkmds

Short Communication SPEC’I’ROPHOTOMETRIC DETERMINATION OF ANIONIC SURFACTANTS BY CONVERSION OF THE LEUCO-BASES OF TRIPHENYLMETHANE DYES

HARVEY

POBINER*

and HENRY

American Can Company, (U.S.A.)

T. HOFFMAN,

Jr.

Princeton Research Center, P-0. Box 50, Princeton, NJ 08540

(Received 11th January 1982)

Summary. A one-phase, one-reagent, spectrophotometric technique is reported for the detection and determination of anionic surfactants of the sulfonate and sulfate types. Both applications are based on conversion of the coIorless leuco-bases of triphenylmethane dyes (Brilliant Green and methyl violet) to colored (quinoid) forms by the anionic surfactank The quantitative method, in the absorption and reflectance modes, is applicable to anionic surfactants in the O-1500 ppm range; the spectrophotometric procedure can be automated. Residual surfactant can be detected at the 0.01% level on the surface of cellulosic products.

A simple spectrophotometric method for detection and determination of anionic surfactants is described. The chemistry is based on the conversion by anionic surfactants of the colorless leuco-base of a triphenylmethane dye to the colored (quinoid) form. The method was developed initially for detecting residual surfactant on a paper substrate in a manufacturing process. It was then developed further for the determination of surfactant in aqueous samples and subsequently for use in an on-line process analyzer. This work is an extension of the Abramovich method [l] which is described in Longman’s text [2] as having possibilities for wide application. The method has a significant advantage compared with the methylene blue procedure of ion pairing with an anionic surfactant [3, 41, in that no extraction of the dyestuff or the complex into a hydrocarbon phase is necessary. This obviates phase separation and is of particular value for on-line determinations. The quantitative procedure described below is based on the use of Brilliant Green (Basic Green I), which was selected from a study of similar dyes for its facility in being converted into a stable, colorless leuco-base that responds instantly to color restoration. Both sulfate (R-GS03Na) and sulfonate (R-S03Na) anionic surfactants respond to the method. Absorption spectrophotometric and reflectance methods are described below. The reactions of the leuco-base conversion for Brilliant Green are probably as follows.

0003-2870/82/0000-0000/$02.75

0 1982 Elsevier Scientific Publishing Company

Na2SO3 at pii 9

N(C2GJ2

I

Quno~d form

Longman [2] suggests that the reaction requires the presence of micelles provided by the dyestuff and the surfactant. Further, the critical micelle concentration of the surfactant is reduced by high concentrations of inorganic salts. The micelles probably e:_ elude sulfite ions and a colored ion-pair forms between the dyestuff and the surfactant. The method was principally used here for two sulfonate types, Arco A-OK (a misture of a hydroxyalkane sulfonate, RCH(OH)S03Na and an alkene sulfonate, RCH=CH
Experimen tai Instrumentation.

A Beckman model 5240 double-beam recording spectroreflectometer was used for method development. In actual practice, a simple calorimeter can be used. For continuous monitoring, the method was successfully tested on an on-line modular process calorimeter (Technicon Industrial Systems, Auto Analyzer II System). Chemicals. The eleven triphenyimethane dyes screened (Aldrich Chemical; Matheson, Coleman and Bell; Fisher Scientific) were used as received. Brilliant Green was Aldrich reagent no_ 86,087-5All solutions were prepared from analytical reagent-grade chemicals. The cr-olefm sulfonate tested is available from Chevron Chemicals and can also be cbtained as Ultrawet A-OK surfactant from Arco Chemical. The sodium dioctyl sulfosuccinate is available as a 75% solution from American Cyanamid. Sodium dodecyl sulfate (SDS) tested was from GallardSchlesinger

431 Chemical. The solid surfactants were dried at 105°C for 2 h before use; the liquid surfactant was used as received. Buffer, pH 9.0. Dissolve 2.384 g of sodium tetraborate hesahydrate in ca. 400 ml of deionized water, pipet in 23.0 ml of 0.1 M HCl and dilute to 500 ml with deionized water. Leuco-bases of Brilliant Green and methyl violet. Dissolve 0.05 g (20.2 mg) of Brilliant Green in 75 ml of methanol. Add 370 ml of deionized water followed by 6.0 g (+l mg) of sodium sulfite; the blue-green solution will become essentially colorless. Use 0.05 IM H2S04 to adjust the solution to pH 9.0. Add 25.0 ml of the pH 9.0 buffer and dilute to exactly 500 ml with deionized water. Store in a low actinic glass-stoppered vessel. The shelf life of this solution exceeds 4 months. Dissolve ca. 0.02 g of methyl violet in 10 ml of methanol and add the solution to ca. 300 ml of deionized water. Add 7 g of sodium sulfite, adjust to pH 9 with ca. 8 ml of 0.05 M H,SOJ and stir in 1 g of activated charcoal (20-40 mesh). Form a charcoal bed on a sintered-glass funnel (no. 40 medium) by adding a suspension of SO.05 g of finely ground activated charcoal in deionized water, allowing the charcoal to settle, and then applying suction. Filter the methyl violet leuco-base solution through this charcoal bed. The filtrate should be colorless; if it is not, repeat the charcoal treatment. Store in a spray bottle for the qualitative use; shelf life is about one month. Calibration procedure with Brilliant Green. Prepare a 2.00 g 1-l solution of the anionic surfactant in deionized water. Pipet the desired amounts (O-20 ml) of this solution into lOO-ml volumetric flasks and add 10.0 ml of the Brilliant Green leuco-base solution; swirl the flasks, and dilute to the mark with deionized water. After about 1 min, measure the absorbance at 630 nm, and set up a calibration graph of the least-squares equation for the linear range. Quantitative procedure for samples. Deliver 20--50 g of filtered aqueous sample into a lOO-ml volumetric flask; pipet in 10 ml of the leuco-base of Brilliant Green and swirl the flask gently. Prepare a control by pipetting 10 ml of the leuco-base into another flask. Dilute both solutions to 100 ml with deionized water and measure the absorbance at 630 nm. Correct for the control, and calculate the concentration of anionic surfactant in the sample from the calibration. If the absorbance is less than 0.1, repeat the procedure on a larger sample or with longer optical paths. Spectroreflectance for opaque samples. A spectroreflectance method is necessary to determine the anionic surfactants added to certain opaque commercial latex formulations. Prepare blends of sodium dioctyl sulfosuccinate at concentrations of O-0.15% (w/w) in latex. Pipet a series of lo-ml aliquots into loo-ml volumetric flasks so that the solutions contain O-00-O-010 g of surfactant. Pipet in 10 ml of the Brilliant Green leuco-base, and proceed as before. Run the absorbance curves in the total spectral reflectance mode. On the Beckman model 5240 spectroreflectometer, the I, is obtained with the control emulsion in a pair of l-in. light scattering cells in position on the barium sulfate-coated integrating sphere.

Qualitatiue test procedures_ The colorless leuco-base solution is most conveniently applied to a paper substrate from a conventional spray bottle. If no characteristic color develops within I min, then no significant anionic surfactant residue is present on the paper. The time restriction is necessary because the leuco-base gradually acquires color on exposure to air. Obviously, it is necessary to take precautions to avoid contamination from the surfactants which are frequently present in laboratory glassware or on bench tops. Results

and discussion

Procedures involving electrical conductivity, surface tension, titrimetry, foaming potential, and zeta potential were investigated, but did not offer the advantages of speed, limit of detection, and ease of on-line control available with the spectrophotometric method. A series of triphenylmethane dyes was evaluated for completeness of conversion to a colorless or lightly colored leuco-base, and for response of the leuco-bases to anionic surfactants. The results of this screening study, expressed as absorptivity relative to surfactant concentration, are listed in Table 1. Brilliant Green met the quantitative selection criteria better than any other dye tested: it provided a colorless and stable leuco-base; it did not form a sediment on standing (a problem with some triphenylmethane dyes); and it did not require filtration on preparation. The Brilliant Green leuco-base solution can replace the methyl violet leucobase solution for qualitative use. The former solution has the advantages that no charcoal treatment is needed for complete decolorization of the leucobase and that shelf life is longer. Because of the lower absorptivity of Brilliant Green, per weight of anionic surfactant, it is potentially less sensitive than methyl violet as a qualitative test solution.

T_\BLE 1 Triphenyhnethane surfactant

dyes -screened for leuco-base

conversion

and response to anionic

Dye

Leuco-base solution

Response to anionic surfactant

Wavelength (nm)

Absorptivity (I g-’ cm’ )

Aniline blue Basic fuchsine Brilliant Green Crystal violet Ethyl violet Malachite green osalate Methylthymol blue Methyl violet New fuchsin (Basic Violet 2) Patent blue Rosolic acid

Colorless Colorless Colorless Light violet Light violet Colorless Dark green Light violet Light yellow Intense blue Light red

None Weak Strong Strong Weak Weak None Strong Weak None Very weak

595 550 630 592 595 630 600 585 555 628 527

0.00 0.55 3.36 6.73 1.02 0.16 0.00 5.80 0.61 0.00 0.05

423 In Grder

to assess the effect of decolorization

of the dye

and

the extent

of restoration of color by surfactant, the sodium sulfite concentration was varied from 0.4% to 7.5% in leuco-base reagent. At the 0.4% concentration, only 91.6% of the color of methyl violet was removed and 97.5% of the color was restored by 0.01% anionic surfactant; at a 1.2% concentration, 95.5% of the color of methyl violet was reduced and 100% of the color was restored by the surfactant. At the highest concentration of sodium sulfite used here (7.5%), the molecular absorption spectra showed a wavelength shift and band broadening, coupled with a decrease in absorbance; this effect was not examined further. At the 1.2% concentration of sodium sulfite, Brilliant Green beczme colorless in solution and 100% of the expected absorbance was realized on reaction; this v concentration was therefore chosen for the recommended method. The color developed from the leuco-base reagent tends to change with time. When methyl violet was used, the absorbance decreased by 2% 5 min after mixing, by 8% after 30 min, and by 24% after 25 h. In a study of the restored Brilliant Green color, the absorbance increased by 1% after the first 10 min, and then decreased by 14% after 18 h. It is recommended that the absorbance be measured immediately following color development and dilution. A representative calibration curve for an alkyl/alkene sulfonate is shown in Fig. 1. Reflectance curves. Spectral reflectance (Fig. 2) proved to be quite useful for determining the anionic surfactant, sodium dioctyl sulfosuccinate, added to certain latex formulations that are white, opaque suspensions. The bluegreen color of Brilliant Green was discernible in the emulsion containing a polymeric binder and an acidic cross-linking agent. The surfactant was readily quantified by the difference technique, where the reference is the colordeveloped emulsion control. Zero suppression and ordinate expanSiOn enhanced the appearance of the reflectance spectra. Repeatability, accuracy and worhin, D range. The standard deviation in measuring 0.01 g of hydroxyalkane/alkene surfactant with four separately prepared solutions of Brilliant Green was kO.005 absorbance and the standard deviation for a single solution measured eight times was iO.019 absorbance. The accuracy of the method based on the quantitation of known concentrations of hydroxyalkane/aIkene sulfonate in the 2OO--1000 ppm range in aqueous solution was found to average 99.8%. Two of the blends were checked by the two-phase titration technique [3] and the differences between the calorimetric and titrimetric methods averaged 5%. The general working ranges of the methods are O-500 ppm of the hydroxyalkane/alkene sulfonate and O-200 ppm of the sodium dodecyl sulfate in 50-g aqueous samples by the absorption spectrophotometric method, and O-1500 ppm of the sodium dioctyl sulfosuccinate in 50-glatex emulsions by the spectroreflectometric method. The detection limit in a l-cm cell is about 1 mg Gf the sodium dodecyl sulfate and about 3 mg of the hydroxyalkane/alkene

424

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0.03 0.04 (g/l00 ml)

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Fig. 1. Calibration curve obtained for hydrosyalkanelalkene sulfonate sutfactant (Ultrawet A-OK) as a function of restoration of color to a leuco-base Brilliant Green solution. Surfactant concentration refers to the final solution measured_ Absorbance refers to 630 nm in l-cm cehs. Fig. 2. Spectroreflectance curves of sodium dioctyl sulfosuccinate in opaque poIymer em&ions as a function of color restoration to leuco-base Brilliant Green. A, Control emulsion without surfactant; B, 0.0067 g of surfactant/lOO ml of final solution; C, 0.0131 g of surfactant/lOO ml of final solution.

surfactant per 100 ml of the final solution. Lower levels of 20-60 ppm can then be determined in 50-g samples. In the reflectance mode, about 3 mg of the sodium dioctyl sulfoauccinate per 100 ml can be determined as a lower limit. Interferences. The interferences of some oxidizing and reducing agents were investigated. Blends were prepared containing surfactant and impurity in weight ratios ranging from l/O.56 to l/2.22. Among the potential impurities studied were KI03, FeCl,, HN03, H2S04, HC104, HCl, NalS203, INH~OH-HCl, and H&O4 - 2H,O. At the concentration level of 0.016 g hydroxyalkane/alkene surfactant per 100 ml, the recoveries ranged from 98.1 to 103.9%. Following the addition of sulfuric acid at the higher end of the weight ratios investigated, it was necessary to readjust the pH to 9.0 for quantitative conversion to the quinoid color. Most anionic surfactants respond to this method much like they would respond to the methylene blue ion-pairing method. Automated procedw-e. The method for anionic surfactant was demonstrated on a modular Technicon Autoanalyzer Model II- Flow-rates were 1 ml min-’ for the leuco-base, 0.14 ml min-’ for the sample line, 2 ml mm-* _ -I for the air line. A linear calibrafor an aqueous diluent, and 0.32 ml mm tion was obtained for the 60-400 ppm range. The cycle time between successive runs was 3.5 min. The technical assistance of Arthur L. Allen and Ronald K. Chen, and the cooperation of Adolph Stnepek, are respectfully acknowledged.

425

REFERENCES 1 E. S. Abramovich, U.S.S.R. Patent No. 122,336, Tekst. Prom&., 20 (1960) 41. 2 G. F. Longman, The Analysis of Detergents and Detergent Products, Wiley, New York, 1975,

Ch. 9,10

and 18.

3 M. J. Rosen and H. A. Goldsmith, Systematic Analysis of Surface-Active Agents, 2nd edn., Wiley, New York, 1972, Ch. 5. 4 J. Kawase, A. Nakae and M. Yamanaka, Anal. Chem., 51(1979) 1640. 5 K. Higuchi, S. Monya, Y. Shimoishi, H. Miyatta and K. Toei, Bunseki Kagaku, 29 (1980) 180. 6 A. Le Bihan and J. CourtotCoupez, Analusis, 6 (1978) 346. 7 C. G. Taylor and J. Waters, Anal. Chim. Acta, 69 (1974) 363. 8 L. K. Wang, S. F. Kao, M. H. Wang, J. F. Kao and A_ L. Loshin, Ind. Eng. Chem., Prod. Res. Dev., 17 (1978) 186. 9 S. Ianeva and R. Borisova-Pangarova, Talanta, 25 (1978) 279. 10 Z. I. Chalaya and 0. M. Voznaya, Zh. Anal. Khim., 27 (1972) 204. 11 G. R. E. C. Gregory, Analyst, 91(1966) 251.