Solid-phase reagents for the determination of anionic surfactants in water O. A. Zaporozhets *a , O. Yu. Nadzhafova a , V. V. Verba a , S. A. Dolenko b , T. Ye Keda b and V. V. Sukhan a a Taras Shevchenko University, Vladimirska str. 64, 252033, Kiev, Ukraine b Institute of Colloid Chemistry and Chemistry of Water of the Ukrainian Academy of Sciences, 2520, Kiev, Ukraine Sorption–spectrometric methods for the determination of anionic surfactants (A Surf) in natural and waste water using silica gel (SG) loaded with ion-pair associates of high molecular weight quaternary ammonium salts (QAS) and anionic dyes were developed. For this purpose, the interaction of Methyl Orange (MO), sodium picrate (Picr) and fluorescein (Fl) with didecylaminoethyl-b- tridecylammonium iodide (I) and N-methyl-N,N,NA,NA,NA- pentadecylethyldiammonium diiodide (II) immobilized on the surface of SG was investigated. SG loaded with I-MO and I-Fl associates was used for the determination of sodium dodecyl sulfate (SDS) in natural and waste water using absorbance, diffuse reflectance spectrometric and luminescence methods. Detection limits were 0.05 and 0.01 ppm, respectively. Humic and fulvic acids at levels < 0.5 ppm did not interfere with SDS determination. Water with higher acid contents was analysed using SG modified with the II-Picr associate at pH 2. Visual test scales for SDS determination in water at one fifth of the maximum admissible concentration (MAC) level was prepared. Keywords: Adsorption onto silica gel; quaternary ammonium salt; anionic dye; anionic surfactant determination Synthetic anionic surfactants (A Surf) are one of the most toxic pollutants in natural water. The spectrophotometric and spectro- fluorimetric methods for A Surf determination at the maximum admissible concentration (MAC) level are based on ion-pair extraction with cationic metal chelates such as bis[2-(5-tri- fluoromethyl-2-pyridylazo)-5-diethylaminophenolato]- cobalt(iii) 1 or basic dyes, e.g., Methylene Blue, Ethyl Violet and Rhodamine B and 6G. 2–6 The sensitivity of these methods is 0.002–0.2 ppm. Inorganic anions, long-chain organic anions, non-ionic and cationic surfactants and especially humic and fulvic acid interfere with A Surf determination. A more selective extraction–spectrophotometric method for A Surf determination at levels > 0.03 ppm has been reported, based on their previous sorption separation using fibrous materials. 7 However, this method is fairly tedious and time consuming (the time for a single analysis is more than 1 h). All the described methods need toxic organic solvents. Rapid techniques for A Surf determination in water that do not require toxic solvents have been developed. For example, filter-paper impregnated with a mixed aluminum–zinc hydrox- ide was used for the analysis of water with > 0.02 ppm A Surf content. 5 The A Surf was collected on the paper as products from their reaction with Rhodamine B, the paper was extracted with ethanol and the extract quantified photometrically. How- ever, humic and fulvic acid interfered with A Surf determina- tion. High molecular weight quaternary ammonium salts (QAS) have found wide application in trace analysis. 8 The flow- injection spectrophotometric determination of A Surf has been described, based on the formation of a stable ion-pair associate with QAS. 9 However, this method is indirect; the detection limit is 0.03 ppm. Didecylaminoethyl-b-tridecylammonium iodide immobilized on silica gel (SG) has been used for the preconcentration and determination of anionic metal com- plexes. 10 This sorbent was not applied to the determination of other anions. In the present work, the possibility of the sorption– spectrometric determination of A Surf in natural and waste water using SG loaded with ion-pair associates of anionic dyes (Methyl Orange, sodium picrate and fluorescein) with QAS (didecylaminoethyl-b-tridecylammonium iodide and N-methyl- N,N,NA,NA,NA-pentadecylethyldiammonium diiodide) was stud- ied. Experimental Apparatus UV/VIS spectra were measured using a Specord M-40 spec- trophotometer (Carl Zeiss Jena, Jena, Germany) and 1 cm quartz cells. A Model EV-74 potentiometer with a glass electrode was used for pH measurements. The luminescence intensity was measured with a Model DRK-120 luminescence spectrophotometer with a mercury vapour lamp (Kiev, Ukraine) and 1 cm quartz cells. Reagents All chemicals were of analytical-reagent grade. Distilled water was further distilled once before use. 11 Methyl Orange (MO), sodium picrate (Picr) and fluorescein (Fl) were obtained from Merck (Darmstadt, Germany); 5.0 mmol l 21 solutions were used. Sodium dodecyl sulfate (SDS) solution (1.0 mmol l 21 ) was prepared by dissolving the reagent (98%) (Merck) in water. Silica gel L 40/100 was from Chemapol (Prague, Czech Republic). The QAS didecylaminoethyl-b-tridecylammonium iodide (I) and N-methyl-N,N,NA,NA,NA-pentadecylethyldiam- monium diiodide (II) were synthesized as described else- where 12 and were used for preparing the modified silica gel. Modified silica gels (I-SG and II-SG) with QAS contents of 0.12 and 0.012 mmol g 21 , respectively, were obtained as described in ref. 10. Humic and fulvic acid obtained by a standard method 13 from river water were used for preparation of their aqueous solutions. Procedure Sorption of anionic dyes onto I-SG and II-SG I-SG or II-SG (0.2 g) was stirred with 50 ml of an aqueous solution containing a known amount of the dye (0.05–0.5 mmol l 21 ) for 10–40 min at the appropriate pH. The mixture was then centrifuged and the solid product dried at 80 °C. The residual MO and Picr in the aqueous solution were measured spectrophotometrically at 440 and 364 nm, respectively. The equilibrium concentration of Fl in solution was determined by measuring the luminescence intensity at 496 nm. Analyst, July 1998, Vol. 123 (1583–1586) 1583