Anal. Chem. 1988, 58, 2265-2269 (14) Hooley, D. J.; Dessy, R. E. Anal. Chem. 1083, 55, 313. E T Re"ea'&; MmPshIre Commodore Publica(15) Hampshire, N. tions.
RECEIVED for review May 6, 1985. Resubmitted March 10,
2285
1986. Accepted April 21,1986. The authors are grateful to the Department of Trade & Industry for provision of a Studentship to T.J.S. and to the British Petroleum Co., Plc, for provision of a Studentship to A.P.W. and an extramural research award to T.J.S.
Indirect Atomic Absorption Determination of Anionic Surfactants in Wastewaters by Flow Injection Continuous Liquid-Liquid Extraction Mercedes Gallego, Manuel Silva, and Miguel Valclrcel* Department of Analytical Chemistry, Faculty of Sciences, University of Cdrdoba, CBrdoba, Spain
A determination of anlonlc surfactants with an automatic contlnuous ilquid-liquid extractor is reported in this paper. The method Involves the formation of the detergent-[l,lOphenanthrolkre-copper( I I)] ton pair and extracHon into methyl isobutyl ketone. The overall concentration of anlonic surfactants can be determined Wectly by measurement of copper present in the organlc layer by atomic absorption spectrometry. A phase separator fttted wtth a PTFE porous membrane was specially deslgned for the determlnation. The optimum experimental condltlons for determining detergents in the concentration range 0.1-5.0 Mg/mL are described. The reiatlve standard devlation found was 0.8%. The method Is highly selective and free from the interference of nonlonlc surfactants and has been applied to the determination of anlonlc surfactants In wastewaters. The results show good agreement with those obtained by the classlcai methylene blue method.
The determination of anionic surfactants in water by atomic absorption spectrometry (AAS)has been reported by several authors (1-7). The methods applied for this purpose are based on the ion-pair extraction of anionic surfactants with a cationic complex such as ethylenediamine-copper(II), 1 , l O phenanthroline-copper(II), thiourea-copper(I), and so on. The flow injection analysis (FIA) technique constitutes a major alternative to manual methods of analysis as it is a simple, inexpensive, versatile, and easy to manipulate continuous-flow analysis mode (8,9).Continuous liquid-liquid extraction based on the flow-injection principle was simultaneously introduced by Karlberg et al. (10) and Bergamin et al. (11). Over 30 papers have been published on this topic up to now (9). Kina et al. (12) have determined anionic surfactants by FIA based on the fluorescence-diminishing reaction of acridine orange-10-dodecyl bromide in an aqueous solution. Other authors have carried out this determination by an extraction spectrophotometric method with a flow injection system. These methods are based on the ion-pair extraction of anionic surfactants and methylene blue (13) or on the ion-pair formation between anionic surfactants and the cationic dye ethyl violet, followed by extraction into toluene (14). The advantages of the AAS-FIA association have been recently praised by several authors (15-18).We have recently made use of the association AAS-FIA in conjunction with 0003-2700/86/0358-2265$01.50/0
liquid-liquid extraction as a separation technique for the indirect determination of perchlorate in human urine and serum samples (19) and nitrate and nitrite in meats (20,21). This paper describes a simple and accurate method for determination of anionic surfactants in wastewaters which involves continuous extraction of the compound formed between the detergent and the sulfate of the 1 , l O phenanthroline-copper(I1) complex into isobutyl methyl ketone and determination of copper in the extract by AAS.
EXPERIMENTAL SECTION Apparatus and Procedure. A schematic diagram of the flow injection system used is shown in Figure 1. A four-channel peristaltic pump (Gilson Minipuls-2)was fitted with Tygon pump tubes for aqueous or organic solutions. For the latter, this pump was coupled on-line with two displacement bottles as shown in Figure 1. Another peristaltic pump was used to transport the organic extract injected as an undispersed plug to the nebulizer of the atomic-absorptionspectrometer (AAS). A Rheodyne 5301 diverting valve was used to change the sample solution in order to avoid the introduction of air into the system. A variable-volume Tecator L100-1 injection valve (I), an A-1OT solvent segmentor (S Sg) (Bifok) and an extraction coil (EC) were also used. The membrane phase separator (PS) (Figure 2) was designed as follows: the Teflon body consists of an inlet and two outlets (bore 0.5 mm i.d.1 and a separatingchamber (approximate volume 60 pL). The three-threaded hole accepted the standard polypropylene end pieces and flaired Teflon tubing. During operation, the two blocks were pressed together with six screws and aluminum end plates. Alignment was achieved with the aid of two stainless steel pins. Two layers of Fluoropore membranes (1.0 pm pore size, FALP 04700, Millipore) permeable to methyl isobutyl ketone (MIBK) but impermeable to the aqueous solution were sandwiched between the two main body pieces. Both porous membranes were wetted with methyl alcohol prior to use according to the manufacturer's instructions. A Perkin-Elmer 380 atomic absorption spectrometer equipped with a copper hollow cathode lamp working at 324.7 nm (spectral band-pass 0.7 nm) and using air-acetylene flame was employed in combination with a multirange Radiometer REC 80 recorder. With the manifold described above and under the optimum instrumental conditions,a sodium dodecyl sulfate sample solution (0.1-5.0 pg/mL) of pH 2.8-10.0 was pumped continuously into the system and mixed with the carrier solution. A stream of MIBK obtained by pumping water into a displacement bottle B, filled with MIBK as well as the mixed sample-carrier solution stream were segmented in the solvent segmentor, resulting in a regular pattern of alternative aqueous and MIBK segments. The extraction process took place in the extraction coil and a fraction of the extract, controlled by bottle B2, was separated in the membrane phase separator. The determination was affected by 0 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58. NO. 11, SEF'TEMBER 1986
mi,n,"
Flpurm 1. Flow dlagram for Lhe delermlnalh of anionic surfaclanls. All tubes are made of Tellon (0.5 mm i.d., except where speclned).
2
888
2. text.
Drawing of Lhe membrane phase separator. Fw details.
injecting a 135-pLorganic extract with injector I into the water stream h m the Ismateeperistalticpump, wbich was subequently ,introduced into the atomic absorption spectrometer. The signal from extracted copper yielded the FIA peaks, the height of which was proportional to the detergent concentration. The calibration curve was obtained by linear regression analysis. Reagents and Chemicals. AU chemicals were reagent grade. A 0.01 M lJ0-phenanthrolinecopper(I1)complex solution was prepared by dissolving 1.760 g of 1,lO-phenanthrolinechloride monohydrate and 0.624 g of copper sulfate pentahydrate in 250 mL of distilled water. This solution remained stable for at least 1month. Sodium dodecyl sulfate (LAS, 9970,Sigma) and sodium dioctyl sulf~uccinate(Manoxol O.T., 9670, Aldrich-Chemie) stock solutions (Loo0 g/L) were prepared in distilled water. All these solutions were stored in a refrigerator to minimize biodegradation, The carrier solution was prepared by mixing 10 mL of 0.01 M complex reagent and 10 mL of 1 M acetic acid/l M sodium acetate buffer (pH 4.75) and diluting to 100 mL with distilled water. RESULTS AND DISCUSSION The automation of the batch procedure for the indirect determination of anionic surfactants by AAS (22,23)associated to a flow injection technique is linked to factors such as the flow injection and chemical variables, which must be fixed and studied in order to obtain more accurate and reproducible results. Influence of Chemical Variables. The plots of ahsorhance vs. pH for LAS and carrier solutions show a plateau over the ranges 2.&10.0 and 4.H.1, respectively (Figure 3). Hence, 10 mL of 1M acetic acid11 M sodium acetate buffer (pH 4.75) was added to 100 mL of carrier solution. The concentration of the 1.10-phenanthrolinecopper(I1) complex in the carrier solution was varied from (0.1 to 5.0)
6
8
10pH
Fbure 3. Influence of Lhe pH on lb absorbance (0) of the sample solution (pH of Lhe cater is kept mnstanl at 5.6) and (0)of lb carrier solution (pH of the sample is kept wnstanl at 4.91. In bolh exmri. menls the LAS concenlratlon was 2.0 pglmL.
lo4 M while the LAS concentration was kept constant a t 2.0 pglmL. Peak height increased rapidly until a constant value was obtained a t about 0.5 X M. At concentrations higher than 2.0 X M the peak height increased owing to the increase in the blank signal, owing to the appreciable extraction of carrier anion (acetate). However, with this or smaller concentrations of the l,l0-phenanthrolineopper(II) complex, the effect of the blank was negligible and its measurement could therefore be avoided in the experimental M of this procedure. Thus, a concentration of 1.0 x variable was selected for use in the determination of surfactant traces. The influence of the chloride ion on the extractability of the ion pair was also studied. In the bath procedure, this variable shows a stronger influence on the extraction of the anionic surfactant. Thus, the chloride concentration in the samples must be above 2.5 X IO-* M when the copper(I1) complex concentration is 1.5 X lo4 M in order to obtain an extraction efficiency of about 95% (22, 23). The effect of chloride ion in this bath procedure can be attributed to its breaking effect on the emulsions originated during the manual extraction proeess. In the FIA technique, the chloride ion does not affect the extraction efficiency even a t 0.25 M concentrations and its addition to the samples is therefore unnecessary. When the surfactant distribution process is carried out by this technique, no emulsions are probably originated between the two phases in the extraction coil since mixing was more gentle. Selection of Flow Injection Variables. The number of possible flow rate combinations in the extraction system is quite large since several of the flows cnn be varied individually. Thus, a systematic investigation was necessary in order to find the optimum conditions for the extraction so that a high concentration ratio of the sample could be achieved without detracting from precision. In this study, the carrier flow rate was kept a t 0.14 of the sample flow rate in order to avoid excessive dilution of the continuously pumped sample. Thus, the liquid-liquid extraction procens w a only ~ linked to the flow rates of the sample and organic phase. Figure 4A shows the effect of varying the sample flow rate while keeping the flow rate of the organic phase constant. This study was carried out by using a membrane phase separator as well as T-shaped separator (A4Tconnector, Technicon, Tarrytown, NY)in order to compare their performance in thisdistribution process. The absorbance increased as the flow ratio of sample to organic phaae increased up to a value of 10 for the membrane phase separator and 7.8 for the T-shaped one and decreased a t higher values. Figure 4B shows the dependence of the absorbance on the flow ratio (aqueous/organic phase) when the aqueous flow rate is kept constant. The absorbance increases as the flow rate of the X
-re
I
ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986 FLOW R A T I O ISarnple/Orgonic phasel 1 9 in 111
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Table I. Features of the Calibration Graphs for the Determination of LAS Using T-Shaped and Membrane Phase Separators
07 OL
06 05
03 U z m
0L
:: 0 2
03
a
4
02 01 01
2 SAMPLE F.0W
1
RATE
6 ImL/minl
5
10
15
20
F L O W R A T I O I A q u e o u s / O r g a n c phasel
Flgure 4. Flow rates in the extraction system and comparison between phase separators: (0)present membrane phase separator: (0)Tshaped phase separator. (A) Organic phase flow rate: 0.41 mL/mln; the carrier flow rate is kept at 0.14 X sample. (B) Sample flow rate: 2.6 mL/min; carrier flow rate 0.36 mL/min. (Extraction coil, 360 cm long, 0.5 mm i.d., coil diameter 4 cm).
organic phase decreases. This decrease is limited by the dissolution of MIBK in the aqueous phase. We chose a sample flow rate of 2.6 mL/min and an organic-phase flow rate of 0.41 mL/min, taking into account the mutual influence of three factors: reproducibility, concentration ratio, and sampling frequency. Finally, the performance of both phase separators used in this work was compared and the following conclusions can be drawn. (a) Both approaches showed a similar dependence on the flow rates although the separation efficiency was higher when the separation of the phases was carried out with the membrane phase separator (79% for T-shaped with regard to the membrane separator). (b) The introduction of air into the extraction system, for example, when a sample was changed, did not affect the performance of the T-shaped phase separator. However, the performance of the membrane phase separator was greatly affected by this factor. Thus, air goes through the membrane and occupies dead volumes in the joints, injection valve, etc., which results in irreproducible measurement until its complete removal from the system, which in turn redounds to decreased sampling rate. This problem could be easily avoided by using a diverting valve. (c) The total lifetime of the Teflon porous membrane was such that its aging originated a break in the continuous liquidliquid extraction process and therefore had to be changed. In this work two 1.0 pm pore size Fluoropore membranes (total lifetime between 2 and 3 h) were selected instead of two 0.5 pm pore size PTFE membranes (total lifetime between 1and 2 h) because of their longer lifetime. When a single membrane was used, the total lifetime is decreased by one-third in both cases. The influences of other variables related to the solvent extraction process were also investigated: mixing coil and extraction coil characteristics and injected sample volume. In all studies the sample solution used contained 1.0 pg/mL LAS. The tube length between the mixing point for the sample and the carrier stream and the solvent segmentor did not affect the FIA peak at values above 40 cm for an inner diameter between 0.35 and 0.70 mm. The extraction coil length was varied from 40 to 750 cm while the flow rate ratio was maintained constant. A plot of the peak height vs. the extraction coil length showed that the height of the FIA peak increased with increasing extraction coil length up to 250 cm; in the range 250-500 cm, the peak height was not affected by the length of the extraction coil, whereas at larger values the FIA peak started to decreased slightly. In order to keep precision as high as possible, an extraction coil length of 360 cm was selected. Several other characteristics of the extraction coil are also worth mentioning:
phase separator
range,
slope,
intercept,
corr coeff
pg/mL
A mL/pg
A
(n = 7)
T-shaped membrane
0.1-5.0 0.1-7.0
0.0743 0.1019
1.73 X 2.17 X
0.998 0.999
(a) Inner diameters between 0.35 and 0.70 mm did not affect the absorbance when the volume of the extraction coil was kept constant at 700 pL. An inner diameter of 0.5 mm was selected. (b) Coil diameters up to 60 mm had no influence on the absorbance. A coil diameter of 40 mm was chosen for further investigations. The extracted sample volume injected into the water line exerted a significant effect on the absorbance. The signal increased with increasing injected sample volume up to 90 p L and remained constant for larger volumes, up to 360 pL. An injection volume of 135 p L was chosen for all subsequent studies. Neither back-extractionnor dispersion of the organic plug occurred since identical results were obtained when the organic phase was aspirated continuously in other experiments. Determination of Trace Levels of Anionic Surfactants. The features of the calibration graphs (absorbancevs. pg/mL) run for several surfactant standards (LAS and Manoxol O.T.) assayed with the proposed flow injection system and using the T-shaped and membrane phase separators are shown in Table I. The lesser sensitivity obtained with the T-shaped separator is in agreement with the results obtained in the study of the flow rate ratio and depends on its separation efficiency. The extraction efficiency was calculated by means of a series of experiments in which the flow-injection method was compared with the batch method, in which, under the same experimental conditions as in the flow method, both phases (aqueous/organic phase volume ratio = 7.2) were brought to equilibrium in a funnel for 5 min, and the organic extract was aspirated into an air-acetylene flame. The value found was 95% with regard to the batch method. Some other characteristics related to the determination of surfactants were also studied. (a) The detection and quantification limits (45 and 150 ng/mL, respectively) were calculated as %fold or 10-fold, respectively, the standard deviation of the peak height for 30 injections of the same sample (24). (b) The precision of the method was checked on 11 samples containing 1.0 pg/mL each. The relative standard deviation was 0.8%. (c) The sampling frequency was 40 f 5 h-l. Finally, it is noteworthy that the proposed method is suitable for the determination of the overall concentration of unknown anionic surfactant mixtures which is corroborated by two facts (a) the slopes of their calibration curves for LAS and Manoxol O.T. are identical when both are plotted as absorbance vs. molar concentration; and (b) good agreement is found between the methylene blue method and the one proposed herein when the flow injection method is applied to the determination of anionic surfactants in wastewaters. The effect of many common species associated with anionic surfactants in real samples was examined to find possible interferences. The tolerances to the species investigated are given in Table 11. Foreign species were added at a maximum level of 1000 pg/mL (tolerated ratio to LAS, 1000). The tolerance ratio of each foreign species was taken as the largest amount yielding an error less than &3% in the peak height for 1.0 pg/mL of LAS. The substances assayed in this study of interferences can be classified in four groups: (a) ionizable organic compounds such as glutamic acid, succinic acid, phenol, etc., which can
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
Table 11. Tolerance Limits for Foreign Ions in the Determination of 1.0 pg/mL of Anionic Surfactant tolerance limit of ion to LAS 1000
500 250 100 25
foreign ion
Table IV. Determination of Total Anionic Surfactants in Wastewaters by the Proposed FIA Method and the Methylene Blue Method sample
Na(I), K(I), Ca(II), Mg(II),COW), Pb(II),” Al(III),”sod*-,s03’-,szO3’-, Po:-, P z O ~ ~ - , P3010”,C032-,CZO4’-, NO;, F,C1-, Br-, B407’-, ClOc, BrOC, IOc, AsOt-, SZ-,b citrate, tartrate, glutamic acid, succinic acid, phthalic acid, phenol benzoic acid Ni(II), Triton X-100 Brij Zn(II),Fe(III), I-, C 1 0 ~
OUsing a microfilter. *With 3 mL of 30% (v/v) hydrogen per-
oxide. Table 111. Comparison of Interferences between the FIA Technique and the Batch Procedure Using the Copper-Phenanthroline Method
foreign ion PO4“, SO-: NO3BrCo(I1) phthalic acid succinic acid glutamic acid benzoic acid
Ni(I1) Triton X-100 Ic10,-
tolerance ratio of ion to LAS FIA batch method method 1000 lo00 1000 1000 1000 1000 1000 500 250 250 25 25
lo00
