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4.1 Anionic detergents 4.1.1 Natural waters 4.1.1.1 Titration methods

Bock and Reisinger [1] and Wickbold [2] have described a method for the determination of sodium dodecylbenzene sulphonate based on precipitation with p-toluidine and titration with sodium hydroxide.

Wang et al. [3] have described a two-phase titration procedure for the determination of linear alkyl sulphonate and branched-chain alkylbenzene sulphonate in water. The method involves initial treatment of water sample with a known amount of a quaternary ammonium salt, adjusting the pH value and adding chloroform. The two-phase mixture is then titrated with standard sodium tetraphenylboron reagent. The procedure can be used for quantitative determination of anionic surfactants in both fresh and saline water. These workers have also published a method for the analysis of linear alkylsulphonate based on an expected stoichiometric reaction between the anionic surfactant and an added known cationic reagent (cetyldimethylbenzyl ammonium chloride) followed by back-titration of the excess cationic reagent with a standard solution of sodium tetraphenylboron.

Wang et al. [4, 5] discuss indirect two-phase titration methods for the determination of anionic detergents. Wang et al. [6, 7] also describe a direct titration procedure for anionics involving titration with 1,5-dimethyl-1,5-diazoundecamethylene polymethobromide.

Li and Rosen [8] have described a two-phase mixed indicator titration method for anionic surfactants including sodium decanesulphonate, sodium dodecanesulphonate and sodium tetradecanesulphonate. The method was found not to be quantitative for surfactants with alkyl chains containing less than 12 carbon atoms, but could be applied to them using chloroform-1-nitropropane (2:3) as the organic phase and a multiple extraction and titration technique.

Tsubouchi and Mallory [9] describe a differential determination of anionic and cationic surfactants in natural waters by two-phase titration.

4.1.1.2 Spectrophotometric methods

Earlier spectrophotometric methods for the determination of anionic detergents were based on the use of the methylene blue, Rhodamine B, ferroin [10, 11], azure A [12] and methylene green [11].

Stroehl and Kurzak10 studied the absorption spectra and stability of complexes formed between anionic detergents and methylene blue and methyl green. The absorption maxima for the methylene blue and methyl green complexes of seven anionic surfactants were found to lie between 657 and 671nm and between 613 and 628nm, respectively. These workers suggested that in routine determinations the measurements should be made at 660 and 620nm, rather than at 650 and 615nm respectively. The rate of change of extinction with time for the same series of complexes was also measured. Differences due to the anions were insignificant, but fading of the colour is much more rapid in methylene blue than in methyl green complexes.

Wang [13] and Rodier [14] have also used methylene blue for the determination of anionic detergents. The British Standards method on anionic detergents is based on the use of methylene blue [15].

Taylor and Waters [16] studied the extraction constants of association compounds of anionic surfactants with iron (II) chelates of the ferroin type. The extraction constants of these chelates distributed between water and chloroform were correlated with the structure of the surfactant and the ligand. Taylor et al. [17] also studied the use of iron (II) chelates of 1,10-phenantholine and its derivatives, bipyridyl and terpyridyl in the estimation of anionic surfactants in chloroform extracts of water samples. The extent of extraction increases with increasing chain length of surfactants and is characterized by a breakthrough point (i.e. the chain length above which the surfactant is extracted and below which it is not). This point depends on the choice of iron (II) chelate and extracting solvent. A selective procedure for determining surfactants in simulated sewage liquor and river water based on experimental control of the 'breakthrough point' and involving spectrophotometric determination of the extracted complexes, is described. The effect of foreign ions is also discussed by the workers.

Began et al. [18] have described a spectrophotometric method for the determination of anionics based on benzene extraction of a detergent-Rhodamine B complex. Various other dyes have been used for the estimation on anionic detergents in fresh water. Thus Wang and Langley [19] have compared azure A with methylene blue as a reagent for the rapid determination of anionic detergents of the linear alkyl sulphonate type and recommended their method as an alternative to the lengthy standard methylene blue procedure described by the American Public Health Association [20] and ASTM [21]. In the same method they estimate cationic detergents with methyl orange.

In the Wang and Langley method [19] the azure A dye reacts with anionic surfactants to form a chloroform-soluble blue-coloured complex in the presence of chloroform (Fig 4.1). The intensity of blue colour in the chloroform layer is proportional to the concentration of the 'azure A-anionic surfactant complex'. The colour intensity of the azure A-surfactant complex can then be measured by making colorimetric or spectrophometric readings of the chloroform solution at the optimum wavelength 623nm.

Higuchi et al. [22] determined detergents in river water spectrophotometrically using 1-(4-nitrobenzyl)-4-(4-diethylaminophenyl azo)-pyridinium bromide as a cationic dye. This technique is faster and simpler than the methylene blue procedure.

Motomizu et al. [23] describe a spectrophotometric technique for determining anionic surfactants in water using ethyl violet as the reagent. The absorption maximum occurs at 615nm. It was found that traces of anionic surfactants could be extracted into benzene and toluene, and then determined by spectrophotometry. The technique is claimed to be simple, rapid and highly sensitive.

Del Valle et al. [24] described a continuous solvent extraction flow injection analysis automated system for the routine determination of anionic surfactants in river and treated waters in the concentration range 0.04-3.5^g mL-1. The method was based on an ion-pair extraction reaction with methylene blue in chloroform. Results compared

Fig. 4.1 Blue-coloured complex of anionic surfactant with azure A. (Reprinted with permission from Wang and Langley [19]. Copyright (1977) Springer Verlag, New York.)

favourably with those obtained by a standard batch extraction method. Those anions that caused greatest interference (perchlorate, thiocyanate and nitrate) were the same ones interfering with the standard batch method. Interference was generally lower at pH2 than at pH7. Nonionic detergents inhibited the rate of extraction of anionic surfactants.

Evstifeev et al. [25] determined the anionic surfactant sodium lauryl sulphate in natural waters by complexation with a pyrilium salt and subsequent extraction of the resultant complex with toluene. The optical density of the toluene layer was measured at 420nm. Sodium lauryl sulphate concentrations were determined from calibration curves. A linear dependence of sodium lauryl sulphate concentration and light absorption was observed in the concentration range 0.01-0.5mg L-1 and the relative standard deviation within this range did not exceed 7%.

Motomizu et al. [26] determined the anionic surfactant sodium dodecyl sulphate present in river water using a spectrophotometric flow injection analysis system coupled with solvent extraction. Optimization studies conducted with eight cationic azo dyes and several extraction solvent systems showed methylene blue (cationic dye) and 1,2-dichlorobenzene (solvent) to be the most efficient. The ion associate of anionic surfactant and methylene blue was extracted into the organic phase and its absorbance measured at 658nm. A PTFE porous membrane was used to separate the organic phase. A detection limit of 5^g L-1 sodium dodecyl sulphate was obtained. Relative standard deviations were 0.9-1.4% for 10 300^l samples of 610^g L-1 sodium dodecyl sulphate.

4.1.1.3 Gas chromatography

Combined gas chromatography-mass spectrometry has been used [27] to determine trace amounts of the individual components of alkylbenzene sulphonates as their methyl sulphonate derivatives. Gas chromatographic analysis was performed using a gas chromatograph equipped with a flame ionization detector. A silanized glass column (2m x 3mm i.d.) was packed with 1.5% silicone OV-1 on Chromosorb W AW DMCS (80-100 mesh). Nitrogen was used as carrier gas with a flow rate of 40mL min-1. The chromatogram was recorded as the total ion current monitor (TICM) at 20eV. The molecular separator and ion source were maintained at 300°C and 330°C respectively. Mass spectra were taken at 70eV with an accelerator voltage of 3.5kV.

In the work-up procedure a chloroform solution of methylene blue is shaken with a suitable volume (200-300mL) of a sample solution. The combined chloroform extracts containing methylene blue and methylene blue-alkylbenzene sulphonate complexes are evaporated to dryness under reduced pressure, then dissolved in a small amount of ethanol and passed through a column of cation-exchange resin (Dowex 50W x X8), for removal of methylene blue. The ethanol eluate (10mL) is then evaporated and the residue dissolved in 20mL of water. The aqueous solution is washed with chloroform, concentrated, transferred to a 1mL ampoule (preignited 3h 500°C) and then evaporated. After addition of ca. 10mg phosphorus pentachloride the ampoule is sealed and maintained at 110°C for 10min on a hot plate. The sulphonyl chloride derivatives are extracted with n-hexane and the extract is transferred to another 1mL ampoule and evaporated. Methanol (0.5mL) is added to the ampoule, which is sealed and maintained at 70°C for 20min, the methanol solution containing the methylsulphonate derivatives produced is evaporated and the residue dissolved in n-hexane. The hexane solution is applied to the silica gel column. The column is washed with three times its volume of n-hexane and then with one volume of n-hexane-benzene (1:1). The eluate, comprising eight times the column volumes of n-hexane-benzene (1:1) is pooled and evaporated to a definite volume.

Alkylbenzene sulphonate was analysed as its methylsulphonate derivatives by gas chromatography and gas chromatography-mass spectrometry. The concentration of alkylbenzene is determined by measuring the peak areas of the gas chromatogram and/or the mass fragmentogram and comparing these with linear dodecylbenzene sulphonate.

Figure 4.2 shows a gas chromatogram of alkylbenzene sulphonate methyl esters in a river water sample. The pattern of the gas

O 10 20 30

Retention time (min)

Fig. 4.2 Gas chromatogram of ABS as methyl esters in river water. Column temperature, 230°C. (Reprinted with permission from Hon-Nami and Hanya [27]. Copyright (1978) Elsevier Science Publishers BV.)

O 10 20 30

Retention time (min)

Fig. 4.2 Gas chromatogram of ABS as methyl esters in river water. Column temperature, 230°C. (Reprinted with permission from Hon-Nami and Hanya [27]. Copyright (1978) Elsevier Science Publishers BV.)

chromatogram is analogous to that of the linear alkylbenzene sulphonate standard. The assignment of the peaks was performed on the basis of the retention times and mass spectrum, and the individual components of alkylbenzene sulphonate were determined by mass fragmentography. For overlapped peaks on the gas chromatogram, more than two mass spectra were recorded. The total amounts of alkylbenzene sulphonate determined by mass fragmentography were in good accord with those determined by gas chromatography. Desulphonation gas chromatography has been applied [28-31] to the analysis of partially degraded linear alkylbenzene sulphonate mixtures.

Waters and Garrigan [32] have described a microsulphonation-gas chromatographic technique for the determination of linear alkylbenzene sulphonates in UK rivers. Sample clean-up, particularly by extracting the linear alkylbenzene sulphonates as the 1-methylheptyl amine salt into hexane, resulted in gas chromatographic traces free from significant interferences in which individual linear alkylsulphonate isomers can be identified on the basis of relative retention times. The procedure has a limit of 10^g linear alkylsulphonate per litre and permits submicrogram levels of C -C homologues to be quantified.

4.1.1.4 High-performance liquid chromatography

Taylor and Nickless [33] have described a paired-ion high-performance liquid chromatographic technique for the separation of mixtures of linear alkylbenzene sulphonates and p-sulphophenylcarboxylate salts,

in river waters. Partially biodegraded linear alkylbenzene sulphonate was analysed by the same method. Structural information on measurable intermediates formed was provided by stopped-flow ultraviolet spectra, comparison of retention behaviour with that of standards and analysis of collected fractions. Samples (1.5L) were concentrated for analysis by acidification with sulphuric acid to pH2, followed by passage through a column containing 20mL of XAD-4 resin at a flow rate of 7mL min-1. Compounds retained by the resin were eluted with 3 x 25mL portions of methanol and the combined eluates evaporated to dryness then up to 2^L.

Figure 4.3 shows the trace obtained from the analysis of undegraded linear alkylbenzene sulphonate (LAS) mixture (sodium

20 10 0 minutes

Fig. 4.3 Paired-ion HPLC of ungraded LAS. Column: 250 x 4.6mm, bonded C silica, d = 5Dm. Mobile phase: 13.7 x 10-3M (CTMA+) SO 2- in 87.5% methanol, 12.5% water, pH5.4. Ffow rate: 0.8mL min-1; UV detection at 224nrn, 0'5AUFS. (Reprinted with permission from Taylor and Nickless [33]. Copyright (1979) Elsevier Science Publishers BV.)

20 10 0 minutes

Fig. 4.3 Paired-ion HPLC of ungraded LAS. Column: 250 x 4.6mm, bonded C silica, d = 5Dm. Mobile phase: 13.7 x 10-3M (CTMA+) SO 2- in 87.5% methanol, 12.5% water, pH5.4. Ffow rate: 0.8mL min-1; UV detection at 224nrn, 0'5AUFS. (Reprinted with permission from Taylor and Nickless [33]. Copyright (1979) Elsevier Science Publishers BV.)

dodecylbenzene sulphonate). Peak identifications were made by coinjection with pure undegraded linear alkylbenzene sulphonate compounds and confirmed by analysis of the undegraded linear alkylbenzene sulphonate mixture by desulphonation followed by gas chromatography of the resultant alkylbenzenes on a 15m OV-1 support coated open-tubular column.

High-performance liquid chromatography with fluorimetric detection has been used to determine alkylbenzene sulphonates in river waters at the 1)ig L-1 level [34].

Di Corcia et al. [35] have described a liquid chromatographic method for the determination of linear alkylbenzene sulphonates in environmental waters.

4.1.1.5 Infrared spectrometry

Rand et al. [36], Sallee [37] and Nagai et al. [38] have investigated infrared spectroscopic methods for the determination of alkylbenzene sulphonates.

4.1.1.6 Ultraviolet spectroscopy

Ultraviolet spectroscopy has been used for the determination of alkylbenzene sulphonates [39, 40].

4.1.1.7 Polarography

The inhibitory effects of anionic detergents on the adsorptive accumulation of the (dimethyl glyoximate)-nickel complex on a hanging mercury trap electrode is the basis of an indirect method for determining anionic detergents [41].

4.1.1.8 Potentiometric analysis

Tsuji et al. [42] carried out microdeterminations of anionic and nonionic detergents in water by a potentiometric method involving the use of cholinesterase. This method utilizes the phenomena that anionics inhibit the cholinesterase-butyrylthiocholine-enzyme system. A constant current is applied across two platinum electrodes immersed in a butyrylthiocholine-iodine solution containing the sample. Cholinesterase is then added and changes in electrode potential due to the formation of thiol are plotted against time to determine the concentration of anionic detergent.

4.1.1.9 Atomic absorption spectrometry

Gagnon [43] has described a rapid and sensitive atomic absorption spectrometric method for the determination of anionic detergents at the microgram per litre level in natural waters. The method described below is based on determination by atomic absorption spectrometry using the bis(ethylene-diamine) copper II ion. The method is suitable for detergent concentrations up to 50^g L-1 but it can be extended up to 15mg L-1.

The limit of detection is 0.3^g L-1. At detergent concentrations at the milligram per litre level environmental copper concentrations produce no problem, but at the microgram per litre level interference by natural organic chelators can occur. A copper concentration between 1.3 and 2.9^g L-1 is extractable in chloroform. Filtration and subsequent ultraviolet irradiation of water reduce these values. At a detergent concentration of the order of 1^g L-1 the error can be 10-20%. This level can fluctuate considerably depending on the water sample. It is important to measure the chloroform-extractable copper in the sample to evaluate the possible error and therefore to get a better accuracy of the detergent concentrations.

Benoit and Lamathe [44] compared two methods for determining anionic surfactants in natural water by flameless atomic absorption spectroscopy.

4.1.1.10 Miscellaneous

Wang et al. [5-7, 45] discussed carbon absorption for the concentration of linear alkylate sulphonates. Wickbold [46] has discussed the concentration and separation of anionic detergents from surface water on a gas-water interface. In this method a 1L sample in a glass tower (500 x 60mm) is treated with concentrated hydrochloric acid (10mL) and an upper layer of ethyl acetate (10mL) is added. Nitrogen (50-60L h-1) is bubbled up through a glass frit at the base of the tower, and the surfactant is carried by the bubbles into the organic layer, which is replaced after 5min. The combined organic layers are evaporated, the residue is dissolved in water (100mL) and a little methanol, and the surfactant (100-200ug) is determined by the method of Longwell and Maniece [47].

Taylor et al. [17] studied the selective determination of anionic surfactants using ionic association compounds of these compounds with iron (II) chelates. The effects of solution variables including ionic strength, pH and buffer concentration, reagent excess, solution volume, type of solvent, and the selection of reagents and conditions of extraction and separation are examined, and a method is proposed for the selective determination of homologous surfactants. A method is also proposed for the determination of surfactants of various chain lengths.

La Noce [48] in 1969 reviewed methods then available for determining anionic and non-ionic surface active agents in water. He reviewed methods for the determination on anionic surfactants by titration with a cationic reagent colorimetrically by complexing with methylene blue or methylene green or by infrared spectroscopy.

Adachi and Kobayashi [49] have compared various methods for determining anionic and non-ionic detergents in natural waters.

Schneider et al. [50] identified cationic and anionic detergents in the river Rhine by combined field desorption-collisionally activated decomposition mass spectrometry. The results demonstrated that three types of surfactants desorbed at distinct emitter heating currents and a partial separation of non-ionic, cationic and anionic surfactants could be achieved.

Borgerding and Hiles [51] carried out quantitative analysis of alkylbenzene sulphonates in amounts down to 0.5^g L-1 by continuous fast-flow atomic bombardment spectrometry.

Kalenichenko et al. [52] used a spectrophotometric method for estimating alkylbenzene sulphates in their biodegradation studies of these compounds.

4.1.2 Sea waters

4.1.2.1 Spectrophotometric methods

Bhat et al. [53] used complexation with the bis(ethylenediamine) copper II cation as the basis of a method for estimating anionic surfactants in fresh estuarine and sea water samples. The complex is extracted into chloroform and copper measured spectrophotometrically in the extract using 1,2(pyridylazo)-2-naphthol.

Bhat et al. [53] using the same extraction system were able to improve the detection limit of the method to 5^g L-1 (as linear alkylsulphonic acid) in fresh estuarine and sea water samples.

Hon Nami and Hanya [27, 54] have investigated the applicability of a combined gas chromatographic-mass spectrometic method to the determination of linear alkylbenzene sulphonates in chloroform extracts of 5L samples of estuary and bay water samples. They also determined the ratio of the concentration of linear alkylbenzene sulphonates to those of methylene blue active substances. Linear alkylbenzene sulphonates were determined as their methylsulphonate derivatives.

Kazarac et al. [55] have discussed methylene-blue-based methods for determining anionic detergents in sea water samples. The technique is based on solvent extraction and preconcentration of detergents using Wickbold's apparatus [46]. A method using azure A instead of methylene blue has been proposed by Den Tonkelaar and Bergshoeff [56]. Workers at the Water Research Centre [59] have described a methyleneblue-based autoanalysis method for determining 0-1^g L-1 anionic detergents in water and sewage effluents. This method was based on the work of Longwell and Maniece [57] and subsequently modified by Abbot [11] and by Sodergren [58]. The Water Research Centre report describes the method in detail and discusses its precision and accuracy.

4.1.2.2 Polarography

Zvonaric et al. [60] have described a polarographic method for the determination of the surfactant activity of sea water.

4.1.2.3 Atomic absorption spectrometry

Le Bihan and Courtot Coupez [61, 62] analysed fresh water and sea water as follows. To 1L of filtered sea water was added, with shaking, 10mL of hydrochloric acid and 10mL of 0.023M copper 1,10-phenanthroline sulphate. After 5min, 43mL of isobutyl methyl ketone were added, shaken vigorously for 1min, allowed to stand for 5min, and, after separating the phases, re-extracted with 25mL of the ketone. The copper was determined by atomic absorption spectrometry using the 324.7nm line. A blank was prepared from sea water containing about 1% of the amount of detergent to be determined. Calibration graphs must be prepared for each anionic detergent. The method is applicable to fresh water if sodium chloride is added to prevent emulsion formation, and is applicable to cationic detergents by a different method. The following species (wt per litre) do not interfere: Co11 and Ni11 (10mg), HSO - (3g), H PO - (0.2g), Br- (10mg), and I- (2mg), Fe3+ (30mg), CrO (1mg), SCN- (20irig) 4and NO - (1mg) produce a small constant increase4 in the atomic absorption, a3 nd hydrogen sulphide must be eliminated by oxidation. Non-ionic detergents do not interfere. Le Bihan and Courtot Coupez [63] used the same complex and nameless atomic absorption spectroscopy to determine anionic detergents. Crisp et al. [64] were the first to use bis(ethylenediamine) Cu(II) ion for the determination of anionic detergents. They determined the concentration of detergents by flame atomic absorption spectroscopy or by a colorimetric method. The colorimetric method was more sensitive with a limit of detection of 0.03^g L-1 (as linear alkyl sulphonic acid) compared to 0.06^g L-1 for atomic absorption spectroscopy. Their method is applicable to fresh and sea water. Crisp et al. [65] determined anionic detergents in fresh estuarine and sea water, at the ppb level. The detergent anions in a 750mL water sample are extracted with chloroform as an ion association compound with the bis(ethylenediamine) Cu(II) cation and determined by atomic absorption spectrometry using a graphite furnace atomizer. The limit of detection (as linear alkylsulphonic acids) is 2^g L-1.

Gagnon [43] has described a rapid and sensitive atomic absorption spectrometric method developed from the work of Crisp et al. [64] for the determination of anionic detergents at the ppb level in sea waters. The method is based on determination by atomic absorption spectrometry using the bis(ethylenediamine) Cu(II) ion. The method is suitable for detergent concentrations up to 50^g L-1, but it can be extended up to 15mg L-1. The limit of detection is 0.3^g

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