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Ind. Eng.
Chem. Prod. Res. Dev..
Vol. 17, No. 3. 1978
TECHNICAL REVIEW Application and Determination of Anionic Surfactants Lawrence K. Wang’ Stevens Instiiute of Technology, Hoboken, New Jersey 07030
Shinn-Fwu Kao, Mu Hao Wang, and Jao-Fuan Kao LWarfmenf of Environmental Engineering. National Cheng Kung Univemw, Tainan, Taiwan, China
Andrea L. Loshln Rensselaer Pobtechnic Institute, Tmy, New York 12181
.
....
Lawrence K . Wangis an ASSO-, ciate Professor of Civil Engineering at Stevens Institute of Technology and an Adjunct Associate Professor o f Chemical and Enuironmental Engineering at Rensselaer Polytechnic Institute. He received his Ph.D. degree in Environmental Engineering from Rutgers University in 1972. He is the author of a book, “Environmental Engineering Glossary”, some 120 research papers, .and two US. Patents. His main research area is unit operations, unit processes, environmental chemistry, and mathematical modeling. Dr. Wang holds memberships in AWWA, NYWPCA, IES, Sigma Xi, DAPE, AEEP, ACS ( N U , and AIChE (Environ. Diu.).
Anionic surfactants are extensively used a s wetting agents, emulsifiers,cleaning agents, flotation agents, and oil extracting agents. Their applications are described and analytical determinations are reviewed and discussed. The standard and modified methylene blue methods and the modified azure A method were evaluated experimentally for analyzing anionic nonsoapy surfactants. Experiments were also conducted to evaluate a titration method for quantitative determination of anionic soapy surfactants. The relative effectiveness, precision, accuracy, and resistance to interference of these methods were documented. Introduction A surface active agent, or surfactant, is an organic molecule which contains both hydrophilic and hydrophobic functional groups (the latter usually a very long carbon skeleton). The hydrophobic end of the surfactants can combine with greasy dirt and fats. These molecules lower the surface tension (7)of the water, and the surfactant can pull the dirt away from the hody, leaving it clean, and will 0019-7890/78/1217-0186$01.00/0
Shinn- F w u K a o is an Environmental Engineer in Taipei, Taiwan, Republic of China. He received his M.E. degree in Environmental Engineering from National Cheng Kung University, Tainan, Taiwan.
Mu Ha0 Wangis a Sanitary Engineer at New York State Department of Environmental Conservation and an Adjunct Associate Professor of Environmental Engineering at National Cheng Kung University. She became Mrs. Lawrence K . Wang in 1968, and received her Ph.D. degree i n Environmental Engineering from Rutgers University in 1972. She has over 30 publications in the fields of environmental engineering, process control, project management, and systems analysis. She is a member of WPCF, NEWPCA, Sigma X i , and AIChE (Enuiron. Diu.). J a w F u a n Kao is Professor and Head of the Department of Environmental Engineering, National Cheng Kung University, Tainan, Taiwan, China. Professor Kao received his M.S. degree in Civil Engineering from Purdue University. He has published nearly 100 technical papers, books, and reports in his fields of environmental microbiology and water quality control. Professor Kao belongs to numerous Chinese and international professional societies.
Andrea L. Loshin is a law school student at Albany Law School, Union University, Albany, N.Y. She graduated from Rensselaer Polytechnic Institute with her B.E. degree in Environmental Engineering in 1977. Miss Loshin, a former student of Professor Lawrence K. Wang, received an honorable academic award at the Commencement due to her excellent performance at R.P.I. collect at the surface or interface of the liquid and air. Thii dual nature of~tbemolecule enables the surfactant to be an excellent cleansing agent. There are three kinds of surfactants, as follows: non0 1978 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 3, 1978
ionic, cationic, and anionic. Nonionic types are primarily used as detergents. Cationic surfactants are useful in fabric softening, corrosion inhibition, emulsion compounding, and disinfection (34). Anionic surfactants are mainly used as wetting agents, emulsifiers, and detergents. Cationic surfactants have positively charged organic ions when dissociated in water. Anionic surfactants, on the other hand, have negatively charged ions. Anionic surfactants can further be divided into two categories-soapy and nonsoapy. Soapy surfactants are the salts of naturally occurring fatty acids. Nonsoapy surfactants include synthetic surfactants composed of long-chained hydrocarbons which have been sulfonated or sulfated. There are many varieties of nonsoapy surfactants since they can contain many types of functional groups-aromatics, esters, amides, or any combination of the above. Anionic soapy surfactants, such as carboxylate soaps, are extensively used in household washing and in the mining industry as promoters for the separation of minerals by flotation. Anionic nonsoapy surfactants, such as alkyl benzenesulfonate (ABS) and linear alkylate sulfonate (LAS), are principal active components of synthetic detergents. Most of the synthetic detergents contain 20-30% anionic nonsoapy surfactant and 70-8070 builders and other components that enhance the detergent properties of the active ingredients. Synthetic detergents first came into extensive use during World War I1 in Germany where soap was becoming more and more scarce due to the ravages of war. Since 1950, synthetic detergents have grown to such a point where they constitute more than 90% of all washing and laundering additives. An anionic nonsoapy surfactant, sodium dodecyl sulfate, has been demonstrated by Chaine and Zeitlin (2) to be an excellent flotation agent for the separation of phosphate and arsenate from seawater by adsorption colloid flotation. Many other anionic surfactants have been used successfully by Matsuzaki and Zeitlin (21) for the separation of trace elements from water by flotation. In the petroleum industry, anionic surfactants, such as petroleum sulfonates, are used in their modern research of surfactant systems for oil production. Typically (26), these engineering projects consist of injecting surfactant formulations into sandstone cores which have been waterflooded to residual oil saturation to determine the efficiency of oil displacement by the anionic surfactant. After the operation, it is desirable to flush the core with isopropyl alcohol to obtain a material balance. Effluent samples are monitored for surfactant concentration. Anionic surfactants can also be used as emulsifiers in food. The Food Protection Committee of the National Research Council (24) researched possible dangers caused by anionic and cationic surfactants used as emulsifiers in food, since this was becoming a popular practice in the middle-1950’s. It was found that anionic surfactants enhanced the toxicity of microorganisms in food and that they were able to denature proteins by disrupting the cross-linkage in the protein structure (24). This would support the movement of advocating control of the substance in food and water. Objectives. Increased awareness of the presence of anionic surfactants in food and water led analysts to develop methods to detect concentrations of synthetic surfactants and soaps in water and wastewater accurately, quickly, and inexpensively. There have been developed, since the middle-l95O’s, several determinations of the presence of nonsoapy anionic surfactants, as follows (1,3-9, 13-20,25-29,25-43): titration against cationic surfactant, the flotation method, the infrared method, gas, paper, or
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thin-layer chromatography, and, finally, colorimetric methods. Methods by which to detect soaps are (5,6,11, 12,22,25,30): paper chromatography, two-phase titration method, colorimetric method, and, possibly, a flotation method. Our purpose is to review methods which have been proposed in the past and to analyze data from an investigation of the effectiveness of modified colorimetric methods (32, 35, 39) for nonsoapy surfactant determination, as well as to attempt to analyze data from a two-phase titration method (22) for the determination of soaps.
Determination of Anionic Nonsoapy Surfactants In the early 1950’s, a general method was devised. It involved the complexing of the anionic surfactant present with methylene blue dye, extracting the complex with a nonpolar solvent, and finally measuring the intensity of the dye in the extract with a colorimeter. The colorimetric method was hampered by “interference” caused by ions present in the solution which complexed with the dye or the surfactant. Ions complexing the dye were extracted with the surfactant-dye complexes into the nonaqueous phase and caused positive interference, Le., an increase in the measured amount of surfactant. Ions complexing with the surfactant, retaining it with aqueous phase, caused negative interference, or a decrease in the measured amount of surfactant from the actual amount present. In addition to interferences, the method was plagued by the difficulty of purifying and extracting all the surfactant present in the original sample. Consequently, many variations on the general methylene blue method were developed purporting to improve upon it. These variations included one or more extractions of the surfactant sample, designed to eliminate the interfering ions. Among these variants was the method devised by Longwell and Maniece (17). Using Manoxol O.T. as a standard anionic surfactant, they established a calibration curve plotting absorbance against concentration of surfactant. Six extractions were required to purify the sample and eliminate inorganic ions, dissolved solids, and proteins. This method, though it recovered much of the original surfactant present, did little to eliminate interfering ions. Additionally, the many extractions were time-consuming. To eliminate interfering ions, Webster and Halliday (43) attempted to develop a method eliminating all interfering ions prior to the addition of dye in order to reduce positive interference. This was in contrast with Longwell and Maniece’s method, which attempted to reduce negative error. While Longwell and Maniece’s method proved to be more accurate than others, since it removed all hydrolyzable materials and isolated the surfactant dye complex from proteins (a negative interference), it was time-consuming and not specifically designed to test for one particular substance. In the early 1960s, ABS was causing much concern due to its lack of biodegradability. Controlling limits were placed upon its presence in water, and it became necessary to detect ABS specifically. This could be accomplished by infrared spectroscopy, or paper or gas chromatography. Hummel (13) developed a method by which he recorded the ability for a specific surfactant to absorb infrared radiation, and thus the surfactant was identified and its concentration determined. Results were plotted in percent transmittance or absorbance vs. the wavelength of infrared radiation to which it was exposed, in microns. The resulting spectrum was derived from the sum of all possible vibration phenomena experienced by the various functional groups as they reacted to the impingement of the radiation.
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This method, however, was limited due to the fact that as the number of vibrating functional groups increased, the resulting spectra emitted would obscure each other and interfere with each other. The resultant confusion was a serious problem as many types of surfactants had several different kinds of functional groups. Hummel suggested wet chemical analytical methods be used to determine the exact nature of the surfactant if this occurs. In 1964, Frazee and Crisler (9) developed a method to determine the relative amounts of branched and straight-chained isomers of ABS by estimating their relative absorption at two infrared frequencies. This enabled more specificity with the infrared method, since these measurements could be compared to calibration graphs made with standard types of ABS which approximated the various types of commercially available detergents present in waste water. This method, however, still ultimately required chemical analyses for actual determination of amount present. Although the infrared methods are more specific than the dye-complexing methods, their use is quite limited. They are costly since the infrared equipment is expensive. The techniques require several extractions for the isolation of the ABS or other surfactant from its solution. This entails loss of surfactant and increases error. In addition, the length of time required for extraction allows some biodegradation of surfactant. As LAS is more susceptible to microbial action than ABS, these methods allow much more degradation and loss. Swisher (28) designed a method using gas chromatography for the specific detection of LAS. His method entails concentrating the LAS by reaction with methylene blue and chloroform extraction from the solution, followed by desulfonation to give the parent alkylbenzene, which is analyzed by gas chromatography. Drewry (5, 6) proposed another method using paper chromatography for the detection of LAS. His paper chromatography also involved special treatment of the sample. The method requires that the paper be dried and fixed in such a way as not to disturb the sample, which was a problem for Drewry and others proposing to use this method. In most methods presented thus far, some type of extraction with an organic solvent is performed. The methylene blue active substances (MBAS) combine with the dye and form chloroform-soluble salts which can be removed from water by adding chloroform to the sample and agitating. Unfortunately, a negative error can result since some surfactant stays in the aqueous phase or adheres to the glass beaker or cylinder in which it is being prepared. In addition, water may appear in the organic phase-this can be filtered out with glass wool. However, some surfactant is lost in the filter as it passes through. Many extractions and filterings may cause severe error in results. Paper and gas chromatography not only involve these, but are also expensive in terms of equipment, as well as being time-consuming. The large time factor involved with both chromatography methods results in biodegradation and negative error. Therefore, an inexpensive, yet accurate and quick method was sought. The Standard Methylene Blue Method (SMBM) was inexpensive but time-consuming, using six extractions with chloroform (I). In 1975, Wang (32, 39) proposed a Modified Methylene Blue Method (MMBM) requiring two extractions after the methylene blue addition to the solution. A calibration curve is prepared using LAS as the standard. The LAS is complexed with methylene blue for chloroform extraction. The chloroform layer is passed through glass wool into a
spectrophotometer cell. The absorbance or percent transmission is measured and compared to values obtained on the calibration curve. In an attempt to discover a reagent which would form a complex with anionic nonsoapy surfactants and not be affected by other components of water and waste water, the reagent Azure A was investigated by Steveninck and Riemersoma (27). They hypothesized that Azure A would be more resistant to reaction with interfering ions than was methylene blue. This hypothesis stimulated another modified colorimetric method (35) similar to the MMBM except for the dye used in the determination (35). Anionic nonsoapy surfactants can also be quantitatively determined by the indirect two-phase titration method (33, 41), in which an appropriate amount of cationic surfactant is added to ensure that the sample contains an excess amount of cationic surfactant. The amount of excess cationic surfactant is then quantitatively determined by titration with a standard anionic titrant in the presence of methyl orange dye, buffer reagent, and chloroform. If the original surfactant content (pM) is anionic, the anionic surfactant content (pM) is equal to the known amount (pM) of the added cationic surfactant minus the amount (pM) of standard anionic titrant spent in the neutralization. The method is very useful for quantitative measurement of unknown water sample (i.e., the charge condition, cationic or anionic, is unknown), in saline water (4I),or in the field, for either anionic or cationic surfactants (33). Wang and Panzardi (37) developed a direct two-phase titration method for the determination of anionic nonsoapy surfactants. The anionic surfactant is directly titrated with a standard cationic titrant (cetyldimethylbenzylammonium chloride) in the presence of dye (Azure A or methylene blue), buffer reagent, and chloroform. The method may be used for analyzing both fresh and saline water samples (38);however, it is less accurate than the two modified colorimetric methods, MMBM and MAAM, for analyzing fresh water samples. The aforementioned test procedures are used for the determination of anionic surfactants in multicomponent systems. Certain pure anionic surfactants with benzene rings, however, can be directly and rapidly measured by a UV method (23). The long-chain alkyl benzenesulfonate shows an absorption maximum at 225 nm with molar absorptivities equal to 1.29 X lo4 cm*/mmol. The absorption maximum for p-toluenesulfonate occurs at 200 nm, with an absorptivity of 1.09 X lo4 cm2/mmol, and phenol has an absorptivity of 1.44 X lo3 at 271 nm, an absorption maximum, and thus neither interferes.
Determination of Anionic Soapy Surfactants Anionic soapy surfactants have not received as much attention as nonsoapy ones due to their relative innocuousness in comparison to anionic nonsoapy surfactants. Soapy surfactants are readily decomposed by microorganisms and thus do not pose as much of a persistent problem as do nonsoapy surfactants. However, because petroleum products from which detergents are synthesized are becoming increasingly rare, soaps may be used with increasing frequency. Therefore, a standard method for the determination of soaps should be developed. Development of soapy surfactant analyses has followed much the same route. Attempts have been made to develop a flotation method, colorimetric methods, two-phase titration methods, paper chromatography methods, etc. (25). Tomlinson and Sebba (30) analyzed traces of soaps in neutral solution with the cationic dye crystal violet in 1962.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 3, 1978
The adduct formed was subsequently removed by “ion flotation” and the optical density of the residual dye solution was measured. The chemical reaction of Sebba’s method is nonstoichiometric; therefore, the method requires precise control of conditions and is very susceptible to interference from dissolved inorganic ions. Later in 1963 and 1964, Drewry ( 5 , 6 )developed a paper chromatography technique for separating and identifying soaps as well as synthetic nonsoapy surfactants. By a method of consecutive spraying, surfactants can be detected on a single chromatogram for quantitative analysis. A colorimetric method was described by Gregory (12) in 1966 for the determination of anionic soapy surfactants as well as anionic nonsoapy surfactants. The reagent used is the cationic copper(I1) triethylene-tetramine complex, which reacts in alkaline solution with anionic surfactants to give an adduct that can be extracted into a 2-butanol-cyclohexane mixture. The copper associated with the surface-active anion in the extract is then determined spectrophotometrically as the colored complex using diethylammonium diethyldithiocarbamate. According to Gregory (12),long-chain carboxylates with carbon numbers from CI4to CZ2,as well as anionic nonsoapy surfactants, may be determined in the range 0.2 to 10 mg/L. Titration of cationic surfactant into the soap, with an indicator dye, has been studied. This method, however, is very much dependent upon the maintenance of the pH of the soap solution above 10. Unlike nonsoapy surfactants, which remain ionized in the low pH ranges, the ionized portions of the salts which form soaps form insoluble fatty acids when exposed to an excess of H30+ions. This soap component would then be unavailable to complex with the cationic surfactant and this would distort results. A direct two-phase titration method using cationic titrant and methylene blue as dye was shown by Glazer and Smith (11) to be accurate to f l % . Methylene blue, however, is blue only in an acidic solution. Since the pH of the soap solution must be kept above 10, the dye turns pink in the chloroform-soap phase and masks the end point of the titration. As in other two-phase titration methods, this method is imprecise due to the reflection of color from the brighter aqueous phase into the duller chloroform phase. To remedy the problems encountered by using methylene blue as indicator, bromocresol green has been used in conjunction with a disodium/trisodium phosphate buffer (22). This dye, it was said, maintains its color in both aqueous and chloroform phases. There are difficulties with this method, a two-phase titration, and therefore it is undesirable. In addition, the procedure as written is time-consuming and complex. This particular method (22) was evaluated experimentally in 1977 (18)for its precision, accuracy, and feasibility as a possible standard method. The results of this experimentation are recorded in the following sections of this paper.
Evaluation of Three Colorimetric Methods for the Determination of Anionic Nonsoapy Surfactants General Description. The standard methylene blue method recommended by the American Public Health Association, American Water Works Association, and Water Pollution Control Federation (1)and the modified methylene blue method (32) and the modified azure A method (35) suggested by Wang were evaluated experimentally under this program ( 2 4 ) . The authors compared interference produced by substances which complex with methylene blue and azure A, which are the following: sulfates, sulfites, sulfonates, bicarbonates, carboxylates,
189
ill0 80
60
Y
4U
zn 0
1
3
us
COYCEITRAIIOY. MG/L
Figure 1. Calibration curve of LAS.
phosphates, phenol, urea, amine, nitrites, nitrates, peptone, polypeptone, chlorides, thiocyanates, and cyanates. These were interpreted in change in aborbance between the blank and the new reading. Calibration curves of the three colorimetric methods (see Figure 1) were also prepared using 15 replicate LAS samples (in distilled water) of a t least seven concentrations between 0.1 and 6.0 mg/L. Precision and accuracy of the three methods were determined. Standard Methylene Blue Method (SMBM). Required reagents, apparatus, and analytical procedures can be found from Standard Methods ( I ) . Modified Methylene Blue Method (MMBM). A. Reagents. (a) Methylene Blue Reagent: Dissolve 625 mg of methylene blue in 400 mL of distilled water. Gradually add 10.0 mL of concentrated sulfuric acid to the 400 mL mixture, shake until dissolution is complete, and dilute to 500 mL. (b) Stock LAS Solution: Weigh an amount of the reference material equal to 1.000 g of LAS on a 100% active basis. Dissolve in distilled water and dilute to 1 L to obtain a concentration of 1.00 mL = 1.00 mg of LAS. This solution should be stored in a refrigerator to minimize biodegradation. (c) Standard LAS Solution: Dilute 50.0 mL of stock LAS solution to 1 L with distilled water to obtain a concentration of 1.00 mL = 0.05 mg of LAS. (d) Buffer Solution: Mix 250 mL of 0.5 M citric acid and 250 mL of 0.2 M disodium hydrogen orthophosphate together. (e) Chloroform, anhydrous. (f) Glass wool. B. Calibration Curve Preparation. Prepare a series of seven separatory funnels with 0, 25,50,75,100,125, and 150 pg of the standard LAS solution. Add water to make the total volume 50 mL in each separatory funnel. Treat each standard as described in steps (b) to (d) below. Plot a calibration curve of mg of LAS vs. absorbance (or YO transmittance), or as mg/L of LAS vs.. absorbance. C. Analysis. (a) Pipet an aliquot amount of water sample into a separatory funnel; dilute to 50 mL with distilled water. (b) Add 1 mL of methylene blue reagent, 5 mL of buffer solution, and 25 mL of chloroform to each separatory funnel. Stopper the separatory funnel and shake it vigorously for 30 s. Allow each sample to stand undisturbed for 5 min after shaking. The chloroform will separate from the water and settle as a lower layer. (c) Wedge a small plug of glass wool in the stem of a filtering funnel. Place the filtering funnel above a clean, dry glass test cell (1-cm light path) and filter the chloroform layer through the glass wool to remove the water therefrom; collect the treated chloroform in the cell. (d) Determine the absorbance (or percent transmittance) of the chloroform solution at 652 nm against a blank of chloroform. (e) Determine the equivalent LAS content from the calibration curve of anionic surfactant. Modified Azure A Method (MAAM). A. Reagents.
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(a) Azure A Reagent: Dissolve 400 mg of Azure A and 5 mL of 1.0 N sulfuric acid in 500 mL of distilled water and then make up to 1000 mL with distilled water. (b) Stock LAS Solution: same as that of the MMBM. (c) Standard LAS Solution: same as that of the MMBM. (d) Buffer Solution: same as that of the MMBM. (e) Chloroform; anhydrous. (f) Glass wool. B. Calibration Curve Preparation. Same as that of the MMBM. C. Analysis. Same as that of the MMBM, except that Azure A Reagent should be used instead of Methylene Blue Reagent, and the absorbance (or percent transmittance) should be measured at 623 nm instead of 652 nm.
Experimental Results and Discussion of Three Colorimetric Methods for the Determination of Anionic Nonsoapy Surfactants A. Precision and Accuracy. The precision and accuracy of the standard methylene blue method (SMBM) has been investigated by 110 water-quality laboratories in this country (see Standard Methods, 13th edition). In those studies, three determinations were made: (a) a synthetic unknown sample containing 270 pg/L of LAS in distilled water was analyzed, and the relative standard deviation and the relative error were 14.8% and 10.6%, respectively; (b) a tap water unknown sample to which was added 480 pg/L of LAS was analyzed, and the relative standard deviation and the relative error were 9.9% and 1.3%, respectively; and (c) a river-water unknown sample to which was added 2.94 mg/L of LAS was analyzed, and the relative standard deviation and the relative error were 9.19% and 1.470, respectively. Evaluation of the SMBM was also conducted by the Department of Environmental Engineering, National Cheng Kung University, Taiwan (14, 15). Results are summarized here: (a) a synthetic unknown sample containing 0.5 mg/L of LAS in distilled water was analyzed 15 times, and the relative standard deviation and the relative error were 2.1% and 5.5%, respectively, and (b) a river-water unknown sample to which was added 0.5 mg/L of LAS was analyzed 15 times, and the relative standard deviation and the relative error were 0.8% and 5.670, respectively. For the modified methylene blue method (MMBM), the following are the results obtained by National Cheng Kung University (14): (a) a synthetic unknown sample containing 0.5 mg/L of LAS in distilled water was analyzed 20 times and the relative standard deviation and the relative error were 2.3% and 0.1%, respectively; and (b) a river-water unknown sample to which was added 0.3 mg/L of LAS was analyzed 20 times, and the relative standard deviation and the relative error were 0.7% and 1.4%, respectively. There are also the experimental results obtained by National Cheng Kung University (14) for evaluation of the modified azure A method (MAAM): (a) a distilled water unknown sample to which was added 0.5 mg/L of LAS was analyzed 20 times, and the relative standard deviation and the relative error were 2.5% and l.l%,respectively; and (b) a river-water unknown sample to which was added 0.3 mg/L of LAS was analyzed 20 times, and the relative standard deviation and the relative error were 0.7% and 2.5%, respectively. These results are summarized in Table I for the purpose of comparison. According to the experimentation at National Cheng Kung University (14), it can be seen that the colorimetric method which had the smallest relative standard deviation and relative error was the MMBM proposed by Wang (32). It was therefore the most precise and most accurate of the
Table I. Precision and Accuracy of LAS Determination by Three Colorimetric Methods precision and accuracy SMBM MMBM MAAM
sample distilled water
RSW
river water
RSD RE
RE^
2.1% 5.5% 0.8% 5.6%
RSD = relative standard deviation. error.
2.3% 0.1% 0.7% 1.4%
2.5% 1.1% 0.7% 2.5%
RE = relative
three colorimetric methods involved in this analysis. The MAAM was also more precise and accurate than the SMBM for distilled water and river-water samples. In addition, the calibration curve that the MMBM yielded had the steepest slope when it was plotted as percent transmittance vs. LAS concentration in mg/L (see Figure 1). This indicates (10) that it is the MMBM that is the most useful method among the three analyzed for determining very small concentration of LAS in these samples. B. Interference. Both organic and inorganic compounds interfere with the determination of LAS by colorimetric methods. The following are the interferencecausing substances investigated by National Cheng Kung University (14): sulfate (K,SO,), sulfite (Na2S03),sulfonate (HOCGH4SO3H),bicarbonate (NaHC03), carboxylate (CH3COOH), phosphate (K2HP04),phenol (C6H5OH), urea (H2NCONH2),amine ((CH2CH20H)2NH),nitrite (NaNO,), nitrate (NaN03), polypeptone, chloride (NaCl), thiocyanate (KSCN), and cyanate (NaCNO). Tables I1 and I11 indicate their experimental results found for interfering substances interpreted in percent change in absorbance between the blank and the new reading. Possible interference caused by foreign compounds to the SMBM has been studied by many researchers (1,17, 31). According to the Standard Methods ( I ) , organic sulfates, sulfonates, carboxylates, phosphates, and phenols-which complex methylene blue-and inorganic cyanates, chlorides, nitrates, and thiocyanates-which form ion pairs with methylene blue-are among the positive interferences. Organic materials, especially amines, which compete with the methylene blue in the reaction, can cause low results. Inorganic nitrites and chlorides in water samples caused positive errors, and organic amines, indeed, caused low results for a 0.5 mg/L LAS sample (14). Based on the chloride interference data, one can conclude that the SMBM is not applicable to measurement of anionic nonsoapy surfactants in saline waters, in agreement with the U S . Environmental Protection Agency (31) finding. Effects of some inorganic and organic substances on the MAAM and MMBM can be seen from Tables I1 and 111. All samples were prepared with distilled water containing 0.5 mg/L of LAS. The tables indicate that inorganic sulfates, sulfites, sulfonates, cyanates, thiocyanates, phosphates, nitrates, and chlorides all formed ion pairs with both azure A and methylene blue which produced positive interferences for the concentration ranges studied. Inorganic nitrites, however, gave positive interference to the MMBM and negative interference to the MAAM. Prins and Spaander ( 3 ) also found that nitrites (5-100 mg/L) gave negative interference to the azure A method, and such interference could be prevented by waiting 25 min between shaking and running off the extract. In view of the negligible amounts of nitrite in drinking and surface waters, this waiting time could be omitted for surface water determinations. Bicarbonates produced positive interferences to the MAAM,
Ind. Eng. Chern. Prod. Res. Dev., Vol. 17, No. 3, 1978
Table 11. Effects of Some Foreign Substances on MAAM and MMBM % change in absorbance of 0.5 mg/L of LAS sample impurities
SO,
mg/L 0.0 0.6 3.0 6.0 30.0 60.0 SO, *-,mg/L 0.0 0.6 3.0 6.0 30.0 60.0 -SO,H, mg/L 0.0 1.0 5.0 10.0 50.0 100.0 COOH-, mg/L 0.0 1.0 5.0 10.0 50.0 2-,
100.0 CNO-, mg/L 0.0 1.0 5.0 10.0 50.0 100.0 SCN-, mg/L 0.0 0.5 1.0 5.0 10.0 50.0 100.0
MAAM
-
3.2 4.8 16.9 27.3 32.5 L
8.8 11.1 13.0 17.4 26.7
-
3.0 3.3 4.6 3.4 2.1 L
4.1 6.1 7.8 10.5 18.8 L
1.5 5.8 9.1 21.1 27.6
-
-
1.3 3.3 8.0 27.0 52.7
MMBM
-
0.2 4.0 6.9 14.4 23.6
3.1 4.4 5.6 7.9 21.5
-
4.7 6.7 7.8 11.1 14.0
1.9 6.1 8.2 12.3 14.0
-
2.5 5.7 6.3 8.6 18.2
-
6.3 13.6 57.4 105.5 420.3
but their effect on the MMBM was unclear according to Table 11. Tables I1 and I11 also indicate the effects of some organic substances on the MAAM and the MMBM. It seems that organic phenols, ureas, and carboxylates complexed methylene blue and azure A, and thus produced positive interferences. Polypeptones gave positive interferences to the MMBM, but gave negative interferences to the MAAM. Den Tonkelaar and Bergshoeff ( 3 ) also found that there was negative interference when nonsoapy surfactants were analyzed in the presence of azure A dye and 20 mg/L of polypeptone. It should be noted that organic amine gave negative interferences to the SMBM, but gave positive interferences to the MMBM and the MAAM, as indicated in Table 111. Although both the SMBM and the MMBM use methylene blue as an indicator dye, the effect of organic amine on the two colorimetric methods was different. This could be due to the fact that the SMBM’s methylene blue reagent and wash solution (1) are different from the MMBM’s methylene blue reagent and buffer solution (32). Den Tonkelaar and Bergshoeff ( 3 ) stated that the naturally occurring cations, such as Mg2+,Fe2+,Fe3+,Mn2+, Ca2+,and NH4+,in drinking and river waters do not interfere with azure A. The methylene blue method of Longwell and Maniece (17) gave closely similar results, but
lQl
% change in absorbance of
0.5 mg/L of LAS sample
impurities 0.0 1.0 5.0 10.0 50.0 100.0 PO, 3 - , mg/L 0.0 1.0 5.0 10.0 50.0 100.0 NO,-, mg/L 0.0 1.0 5.0 10.0 50.0 100.0 urea, % 0.0 0.5 1.0 5.0 polypetone, mg/L 0.0 1.0 5.0 10.0 50.0 100.0 phenol, mg/L 0.0 1.0 5.0 10.0 50.0
100.0
MMBM
MAAM
HCO,-,mg/L
-
-
6.0 9.0 17.0
3.0 2.7 - 0.2 8.0 7.0
-
-
1.8 2.1
1.0 5.5 7.2 11.1 21.6
1.3 6.7 7.8 10.5 12.2
-
-
1.3 6.0 12.5 30.8 52.1
6.7 9.5 16.0 23.2 31.7
-
-
11.5 19.1 30.0
7.0 11.4 16.1
-
- 2.0
1.3 3.7 6.1 8.7 15.1
- 4.5 - 6.1 - 14.2 - 21.6
-
8.3 9.1 9.6 11.1 21.6
4.7 10.9 13.2 23.3 27.1
Table 111. Effects of Chlorides, Nitrites, and Amines on MAAM, MMBM, and SMBM % change of absorbance of
impurities
0.5 mg/L of LAS sample MAAM MMBM SMBM
NO;,
mg/L 0.0 0.7 7.0 15.0 30.0 70.0 c1-, % 0.0 0.5 1.0 2.0 5.0 >NH, mg/L 0.0 1.0 5.0 10.0 50.0 100.0
3.6 4.0 -3.9 -15.9 - 33.4
6.9 9.7 12.4 18.0 24.6
4.5 8.5 11.2 15.4 17.1
8.9 10.5 12.9 20.4
3.3 15.8 27.6 63.3
3.2 8.3 15.3 33.2
7.2 12.0 12.5 13.0 18.2
2.4 5.2 6.2 11.7 14.5
- 2.8 -8.1
- 12.4
precipitation occurred in the presence of calcium. Cationic surfactants will give negative interference by
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Table IV. Required Time and Cost for Each Analysis analytical required required method time, min cost, NT$ SMBM 12- 20 $26.17 MMBM 6-7 6.88
MAAM
6-7
6.70
competing with the cationic methylene blue dye or azure A dye in joining up with the anionic surfactant (36). All three colorimetric methods studied (SMBM, MMBM, and MAAM) can only measure the free anionic nonsoapy surfactants in aqueous solution (36). In the presence of chlorine, the color of the MAAM’s chloroform extract was pink instead of blue (one test). Therefore, the MAAM is not recommended for LAS determination in tap water or any waste water containing chlorine. The SMBM and MMBM may be used. C. Development and Stability of the Color. The rapid development of the blue color allows the absorbance to be measured almost immediately after chloroform extraction. The absorbance obtained by the SMBM, the MMBM, and the MAAM remain constant for 4 h. After 24 h, the average percent change in absorbance was found to be -3.2%, -9.670, and 0.53% respectively. D. pH Effect. Effects of pH on the methylene blue and azure A methods were not investigated by the authors. According to Den Tonkelaar and Bergshoeff ( 3 ) ,pH between 1.6 and 3.0 has no effect on the absorbance of the blue chloroform extract by the azure A method. At higher pH values, the color changed from blue to violet as a result of donating a proton, and azure A (in contrast to methylene blue) thus became soluble in chloroform. Azure A is also oxidized in alkaline solution to dimethylthionoline. Extraction of the azure A complex from alkaline solution (as in the case of methylene blue) was therefore impossible (3).
E. Time and Cost Required for Each Analysis. Analyzing anionic nonsoapy surfactants by either the MMBM or the MAAM will require less time, reagents, and apparatus for each test than the SMBM method. The required time and cost for each analysis were estimated as listed in Table IV. It should be noted that the required test time was estimated assuming all apparatus was clean and ready for use, all reagents were prepared, and necessary calibration curves were available. The cost of each test included only the reagent cost in terms of NT$. (Note: U S . $1.00 = N T $40.00). Labor cost in the U S A . is much more expensive than the reagent cost. F. Report of Results. The environmental chemist or engineer should report the molecular weight (MW) of LAS to be used as standard anionic nonsoapy surfactant. When the SMBM or the MMBM are used, report the results as mg/L of methylene-blue-active substances (i.e., MBAS) calculated as LAS, MW.-. When the MAAM is used, report the results as mg/L of azure-A-active substances (Le., AAAS) calculated as LAS, MW.-. Evaluation of a Direct Two-Phase Titration Method for the Determination of Anionic Soapy Surfactants General Description. A direct two-phase titration method originally proposed by Milwidsky and Holtzman (22)was evaluated experimentally under this program (18). This method utilizes bromocresol green (BCG) as indicator dye. BCG can be used in high pH environments and it was proposed (22)that a two-phase titration using this dye would be particularly applicable to soap determination. Ivory soap (99.4% pure) and cetyldimethylbenzylammonium chloride are used as the standard anionic soapy
surfactant and the cationic titrant, respectively. Disodium and trisodium phosphate are used as buffer agents in the BCG indicator solution. The following sections present the required reagents, apparatus, analytical procedure, and experimental data. The precision and accuracy of the test method were calculated using the statistical equations and Fortran IV computer program introduced in the literature (40).
Direct Two-Phase Titration Method. A. Reagents. (a) Chloroform-alcohol solution: Add 200 mL of chloroform to 100 mL of 2-propanol in a 500-mL glass bottle. Stopper, shake, and store it. (b) Indicator solution: Combine 0.050 g of bromocresol green (BCG) with 10 mL of 2-propanol. Then add the BCG solution to a 1-L volumetric flask. To this add 50 g of NaC1, 14.98 g of NazHP04-7Hz0,and 50 g of Na3PO4.12Hz0. Dilute to 1 L and store in a glass-stoppered bottle. (c) Cationic quaternary ammonium titrant: Mix 0.8260 g of CDBAC (cetyldimethylbenzylammonium chloride, 100%) with 200 mL of 2-propanol in a 1-L flask. Dilute to 1L with warm distilled water. Magnetic stirring might be necessary. The concentration of the cationic titrant is 0.002 M. (d) Stock soap solution: Dissolve 500 mg of pure Ivory brand soap in 1 L of water. Refrigerate to prevent excessive biodegradation. It might be necessary to heat the solution in order to dissolve the soap. The concentration of the stock soap solution is 500 mg/L (1.00 mL = 0.50 mg). B. Calibration Curve Preparation. Prepare a series of separatory funnels with 0, 0.6, 3, 15, and 30 mL of the stock soap solution. Add water to make the total volume 30 mL in each separatory funnel. The corresponding concentrations of the soap standards are 0, 10,50,250,and 500 mg/L. Then treat each standard as described in steps (b) and (c) below. Plot a calibration curve of mg/L of soap vs. mL of CDBAC titrant spent in titration. C. Analysis. (a) Pipet an aliquot amount (30 mL or less) of water sample into a separatory funnel; dilute to 30 mL with distilled water if the original sample size is less than 30 mL. Record the sample size in milliliters. (b) Add 30 mL of chloroform-alcohol solution and 20 mL of indicator solutions to the separatory funnel, stopper, and shake vigorously for 30 s. (c) The 0.002 M CDBAC titrant is run into the funnel slowly from a buret. After a very small amount (e.g., less than 0.5 mL) of titrant is added, restopper, shake vigorously, and wait for phase separation. The color of the bottom chloroform phase will turn bluish. Continue titration until the end point is reached when the chloroform phase is the same color and intensity as the aqueous phase. (Note: The separatory funnel has a poor shape to properly observe the color of the layers. It should have parallel instead of divergent sides.) Record the amount of cationic surfactant (Le., 0.002 M CDBAC) required for titration. (d) Determine the measured amount of anionic soapy surfactant from the calibration curve prepared previously. Calculate the concentration of anionic soapy surfactant using the following equation mg/L of anionic soapy surfactant = (mg/L of soapy surfactant read from calibration curve) X (30/sample size) Experimental Results and Discussion of a Direct Two-Phase Titration Method for the Determination of Anionic Soapy Surfactants Five concentrations (0, 10, 50, 250, and 500 mg/L) of soap were prepared from the stock soap solution. Nine samples of each concentration were tested. Experimental results are listed in Table V and Figure 2. The calibration
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Table V. Preparation of Calibration Curve of Soap soap, mg/L 4.2 7.8 8.6 21.8 35.0
0
10 50 25 0 500
3.6 7.0 7.0 18.2 41.9
volume of 0.002 M CDBAC solution spent in titration, mL 4.6 6.4 5.1 4.3 5.3 6.5 6.4 6.3 6.6 6.8 7.4 7.6 7.4 7.5 8.2 17.7 17.3 17.6 19.1 17.9 34.2 37.2 36.5 33.8 35.5
4.4 6.3 7.4 17.3 36.5
4.4 5.8 7.5 18.3 36.0
Table VI. Precision and Accuracy of Soap Determination by Direct Two-Phase Titration Method ~~
true values, melL of soaD 0
10 50 250 500
std dev, mg/L 12.82 9.04 7.70 22.66 38.15
re1 std dev, % 7.65 28.14 16.13 10.32 7.51
curve (see Figure 2 ) of soap can be expressed by the following linear regression line X = 16.03Y - 73.74 in which X = anionic soapy surfactant concentration in mg/L, and Y = volume of 0.002 M CDBAC titrant spent in titration. The standard deviation and relative standard deviation were calculated and are listed in Table VI. The direct titration method originally proposed by Milwidsky and Holtzman (22) was shown to be inaccurate and imprecise in the low concentration range (Le., 0-50 mg/L) by experimental evaluation. The results are not easily reproducible, as in other two-phase titration methods, since there is reflection from the aqueous phase into the chloroform phase, making end point detection difficult. The slope of the calibration curve (Figure 2 ) has virtually no difference between a 10 mg/L sample and a 50 mg/L sample in color. Standard deviations and relative standard deviations were very high with respect to the amount of soap in the samples. This is in the lower concentrations ranges, since the error was greater than the total amount of soap in the sample. This method may be used in situations where soap concentrations exceed 50 mg/L. The effective work range extends to the greater concentration ranges with a relatively low standard deviation. For determination of anionic soapy surfactant in the low concentration range, a search for a more reliable dye which may be used under high-pH conditions and which can distinguish among low concentrations is suggested. With a dye such as this, a less tedious colorimetric method might be established as a standard method for the accurate determination of soap in water and waste water. Accordingly, a colorimetric method described in 1966 by Gregory (12) is suggested to be evaluated experimentally for its possible adoption as a standard method.
Summary and Conclusions 1. Anionic surfactants are used extensively as wetting agents, emulsifiers, cleaning agents, flotation agents, and oil extraction agents. Rapid and accurate determination of anionic surfactant concentrations in the process water, plant effluent, and receiving water is important. This paper reviews the current analytical techniques available for the determination of anionic surfactant and discusses each method’s advantages, disadvantages, interferences, and applicability. The standard methylene blue method (SMBM), the modified methylene blue method (MMBM), and the modified azure A method (MAAM) were experimentally evaluated under this program for anionic nonsoapy surfactant determination. A two-phase titration
so 60
40
30
20 10
0 0
100
200
300
SOAP CGNCE!CPATIGN,
400
so0
600
YG/L
Figure 2. Calibration curve of soap.
method was also evaluated for its applicability to the anionic soapy surfactant determination. 2. Spectroscopic determination in conjunction with carbon adsorption (carbon adsorption method, or CAM) is recommended by the academic and professional associations ( I ) as a standard method for determining the linear alkylate sulfonate (LAS) content of water. This method involves the collection and isolation of a few milligrams of LAS and its quantitative determination based on infrared adsorption of an amine complex of LAS. Compared to the SMBM, the MMBM, and the two-phase titration method, the big drawback to the infrared CAM is complication and tedium. Though lengthy, this method is specific and accurate for low LAS concentrations in water and it eliminates alkyl sulfates. When an infrared spectrophotometer is not available, a colorimetric determination may be substituted by recovering the purified LAS and applying the SMBM (1,31) or the MMBM (32). The CAM is subject to interference ( I ) and is not applicable to the analysis of sewage or industrial waste waters. 3. The standard methylene blue method (SMBM) is widely used for the determination of free anionic nonsoapy surfactants in terms of “mg/L MBAS” present in fresh water (1,31). Organic sulfates, sulfonates, carboxylates, phosphates, and phenols, and inorganic cyanates, chlorides, nitrates, and thiocyanates give positive interferences. Cationic surfactants and amines cause low results. For water quality control, the environmental scientist should analyze the sample by either the SMBM (31) or the MMBM (32). If the results are lower than or equal to the Drinking Water Standard (0.5 mg/L of MBAS) no further analysis will generally be required. Should the MBAS concentration be high, sometimes it is necessary to use the CAM ( I ) to determine how much represents true anionic surfactant and how much interferences. Compared to the CAM, the SMBM is less reliable (because methylene blue reacts with not only alkyl benzenesulfonate but also alkyl sulfates), but less tedious (because no pretreatment of sample is needed). The SMBM uses a lengthy multiple chloroform extraction and multiple washing procedure for the elimination of some possible interferences. However, the method is still not applicable to measurement of
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anionic nonsoapy surfactants in saline waters. 4. The modified methylene blue method (MMBM) is a simplified version of the SMBM. The MMBM is applicable to measurement of free anionic nonsoapy surfactants, in terms of “mg/L MBAS” present in fresh water (321, and is accureate to fO.01 mg/L. The SMBM is extremely time-consuming, using six chloroform extractions and multiple washing, while the MMBM requires only two chloroform extractions (one for blank, one for sample) and no washing. For analyzing LAS in distilled water and river water, the MMBM’s relative standard deviation and relative error are smaller than that of the SMBM. Phenols, ureas, carboxylates, polypeptones, amines, cyanates, thiocyanates, nitrates, nitrites, chlorides, phosphates, sulfates, sulfites, and sulfonates give positive interferences to the MMBM. Cationic surfactants, if present, will give negative interference to the modified method. Compared to the SMBM, the MMBM is simpler, less time-consuming, less expensive and more accurate for fresh water (chloride concentrations
