5540 SURFACTANTS*

5540 A. Introduction

1. Occurrence and Significance

Surfactants enter waters and wastewaters mainly by discharge

of aqueous wastes from household and industrial laundering and

other cleansing operations. A surfactant combines in a single

molecule a strongly hydrophobic group with a strongly hydrophilic one. Such molecules tend to congregate at the interfaces

between the aqueous medium and the other phases of the system

such as air, oily liquids, and particles, thus imparting properties

such as foaming, emulsification, and particle suspension.

The surfactant hydrophobic group generally is a hydrocarbon

radical (R) containing about 10 to 20 carbon atoms. The hydrophilic groups are of two types, those that ionize in water and

those that do not. Ionic surfactants are subdivided into two

categories, differentiated by the charge. An anionic surfactant

ion is negatively charged, e.g., (RSO3)

Na, and a cationic one

is positively charged, e.g., (RMe3N)Cl. Nonionizing (nonionic) surfactants commonly contain a polyoxyethylene hydrophilic group (ROCH2CH2OCH2CH2......OCH2CH2OH, often abbreviated REn, where n is the average number of -OCH2CH2-

units in the hydrophilic group). Hybrids of these types exist also.

In the United States, ionic surfactants amount to about two

thirds of the total surfactants used and nonionics to about one

third. Cationic surfactants amount to less than one tenth of the

ionics and are used generally for disinfecting, fabric softening,

and various cosmetic purposes rather than for their detersive

properties. At current detergent and water usage levels the surfactant content of raw domestic wastewater is in the range of

about 1 to 20 mg/L. Most domestic wastewater surfactants are

dissolved in equilibrium with proportional amounts adsorbed on

particulates. Primary sludge concentrations range from 1 to

20 mg adsorbed anionic surfactant per gram dry weight.1 In

environmental waters the surfactant concentration generally is

below 0.1 mg/L except in the vicinity of an outfall or other point

source of entry.2

2. Analytical Precautions

Because of inherent properties of surfactants, special analytical precautions are necessary. Avoid foam formation because the

surfactant concentration is higher in the foam phase than in the

associated bulk aqueous phase and the latter may be significantly

depleted. If foam is formed, let it subside by standing, or collapse

it by other appropriate means, and remix the liquid phase before

sampling. Adsorption of surfactant from aqueous solutions onto

the walls of containers, when concentrations below about 1 mg/L

are present, may seriously deplete the bulk aqueous phase.

Minimize adsorption errors, if necessary, by rinsing container

with sample, and for anionic surfactants by adding alkali phosphate (e.g., 0.03N KH2PO4).3

3. References

1. SWISHER, R.D. 1987. Surfactant Biodegradation, 2nd ed. Marcel

Dekker, New York, N.Y.

2. ARTHUR D. LITTLE, INC. 1977. Human Safety and Environmental

Aspects of Major Surfactants. Rep. No. PB-301193, National Technical Information Serv., Springfield, Va.

3. WEBER, W.J., JR., J.C. MORRIS & W. STUMM. 1962. Determination of

alkylbenzenesulfonate by ultraviolet spectrophotometry. Anal. Chem.

34:1844.

5540 B. Surfactant Separation by Sublation

1. General Discussion

a. Principle: The sublation process isolates the surfactant,

regardless of type, from dilute aqueous solution, and yields a

dried residue relatively free of nonsurfactant substances. It is

accomplished by bubbling a stream of nitrogen up through a

column containing the sample and an overlying layer of ethyl

acetate. The surfactant is adsorbed at the water-gas interfaces of

the bubbles and is carried into the ethyl acetate layer. The

bubbles escape into the atmosphere leaving behind the surfactant

dissolved in ethyl acetate. The solvent is separated, dehydrated,

and evaporated, leaving the surfactant as a residue suitable for

analysis. This procedure is the same as that used by the Organization for Economic Co-operation and Development (OECD),1

following the development by Wickbold.2,3

b. Interferences: The sublation method is specific for surfactants,

because any substance preferentially adsorbed at the water-gas

interface is by definition a surfactant. Although nonsurfactant substances largely are rejected in this separation process, some amounts

will be carried over mechanically into the ethyl acetate.

c. Limitations: The sublation process separates only dissolved

surfactants. If particulate matter is present it holds back an equilibrium amount of adsorbed surfactant. As sublation removes the

initially dissolved surfactant, the particulates tend to reequilibrate

and their adsorbed surfactants redissolve. Thus, continued sublation

eventually should remove substantially all adsorbed surfactant.

However, if the particulates adsorb the surfactant tightly, as sewage

particulates usually do, complete removal may take a very long

time. The procedure given herein calls for preliminary filtration and

measures only dissolved surfactant. Determine adsorbed surfactant

* Approved by Standard Methods Committee, 2000. Editorial revisions, 2011.

1
content by analyzing particulates removed by filtration; no standard

method is available now.

d. Operating conditions: Make successive 5-min sublations from

1 L of sample containing 5 g NaHCO3 and 100 g NaCl. Under the

conditions specified, extensive transfer of surfactant occurs in the

first sublation and is substantially complete in the second.2– 4

e. Quantitation: Quantitate the surfactant residue by the procedures in 5540C or D. Direct weighing of the residue is not

useful because the weight of surfactant isolated generally is too

low, less than a milligram, and varied amounts of mechanically

entrained nonsurfactants may be present. The procedure is applicable to water and wastewater samples.

f. Quality control (QC): The QC practices considered to be an

integral part of each method are summarized in Table 5020:I.

2. Apparatus

a. Sublator: A glass column with dimensions as shown in

Figure 5540:1.* For the sintered glass disk use a coarse-porosity

frit (designation “c”–nominal maximum pore diam 40 to 60 m

as measured by ASTM E-128) of the same diameter as the

column internal diameter. Volume between disk and upper stopcock should be approximately 1 L.

b. Gas washing bottle, as indicated in Figure 5540:1, working

volume 100 mL or more.

c. Separatory funnel, working volume 250 mL, preferably

with inert TFE stopcock.

d. Filtration equipment, suitable for 1-L samples, using medium-porosity qualitative-grade filter paper.

e. Gas flowmeter, for measuring flows up to l L/min.

3. Reagents

a. Nitrogen, standard commercial grade.

b. Ethyl acetate: CAUTION: Ethyl acetate is flammable and

its vapors can form explosive mixtures with air.

c. Sodium bicarbonate, NaHCO3.

d. Sodium chloride, NaCl.

e. Water, surfactant-free.

4. Procedure

a. Sample size: Select a sample to contain not more than 1 to

2 mg surfactant.4 For most waters the sample volume will be

about 1 L; for wastewater use a smaller volume.

b. Filtration: Filter sample through medium-porosity qualitative filter paper. Wash filter paper by discarding the first few

hundred milliliters of filtrate.

c. Assembly: Refer to Figure 5540:1.

Connect nitrogen cylinder through flowmeter to inlet of gas

washing bottle. Connect gas outlet at top of sublator to a gas

scrubber or other means for disposing of ethyl acetate vapor

(e.g., vent to a hood or directly outdoors). In the absence of a

flowmeter, ensure proper gas flow rate by measuring volume of

gas leaving the sublator, with a water-displacement system.

d. Charging: Fill gas washing bottle about two-thirds full with

ethyl acetate. Rinse sublation column with ethyl acetate and

discard rinse. Place measured filtered sample in sublator and add

5 g NaHCO3, 100 g NaCl, and sufficient water to bring the level

up to or slightly above the upper stopcock (about 1 L total

volume). If sample volume permits, add salts as a solution in

400 mL water or dissolve them in the sample and quantitatively

transfer to the sublator. Add 100 mL ethyl acetate by running it

carefully down the wall of the sublator to form a layer on top of

the sample.

e. Sublation: Start the nitrogen flow, increasing the rate carefully to 1 L/min initially but do not exceed a rate at which the

liquid phases begin vigorous intermixing at their interface.

Avoid overly vigorous intermixing, which will lead to backextraction of the surfactant into the aqueous phase and to dissolution of ethyl acetate. Continue sublation for 5 min at 1 L/min. * This apparatus is available from Kontes at kontes.com; Cal Glass, Costa Mesa,

CA; or similar suppliers.

Figure 5540:1. Sublation apparatus.1 See 5540B.2a and b and 4c. Bottom

stopcock: TFE plug, 4-mm bore; side stopcocks: TFE plug,

2-mm bore.

SURFACTANTS (5540)/Surfactant Separation by Sublation

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If a lower flow rate is necessary to avoid phase intermixing,

prolong sublation time proportionally. If the volume of the upper

phase has decreased by more than about 20%, repeat the operation on a new sample but avoid excessive intermixing at the

interface. Draw off entire ethyl acetate layer through upper

stopcock into the separatory funnel; return any transferred water

layer to the sublator. Filter ethyl acetate layer into a 250-mL

beaker through a dry, medium-porosity, qualitative filter paper

(prewashed with ethyl acetate to remove any adventitious surfactant) to remove any remaining aqueous phase.

Repeat process of preceding paragraph with a second 100-mL

layer of ethyl acetate, using the same separatory funnel and filter,

and finally rinse sublator wall with another 20 mL, all into the

original beaker.

Evaporate ethyl acetate from the beaker on a steam bath in a

hood, blowing a gentle stream of nitrogen or air over the liquid

surface to speed evaporation and to minimize active boiling.

Evaporate the first 100 mL during the second sublation to avoid

overfilling the beaker. To avoid possible solute volatilization,

discontinue heating after removing the ethyl acetate. The sublated surfactant remains in the beaker as a film of residue.

Draw off aqueous layer in the sublator and discard, using the

stopcock just above the sintered disk to minimize disk fouling.

5. Precision and Bias

Estimates of the efficiency of surfactant transfer and recovery

in the sublation process include the uncertainties of the analytical

methods used in quantitating the surfactant. At present the analytical methods are semiquantitative for surfactant at levels below 1 mg/L in environmental samples.

With various known surfactants at 0.2 to 2 mg/L and appropriate analytical methods, over 90% of added surfactant was

recovered in one 5-min sublation from 10% NaCl. Without

NaCl, recovery of nonionics was over 90% but recovery of

anionics and cationics was only 2 to 25%.4

Five laboratories studied the recovery of five anionic surfactant types from concentrations of 0.05, 0.2, 1.0, and 5.0 mg/L in

aqueous solutions.5 The amount in each solution was determined

directly by methylene blue analysis and compared with the

amount recovered in the sublation process, also analyzed by

methylene blue. The overall average recovery was 95.9% with a

standard deviation of  7.4 (n  100). The extreme individual

values for recovery were 65% and 115% and the other 98 values

ranged from 75% to 109%. Recovery did not depend on surfactant concentration (average recoveries ranging from 94.7% at

5.0 mg/L to 96.8% at 1.0 mg/L) nor on the surfactant type

(average recoveries ranging from 94.7% to 96.6%). Average

recoveries at the five laboratories ranged from 90.0% to 98.0%.

Application of the sublation method in three laboratories to

eight different samples of raw wastewater in duplicate gave the

results shown in Table 5540:I. Methylene blue active substances

(MBAS) recovery in double sublation averaged 87  16% of

that determined directly on the filtered wastewater; these results

would have been influenced by any nonsurfactant MBAS that

might have been present. Repeating double sublation on the

spent aqueous phase yielded another 0.02 mg MBAS and another

0.08 mg cobalt thiocyanate active substances (CTAS). Adding

0.05 to 0.10 mg of known linear alkylbenzene sulfonate (LAS)

or 0.50 to 0.67 mg of known linear alcohol-based C1218E11 to

the same sublator contents and again running double sublation

resulted in over 90% recovery of the amount added.

6. References

1. ORGANIZATION FOR ECONOMIC CO-OPERATION AND DEVELOPMENT ENVIRONMENTAL DIRECTORATE. 1976. Proposed Method for the Determination of the Biodegradability of Surfactants Used in Synthetic Detergents. Org. for Economic Co-Operation & Development, Paris.

2. WICKBOLD, R. 1971. Enrichment and separation of surfactants from

surface waters through transport in the gas/water interface. Tenside

8:61.

3. WICKBOLD, R. 1972. Determination of nonionic surfactants in riverand wastewaters. Tenside 9:173.

4. KUNKEL, E., G. PEITSCHER & K. ESPETER. 1977. New developments in

trace- and microanalysis of surfactants. Tenside 14:199.

5. DIVO, C., S. GAFA, T. LA NOCE, A. PARIS, C. RUFFO & M. SANNA.

1980. Use of nitrogen blowing technique in microdetermination of

anionic surfactants. Riv. Ital. Sost. Grasse 57:329.

5540 C. Anionic Surfactants as MBAS

1. General Discussion

a. Definition and principle: Methylene blue active substances

(MBAS) bring about the transfer of methylene blue, a cationic

dye, from an aqueous solution into an immiscible organic liquid

upon equilibration. This occurs through ion pair formation by the

MBAS anion and the methylene blue cation. The intensity of

the resulting blue color in the organic phase is a measure of

MBAS. Anionic surfactants are among the most prominent of

many substances, natural and synthetic, showing methylene

blue activity. The MBAS method is useful for estimating the

anionic surfactant content of waters and wastewaters, but the

TABLE 5540:I. SURFACTANT RECOVERY BY SUBLATION

Variable MBAS CTAS

Sample volume, mL 200–300 500

Concentration without sublation, mg/L 2.2–4.7 —

Concentration found in sublate,* mg/L 1.8–4.4 0.3–0.6

Recovery in sublate, % 87  16† —

Amount in second sublate,‡ mg 0.02  0.02† 0.08  0.01†

Amount added, mg 0.05–0.10§ 0.50–0.67

Recovery in sublation,# % 94  17† 92  6†

* Two 5-min sublations.

† Average  SD (n  8).

‡ Two more 5-min sublations.

§ Reference LAS.

 Linear alcohol ethoxylate C12–18E11.

# Fifth and sixth 5-min sublations.

SURFACTANTS (5540)/Anionic Surfactants as MBAS

3
possible presence of other types of MBAS always must be

kept in mind.

This method is relatively simple and precise. It comprises

three successive extractions from acid aqueous medium containing excess methylene blue into chloroform (CHCl3), followed by

an aqueous backwash and measurement of the blue color in the

CHCl3 by spectrophotometry at 652 nm. The method is applicable at MBAS concentrations down to about 0.025 mg/L.

b. Anionic surfactant responses: Soaps do not respond in the

MBAS method. Those used in or as detergents are alkali salts of

C10 –20 fatty acids [RCO2]

Na, and though anionic in nature

they are so weakly ionized that an extractable ion pair is not

formed under the conditions of the test. Nonsoap anionic surfactants commonly used in detergent formulations are strongly

responsive. These include principally surfactants of the sulfonate

type [RSO3]

Na, the sulfate ester type [ROSO3]Na, and

sulfated nonionics [REnOSO3]

Na. They are recovered almost

completely by a single CHCl3 extraction.

Linear alkylbenzene sulfonate (LAS) is the most widely used

anionic surfactant and is used to standardize the MBAS method.

LAS is not a single compound, but may comprise any or all of

26 isomers and homologs with structure [RC6H4SO3]

Na,

where R is a linear secondary alkyl group ranging from 10 to 14

carbon atoms in length. The manufacturing process defines the

mixture, which may be modified further by the wastewater

treatment process.

Sulfonate- and sulfate-type surfactants respond together in

MBAS analysis, but they can be differentiated by other means.

The sulfate type decomposes upon acid hydrolysis; the resulting

decrease in MBAS corresponds to the original sulfate surfactant

content while the MBAS remaining corresponds to the sulfonate

surfactants. Alkylbenzene sulfonate can be identified and quantified by infrared spectrometry after purification.1 LAS can be

distinguished from other alkylbenzene sulfonate surfactants by

infrared methods.2 LAS can be identified unequivocally and its

detailed isomer-homolog composition determined by desulfonation-gas chromatography.3

c. Interferences: Positive interferences result from all other

MBAS species present; if a direct determination of any individual MBAS species, such as LAS, is sought, all others interfere.

Substances such as organic sulfonates, sulfates, carboxylates and

phenols, and inorganic thiocyanates, cyanates, nitrates, and chlorides also may transfer more or less methylene blue into the

chloroform phase. The poorer the extractability of their ion pairs,

the more effective is the aqueous backwash step in removing

these positive interferences; interference from chloride is eliminated almost entirely and from nitrate largely so by the backwash. Because of the varied extractability of nonsurfactant

MBAS, deviations in CHCl3 ratio and backwashing procedure

may lead to significant differences in the total MBAS observed,

although the recovery of sulfonate- and sulfate-type surfactants

will be substantially complete in all cases.

Negative interferences can result from the presence of cationic

surfactants and other cationic materials, such as amines, because

they compete with the methylene blue in the formation of ion

pairs. Particulate matter may give negative interference through

adsorption of MBAS. Although some of the adsorbed MBAS

may be desorbed and paired during the CHCl3 extractions,

recovery may be incomplete and variable.

Minimize interferences by nonsurfactant materials by sublation if necessary (5540B). Other countermeasures are nonstandard. Remove interfering cationic surfactants and other cationic

materials by using a cation-exchange resin under suitable conditions.3 Handle adsorption of MBAS by particulates preferably

by filtering and analyzing the insolubles. With or without filtration, adsorbed MBAS can be desorbed by acid hydrolysis; however, MBAS originating in any sulfate ester-type surfactant

present is destroyed simultaneously.1 Sulfides, often present in

raw or primary treated wastewater, may react with methylene

blue to form a colorless reduction product, making the analysis

impossible. Eliminate this interference by prior oxidation with

hydrogen peroxide.

d. Molecular weight: Test results will appear to differ if

expressed in terms of weight rather than in molar quantities.

Equimolar amounts of two anionic surfactants with different

molecular weights should give substantially equal colors in the

CHCl3 layer, although the amounts by weight may differ significantly. If results are to be expressed by weight, as generally is

desirable, the average molecular weight of the surfactant measured must be known or a calibration curve made with that

particular compound must be used. Because such detailed information generally is lacking, report results in terms of a suitable

standard calibration curve, for example “0.65 mg MBAS/L (calculated as LAS, mol wt 318).”

e. Minimum detectable quantity: About 10 g MBAS (calculated as LAS).

f. Application: The MBAS method has been applied successfully to drinking water samples. In wastewater, industrial wastes,

and sludge, numerous materials normally present can interfere

seriously if direct determination of MBAS is attempted. Most

nonsurfactant aqueous-phase interferences can be removed by

sublation. The method is linear over an approximate range of 10

to 200 g of MBAS standard. This may vary somewhat, depending on source of standard material.

g. Quality control (QC): The QC practices considered to be an

integral part of each method are summarized in Table 5020:I.

2. Apparatus

a. Colorimetric equipment: One of the following is required:

1) Spectrophotometer, for use at 652 nm, providing a light

path of 1 cm or longer.

2) Filter photometer, providing a light path of 1 cm or longer

and equipped with a red color filter exhibiting maximum transmittance near 652 nm.

b. Separatory funnels: 500-mL, preferably with inert TFE

stopcocks and stoppers.

3. Reagents

a. Stock LAS solution: Weigh an amount of the reference

material* equal to 1.00 g LAS on a 100% active basis. Dissolve

in water and dilute to 1000 mL; 1.00 mL  1.00 mg LAS. Store

in a refrigerator to minimize biodegradation. If necessary, prepare weekly.

* For sources of suitable reference material, contact Standard Methods tecnical

information manager at www.standardmethods.org/contact/.

SURFACTANTS (5540)/Anionic Surfactants as MBAS

4
b. Standard LAS solution: Dilute 10.00 mL stock LAS solution to 1000 mL with water; 1.00 mL  10.0 g LAS. Prepare

daily.

c. Phenolphthalein indicator solution, alcoholic.

d. Sodium hydroxide, NaOH, 1N.

e. Sulfuric acid, H2SO4, 1N and 6N.

f. Chloroform, CHCl3: CAUTION: Chloroform is toxic and a

suspected carcinogen. Take appropriate precautions against

inhalation and skin exposure.

g. Methylene blue reagent: Dissolve 100 mg methylene blue†

in 100 mL water. Transfer 30 mL to a 1000-mL flask. Add

500 mL water, 41 mL 6N H2SO4, and 50 g sodium phosphate,

mono- basic, monohydrate, NaH2PO4 H2O. Shake until dissolved. Dilute to 1000 mL.

h. Wash solution: Add 41 mL 6N H2SO4 to 500 mL water in

a 1000-mL flask. Add 50 g NaH2PO4 H2O and shake until

dissolved. Dilute to 1000 mL.

i. Methanol, CH3OH. CAUTION: Methanol vapors are flammable and toxic; take appropriate precautions.

j. Hydrogen peroxide, H2O2, 30%.

k. Glass wool: Pre-extract with CHCl3 to remove interferences.

l. Water, reagent-grade, MBAS-free. Use for making all reagents and dilutions.

4. Procedure

a. Preparation of calibration curve: Prepare an initial calibration curve consisting of at least five standards covering the

referenced (5540C.1f) or desired concentration range. Provided

that linearity is demonstrated over the range of interest (r 

0.995 or better) run daily check standards at the reporting limit

and a concentration above the expected samples’ concentration.

Check standard results should be within 25% of original value at

the reporting limit and 10% of original value for all others.

Otherwise, prepare a new calibration curve.

Prepare a series of separatory funnels for a reagent blank and

selected standards. Pipet portions of standard LAS solution

(5540C.3b) into funnels. Add sufficient water to make the total

volume 100 mL in each separatory funnel. Treat each standard as

described in ¶s d and e below, and plot a calibration curve of

absorbance vs. micrograms LAS taken, specifying the molecular

weight of the LAS used.

b. Sample size: For direct analysis of waters and wastewaters,

select sample volume on the basis of expected MBAS concentration:

Expected MBAS

Concentration

mg/L

Sample

Taken

mL

0.025–0.080 400

0.08 –0.40 250

0.4 –2.0 100

If expected MBAS concentration is above 2 mg/L, dilute

sample containing 40 to 200 g MBAS to 100 mL with water.

For analysis of samples purified by sublation, dissolve sublate

residue (5540B.4e) in 10 to 20 mL methanol, quantitatively

transfer the entire amount (or a suitable portion if more than

200 g MBAS is expected) to 25 to 50 mL water, evaporate

without boiling until methanol is gone, adding water as necessary

to avoid going to dryness, and dilute to about 100 mL with water.

c. Peroxide treatment: If necessary to avoid decolorization of

methylene blue by sulfides, add a few drops of 30% H2O2.

d. Ion pairing and extraction:

1) Add sample to a separatory funnel. Make alkaline by

dropwise addition of 1N NaOH, using phenolphthalein indicator.

Discharge pink color by dropwise addition of 1N H2SO4.

2) Add 10 mL CHCl3 and 25 mL methylene blue reagent.

Rock funnel vigorously for 30 s and let phases separate. Alternatively, place a magnetic stirring bar in the separatory funnel;

lay funnel on its side on a magnetic mixer and adjust speed of

stirring to produce a rocking motion. Excessive agitation may

cause emulsion formation. To break persistent emulsions add a

small volume of isopropyl alcohol (10 mL); add same volume

of isopropyl alcohol to all standards. Some samples require a

longer period of phase separation than others. Before draining

CHCl3 layer, swirl gently, then let settle.

3) Draw off CHCl3 layer into a second separatory funnel.

Rinse delivery tube of first separatory funnel with a small

amount of CHCl3. Repeat extraction two additional times, using

10 mL CHCl3 each time. If blue color in water phase becomes

faint or disappears, discard and repeat, using a smaller sample.

4) Combine all CHCl3 extracts in the second separatory funnel.

Add 50 mL wash solution and shake vigorously for 30 s. Emulsions

do not form at this stage. Let settle, swirl, and draw off CHCl3 layer

through a funnel containing a plug of glass wool into a 100-mL

volumetric flask; filtrate must be clear. Extract wash solution twice

with 10 mL CHCl3 each and add to flask through the glass wool.

Rinse glass wool and funnel with CHCl3. Collect washings in

volumetric flask, dilute to mark with CHCl3, and mix well.

e. Measurement: Determine absorbance at 652 nm against a

blank of CHCl3.

5. Calculation

From the calibration curve (5540C.4a) read micrograms of

apparent LAS (mol wt ) corresponding to the measured

absorbance.

mg MBAS/L  g apparent LAS

mL original sample

Report as “MBAS, calculated as LAS, mol wt .”

6. Precision and Bias

A synthetic sample containing 270 g LAS/L in distilled

water was analyzed in 110 laboratories with a relative standard

deviation of 14.8% and a relative error of 10.6%.

A tap water sample to which was added 480 g LAS/L was

analyzed in 110 laboratories with a relative standard deviation of

9.9% and a relative error of 1.3%.

A river water sample with 2.94 mg LAS/L added was analyzed

in 110 laboratories with a relative standard deviation of 9.1% and

a relative error of 1.4%.4 † Eastman No. P573, or equivalent.

SURFACTANTS (5540)/Anionic Surfactants as MBAS

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7. References

1. AMERICAN PUBLIC HEALTH ASSOCIATION, AMERICAN WATER WORKS

ASSOCIATION & WATER POLLUTION CONTROL FEDERATION. 1981. Carbon adsorption-infrared method. In Standard Methods for the Examination of Water and Wastewater, 15th ed. American Public Health

Assoc., Washington, D.C.

2. OGDEN, C.P., H.L. WEBSTER & J. HALLIDAY. 1961. Determination of

biologically soft and hard alkylbenzenesulfonates in detergents and

sewage. Analyst 86:22.

3. OSBURN, Q.W. 1986. Analytical methodology for LAS in waters and

wastes. J. Amer. Oil Chem. Soc. 63:257.

4. LISHKA, R.J. & J.H. PARKER. 1968. Water Surfactant No. 3, Study No.

32. U.S. Public Health Serv. Publ. No. 999-UIH-11, Cincinnati,

Ohio.

8. Bibliography

BARR, T., J. OLIVER & W.V. STUBBINGS. 1948. The determination of

surface-active agents in solution. J. Soc. Chem. Ind. (London)

67:45.

EPTON, S.R. 1948. New method for the rapid titrimetric analysis of

sodium alkyl sulfates and related compounds. Trans. Faraday Soc.

44:226.

EVANS, H.C. 1950. Determination of anionic synthetic detergents in

sewage. J. Soc. Chem. Ind. (London) 69:Suppl. 2, S76.

DEGENS, P.N., JR., H.C. EVANS, J.D. KOMMER & P.A. WINSOR. 1953.

Determination of sulfate and sulfonate anion-active detergents in

sewage. J. Appl. Chem. (London) 3:54.

AMERICAN WATER WORKS ASSOCIATION. 1954. Task group report. Characteristics and effects of synthetic detergents. J. Amer. Water Works

Assoc. 46:751.

EDWARDS, G.P. & M.E. GINN. 1954. Determination of synthetic detergents in sewage. Sewage Ind. Wastes 26:945.

LONGWELL, J. & W.D. MANIECE. 1955. Determination of anionic detergents in sewage, sewage effluents, and river water. Analyst 80:167.

MOORE, W.A. & R.A. KOLBESON. 1956. Determination of anionic detergents in surface waters and sewage with methyl green. Anal. Chem.

28:161.

ROSEN, A.A., F.M. MIDDLETON & N. TAYLOR. 1956. Identification of

synthetic detergents in foams and surface waters. J. Amer. Water

Works Assoc. 48:1321.

SALLEE, E.M., et al. 1956. Determination of trace amounts of alkyl

benzenesulfonates in water. Anal. Chem. 28:1822.

AMERICAN WATER WORKS ASSOCIATION. 1958. Task group report. Determination of synthetic detergent content of raw water supplies.

J. Amer. Water Works Assoc. 50:1343.

MCGUIRE, O.E., F. KENT, LAR. L. MILLER & G.J. PAPENMEIER. 1962.

Field test for analysis of anionic detergents in well waters. J. Amer.

Water Works Assoc. 54:665.

ABBOTT, D.C. 1962. The determination of traces of anionic surfaceactive materials in water. Analyst 87:286.

ABBOTT, D.C. 1963. A rapid test for anionic detergents in drinking water.

Analyst 88:240.

REID, V.W., G.F. LONGMAN & E. HEINERTH. 1967. Determination of

anionic-active detergents by two-phase titration. Tenside 4:292.

WANG, L.K., P.J. PANZARDI, W.W. SHUSTER & D. AULENBACH. 1975.

Direct two-phase titration method for analyzing anionic nonsoap surfactants in fresh and saline waters. J. Environ. Health

38:159.

5540 D. Nonionic Surfactants as CTAS

1. General Discussion

a. Definition and principle: Cobalt thiocyanate active substances (CTAS) are those that react with aqueous cobalt thiocyanate solution to give a cobalt-containing product extractable

into an organic liquid in which it can be measured. Nonionic

surfactants exhibit such activity, as may other natural and synthetic materials; thus, estimation of nonionic surfactants as

CTAS is possible only if substantial freedom from interfering

CTAS species can be assured.

The method requires sublation to remove nonsurfactant interferences and ion exchange to remove cationic and anionic surfactants, partition of CTAS into methylene chloride from excess

aqueous cobalt thiocyanate by a single extraction, and measurement of CTAS in the methylene chloride by spectrophotometry at 620 nm. Lower limit of detectability is around 0.1 mg

CTAS, calculated as C1218E11. Beyond the sublation step the

procedure is substantially identical to that of the Soap and

Detergent Association (SDA).1

b. Nonionic surfactant responses: For pure individual molecular species the CTAS response is negligible up to about RE5,

where it increases sharply and continues to increase more gradually for longer polyether chains.2,3 Fewer than about six oxygens in the molecule do not supply enough cumulative coordinate bond strength to hold the complex together. Commercial

nonionic surfactants generally range from about RE7 to RE15;

however, each such product, because of synthesis process constraints, is actually a mixture of many individual species ranging

from perhaps RE0 to RE2n in a Poisson distribution averaging

REn.

The hydrophobes used for nonionic surfactants in the U.S.

household detergent industry are mainly linear primary and

linear secondary alcohols with chain lengths ranging from about

12 to about 18 carbon atoms. Nonionics used in industrial

operations include some based on branched octyl- and nonylphenols. These products give strong CTAS responses that may

differ from each other, on a weight basis, by as much as a factor

of 2. Specifically, eight such products showed responses from

0.20 to 0.36 absorbance units/mg by the SDA procedure.1

As with anionic surfactants measured as MBAS, the nonionic

surfactants found in water and wastewater might have CTAS

responses at least as varied as their commercial precursors because the proportions of the individual molecular species will

have been changed by biochemical and physicochemical removal at varied rates, and further because their original molecular structures may have been changed by biodegradation processes.

c. Reference nonionic surfactant: Until it is practical to determine the nature and molecular composition of an unknown

mixed CTAS, and to calculate or determine the CTAS responses

SURFACTANTS (5540)/Nonionic Surfactants as CTAS

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of its component species, exact quantitation of uncharacterized

CTAS in a sample in terms of weight is not possible. Instead,

express the analytical result in terms of some arbitrarily chosen

reference nonionic surfactant, i.e., as the weight of the reference

that gives the same amount of CTAS response. The reference is

the nonionic surfactant C1218E11, derived from a mixture of

linear primary alcohols ranging from 12 to 18 carbon atoms in

chain length by reaction with ethylene oxide in a molar ratio of

1:11. C1218E11 is reasonably representative of nonionic surfactants in commercial use; its CTAS response is about 0.21 absorbance units/mg.

If the identity of the nonionic surfactant in the sample is

known, use that same material in preparing the calibration curve.

d. Interferences: Both anionic and cationic surfactants may

show positive CTAS response1,4 but both are removed in the

ion-exchange step. Sublation removes nonsurfactant interferences. Physical interferences occur if some of the CTAS is

adsorbed on particulate matter. Avoid such interference by filtering out the particulates for the sublation step; this will measure only dissolved CTAS.

e. Minimum detectable quantity: About 0.1 mg CTAS, calculated as C1218E11, which corresponds to 0.1 mg/L in a 1-L

sample.

f. Application: The method is suitable for determining dissolved nonionic surfactants of the ethoxylate type in most aqueous systems.

g. Quality control (QC): The QC practices considered to be an

integral part of each method are summarized in Table 5020:I.

2. Apparatus

a. Sublation apparatus: See 5540B.2.

b. Ion-exchange column, glass, about 1-  30-cm. Slurry

anion-exchange resin in methanol and pour into column to give

a bed about 10 cm deep. Insert plug of glass wool and then add

a 10-cm bed of cation-exchange resin on top in the same manner.

One column may be used for treating up to six sublated samples

before repacking.

c. Spectrophotometer and 2.0-cm stoppered cells, suitable for

measuring absorbance at 620 nm.

d. Separatory funnels, 125-mL, preferably with TFE stopcock

and stopper.

e. Extraction flasks, Soxhlet type, 150-mL.

3. Reagents

a. Sublation reagents: See 5540B.3.

b. Anion-exchange resin, polystyrene-quaternary ammoniumtype,* 50- to 100-mesh, hydroxide form. To convert chloride

form to hydroxide, elute with 20 bed volumes of 1N NaOH and

wash with methanol until free alkali is displaced.

c. Cation-exchange resin, polystyrene-sulfonate type,† 50- to

100-mesh, hydrogen form.

d. Cobaltothiocyanate reagent: Dissolve 30 g Co(NO3)2 6H2O

and 200 g NH4SCN in water and dilute to 1 L. This reagent is

stable for at least 1 month at room temperature.

e. Reference nonionic surfactant, C1218E11: Reaction product

of C1218 linear primary alcohol with ethylene oxide in 1:11

molar ratio.‡

f. Reference nonionic surfactant stock solution, methanolic,

approximately 2 mg nonionic/mL methanol: Quantitatively

transfer entire contents (approximately 1 g nonionic) from preweighed ampule into 500-mL volumetric flask, thoroughly rinse

ampule with methanol, make up to volume with methanol, and

reweigh dried ampule. Calculate concentration in milligrams per

milliter as in 5540D.5a. Because of possible phase separation,

use all material in the ampule.

g. Reference nonionic surfactant standard solution, methanolic, approximately 0.1 mg nonionic/mL methanol: Dilute 10.00

mL stock solution to 200 mL with methanol. Exact concentration

is 1/20 that of the stock solution.

h. Sodium hydroxide, NaOH: 1N.

i. Glass wool: Pre-extract with chloroform or methylene chloride.

j. Methanol, CH3OH: CAUTION: Methanol vapors are flammable and toxic; take appropriate precautions.

k. Methylene chloride, CH2Cl2: CAUTION: Methylene chloride vapors are toxic; take adequate precautions.

l. Water: Use distilled or deionized, CTAS-free water for

making reagents and dilutions.

4. Procedure

a. Purification by sublation: Proceed according to 5540B,

using sample containing no more than 2 mg CTAS. (NOTE: For

samples of known character containing no interfering materials,

omit this step.)

b. Ion-exchange removal of anionic and cationic surfactants:

Dissolve sublation residue in 5 to 10 mL methanol and transfer

quantitatively to ion-exchange column. Elute with methanol at 1

drop/s into a clean, dry 150-mL extraction flask until about 125 mL

is collected. Evaporate methanol on a steam bath aided by a

gentle stream of clean, dry nitrogen or air, taking care to avoid

loss by entrainment; remove from heat as soon as the methanol

is completely evaporated. (NOTE: With samples of known character containing no anionic or cationic materials, omit Step b.)

c. CTAS calibration curve: Into a series of 150-mL extraction

flasks containing 10 to 20 mL methanol place 0.00, 5.00, 10.00,

20.00, and 30.00 mL reference nonionic surfactant standard

solution and evaporate just to dryness. Continue as in ¶s d and e,

below, and plot a calibration curve of absorbance against milligrams of reference nonionic taken, specifying its identity (e.g.,

C1218E11 and lot number).

d. Cobalt complexing and extraction: Charge a 125-mL separatory funnel with 5 mL cobaltothiocyanate reagent. With precautions against excessive and variable evaporation of the methylene chloride, dissolve residue from ion-exchange operation

(¶ b above) by adding 10.00 mL methylene chloride and swirling

for a few seconds. Immediately transfer by pouring into the

separatory funnel. Do not rinse flask. (NOTE: Because of the

volatility of methylene chloride, rigidly standardize these operations with respect to handling and elapsed time; alternatively,

evaporate the methanol in 200-mL erlenmeyer flasks to be stop-

* Bio-Rad, AGl-X2, or equivalent.

† Bio-Rad AG 50W-X8, or equivalent.

‡ For sources of suitable reference material, contact Standard Methods technical

information manager at www.standardmethods.org/contact/.

SURFACTANTS (5540)/Nonionic Surfactants as CTAS

7
pered with glass or TFE stoppers during dissolution. Transfer as

directed here is incomplete, but in this case it will not introduce

error because the loss of nonionics is exactly compensated for by

the diminished volume of the organic layer in the extraction.) Shake

separatory funnel vigorously for 60 s and let layers separate. Run

lower layer into a 2.0-cm cell through a funnel containing a plug of

pre-extracted glass wool and stopper. Be sure filtrate is absolutely

clear. (NOTE: If desired, clarify by running the lower layer into a

12-mL centrifuge tube, stopper, spin at or above 1000  g for

3 min, and transfer to the cell by a Pasteur pipet; use same procedure

for both calibration and samples.)

e. Measurement: Determine absorbance at 620 nm against a

blank of methylene chloride. (NOTE: If haze develops in the cell,

warm slightly with a hot air gun or heat lamp to clarify.)

5. Calculations

a. Nonionic surfactant in reference nonionic stock solution

5540D.3f:

mg nonionic/mL methanol  mg reference sample/500 mL

b. Nonionic surfactant in sample: From the calibration curve

read milligrams of reference nonionic corresponding to the measured absorbance:

mg CTAS/L  mg apparent nonionic/L sample

Report as “CTAS, calculated as nonionic surfactant

C1218E11.”

6. Precision and Bias

Twenty-four samples of 6.22% w/v solution of reference nonionic surfactant C1218E11 were analyzed in three laboratories by

CTAS alone, without sublation or ion exchange. The overall

relative standard deviation was about 3%. Results of the three

laboratories individually were:

Laboratory % w/w  SD

A 6.08  0.14 (n  36)

B 6.56  0.17 (n  6)

C 6.25  0.14 (n  36)

Overall 6.20  0.19 (n  78)

Samples of raw wastewater were freed of surfactants by four

successive sublations, then 0.50 or 0.67 mg reference nonionic

surfactant C1218E11 was added and carried through the entire

sequence of sublation, ion exchange, and CTAS extraction.

Recoveries averaged 92% with overall standard deviation around

6%:

Laboratory % Recovery  SD

A 87  4 (n  4)

B 97  1 (n  4)

Overall 92  6 (n  8)

The above data relate to the bias and precision of the method

when applied to a known nonionic surfactant. When the nature of

the nonionic surfactant is unknown, there is greater uncertainty.

The response of the reference C1218E11 is about 0.21 absorbance units/mg, while that of the eight nonionic types mentioned

under 5540D.1b ranged from 0.20 to 0.36, and environmental

nonionics might differ still more. If the nonionic surfactant in the

sample has a response of 0.42, the result calculated in terms of

milligrams C1218E11 would be double the actual milligrams of

the unknown nonionic.

7. References

1. SOAP AND DETERGENT ASSOCIATION ANALYTICAL SUBCOMMITTEE. 1977.

Analytical methods for nonionic surfactants in laboratory biodegradation and environmental studies. Environ. Sci. Technol. 11:1167.

2. CRABB, N.T. & H.E. PERSINGER. 1964. The determination of polyoxyethylene nonionic surfactants in water at the parts per million

level. J. Amer. Oil Chem. Soc. 41:752.

3. CRABB, N.T. & H.E. PERSINGER. 1968. A determination of the apparent

molar absorption coefficients of the cobalt thiocyanate complexes of

nonylphenol ethylene oxide adducts. J. Amer. Oil Chem. Soc. 45:611.

4. GREFF, R.A., E.A. SETZKORN & W.D. LESLIE. 1965. A colorimetric

method for the determination of parts per million of nonionic surfactants. J. Amer Oil Chem. Soc. 42:180.

8. Bibliography

TABAK, H.H. & R.L. BUNCH. 1981. Measurement of non-ionic surfactants

in aqueous environments. Proc. 36th Ind. Waste Conf., p. 888.

Purdue Univ., Lafayette, Ind.

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