indirect tensammetric method for the determination of non-ionic `surfactants: part 2. investigation...

kzalyttca Chmuca Acta, 273 (1993) 313-321 Elsevler Science Publishers B V , Amsterdam

313

Indirect tensammetric method for the determination of non-ionic surfactbts

Part 2. Investigation and improvement of tolerance to man-made anionic surfactants

AndrzeJ Szymanslu and Zenon Lukaszewslu

Techmcal Unrvers~ of Poznan, Insrrture of Chemutly, 60-965 Poznan (Poland)

(Recerved 9th Apnl1992, revised manuscript received 1st June 1992)

Non-ions surfactants can be detemuned by means of their lowering effect on the tensammetnc peak of ethyl acetate Representatwe man-made anionic surfactants were mvestlgated m order to check the tolerance to this group dunng the determination of non-ionic surfactants Both the “normal” procedure (recording m the cathodic direction startmg from - 1200 V) and “reverse” recording (scanning m the anodlc dIrectIon starting from - 1400 V) were used Sodium salts of dodecylbenzene sulphonate (DBS), dodecyl sulphate (DSA), dodecyl sulphonate (DSO) and stearate (S) and the commercial products Kosulfonat 40 (KOS) and Sulforokanol L3 (lauryl ether suiphate) (LES) were investigated The interference of the anionic surfactants increases m the order DSA, DBS < LES < DSO, S < KOS with “normal” recording and in the order DSA, DBS, S < LES, DSO < KOS with “reverse” recordmg The tolerance to anionic surfactants IS substantially better if “reverse” recordmg IS used On the other hand, the signals of non-ionic surfactants havmg l-3 oxyethylene subunits are sbghtly lower m this case The real tolerance to anionic surfactants IS better than expected presuming addltlwty of analytical signals A 2-pg amount of Triton X-100 can be determmed m the presence of 20 and 50 pg of DBS with “normal” and “reverse” recording respectively However, only 2 7 and 7 pg of KOS, respectively, are tolerated under the same conditions

Keywords Polarography, Surfactants, Tensammetry

The indirect tensammetrx method (ITM) of- fers new posslbrhtles for the determmatlon of non-lomc surfactants [l] The method also works m combmatlon with gas strlppmg separation of non-ionic surfactants from envnonmental matn- ces, e g , surface water Durmg gas strlppmg sep- aratlon non-ionic surfactants are separated and concentrated m an ethyl acetate layer A small test sample of this ethyl acetate layer 1s dissolved m a supportmg electrolyte to perform the tensammetnc measurement No preconcentratlon 1s

Correspondence to Z Lukaszewskr, Technical University of Poznan, Institute of ChenustIy, PMO-965 Poznan (Poland)

required The difference of the heights of the tensammetnc peaks of ethyl acetate alone and that m the presence of surfactants 1s used as the analytical signal The lowermg of the peak of ethyl acetate 1s caused by competitive adsorptlon of surfactants with respect to the adsorption of ethyl acetate ITM IS used wlthm a sumlar range of concentration of non-lomc surfactants to that m W&bold’s method, commonly used for this purpose [2,3] However, ITM 1s much sunpler and substantially less time consummg and allows the determination of a broader range of ethoxylates The slopes of the cahbratlon graphs for the van- ous surfactants are much closer to that of the

0003-2670/93/$06 00 0 1993 - Elsevler Science Pubbshers B V All nghts reserved

314 A Szymanskl and Z Lukaszewskt /Anal Chun Acta 273 (1993) 313-321

surfactant normally used for calibration (Triton X-100) than It 1s m Wickbold’s method

Man-made anionic surfactants seem to be the most serious source of interferences These surfactants are usually present in excess over non- ionic surfactants m environmental matrices An- lomc surfactants partially remam 111 the water phase durmg gas stripping separation but they partially pass mto the ethyl acetate phase Small analytical signals of amomc surfactants were ob- served durmg prehmmary expernnents As these signals do not differ from the signals of non-ionic surfactants, they can produce a posltrve error m determmatlon

The aim of this work was to study the tolerance of ITM to man-made anionic surfactants, because this tolerance determmes the degree of separation that should be achieved Improvement of this tolerance by using the posslblhtles offered by tensammetry itself was also an aim of this investigation

Surfactants representative of the mam classes of amomc surfactants were investigated Atten- tion was focused on sodium dodecylbenzene sulphonate because this surfactant 1s a dominant product m surfactant manufacture [4] and it 1s a malor constituent m the group of surfactants normally found m environmental samples

EXPERIMENTAL

A Radellus OH-105 polarograph was used with a voltage scan rate of 400 mV mm-’ The apphed amplitude of the altematmg voltage was 2 mV Controlled-temperature hangmg mercury drop electrode (HMDE) equipment (Radiometer), havmg an additional platmum wire auxlhary electrode, was used All potentials cited were meas- ured versus a saturated calomel electrode (SCE) The beaker of the measurmg cell was replaced with a quartz beaker The ceramic fret on the end of the salt bridge was protected wrth a polyethyl- ene tube, which reduces the adsorptrve loss of surfactant [5]

The followmg surfactants were used unthout addItiona purlficatlon sodmm dodecylbenzene sulphonate (DBS), sodmm dodecyl sulphonate

(DSO), sodmm dodecyl sulphate (DSA), sodmm stearate (S) (all from BDH), oxyethylated alkylphenols (number of oxyethylene subumts m parentheses) Rokafenol N-l (1) (KS0 Blachow- ma, Poland), Rokafenol N-3 (3) (Roluta, Poland), Tnton X-100 (9 5) and Tnton X-305 (30) (both from Rohm and Haas), oxyethylated 0x0 alcohols havmg a C10_13 hydrophobic part of the molecule Oxetal D104 (4) and Oxetal Cl 14 (14) (Zschnnmer und Schwarz), oxyethylated alcohols havmg mainly an n-C,, hydrophobic part of the molecule Rokanol KO-4 (4) (Roluta) and Bt-1~~35

(20) (Atlas), oxyethylated alcohols havmg a C,,_,, hydrophobic part of the molecule surfactant 18-2 (2) (Technical Umverslty of Poznan), Marhpal 1618/18 (18) (Huls) and Rokanol O-30 (30) (Roluta), oxyethylated ammes havmg a C,,_,, hydrophobic part of the molecule Rokamm R-3 (3) (ICSO) and Rokamm S-22 (22) (Roluta), oxyethylene-oxypropylene block copolymers Rokopol 3Op5 (5) and Rokopol 3Op27 (27) (both

I

a b c

Rg 1 Tensammetnc curves of ethyl acetate m the presence of (b) Tnton X-100, Cc and d) DES and (e) KOS urlth “normal” recordmg (a) Tensammetnc peak of ethyl acetate alone Arrows show dlrectlon of recordmg Concentration of surfactant [pg m the sample (25 ml)] (a) 0, (b) 25, (c) 25, (d) 75 and (e) 100 Concentration of ethyl acetate 15 ml m the sample (25 ml) (0) Zero of current for respective tensammetric curves

A Szymanskt and 2 Ltdasxws~ /Anal Chtm. Acta 273 (1993) 313-321 315

from Roluta), havmg an average number of oxypropylene subunits of 30 m both instances Two commercial products Kosulfonat 40 (In- stytut Chemn PrzemysloweJ, Poland) (KOS) and sulforokanol L-3 (Roluta) (LES) were used after punficatlon Kosulfonat 40 IS a 2 3 nuxture of sodmm salts of alkyl sulphate and alkylbenzene sulphonate (havmg mainly a C1s hydrophobic part of surfactant), and Sulforokanol G3 1s the sodium salt of lauryl trlethoxy sulphate Non-lomc nnpu- ntles of these two surfactants were extracted with ethyl acetate

Ethyl acetate (POCh, Poland), pure for gas chromatography, was used The sodium sulphate used for the preparation of the base electrolyte was purified by double recrystalllzatlon and heated at 600°C All solutions were prepared m water trlply dlstdled from quartz Only freshly dlstllled water was used The supportmg electrolyte m all the studies was 0 5 M sodnun sulphate solution

RESULTS AND DISCUSSION

Analytial srgruds of anloruc surJactants with recorduzg towar& the cathodzc (“normal”‘) dzrec- tlon

The analytical signals gwen by representative man-made anionic surfactants were mvestlgated under the same condltlons as those used m Part 1 [ll Under these condltlons, the tensammetrrc curves were recorded towards the cathodic dlrectlon usually used m adsorptive stnppmg tensammetry Thus directton of recordmg 1s called “normal”, m contrast to “reverse” recordmg These two terms wdl be used further m this paper Sodium salts of DBS, DSA, DSO and S and the commercial products KOS and LES were mvestlgated Results for Trlton X-100 as a representa- tlve non-lomc surfactant, used for calibration m ITM, were added The concentration was vaned wlthm the range 2 5-100 pg m the sample, corresponding to 1004000 I.cg 1-l

Several tensammetrlc curves are given as examples m Fig 1 Curve a shows the peak of ethyl acetate alone and curves b and c show this peak in the presence of 25 pg of Tnton X-100 and

16 I’

50 ‘tJ0 H

2 I mgP

Fig 2 Dependence of analytml slgnal on the surfactant concentratton for (a and a’) DSA, (b and b’) DBS, (c and c’) LES and (d, dashed hne) Tnton X-100 with “nom& recordmg (a-c) and “reverse” recording (a’-c’) Upper scale of concentration shows the amount of surfactant m the sample (25 ml) Concentration of ethyl acetate 15 ml m the sample (25 ml) Imtml potential and direction of recordmg (a-c) - 1200 V, cathodic dlrectlon, (a’-~‘) - 1400 V, anodlc dlrec- tion

DBS, respectively The decrease m the peak height of ethyl acetate 1s the analytlcal slgnal m the reported method The decrease m the ethyl acetate peak caused by 25 pg of Tnton X-100 (expected useful signal) 1s large whereas that caused by 25 pg of DBS (expected mterference) IS small, although vlslble

The analytical agnals of the mvestlgated an- lomc surfactants Hrlthm the concentration range 12 5-100 pg 111 the sample are shown m Frgs 2 and 3, curves a, b and c Although the signals of most of the mvestlgated amomc surfactants are much lower then that of Trlton X-100, they are slgmficant and their presence can be a source of error These signals mcrease m the order DSA, DBS < LES < DSO, S < KOS Fortunately, the most frequently used DBS produces only a weak signal On the other hand, the signals of KOS and LES, also commonly used surfactants, are hrgh

Most of the cahbratton graphs m Figs 2 and 3 mdlcate manma These maxnna are connected mth the appearance of specific peaks of amomc

A Szymansk~ and Z Lukaszewskt /Anal Chtm Acta 273 (1993) 313-321 316

4/ra

lb-

l?6-

dhmm

200

100

IJg

2 I mgi’

Fig 3 Dependence of analytical signal on the surfactant concentration for S (a and a’), DSO (b and b’), KOS (c and c’) and Trlton X-100 (d, dashed hne) unth “normal” recordmg (a-c) and “reverse” recordmg (a’-~‘) Upper scale of concen- tratlon shows the amount of surfactant m the sample (2.5 ml) Concentration of ethyl acetate 15 ml m the sample (25 ml) Imtlal potential and dlrectlon of recordmg (a-c) - 1200 V, cathodx direction, (a’-~‘) - 1400 V, anodlc dlrectlon

surfactants themselves m ad&tlon to the ethyl acetate peak Examples of such curves havmg both an ethyl acetate peak and a specific peak of surfactant Itself are vlslble m Fig 1, curves d and e, for DBS and KOS, respectively Comcldence of these two peaks leads to irregular behavlour of the analyt~~l signal It should be stressed that the comphcatlon dtscussed appears only for com- paratively high concentrations of amomc surfactants (3-4 mg l-l), and not over the range of usual concentrations of amomc surfactants in surface water With lower concentrations only the suppressed peak of ethyl acetate appears (corn- pare curves c and d 111 Fig 1 for 25 and 75 pg of DBS, respectwely)

“‘Reverse ” recorduzg of temammetnc curves

It 1s obvious from the reported expernnents that only a certam excess of anionic over non-iomc surfactants 1s tolerated m ITM under the condo- tlons used 111 Part 1 [l], 1 e , using an mltlal potential of - 1200 V vs SCE This 1s not satls- factory Shlftmg of the rnttlal potential m the negative direction could be a possible way of

nnprovmg the tolerance to anionic surfactants, because the range of adsorptlon of amomc surfactants 1s narrower than that of non-Ionic surfactants Using a properly selected mltlal potentlal it 1s possible to dlfferentlate between these two groups of surfactants Thus effect 1s shown m Fig 4, which shows varlatlons m the ranges of adsorption at the cathodic side for the tested non-Ionic and amomc surfactants, caused by varlatlons m the concentration of surfactants In addltlon to the surfactants tested m this work (bars a-m), the variations m the adsorption range for oxyethylated alcohols, alkylphenols, ammes and oxypropylene-oxyethylene block copolymers, 1 e , sur-

-10 -18 tgv

Fu 4 Fluctuations of adsorption range from cathodic side for amomc (a-f) and non-lomc surfactants (g-s) Lower boundary of adsorption range corresponds to a concentration of surfactant of 200 pg I-’ and the higher boundary to a concentration of 1000 pg 1-l Bars correspondmg to amomc surfactants (a) DSA, (b) DBS, (c) DSO, (d) S, (e) LES and (fI KOS Bars correspondmg to non-ions surfactants (B) Rokafenol N-l, (h) Rokamm R-3, (1) Rokafenol N-3, 0) Tnton X-100, (k) Oxetal C114, (1) Surfactant 18-2 and (m) Marhpal 1618/18, and to groups of non-iomcs (n) oxyethylated alkylphenols (EO l-301, (0) oxyethylated ammes (EO 3-22), (p) oxyethylated 0x0 alcohols havmg a C,,_,, hydrophobic part of the molecule (EO 4-14), (q) oxyethylated n-alcohols havrng mamly a CIz by- drophoblc part of the molecule (EO 4-u)), (r) oxyethylated alcohols havmg a C16_1s hydrophobic part of the molecule (EO 2-30) and (s) oxyethylene-oxypropylene block copolymers (EO 5-27) Vertical lme A mdlcates the mltlal potential for “normal” recordmg and hne B that for “reverse” recordmg Arrow mdxates potential of the peak of ethyl acetate

A Szymamh and Z Lukaszewskz/Anal Chum Acta 273 (1993) 313-321 317

factants tested m Part 1 [l], have also been m- eluded (bars n-s) The adsorptlon range of a particular surfactant was defined as their range of mltlal potentials at which a specfic peak of the mvestlgated surfactant appears It 1s known that the range of adsorptlon of surfactants 1s concentration dependent This 1s why each surfactant 1s represented m Fig 4 by two values a lower one for a concentration of 200 pg 1-l (5 pg m the sample) and a higher one for a concentration of 1000 pg 1-l (25 pg m the sample) Of course, Fig 4 1s only a rough approxunatlon of real conditions because the boundaries of adsorption indicated were determined m the absence of ethyl acetate However, Fig 4 1s very useful m explam- mg the role of the mltlal potential m mmmnzmg the signal of anionic surfactants It also explains the effect of the mitral potential on the changes m the signals of some non-lomc surfactants

With “normal” recording using an mltlal potential of - 1 200 V (Fig 4, vertical line A), all the tested non-lonrc surfactants (bars g-s) are still located wlthm the range of their adsorption These surfactants replace ethyl acetate on the electrode surface, producmg an analytical signal m this way Under these conditions the anionic surfactants DSA (a) and DBS (b) are outside their adsorption range On the other hand, KOS (f) and LES (e) and partially S Cd) and DSO Cc) are still located within their adsorption ranges and produce analytical signals

Shifting the mitral potential to more negative value would be desirable for decreasing the mterference of aruomc surfactants However, the ml- teal potential used ( - 1200 V) 1s almost a boundary value from the pomt of view of recording the peak of ethyl acetate (E, = - 131 V) m the cathodic direction This 1s why attempts were made to record the negative peak of ethyl acetate from the opposite side, 1 e , from an mltlal potential more negative than - 1300 V m the anodlc dlrectlon The recording of a peak m such a “reverse” manner is possible An example of such a peak 1s shown m Fig 5, curve a, and the dependence of its height on the mltlal potential IS shown m Fig 6 The results concerning the peak recorded m the cathodic direction (“normal” recording) were also added for comparison The

;

L i

.

d

1

I

e

I I

. .

Fig 5 Tensammetnc curves of ethyl acetate m the presence of (b) Tnton X-100, (c and d) DBS and (e) KOS wrath “reverse” recordmg (a) Tensammetnc peak of ethyl acetate alone Arrows show dIrectIon of recordmg Concentration of surfactant [pg m the sample (25 ml)] (a) 0, (b) 25, (cl 25, Cd) 75 and (e) 100 Concentration of ethyl acetate 15 ml m the sample (25 ml) (0) Zero of current for respectwe tensammetnc curves

heights of the peaks of ethyl acetate obtamed v&h “normal” and “reverse” recording are sumlar with the exception of the range of mltlal potentials adjacent to the potential of the peak of ethyl acetate It should be stressed that the use of

b b/mm ”

-

16. 700

0 OB-

-10 -24%

Fig 6 He&t of peak of ethyl acetate versus the mlhal potentml Hnth recordmg towards (a) cathodx (“normal”) and (b) anodlc (“reverse”) direction Concentration of ethyl acetate 15 ml m the sample (25 ml)

318 A Szymansh and Z. Lukaszewskt /AnaL Chmt Acta 273 (1993) 313-321

such a recording with respect to the tensammetrlc peak located on the cathodic side 1s a new approach The recordmg of a tensammetrlc curve m the anodlc direction IS usually used for recording anodlc tensammetrlc peaks

Anaiytrcal sagnals of anwruc surjactants with “reverse” recordzng

The analytical signals of the same anionic surfactants as m the case of “normal” recording were investigated usmg “reverse” recordmg starting from - 1400 V The concentration was changed within the range 125-100 kg m the sample, 1 e , NO-4000 pg I-’ Examples of decreasing the peak height of ethyl acetate (recorded m the “reverse” manner) are shown m Fig 5 and the complete results are shown m Figs 2 and 3, curves a’, b’ and c’ The cahbratlon graphs obtained with “reverse” recording have been added to the results concernmg “normal” recordmg The advantage of usmg “reverse” recordmg IS clearly vlslble on comparing Figs 1 and 5 The useful analytical slgnal, I e , that represented by Tnton X-100, remains almost un- changed (see curves b) The analytical signal of DBS 1s reduced at both concentrations (curves c and d) With KOS the nnprovement IS smaller (curves e) However, the specific peaks of DBS and KOS disappeared with “reverse” recordmg (compare curves d and e m Figs 1 and 5) It 1s easy to explam this effect by means of Fig 4 In the case of “reverse” recording (starting from vertical line B) the adsorption ranges of all the anionic surfactants (bars a-f) are almost com- pletely located at the posltrve side of the ethyl acetate peak

From Figs 2 and 3 it 1s obvious that with “reverse” recording a substantial improvement m

TABLE 1

tolerance to anionic surfactants, i e , lowering of their signals, has been achieved for DSA (Fig 2, curves a and a’) DBS (Fig 2, curves b and b’) and S (Fig 3, curves a and a’) With LES (Fig 2, curves c and c’> this lowermg IS vlslble mostly for the higher concentration range, whch 1s less valu- able from an analmcal pomt of view With KOS and DSO (Fig 3, curves c and c’ and curves b and b’, respectively), the lowermg of then signals caused by the change m recordmg dlrectlon 1s small The signals of the surfactants increase m the order DSA, DBS, S < LES, DSO < KOS, 1 e , m a different sequence than vvlth “normal” recording

Compamon of “noml” and “reverse” recorduzg zn ITM

Results obtained with both recording modes were compared with respect to tolerance to an- lomc surfactants and with respect to the analytlcal signals of non-ionic surfactants

The mmunum amounts of the anionic surfactants mvestlgated which cause the appearance of an analytical signal were established for both “normal” and “reverse” recording This amount should be the nummum concentration mterfermg m the determination of non-lomc surfactants, presuming addltlvlty of both signals The muu- mum amount was defined as that glvmg a signal correspondmg to the standard devlatlon of the peak height of pure ethyl acetate (S, = 0 012, n = 7) This value corresponds to a 3 mm peak height or 24 nA under the condltlons used m this work The results are shown m Table 1 They agree wrth the conclusions drawn on the basis of Figs 2 and 3 An nnprovement of the tolerance to all the investigated surfactants except LES is apparent with “reverse” recording

Tolerance to different man-made amomc surfactants wth “normal” and “reverse” recordmg [defined as the amount (pg) of surfactant m the sample (25 ml) producmg a slgnal corresponding to the standard dewatlon of the ethyl acetate peak (3 mm, n = 7)]

Recordmg mode Surfactant

DSA DBS LES S DSO KOS

“Normal” 71 61 37 27 25 17 “Reverse” 19 6 178 34 144 46 25

A Szyntansk~ and Z Lukaszewska/Anal. Chm Acta 273 (1993) 313-321 319

100

50

JY mgr’

Fig 7 Dependence of analytical slgnal on the surfactant concentration for (a-c) DBS and for Tnton X-100 (d) alone and (e-h) III the presence of excess of DBS Dashed curves (g and h) show values of the signal assunung ad&tmty of the signals of Tnton X-100 and DBS Upper scale of concentra- tlon shows the amount of surfactant m the sample (25 ml) Concentration of DBS (mg 1-l) (e and g) 10, (f and h) 2 0 Concentration of ethyl acetate 15 ml m the sample (2.5 ml) Imtlal potential (V vs SCE) and dlrectlon of recordmg (a) - 0 800, cathodic, (b), Cd, 0 1, (e) and (gl - 1200, cathodic, (c), (d, l ), (f) and (h) - 1400, anodlc directron

It should be stressed that the amounts determmed above can be only roughly considered as a hmlt of tolerance to anionic surfactants For a more precise approach the addltlvtty of signals should be checked The analytical signal of Trrton X-100 m the presence of DBS was mvestlgated mth this arm A cahbratlon graph of Trlton X-100 was obtained with and without the presence of an excess of DBS “normal” and “reverse” recordmg were used The results are shown m Fig 7 Cah- bration graphs of DBS obtained using different mmal potentials were added for comparison Dashed hnes correspondmg to the algebraic sum of the signals of Tnton X-100 and DBS were also added It can be seen that the signal with mterfermg amomc surfactants 1s much lower than that expected presummg addltlvlty of signals That 1s because of competltlon between amomc and non-lomc surfactants on the electrode surface, m

which the non-lomcs are stronger surfactants wtthm the range of potential used This effect IS vlslble both mth “normal” recordmg of the Trt- ton X-100 agnal m the presence of excess of DBS (see Fig 7e) and Hrlth “reverse” recordmg (Fig 7d, f and h) With “reverse” recordmg, the signal of Trlton X-100, recorded over the concentration range SO-1000 pg I-‘, 1s only slightly higher m the presence of 2000 pg 1-l of DBS than for Tnton X-100 alone (Fig 7d)

Series of experunents with bmary mixtures of Tnton X-100 and DBS or KOS were performed wth the aim of evaluating the Improvement of the tolerance to anionic surfactants The mml- mum amounts of DBS and KOS that cause the appearance of mterference with the analytical signal of Trlton X-100 were determmed, DBS represented the most commonly used amomc surfactant and KOS the most troublesome surfactant wth the worst tolerance This mmnnum amount was defined as that gwmg a signal correspondmg to the standard devlatlon of the signal of 2 pg of Trlton X-100 m the sample (80 pg 1-l) This value was determmed from seven measurements as 3 mm peak height (24 nA) under the condo- tlons used m thrs work In the presence of 2 ,ug of Trlton X-100 m the sample, an increase m the signal by 3 mm requires 20 5 pg of DBS with “normal” recording and 50 0 pg with “reverse” recordmg, 1 e , 2000 pg I-’ of DBS 1s tolerated If 80 pg 1-l of Trlton X-100 are determined using “reverse” recording From Table 1 it can be seen that a 6 1 fig of DBS alone gives such a 3 mm signal if “normal” recording 1s used and 17 8 pg with “reverse” recording Hence the real tolerance to DBS 1s roughly three times better than that expected presuming addltrvlty of signals It 1s of the order of 1 25 A recently reported ratio of non-lomc and amomc surfactants m rivers m West Germany 1s ca 1 10 [6] Hence, the tolerance achieved 1s better than the expected excess of aniomc surfactants m surface water

The tolerance of KOS 1s worse In the presence of 2 pg of Trlton X-100 the signal Increases by 3 mm if 2 7 pg of KOS are added with “normal” recordmg and 7 0 pg of KOS with “reverse” recording This corresponds to 280 pg I- ’ of KOS as the tolerated concentration m the deter-

320 A Szymans~ and Z Lukaszewskt /Anal Chtm Acta 273 (1993) 313-321

mmatlon of 80 pg 1-l of Trlton X-100 From Table 1 rt can be seen that 17 pg of KOS alone produced a 3 mm signal with “normal” recordmg and 2 5 pg with “reverse” recordmg Although the real tolerance to KOS 1s also better than that expected presuming addltlvlty of signals, only an excess of 1 3 5 of this surfactant 1s tolerable under the most favourable conditions Fortu- nately, the mam amomc surfactant component m surface water 1s DBS [4] and the fraction of KOS-type surfactants 1s much smaller

Comparing the results obtained with “normal” and “reverse” recording, it 1s clear that an m- crease m the tolerance to anionic surfactants was achieved by using “reverse” recordmg However, variations m the mltlal potential and m the dlrectlon of recording could also change the condltlons for the determmatlon of non-lomc surfactants by ITM because with “reverse” recordmg (mltlal potential - 140 V) some of the mvestlgated non-lomc surfactants [Rokafenols N-l (g) and N-3 (1) and Rokamm R-3 (h)] are also partially located to the positive side of the ethyl acetate peak and their analytical signals are lower (see Fig 4) Senes of expernnents with representative non-ionic surfactants using both “normal” and “reverse” recording were performed Trlton X-100 was used for cahbratlon Oxetal Cl14 and

Marhpal 1618/18, 1 e , representative oxyethylated alcohols havmg a typical oxyethylene chain length, and Rokafenol N-l, Rokafenol N-3, Rokamm R-3 and surfactant 18-2, representative of the group of surfactants havmg extremely short oxyethylene chains, were investigated The results are given m Table 2

The results for Trlton X-100, Oxetal C114, Marhpal 1618/18 and surfactant 18-2 remain un- changed with “normal” and “reverse” recordmg Some decrease m the analy&al signal 1s vlslble with Rokafenol N-3 and Rokamm R-3, if “reverse” recording 1s used, but there 1s a substantial decrease with Rokafenol N-l It should be stressed that Rokafenol N-l, Rokafenol N-3, Rokamm R-3 and surfactant 18-2 belong to the group of surfactants which are not determined by the methods currently used [2,3,71 Certainly It would be better to mamtam all advantages of ITM over Wlckbold’s method The use of “reverse” recording IS Justified only with a certain excess of amomc surfactants “Normal” recordmg could be used if the separation stage preceding the determination (sublatlon or extraction) reduces the concentration of amomc surfactants below the level of their influence on the results of determmatlon, 1 e , below 50-100 kg 1-l This will be the sublect of further work

TABLE 2

Analytical signal (mm) of non-lomc surfactants with “normal” (N) and “reverse” (RI recordmg ’

Surfactant Recordmg mode

Concentration of surfactant (pg 1-l)

100 200 300 400 500 600 700 800 1000

Tnton X-100 N 13 26 38 51 64 77 - 99 119 R

Oxetal Cl14 N R

Marhpal 1618/18 N R

Rokafenol N-3 N R

Rokafenol N-l N R

Rokamm R-3 N R

Surfactant 18-2 N R

13 26 42 52 - 74 - 95 118 15 30 43 - 71 80 - 101 122 15 - 44 57 69 81 93 103 119 13 26 38 50 62 73 - 88 102 13 27 - 49 61 72 - 87 100 12 24 36 47 58 68 - 83 95 13 - 36 47 57 - - 76 86 10 20 28 36 42 47 - 57 65 5 9 13 - 16 - 20 - 22

13 26 38 - 54 - 64 - 17 6 18 25 - 35 - 54 - 68

11 23 36 - 48 - 61 - 72 12 24 37 - 49 - 62 - 74

’ India1 potentml v&h “normal” recordmg - 1200, and with “reverse” recordmg - 1400 V vs SCE

A Szymansh and Z Lukaszewskz/Anal Chun. Acta 273 (1993) 313-321 321

The results obtamed also give rough mforma- tlon about the posslblllty of the determnatlon of anlonlc surfactants by ITM It 1s possible to ob- tam a substantial analytical slgnal of DBS by shifting the uutml potential towards a less negative value (see Fig 7c)

This work was supported by the Technical Uruverslty of Poznan (grant No R3KB/ 500/ 27/ 8/91)

REFERENCES

1 A. Szymansk~ and Z Lukaazewslu, Anal Chun Acta, 260 (1992) 25

2 R WIckbold, Tenslde Deterg , 9 (1972) 173 3 D Brown, H de Henau, J T Gangan, P Genke, M Holt,

E Keck, E Kunkel, E Matthlp, J Waters and J Watkm- son, Tenslde Deterg , 23 (1986) 190

4 L Noll, Tenslde Surfact Deterg , 28 (1991) 90 5 A Szymanslu and Z Lukaszewslu, Anal Chun Acta, 231

(1990) 77 6 H Hellmann, Tenslde Surfact Deterg ,27 (1990) 318 7 TM Schnutt, MC Allen, DK. Brain, KF Gum, DE

Lemmel and Q W Osbum, J Am 011 Chem Sot, 67 (1990) 103

indirect tensammetric method for the determination of non-ionic `surfactants: part 2. investigation...

Documents