Tensânmetry:

Revterv of Literature and ResuLts of Initial Experinents

for

The Adsorption of SurfacLants

A Research Project Sponsored by

The Soap and Detergent Association

475 Park Avenue Souch

Nenr York, New York L001-6

Fred B. Rea and John C. fJestall

Departrnent of ChemisÈryOregon StaÈe Unlversity

Corvallís, Oregon 97331-4003(503) 754-259L

March 1, L989

by

INTRODUCÎION

Here we present a brÍef overvÍew of our work on tensammetry for the

determination of the actlvlty of surfactants ln solution. This report consists

of two parts:

Review of 1Íterature

Results of Ínltíal experinents

l{e have not lncluded in thís report a thorough description of just how

tensammetry works. For those unfamilÍar with tensannetry, r're recommend reading

the literature review, skimming the tltles of the references, and then skimming

Èhe results sectlon, checking the calibratíon curves (e.g., Figure 7) and the

kinetics (e.g. Figure 3). There will be an opportunity to discuss tensammetry

in detail at the neeting ln New York at the end of March.

I.le are optimlstic about the appllcatlon of tensammetry to problens of

environmental chemistry. Considerable progress has already been nade in thís

direction by Cosovic and co-workers.

Llterature revLes -l--

A survey of the literature concerning tensanrnetry was undertaken to answerseveral questlons. The apparatus and nethods in use in other laboratories wereof primary fnterest. Also of speciffc interest ls the experimental data bestsuÍted to quantltatÍve analysís.

InstrumentatfonThe instruments in use by various research groups are su¡nmarLzed in Table

1, Specifie attentlon was pald to the potentlostat, as lts speed is a prirnarylirniting faetor ín the success of the technique.

Table 1. The breakdown gf potentíosLats ln use 1s as follows:Instrumentt- Number in uset'Homemadç " 5

PAR 174A¿ 4PAR 170 2

RadelkÍs 0H105 2

Miscellaneous L eachBAS LC-4GI.IP 563 (Deutsche Akademie der l{issenschaften)Metrohm E26L/E393MeÈrohm 850Pþß. L73/I79UnÍrra PPo4

Notes:L In most, but not all, cases the apparatus is controlled with a

computer.2 In cases where the PAR L74A is used, the instrurnent has been

altered to allow an external signal to be added at the sumningpoint of the nnain potentiostat.

3 Although not tensammetry, similar klnds of measurements lrereperformed with a PAR 1-73/179 (Hindagolla,1985)

Chemical ourltvAll authors took some measures to ensure the purity of blanks, wíth widely

varying degrees of concern over sample cleanliness. The urajor materials ofnote were lrater, mercury, purging gas, and supporting electrolyte.

The widest variance in purity assurance ï¡as found in treatment of vrater.One group (Bednarkiewícz, 1981) did an extensive study of several methods ofwater purlfication, from doubly distilling tap water to a lengthy processínvolving mulciple distillations at atmospheríc and reduced pressures,Amberlite XAO2 filtratlon, and actlvaÈed charcoal filtration. Most otherauthors used twice- or thrice-quartz distilled water, with thro exceptions.Hunter (1981) irradiaced supporting electrolytes and solvents with W light toremove organics, and Bos (1982) used water from a Millípore Q2 system.

Nearly all authors used triple distilled polarography grade mercury.Cosovic indicated the use of only double distilled mercury, and Gupta (1965)washed, filtered, and vacuum dlsttlled his own.

Most authors used N2 purglng of samples to remove O2. Those whospecified N2 purítÏ lndlcated high quality or ultrapure N2. Bos (1982)

Llterature review -2-

took the additional precauÈlon of passlng his N2 through copper tubing at450 C. Bednarklewicz (1985) purged hfs solutlons wlth argon, and Cosovic(L972) cornmented that better results were obtained for some samples if nopurging was done.

Some varlation ín supporting electrolyte purity requirements was found.Bednarklewicz (1-985) and Cosovlc (L982) purifled some supportíng electrolyteswith activated charcoal. In addltlon, Cosovlc (L977,L982) baked NaCl at 450 Ç

for several hours. Pawlak (L987) purtfled Na2S04 by doublerecrystal-Lization and subsequent heatlng at 600 C. Most other authorsspecified reagent or analytlcal grade eleetrolytes.

An additional procedural precautíon was temperature control. Two authorslndÍcated work was done at 20+1 C and another preferted 25.0 C. Cosovicindicated that experiments were performed at room temperature withoutthermostatic control. Over half of the authors cited did not speeify anytemperature control precautÍons.

Experimental measurementsA general tensammetric experiment lnvolves application of a changing

potential to a hanging or dropping mercury electrode and processing themeasurÍng the output currenÈ to determine the double layer capacitance. Thespecifie method of censammetry is characterlzed by waveform of the appliedpotential. The most popular techniques fall into two najor categories: actensamnetry and differential pulse Eensammetry.

AC Ëensamrnetry employs a sma11 sinusoidal potentlal superimposed on a DC

ramp as the applied potential. The quantity measured is the ac amplitude ofthe cell current. This is achieved with a lock-in arnplifier in some cases. Inothers, the buffered ac current is rectified and the rms amplitude measureddirectly wiuh a volÈmeter. One author (Gupta,l-965) measured the amplitude atdiscrete points with an oscilloscope. AC tensammeEry may be furthereategotLzed by the relation between the exclcation frequency and themeasurement frequency, Ln cases where a lock-ln anplÍfier is used.

The most common tensannetric nethod is ac fundamenÈal tensammetry (acft).Here Èhe cell current 1s monitored with a lock-ln amplifier or a phase nullingdevice tuned to the fundamental ac frequency. This method ís prone to havinglarge background, due to background Faradlc processes.

The l-arge background of ACHI rnay be avoided wlth ac second harmonicÈensammetry (acsht). Here the reference signal for detection has a frequencytv¡ice that of the excitation waveform. The naJor advanÈage of this nethod isits low background current. A Faradic process wíl1 be responding to theapplied potential at any glven moxnent, and will therefor have a vanishinglysmall second harmonic contribution. The capaclty current, however, will have a

second harmonic component. The prlmary disadvantage of acsht is the smallmagnitude of the second harmonic signal. This need not be a major limitation,however, as many lock-in amplifiers are capable of isolating and measuringsinusoidal signals with amplÍtudes ln the microvolt range.

Differential pulse tensammetry (dpt), Ís acÈually a collective term forseveral cechniques which employ applied potentlal waveforms other than asinusoid. These methods may be used in a single drop (Hl,fDE) mode or with aDME. Some authors employing thís technique are using simple DP polarographs\.¡ith little or no modiflcatlon. Others, most notably Cosovic, are using awaveform generator known as a Kalousek commuÈator. Here the applied potentialis nechanically chopped becween a llnear ramp and a constant potential, usuallyche electrocaplllary maximum potential, Er. Some authors are working withdifferential step waveforms (as in square wave voltanmetry), due to the ease

Llterature review -3-

with these forns nay be dlgltally generated. In all cases, the DC current issarnpled after a dlscontinuous change in the applled potenÈial. In the absenceof Faradic processes, the DC current ls due only Èo charging of the doublelayer.

Interpretation of dataThe data obtained by either method ls a curve of dlfferential capacitance

as a function of applted dc potential. In pure supportlng electrolyte thiscurve generally exhibits a single broad peak. Ihe maximum of this peak, theeLectrocaplllary maximr¡n, is at the polnÈ of zero surface charge on the mercurydrop; its potentlal is deslgnatedE", In Lhe presence of a surface actlvesubstance, the curve of differenÈial capacity versus applied potential displaystwo peaks separated by a trough. One peak 1s at a poÈential more negative thanE" and is designated s-. The other peak, at a potential more positive thanE;, ls designaied s+. The broad p""L drr" to Lhe supporting electrolyte isgenerally not observed in the presence of surface active substances.

Ideally the parameter selected for quantítative analysis would have arelatíveLy large region of a linear dependence on concenLration and would bereadily and reproducibly obtainable from the raw experimental data. Theposition (Es-, E"+) and height (C(Es-), C(E"+)) of the desorptlon peaksand the magñitudã of the capacitancã at E" éxhibit some dependence on thebulk activity of surface active substancei in the solution. The s- and s*peaks increase in amplitude and separation from E" with lncreasíng bulkconcentration. The ac current aË E, decreases wlEh increasing bulkconcentration. All five of these qüantities are readily avallable from theexperimental data.

Measurements on the electrocapÍllary maximum are normally done assuppression of io^ at Eo from the blank (supporting electrolyte).Bednarkiewicz (19ð1) inãlcated a llnear relatíonship between the suppression ofthe ac current al E" and the surfactant concentratÍon. Ulrich (1986,1-988)derives a more complex relationship between i"" at E. and C throughconslderatlons of therrnodynamic treatment of surface coverages. The totalcapaeitance, CT, aÈ low values of the surface coverage fraction is:

cr-cn +co(l_ )where CR is the--capacltance of the mercury surface with a monolayer of a

surfacÈánt R and Cg is the capacÍtance of the surface with only supportingelecErolvte and solvent adsorbed.

the s+ peak fs not consLdered by most authors. The scan normally beginsat a potential more positive than E"+ and proceeds Ín a negative direction.SÍnce the surface active substance õoes not adsorb significantly at potentialsmore positive than E"+, observation of a clear s+ peak is rare and notexpected. A notable-exceptíon is Britz (L98L), who was workÍng in low ionicstrengths (0.05 U). Bednarkiewicz (1981) managed to produce data for this peakby performing the tensammetric scan ln two segmenËs -- one scan loward E"-from a resÈ potential near E" and Èhen a separate scan Èoward-E"+'- "9"11 -from near 8". The difficult| in reproduclbly observing the sÌ þeak led hiurto use current suppression at E. as hls quantitative signal.

The s- peak is the third poõsible point of analytlcal interest. Thispeak ls generally well-resolved from others, and lts hefght and position haveboth been used as analytical signals. According to Britz (L980), is- is morelinear at low concenÈrations, while E"_ is preferable when concentrations arehigher.

Llterature revlew -4-

REFERENCES

Anderson, J.E., A.M. Bond, I.D. Herftagê, R.D. Jones, and G.G. I{allace,"Transient electrochenical technlques ln llquld chronatography withmlcroprocessor-based fnstrumentatlon, " AÊgL !.hgg-, 54 (L982) 1702-1-705.

Bard, 4.J., Electroanalytical Chemfstr.'¡: Vol-ume !, llarcel Dekker, Inc.: NewYork, L966,

Bard, Al,len J., and Larry R. Faulkner, Electrochemicalt Methods, John l.Iiley &Sons: New York, 1980.

Standard Potentials in Aqueous Solutlon, A.J. Bard (ed.), Dekker: New York,198s.

Bednarklewlcz, 8., and Z. Kublik, "ComparÍson of the uÈillty of severaltensammetric techniques for the determination of Ëraces of surfactantsafter preconcentration at the hanging mercury drop electrode," &.L- Chim.

, 176 (l_985) 133-L42.

Bednarkievtlcz, E., M. Donten, and Z. Kublik, "DeÈermination of traces ofsurfactants ln distílled, potable and untreated waters and in supportingelecËrolytes by tensanmetry v¡ith acctrnulatlon on the HMDE," L- Electroanal .

Chem. , L27 (1981) 241-253.

Bersier, P., and J. Bersler, "Polarographlc adsorption analysis andtensammetry: Toys or tools for day-to-day routlne analysls?", Ag!5.!, 1L3.(1988) 3-14.

Bockris, J.0'M., B.E. Conway, and E. Yeager, Comorehensive Treatise ofElectrochemlstry, Plenum: New York, 1-980.

Bond, 4.M., and R.D. Jones, "Microprocessor-based tensanmetric detection forliquid chromarography," AteL Chim. A@, l-52 (1983) L3-24.

Bos, M., "The tensammetric titration of ionic detergents," @L- Chim. &jEê,13s (L982) 249-26L.

Bos, M., "The computerized determination of double-layer capacitance with theuse of Kalousek-type !¡aveforms and iÈs application in titrimetrÏ," Anal.Chim. As-E, 122 (1980) 387-394.

Breyer, 8., and S. Hacobian, "Tensammelry: a u¡ethod of lnvestigatíonsurface phenomena by A.C. current measurements,rr Australian J. ScÍ.Research (L952) 500-520.

BriLz, D., and J. Mortensen, "Computer-aided staircase tensanmetrictitration for the accurate rneasurement of crltÍcal micelle concentration.Measurements on sodiu¡n dodecyl sulphate ln sodiun chloride sol-uËions, " J*Eleetroanal. Chem., 1,27 (1981) 23L-240.

Brltz, D., "Single drop tensammeËry," AIêL Chim. 4.9,!,ê, 115 (L980) 327 -330.

LiÈerature review -5-

Canterford, D.R., "Tensammetric analysls of surfactant nixtures," I_Electroanal. Chem., 118 (1981) 395-403.

Canterford, 0.R., "Tensanmecric analysls of surfactant mixtures," I-Electroanal. Chen. , 11-l- (1980) 269-278.

Canterford, D.R., uA.C. polarographlc deternlnaÈlon of polyethylene glycolsapplication Èo analysis of photographic processfng solutions," @1.- Chim.Acta, 94 (L977) 377-384.

Canterford, D.R., "Tensammetríc peaks of some p-phenylenedlanínes," J*Electroanal. Chem., 73 (L976) 247-252.

Cosovic, B. and V. Vojvodlc, "Direct determinatlon of surface active substancesin natural waters, n Marine Chemistry , 22 (L987) 363-373.

Cosovic, 8., "Aqueous surface chemistry: assessment of adsorptioncharacterlstles of organic solutes by electrochemical rnethods, " in ChemicalProcesses in Lakes, Interscience: New York, l-985.

Cosovic, 8., V. Vojvodic, and T. Plese, "Electrochemical determinatíon andcharacterízatíon of surface active substances in freshwaters, " ÞEgResearch, L9(2) (1985) 175-L83.

Cosovic, B. and V. Vojvodic, "Application of AC polarography to thedetermination of surface-active substances in seawater," t!ryþ€J, andoceanography, 27 (2) (l-982) 361-369.

Cosovic, 8., N. Batlna, and Z. Kozaxac, "Adsorption of some mixtures of surfaceactive substances aÈ the mercury elecÈrode. Kalousek couunutatormeasurements," f, Electroanal. Chem., 113 (1980> 239-249.

Cosovic, 8., and M. Branlca, "Study of the adsorption of organic subsÈances ata mercury electrode by the Kalousek commutaÈor techniquê," J* Electroanal.Chem., 46 (1-973) 63-69.

DamaskÍn, 8.8., O.A. Petrli, V.V. Batrakov, &SeIBfu of Organic ComErounds onElectrodes, Plentrn: New York, L97L.

Davison, I.¡., and M. llhicfleld, "Modulated polarographic and voltarnmetrictechniques in the study of natural waËer chemistry," L_ Electroanal. Chem.,7s (L977) 763-789.

de Jong, H.G., I,l .H. Voogt, P. Bos, and R.I{. Frei, "TensammetrÍc detection inhigh performance liquid chromatography. Application Èo lynestrenol and somecardiac glycosides," J* Liquid Chromatog*, 6(L0) (L983) L745-L758.

Delahey, P., Double Layer and Electrode Kinetics, Interscience, 1965,

Dobos, D., Electrochemical Data: { handbook for ElecÈrochernists in Industry andUniversities, Elsevier, L975.

Dryhurst, G., ElectrochemÍstry of Biological Molecules, Academic Press: NewYork, L977,

Licerature revlew -6-

Eberson, Lennant, Electron Transfer Reactfons in Organic ç¡gnlg-g5y,Sprfnger-Verlag: Berlln, L987 .

Frtrnnkín, 4.N., and B.B. Damaskln, nThe so-cal-led tens.âmmetric l¡aves, " I-Electroanal. Chem., 2 (L962) 36-44.

Grahame, D. nThe electrical double layer and the theory of electrocaplllarity,"Chernical &¡¿!ews,, 4L (L947) 441--501.

Gupta, S.L., and S.K. Sharma, nAC polarographic studies of the influence oftensaru[etricwavesononeanother,nE.t.@@,10(1965)151-158.

Heyrovsky, J., and J. Kuta, Principles of Polarography, Academic Press: NewYork, L966.

Hindagolla, Suraj L., A Study of the Oxide/Solutlon Interface þy CapacitanceMeasurements of Electrolyte/Oxide/Semiconductor Structures andPotentiometrlc Titrations of Colloidal Oxide Suspensions, a doctoral thesissubrnitted Èo Oregon State University, Corvallls, oregon, L985.

Hunter, K.4., and P.S. Liss, "Polarographie measurement of surface-activematerial ln natural waters, " I.Ia¡Eer BCg@,, l-5 (1981) 203-2L5.

Japaridze, J.I., V.A. Chagelishvili, and Z.A. I(hutzishvill, "Interfacialbehavlor of tetraalkylammonium ions at Èhe Hg eleetrode in glycols, "Colloíds and Surfaces, 28 (L987) L35-L46.

Jehring, H., Nguyen Vlet Huyen, Trinh Xuan Gian, E. Horn, and C. Hirche, "Arethere any analogies between electrosorption layers of organic comPounds andsignal-transmiEttng biomembranes?", J- Electroanal. Chem., 100 (I979)L3-22.

Jehring, H., Elektrosorptionsanalys mit der t{echselstrompolarographie, AkademieVerlag: Berlln, L974.

Jehring, H., "Untersuchungen mít der llechselstrompolarographie: VIIU, Z. Phys.Chemle: Leípzí-g, 246 (L97L) L-24.

Jehring, H. , "Doppelschichtskapazittsmessung mit der Breyer-l.Iechselstrom-polarographie (Tensarffnetrie)," I- ElectroanaL. Chem., 2L (L969) 77-98.

Jehring, H., ,Untersuchungen mit der l,lechselstrompolarographie: IV," J.Electroanal. Chem., 20 (1969) 33-46.

Kalvoda, R., and L. Novotny, "Polarographlc behavlour of petroleum componentsin aqueous solutions: a study using DPP and convective adsorptionaccumulation," Coll. Czech. Chem. @-,'51 (1986) l-587-L594.

Kozarac, 2., B. Cosovic, and V. Vojvodic, "Effects of natural and syntheticsurface actlve substances on the eleeÈrochemlcaL reduction of cadrnium innatural Ì{aters, " !þggE Research, 20(3) (L986) 295-300.

Llterature review -7 -

Kozarac, 2., B. Cosovic, and M. Branica, nEstl¡natlon of surfactant actlvfty ofpolluted sear^rater by Kalousek commutator technique,' l-- Electroanal . Chem.,68 (1976) 7s-83.

Krznaric, D., and B. Cosovf.e, nThe lnteractfon wlch llnolelc acld and ltsínfluence upon the adsorbed film structure at the polarized mercury / watersolutlon interface,n J* Colloid and Interface Sclence, 96(2) (1983)425-436.

Krznaric, D., B. Cosovic, and Z. Kozatac, "Adsorptlon and interaction oflong-chaln fatty aclds and heawy netals at the mercury electrode / sodiumchlorlde solution interface,n U4!g Chemistry, 14 (1983) L7-29.

Lukaszewski, 2., and M.K. Pawlak, "The behaviour of oxyethylated alcohols undercondltions of tensanrnetry s¡lth adsorptive accumulation on HMDE," inElectrochemístry. Sensors. and Analysls, by M.R. Snyth and J.G. Vos,Elsevler Science Publishers: Amsterdan.

Macdonald, J. Ross, Impedance Spectroscopy, John lJiley & Sons: New York, L987.

Y[iLLazzo, G. and S. Caoli, Tables of Standard Electrode Potentials, I,triley: NewYork, L978.

Navotny, L., and R. Kalvoda, "Polarographic behaviour of petroleum componentsÍn aqueous solution: applicatlon of electrocaplllary measurements andconvective adsorpEion accumulation," Coll. Czech. Chem. 8-@-, 51 (1986)1s9s-1603.

Pawlak, M.K., and Z. Lukaszewski, "Tensammetry wlth accumulation on thehangíng mercury drop electrode, Part 4," Anal. Chím. þþ, 202 (L987)85-96

Rosen, M., Xi-yuan Hua, P. BratÍn, and A. Cohen, nTensanmetric determination ofpolyoxyethylenated alcohols wfUh low oxyethylene content aE parts-per-mill1on eoncentrations," &Lyglggt Chemlstry, 53 (L981) 232-236.

Ulrich, H., W. Stumn, and B. Cosovlc, "Adsorption of aliphatic facty acids onaquatic interfaces. Comparison between two rnodel surfaces: the mercuryelectrode and d-41203 Colloids," Env. Sci. Tech.,22(L) (1988) 37-41.

Ulrích, Hans-Jakob, The Adsorption of Aliphatic Fatty Acids at AquaticInterfaces: A Comparison Between Polar and Nonpolar Surfaces, a doctoralthesis submitted to ühe Swiss Federal Institute of Teehnology Zurich, 1986.

Vlcek, 4.4., Chem. Listy., 50 (l-956) 400.

Zrr1cLc, V., B. Cosovic, and Z. Kozatac, "Electrochemical determination ofsurface active substances in natural waters on the adsorption of petroleumfractLons at mercury electrode / seawater interface," L Electroanal.Chem., ß (L977) l-L3-121.

Experimental procedure for tensanmetry -l--

ApparatusThe apparatus for tensanmetric analysls consists of a potentiostat, a

current-co-voltage convertor, a dlgltal voltneter, a lock-1n anplifier, anelectrochenical- cell, and a computer.

The potentlostaÈ and current-to-voltage convertor are both housed in an IBMEC/225 Voltanmetric Analyzer. Its potentiosÈat and currenÈ-to-voltageconverÈor are both fast enough to allow use of ac frequencies up to 500 Hz.The current to voltage convertor output fs dlrected to the input of the lock-inamplifier.

The applled potential from xt.e EC/225 ls measured wlÈh the Keithley Model617 Programmable ELectrometer. Thls provides a dlgital signal to the computer,as the only outputs from tl:.e EC/225 are analog.

The PAR 5301- Lock-In tunplifier serves both as the source of the ac signaland the detector for the ac current amplitude. The Ínternal oscillator of the5301 is set to l-05 Hz at 30 mV peak to peak amplitude. The oscillator outputis sent ínto the auxiliary input of the TBI{ EC/225.

The computer, a Cordata PPC-400, is interfaced to the Keithley 617, the PAR5301-, and the T-BytEC/225. The KeirhLey 6L7 and the PAR 5301- Ínterface is aNational Instruments GPIB-PCIIA card. The EC/225 is conËrolled via a MetraBytePIO12 Parallel Interface card.

The analog data are recorded on a Heath Model SR-207 X-Y recorder. Thetraces from the X-Y recorder are used prirnarily for diagnostic purposes. Allnumerical data are taken from the digltal record.

The electroche¡nical cell is the PAR Model 303 Static Mercury Drop Electrodeequipped wíth a borosilicate glass sample cup. A Model 305 Stirrer is alsoused. The PAR 303 ls used in three-electrode mode, with a eglegCl referenceelectrode.

ProcedureThe çell and all glassware are cleaned with HPLC grade methanol acidified

with 10-¿ tt Hcl.Solutlons are prepared fron water freshly delivered from a Millipore

Milli-Q Q2 system. The specffic resistance of the nater does not drop below17 Mohm.cn. Stock solutions are refrigerated at 4 C when not in use.

Each sample solution 1s prepared individually in a 10-nL volumetric flask.A eonstant concentratlon of the supportlng elecÈrolyte Is mafntained in allsolutíons. The desired concentration of a surface actlve substance is preparedby transferríng by Eppendorf pipet a stock solutlon (with supportingelectrolyte) into the 10-url, flask and diluËing wlth supporting electrolyte.

The contents of the volumetric flask are transferred to the PAR 303 samplecup, which is fitted into position. The solenoid Ín the 303 which controls theflow of purging gas is actuaÈed fron the computer, allowing a purge time of 10minutes, rather than the maximum of 4 mlnutes available from the 303 frontpanel.

The purglng gas 1s standard grade nítrogen which ls passed chrough avanadous chloride scrubber to remove oxygen. A bubbler of the supportingelectrolyte dor+nstream of the vanadous chloride scrubber protects the cel1 fromany carryover from the scrubber.

Experimental procedure for tensammetry

The initial potential of E}¡e EC/225 sweep ls set Eo Ez, theelectrocaptllary maxlmtm as deternlned fron a run of supporting eleclrolytealone. Two drops are extruded end dlslodged fro¡n the electrode. A third dropLs extruded buË not dislodged. i.¡lth the stirrer running, a predeternlnedaccumulation tlme is allosed to pass before inltlatlon of the scan.

After the accumulation time has passed,. the scan toward positíve potentialsís initiated. The dc scan rate is 10 nV.s-r. The dc potentlal is monitored,and the scan fs reset when the potential reaches -0.1 V vs. AglngCf. Threemore drops are extruded, the thírd of r¡hich is retalned for the accumulationand scan ln the negatlve direetion. The negative scan is terminated at apotential of -1.3 V vs. eglagCl.

ResultsFigure L shows the tensammogram of a 1 M Na2SO4 blank. Accumulation

condltions lrere 60 s at -0.6 V vs. AglAgCl. NoÈe thaÈ the potentlal axis is-E(dc), with negative values on the righc. The E" estinated frorn this curveis -0.25 V.

Figure 2 shows the tensammogram of 1x10-5 tt l--¿odecylpyridinium bromide(C12PYR) in l- M Na2SO4. Thg accumulation tirne was 60 s, and the accumulationpoËåntia1 r." -0.6"v T". eglagcf. The large peak at -60 nV is the s+ peak,and the smaller feature at -l-.25 V is Èhe s- peak. The magnitude of i(aqc)at -0.25 V is lnterpolated from the data to give the suppression at E".

It has been found that, at low concentrations of surfacLanEs, it issometimes dífficult to achieve reproduclble results from ostensibly ídenticalconditions. The variabilÍty has been attributed to variable adsorption on thewalls of the cell, resulting from variable pretreatments of the cell. Normallythree or four pre-equilibrations are sufficient to get reproducible scans atconcentrations as low as 0.1- M,

Figure 3 shows the decrease in the ac componenc of the cell current atconsuant potentlal with time. The sample solution is C12PYR in 1 M Na2SO4.The four curves correspond to bulk concenÈrations of 100, 10, l-, and 0.1 M.

Figure 4 shows the analogous curves for sodiun 4[(2'-dodecyl)benzene] sulfonate(C12I^A,S) ln 0.1 M KNO3. Due torthe dlfficulty ln working at lowconcentrations of C12LAS, no 10-'l,l curve ls shown in Figure 4.

The surface conõõntration, calculated from the ac suppression at E", hasbeen plotted against bulk concentration for C12P1'R ln Figures 5 throu9}:. 7,Figure 5 covers Ëhe entire range of concentraÈlons ln the experiment. Figure 6

is a more detail-ed look at the lower concentration range. Figure 7 is a plotof the same data on a log concentration scale.

Figure 8 shows the data taken so far for C12LAS in 0.1 M KNO3. Agaín,difficulty in obtaining reproducible results at low concentrations has slowedextension of C12LAS work into sub-micromolar concentrations.

-2-

-3-

tE

5

oo:/

Tensammogram of 1M NaZSO4 Blank

0.4 0.8 0.t

-E/VYr.ACAscl

-o.2 0.0 02

6O ¡ oooumulotlon of -0.6 V

Figure l. Tensammoqram of I I Na¡SO¡ blank.

-4-

IE

t

oú:f

Tensammogram of 10 uM CI?PYR

a4 0.¡ 0.t

-E/Yvr.ÂgAs0l

EO r ccournulqtton ol -O.ã V

Figure ê. Tensammogrem of lxl0't I Cn¡pYR in t U N¿rSOr.

-5-

Accumulation at the Hg dropCíZPYRlrr, U ¡¡qaS0l

Ë

¡õbed

t

2.001.90

t.801.701.80

1.60

1.401.80

1.20t.r 01.00

0.s00,800.700.800.600.10o,800.200.100,00

0

t 00 lttl

Figure 3. Kinetics of

curve corresponds

Nae SOr .

120

t+nú,t0 aU

Cr r PYR adsorpt ion

to lO0 Fü. C,IPYR.

, tttl

on the mereury drop. The lowest

The support ing electrolyte is I I'l

180

t

-6-

Accumulation at theCl2t¡S ln 0.1 ll KNOI

Hg drop2.4

l.¡t.t1.7

t.c1.5

1.1

t\ t.zg l'ln t'o

3 o.¡

J o.rgoJ0.t0.t0.40.to.2o.to.0

100 uI

120 tco

llmo ,/ tl0 ull

Figure 4. Kinetics of

curve corresponds

KNO¡.

Cr c LAS adsorpt ion

to IOO l¡[ CI.LAS,

t ¡¡I

on the trereury drop. The lowest

The supportinq electrolYte is O. I

-7-

Io.

otú

Catribration eurve for CI?PYRI lJ No2SO*. e0 r ooosmglollon

40 ¡0

TR(r) ./ uilExp dolo fÏTt Colo'd

Fiqure 5. Calibration eurve

version l. Ê.

for C¡.pYR. The curve is caleulated from FITEOL

-ê-

Caiibration curve for CI?PYRI ll No2SO4, t0 r a¡oumulqllon

TÀ

¡¡¡ü

lr

2

Exp dolc

1

n(r) 1 "!t

FlTl Colo'd

Figure 6. Expanded calibration curve for Cr¡ÞYR.

-9-

Calibration cur\¡e for CI?PYRI U No2SO4. ð0 r oocumulollon

Io.

uTt

loe IR(r)RTt Cotc'd

Figure 7. Calibr¡tion curve for CTTPYR with log coneen'trat íon scale.

-to-

TÈ

o4j

Calibrat,ion curve for CieLAS

Figur"e A. Eal ibrat ion eurve

Fiqure'5 for compari:on.

0.1 H Kl{Ot. 120 r aoqumulollon

for CroLAS. The Eeele is the Båmë á5 thet of