Analyirca Chrmlca Acta, 231 (1990) 77-84 Elsevler Science Pubhshers B.V., Amsterdam - Pnnted m The Netherlands

77

Tensammetry with accumulation on the hanging mercury drop electrode

Part 6. Errors of determination caused by adsorption of non-ionic surfactants on the material of the measuring cell

ANDRZFJ SZYMAfiSKI and ZENON LUKASZEWSKI *

Institute of Chemrstty, Technrcal Unrversrty of Poznati, 60-965 Poznari (Poland)

(Received 1st August 1989)

ABSTRACT

The adsorptive loss of oxyethylated alkylphenols (Tnton X-100 and Rokafenol N-3), oxyethylated alcohols (Bn~-35,

Marhpal 1618/40 and surfactant 18-10) and poly(ethylene glycol) of MW 9000 on cells constructed from quartz, glass,

glassy carbon, PTFE, polyethylene, polypropylene, polystyrene and alurrumum dunng the measurmg process was

mvestlgated In all Instances very substantial losses were found, whch completely changed the results of the

determmatlon Apart from the vessel Itself, very Important losses on the ceranuc fnt located at the end of the salt

bndge were found

The loss of all the surfactants tested on the quartz vessel 1s not very substantial and, after protectlon of the cerarmc

fnt, a cell with such a construction gves rmmmum losses. The glass cell 1s very sensltlve to the manner of preparation

of its surface. Cells constructed from PTFE and glassy carbon adsorb more speclflcally than others and can be used for

the determmatlon of oxyethylated alcohols but not for the determmatlon of oxyethylated alkylphenols Other mater&

used gave substantial losses of almost all the surfactants tested Particularly bad results were obtained with polystyrene

and alurmmum cells

Tensammetry with adsorptive accumulation on a hanging mercury drop electrode (HMDE) seems to be a promising method for the determination of trace concentrations of surfactants [l-12]. How- ever, the precision of measurements made with thts method is not always satisfactory [11,12] if

not properly controlled. One of the reasons (or even the mam reason) is the adsorption of surfac- tant on the walls of the measuring cell. Such an effect is obvious at a certain concentration of the

surfactant and occurs with a glass or PTFE cell and octadecyloxyethylene glycol monoether with

an average of fourteen oxyethylene units [ll]. In the first measurements in an unsaturated cell the analytical signal completely disappeared owing to the adsorptive loss of surfactant. In model mvestt- gations this effect can be removed by saturation of the cell during successive washing of the cell with a sufficient amount of a solution of the same composition as that to be measured. However, in analyses of real samples such an approach is very

difficult to realize or is even impossible if the volume of the investigated solution is small. The adsorption on the wall of the cell and on the

78 A SZYMANSKI AND Z LUKASZEWSKI

electrodes, stirrer, etc., of a surfactant to be de- terrnmed causes a dramatic decrease m the surfactant concentration during the measurement and hence the analytical signal is much lower than expected. On the other hand, insufficient cleaning of the cell between sequential measurements can cause desorption of the surfactant that was ad- sorbed during the previous measurement if the

new concentration in the cell is lower. As a result, the peak height increases above the expected value. The decreases in concentration due to both ad- sorption and the desorptton of previously ad- sorbed surfactant, tf uncontrolled, can lead to poor precision of measurement [11,12].

It is necessary to point out that such a source of errors is not specific to tensammetry with accu- mulation on an HMDE, but occurs with all other analytical methods or physico-chemical measure- ments operating with solutions in snnilar very low ranges of concentration. This concerns all the methods using preconcentration of surfactant, on the HMDE, e.g., the modified Kalousek method [13-161, the lowermg differential capacity method [17] and differential pulse tensammetry at the HMDE [18], and also measurement of surface tension, light scattering of solutions of surfactants,

etc. The aim of thts work was the investigation of

the loss of surfactant due to its adsorption on the walls of the measuring cell and the error in the determination connected with this phenomenon, and to find, rf possible, an appropriate material for construction of the cell that would minimize

the analytical errors. Most of the mvesttgated materials were those commonly used for the con- struction of laboratory vessels, e.g., glass, quartz,

glassy carbon, polyethylene, polypropylene and PTFE. For comparison, polystyrene and alumin- mm vessels were added.

The surfactants used represent two main classes of non-ionic surfactants: oxyethylated alkylphe- nols (Triton X-100 and Rokafenol N-3) and oxy- ethylated alcohols (Brt~ 35, Marlipal 1618/40 and surfactant 18-10). For each of these groups surfactants were selected that have various ratios of hydrophilic and hydrophobic parts in the mole- cule. Additionally, poly(ethylene glycol) of norm- nal mean MW 9000 (PEG 9000) was investigated

as a representative of surfactants that have been thoroughly investigated and are readily determma-

ble [7-91.

EXPERIMENTAL

Apparatus and reagents

A Radelkis OH-105 polarograph was used with a voltage scan rate of 400 mV mini ‘. The applied amplitude of the alternating voltage was 2 mV. Controlled-temperature HMDE equipment (Ra- diometer) having an additional platinum-wire aux- iliary electrode was used. All potentials cited are vs. the saturated calomel electrode (SCE). The beaker of the measuring cell was replaced with beakers made from different materials. The materials of these beakers were not additionally characterized.

The following surfactants were used without additional purification: the oxyethylated al- kylphenols Triton X-100 (Merck) and Rokafenol

N-3 (Organika-Rokrta, Poland), the oxyethylated alcohols Brij 35 (Riedel-de H&en), Marhpal 1618/40 (Htils) and surfactant 18-10 (decycloxy- ethylene-n-octadecyl monoether), synthesized at the Institute of Chemical Technology and En- gmeering of Poznan University and PEG 9000 (Fluka).

The sodium sulphate used for preparation of the base electrolyte was purified by double re- crystallization and heatmg at 600 o C. All solutions were prepared in water triply distilled from quartz. Only freshly distilled water was used. The sup- porting electrolyte in all the studies was aqueous

0.5 M sodium sulphate.

Typlcal expenment The investtgated cell was previously carefully

cleaned with methanol. The first portion (20 ml) of the investigated solution was added to the cleaned cell and deaerated with purified nitrogen for 15 min. The proper potential of accumulation was set up, the stirrer was switched on and the new drop was formed from the HMDE. From this moment the preconcentration time was counted. The measurements were performed as described previously [ 111 using a 300-s preconcentration time.

TENSAMMETRY WITH ACCUMULATION ON HMDE PART 6

After the measurement the solution was removed and the cell was filled with the new solution of the same composition. The measurement was re- peated, including deaeration. Such repetitions were usually made six times.

RESULTS

Eight materials were used for the construction of measuring cells: glass, quartz, glassy carbon,

alumimum, polyethylene, polypropylene, PTFE and polystyrene. A peak would be expected to have a stable height in sequential experiments in the same cell if there is no adsorption of the surfactant on the walls of the cell. However, owing to the differences m the hydrodynamics of the different cells, on might expect some differences m the peak height. These differences are caused both by the different adhesion of solutions on different materials and some differences in the

geometry of the cells. The tensammetric curves for one series of six

sequential experiments performed in the glass cell with a solution of 50 pg 1-i of Triton X-100 are shown in Fig. 1. The increase in the peak height in

sequential experiments mdicates a loss of surfac- tant owing to its adsorption on the glass cell during the first two experiments and a gradual decrease in tins loss in subsequent experiments.

The complete results for Triton X-100 (50 pg l- ‘) with eight cells are shown in Fig. 2. These results differ considerably from those predicted if

there is no adsorption on the cell walls. With all the cells tested the first measurements give much lower results than those expected. Also, in further measurements, the peak height is usually lower than expected. Only m the third experiment with the quartz cell and the fifth experiment with the glassy carbon cell is the plateau of the curve reached. Such stabilization of the peak height in successive experiments can be considered to be the result of the achievement of a state of adsorptive equihbrmm between the solution and the walls of the cell. For the glass and polyethylene cells, the considered dependences seem to be close to the

saturation state m the last (sixth) experiment. In the SIX successive experiments the equilibrium state

Fig. 1 Tensammetnc curves for Tnton X-100 belongmg to one

senes of successive expenments done m a glass cell. The

numbers denote the sequence of expenments Concentration of

surfactant, 50 pg I-‘; preconcentration potential, -1 20 V,

preconcentratlon time, 300 s

was not reached with cells constructed of polypro- pylene, polystyrene or aluminium. A non-typical sigmoidal dependence was obtained with the PTFE

O+ 2 3 4 5 6 Number of the sepenbal meosum,,ent

Fig 2 “Saturation curves” of cells constructed from (a) quartz,

(b) glass, (c) glassy carbon, (d) FTFE, (e) polyethylene, (f)

polypropylene, (g) polystyrene and (h) alunnmum with Trlton

X-100 The dashed hne denotes the result for a quartz cell with

protectlon of the ceramic fnt at the end of the salt bndge

Concentration of surfactant, 50 pg I-‘, preconcentratlon

potential, - 1 20 V, preconcentratlon time, 300 s, 100 mm = 0 4

PA

80 A SZYMAIhKI AND Z LUKASZEWSKI

1 2 3 I 5 6 Number of the seqwnt~al measurement

Fig 3 As Fig. 2, with Rokafenol N-3 Comhtlons as III Fig 2, except concentration of surfactant, 200 /.~g 1-l

cell. It is not clear whether the plateau observed corresponds to the saturation state of the cell. The results with the polystyrene and aluminium cells

also seem to show a sigmoidal dependence, but only the lower section of the curve is visible. Further experiments with these cells should result m an increase in peak height.

The results concernmg Rokafenol N-3 (200 pg 1-l) shown m Fig. 3, are similar to those with Tnton X-100. All the cells tested indicate adsorp- tion of the surfactant in the first measuring cycle. The quartz cell is saturated relatively quickly (the third measuring cycle). The other cells, except the

polyethylene cell, are not completely saturated during the six measuring cycles. The PTFE cell indicates the same sigmoldal type of saturation curve as with Triton X-100. The similarity of the behaviour of Rokafenol N-3 and Triton X-100 is

caused by the similarity of their structures, both being oxyethylated alkylphenols. The slight dif- ference in behavlour IS caused by the different ratios of hydrophobic and hydrophilic parts in their structures.

Figures 4-6 show the behaviour of oxyethyl- ated alcohoh. F@re 4 shows the b&&our of

Brig 35 (100 pg 1-l). Although all the cells again adsorb the surfactant, the quartz, PTFE and glassy

A9 &

0 1 2 3 I 5 6

Number of the sequenttal mwwrement

fig. 4. As Fig 2, with B~J 35 Conditions as m fig 2, except concentration of surfactant, 100 pg I-‘.

carbon cells are saturated comparatively easily. Saturation of the polyethylene and glass cells needs more measurmg cycles and with the polypropylene cell saturation is not achieved during the six mea- suring cycles. The aluminium cell indicates very strong adsorption and the polystyrene cell adsorbs

BrlJ 35 almost completely.

160

I

IL0

5 80

ii60

0 L, 1 2 3 L 5

Number of the sequent/al measurement 6

fig 5 As fig. 2, with Marhpaf 161g/417. Condltlons as m Rg.

2, except concentration of surfactant, 200 pg l-‘, 100 mm = 1 2

PA

TENSAMMETRY WITH ACCUMULATION ON HMDE PART 6 81

The dependences concerning of Marlipal1618/ 40 (200 pg 1-l) are shown in Fig. 5. The loss due to adsorption on the cell occurs with all the cells tested. The quartz cell is saturated comparatively easily (the third measurement shows a stable re- sult). The cells manufactured from PTFE, glass and polyethylene are saturated comparatively easily, especially the PTFE cell, which is saturated without a Jump in peak height between the first and the subsequent measurements. A small dif- ference between the first and subsequent measure- ments is also characteristic of the glass and

polyethylene cells, but complete saturation of the cell is virtually not achieved (absence of a plateau). The glassy carbon cell achieved a state of adsorp- tive equilibrium in the third measurement, but the

difference between the first measurement and the plateau is very large here. The polypropylene cell behaves similarily, although in this case the equi- librium state is approached only in the sixth mea- surement. The aluminium and polystyrene cells strongly adsorb Marlipal 1618/40 and they were not saturated during the six measurements. How- ever, the final experiments in the series indicate

the first symptoms of saturation of the cells. Surfactant 18-10 (50 pg 1-l) saturates the PTFE

and glassy carbon cells comparatively easily (Fig.

6). The polyethylene cell is also saturated com- paratively easily, although the difference between the first and subsequent measurements is very large. In comparison with the cases of previously considered surfactants, the quartz and glass cells are saturated more slowly, i.e., the achievement of the saturation state needs a larger number of

measurement. The cells manufactured from aluminmm and polystyrene adsorb surfactant 18- 10 very strongly and they are not saturated during the six measurements. The sigmoidal shape of the curves for the polystyrene cell is interesting, al- though the position of plateau of this curve seems to be away from the saturation state.

PEG 9000 (50 pg 1-l) is also adsorbed on the walls of the different cells (Fig. 7). The adsorption of PEG 9000 on the quartz, glass and glassy carbon cells is comparatively low, which explains why adsorptive losses were not observed during previous experiments on the behavlour of PEGS

[7-91 m a glass cell. The polyethylene and poly-

1 2 3 L 5 6 Number of the sequential measurement

Fig 6 As Fig 2, with surfactant 18-10 The dashed hne denotes the result for a quartz cell with protection of the

ceranuc fnt at the end of the salt bndge Condmons as m Fig

2, except preconcentratlon potential, - 1 40 V, preconcentra-

tlon time, 600 s

180~

70+ 2 3 4 5 6 Number of the sequenbcd -surement

Fig 7 As Fig 2, with PEG 9000 Condltlons as in Fig 2

82

MDmV

-1

Ql,UA :I . . . . . . Fig. 8 Tensammetnc curves for Tnton X-100 obtamed m one senes of the successwe expenments done m a quartz cell The

concentration of Tnton X-100 (1 mg 1-l) IS sufficient for the

formation of prenucellar associates The numbers on the curves

denote the sequence of experiments. Preconcentratlon poten-

teal, - 1 20 V, preconcentration time, 300 s l = Zero of cur-

rent for respective tensammetnc curves.

propylene cells indicate moderate adsorption of PEG 9000. The adsorption of PEG 9000 is slightly higher on the polystyrene cell than m the cases

above; specifically, with a larger difference be- tween the first and subsequent measurements. The PTFE cell indicates a substantial adsorptive loss, although the plateau is formed in the fourth mea- surement. The alurmmum cell adsorbs PEG 9000

very strongly. In conclusion, all the surfactants tested un-

dergo adsorption on the cells, although to drffer- ent extents, depending on the material. The changes in the concentration of the surfactant due

to adsorptive losses depend on the concentration of the surfactant, being higher for more dilute solutions. An example is shown m Fig. 8, whtch shows the series of successive tensammetric curves for Triton X-100 (1 mg 1-i) obtained with the glass cell. In contrast to Fig. 2, curve b, i.e., with a 20-fold lower concentration, the peak height changes very slightly here. However, the height of the second peak increases strongly in the succes-

sive experiments. Tlus peak corresponds to the premicellar associate form of Triton X-100. As the

A SZYMAI;ISKI AND Z &UKASZEWSKI

concentration of this form of Triton X-100 1s much lower than that of the monomeric form, its adsorption on the glass is clearly visible. The saturation of the cell with the premicellar associ- ate form of Tnton X-100 in the successive experi- ments leads to stabilization of the height of the

corresponding tensammetric peak. Up to now the total effect of the adsorptive

losses m the cell has been considered. However, apart from the vessel itself, which exposes the largest surface, the adsorption of the surfactant on the stirrer and on the electrodes should be evaluated. The stirrer was constructed from PTFE and the electrodes were mainly made of glass with a ceramic frit on the end of the salt bridge.

Two series of experiments were done with the aim of evaluating the loss of surfactant on the stirrer and on the electrodes. A quartz vessel was

used, with Tnton X-100 in one series and surfac- tant 18-10 in the other. In both series the elec- trodes and the stirrer were preliminarily saturated with the tested surfactant, then a series of tensam-

metric measurements were made as described pre- viously. The results obtained were added to the corresponding figures obtained previously with these surfactants in Fig. 2 for Triton X-100 and Fig. 6 for surfactant 18-10, where the correspond- ing curves are given as dashed lines.

Undoubtedly, substantial adsorption of both surfactants on the electrodes and on the stirrer is visible tf the quartz vessel is used, owing to the very low adsorption of the quartz vessel itself. The

most important conclusion is that, for construc- tion of the cell, rt is possible to find a material that shows minimum adsorption of surfactants on its surface and in tins way to construct a cell that

does not demonstrate serious adsorptive losses. With such an aim, the contributions of the

separate electrodes and stirrer to the total effect of adsorption were examined. The same measuring procedure was used as described previously. It was found that the main loss is caused by the ceramic fnt located at the end of the salt bridge. A com- mercially available salt bridge (Radiometer) was used and the charactenstrcs of its ceramic frit are unknown.

Limitation of the convection of the surfactant to the frit seemed to be the simplest way to reduce

TENSAMMETRY WITH ACCUMULATION ON HMDE PART 6 83

the surfactant losses caused by this frit. For thts purpose a polyethylene ring was fitted to the end of the salt bridge. In this way the ceramic frit

appeared at the bottom of a channel and its surface was not flushed with the solution during the circulation of solution in the cell. The surfac- tant transport to the ceramic frit was restncted to diffusion and thus was substantially reduced. The results of such a simple modification were surpris- ingly successful, as shown in Fig. 9. Two series experiments were made with Tnton X-100 in the quartz cell. In the ftrst series the cerannc frit was protected. Curve b is the cahbration graph ob-

tained under such conditions. In the second series (curve a), the ceramic fnt remained unprotected. The two curves are very different, although they were measured with the same solution and under the same experimental conditions. The difference between curves a and b is a measure of the loss due to the adsorption of Tnton X-100 on the cerarmc frit, because the adsorption of surfactant on the other parts of the quartz cell is negligible.

The state of the surface of a vessel used for a cell construction also plays an important role in the adsorption of the surfactant, as illustrated in Fig. 10, which shows the results of three series of experiments with Triton X-100 (50 pg 1-l) and

the glass cell. The curves of saturation of the cell were measured for three states of the glass cell. In

Fig 9 Cahbratlon graphs for Tnton X-100 obtamed usmg the

glass cell (a) with the ceranuc fnt unprotected and (b) with

protectlon of the ceramic fnt. Preconcentratlon potential,

- 1 20 V, preconcentratlon time, 300 s, 100 mm = 0 4 PA.

1 2 3 4 5 6 Num&r of the sequential measurement

Fig. 10 Influence of the preparation of the surface of the glass

cell on Its ablhty to adsorb Tnton X-100 (a) New cell, (b) cell

prewashed with methanol, (c) cell boiled with concentrated

sodium hydroxide solution for 2 h Concentration of Tnton

X-100, 50 pg l-‘, preconcentratlon potential, -120 V; pre-

concentration time, 300 s, 100 = 0 4 PA

the first series (curve a), the cell was new. In the second series, the cell was washed with methanol prior to the experiments (curve b) (such a prepara-

tion of the cell was used m the prevtously de- scribed experiments). In the third series, the glass

vessel was boiled with concentrated sodium hy- droxide solution for 2 h (curve c). In all these experiments the ceramic frit remained unpro- tected.

The influence of the method of cleaning the glass cell on the results is obvious from these experiments. Taking mto account the adsorption on the ceramic frit, it can be concluded that in the case c the glass vessel adsorbs to a negligible extent.

DISCUSSION

The results of tins work enable the followmg recommendations to be formulated for the selec- tion of materials for the construction of measuring cells. The use of a quartz cell together with protec- tion of the ceramic frit at the end of the salt bridge leads to a substantial reduction m the adsorptive loss and permits the determination of the concentration of surfactants at very low levels.

84 A SZYMANSKI AND Z ISJKASZEWSKI

Slightly worse results were obtained with the glass cell. Specifically, the loss of surfactant 18-10 is large. However, the results could be improved by using a glass cell with the surface pretreated with concentrated sodium hydroxide solution (see Fig. 10). The PTFE cell adsorbs more specifically. Oxyethylated alcohols are adsorbed comparatively weakly. On the other hand, the adsorptive loss of oxyethylated alkylphenols on the PTFE vessel is very substantial. The glassy carbon cell behaves sirmlarily, although generally the results are worse than with the PTFE vessel. Both of these materials

seem to be sultable for the construction of cells for the determination of oxyethylated alcohols, but are unsuitable for cells for the determmation of oxyethylated alkylphenols. The other four materi- als used for construction of the cells are char- acterized by adsorption of almost all the surfac- tants tested. The adsorptive ability of these materials increases in the order polyethylene < polypropylene < polystyrene, alumimum. The very strong adsorption of the surfactants on aluminium seems to be connected with aluminium oxide on the surface.

The specificity with which surfactants are ad- sorbed on glass, glassy carbon and PTFE seems to offer a possibility of using this effect for the selective separation of the surfactants.

These mvestigatlons were done with commonly available matenals. Further investigations should

be based on the use of better characterized maten- als, e.g., kmd of glass, molecular weight of the

polymer used. The results obtained so far are probably too scanty for drawing conclusions with regard to the possible relationship between the adsorption of surfactants on certam materials and their cherrucal structures.

Tl-us work was supported by Research Program CPBP 01.17.

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