EQctroanaIysis, 3 ( 199 1 ) 1 7-25

Adsorptive Stripping Tensammetry of Commercially Available Oxyethylated Alcohols. Part 1 Surfactants Having C,o-C,3 Hydrophobes

Andmej Szymanski and Zenon Lukaszewski' Institute of Cbmkty, Technical University of Poznan, ul. Piotrowo 3, 60-365 Poznan, Poland Received April 24, 1990.

ABSTRACT The behavior of oxyethylated oxoalcohols (e.g., Oxetals D104, T105, and C114, and oxyethylated n-dodecanol BRIJ-35) was investigated. The dependence of peak height on concentration of surfactant, preconcentration time, and preconcentration potential was examined. To improve the analytical signal of Oxetals C114 and T105, the influence of methanol on their tensammetric curves was investigated. Roughly, all the surfactants exhibit two peaks: a wide one, corresponding to the monomer form, which shifts toward the negative direction with an increase in concentration, and a narrow one, corresponding to the associate form, whose position is stable. Oxetals C114 and T105 exhibit additional peaks, which complicate the determination. The addition of 20% of methanol removes such difficulties and only one peak remains.

WTRODUCTION

Oxyethylated alcohols constitute the most frequently used group of nonionic surfactants [ 11. The surfactants having the lauryl group as hydrophobe are commonly used, in addition to those having the cetyl or stearyl group as hydrophobe. For several years oxoalcohols also were used in the manufacture of nonionic surfactants.

Trace analysis of nonionic surfactants is difficult. Ad- sorptive stripping tensammetry (AST)-that is, tensamme- try with the use of adsorptive preconcentration of surfac- tant on the hanging mercury drop electrode (HMDE) holds great promise for this task [2-141. One of the turning points in the development of this method was the elimina- tion of the errors of determination caused by the adsorp- tion of the surfactant that was to be determined on the measuring cell [ 151.

The aim of this work was the investigation of a rela- tionship between the structure of oxyethylated alcohols and their analytical signal in AST and the characterization of this signal. Such characterization of the analytical signal of oxyethylated alcohols is necessary for the future use of AST for the investigation of separation processes of surfactant such as gas stripping or coprecipitation, as well as for identification of surfactants, and for their quantita- tive trace analysis. Such study should comprise the investi- gation of the shape of the peaks and the dependence of peak height on surfactant concentration, as well as the

To whom correspondence should be addressed.

dependence of peak height on preconcentration time and preconcentration potential for the comparatively numer- ous commercially available surfactants, which dlffer from each other in terms of hydrophobes and length of the oxyethylene chain. This article concerns only the oxyeth- ylated oxoalcohols having ClO-Cl3 hydrophobes and, for comparison, one oxyethylated straight-chain dodecanol. Oxyethylated alcohols having C16-Cl, hydrophobes will be considered in Part 2. The pilot investigation of the behavior of oxyethylated straight-chain decanol and hexa- no1 was performed earlier [ 11, 131.

The following oxyethylated alcohols having Clo-Cl3 hydrophobes were selected for the investigation: Oxetal D104, Oxetal C114, and Oxetal T105 (all oxyethylated oxoalcohols: decanol, dodecanol, and tridecanol, respec- tively, having 4, 14, and 5 oxyethylene subunits, respec- tively), and BRIJ-35 (oxyethylated n-dodecanol having 20 oxyethylene subunits).

mERIMmm Apparatus and Reagents A Radelkis OH-105 polarograph was used with a voltage scan rate of 400 mV/min. The applied amplitude of the alternating voltage was 2 mV. Controlled-temperature HMDE equipment (Radiometer), having an additional platinum wire auxiliary electrode, was used. All potentials cited are against the saturated calomel electrode. The beaker of the measuring cell was replaced by a quartz

6 1991 VCH Publishers, Inc 1040-0397/91/$3.50 + .25 17

Szymanski and Lukaszewski 18

i

I

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FIGURE 1. Tensammetric curves for different concentrations of Oxetal D104; solid points denote zero current for the corresponding curve. Surfactant con- centration (mg/L): (a) 0, (b) 0.04, (c) 0.2, (d) 0.6, (e) 1.0, (f) 3.0, (9) 4.0, (h) 4.4, (i) 4.8, and (j) 5.0. Precon- centration potential, - 1.20 V; preconcentration time, 300 seconds.

beaker. The ceramic frit on the end of the salt bridge was protected by the fitting of the polyethylene tube. This ceramic frit indicates very large adsorptive ability [ 151, and the measurements performed very effectively reduce the adsorptive loss of surfactant.

The following oxyethylated alcohols were used with- out additional purification: Oxetal D104, Oxetal T105, and Oxetal C114 (Zschimmer und Schwarz) and BRIJ-35

The sodium sulfate used for the preparation of the base electrolyte was purified by double recrystallization and heated at 600°C. All solutions were prepared in water thrice distilled from quartz. Only freshly distilled water was used. The supporting electrolyte in all the studies was aqueous 0.5 M sodium sulfate.

(Atlas).

PROCEDURE The cell to be investigated was first carefully cleaned with methanol. Then the solution being studied was added to the cell. The proper potential of accumulation was ap-

plied, the stirrer was switched on, and the new drop was formed from the HMDE. From this moment the precon- centration time was counted. The measurements were performed as described previously [ 131, usually using 300 seconds preconcentration time (with the exception of the series of experiments in which preconcentration time is variable). The preconcentration potential was usually - 1.20 V (with the exception of the experiments with variable preconcentration potential).

RESULTS Influence of Surfactant Concentration on Shape and Height of Tensammetric Peaks of Oxyethylated Alcohols Representative tensammetric curves for the surfactants investigated are shown in Figures 1-4; calibration curves for these surfactants are shown in Figure 5.

Oxetal D104 (Figure 1) forms one broad peak in the range of its concentration, 0.01-10 mg/L. The potential of this peak shifts with increase of surfactant concentration toward the negative direction. The changes of the peak potential (Ep) are shown in Figure 6 in the coordinate

FIGURE 2. Tensammetric curves for different concentrations of Oxetal T105. Conditions as in Figure 1, except surfactant concentration (mg/L): (a) 0, (b) 0.01, (c) 0.02, (d) 0.04, (e) 0.07, (1) 0.2, (9) 1.0, (h) 5.0, and (j) 10.

Adsorptive Stripping o f Oxyethylated Alcohols: Part 1 19

0 0 0 0

FIGURE 3. Tensammetric curves for different concentrations of Oxetal C114. Conditions as in Figure 1, except surfactant concentration (mg/L): (a) 0, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.5, (f) 0.7, (9) 1.0, (h) 2.0, (i) 4.0, (I) 7.0, and (k) 100.

system Ec versus the logarithm of the surfactant concentra- tion. The increase in Oxetal D104 concentration over the value of 10 mg/L causes the appearance of the following peak. The new peak grows with the increase of surfactant concentration and, simultaneously, the first peak de- creases. This transformation takes place within a compara- tively narrow range of concentration (i.e., between 30 and 50 mg/L). Finally, with the further increase in surfactant concentration, the first peak completely disappears, and the second peak becomes very narrow. It is interesting to note the very sharp jump of current in the place at which the first peak disappears.

Oxetal T105 (Figure 2) forms one broad bulging peak within the concentration range of 0.01-2 mg/L. This peak seems to be composed of two coinciding peaks, and the ratio changes with the increase of surfactant concentra- tion. The complex nature of this peak is probably the

reason for the great difference in the concentration de- pendence of the peak height of Oxetal T105 from all the other oxyethylated alcohols (Figure 5, curve b). The increase of concentration over 5 mg/L causes the appear- ance and successive growth of the additional separate peak (Ep = - 1.82). Parallel with the growth of the new peak, the “old” one decreases and eventually disappears, and in its place a sharp jump of the baseline remains, as in the case of Oxetal D104.

Both Oxetal C114 and BRIJ-35 form at least two peaks (Figs. 3 and 4, respectively), within the whole investigated range of concentration. The less negative of these peaks is wide and the other is very narrow. The wide peak of Oxetal C114 seems to have a complex nature for the low concentration (see curves b-d), as discussed below. The increase in concentration of both Oxetal C114 and BRIJ- 35 makes both peaks (wide and narrow) coincide more

FIGURE 4. Tensammetric curves for different concentrations of BRIJ-35. Conditions as in Figure 1, except surfactant concentration (mg/L): (a) 0, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.5, (f) 0.7, (9) 1.0, (h) 2.0, (i) 5.0, and (1) 10.

k

0 b a

I

20 Simanski and Lukaszewski

FIGURE 5. Calibration graphs for Oxetal D104 (a), Oxetal TI05 (b), Oxetal C114 (c), and BRIJ-35 (d). Preconcentration potential, - 1.20 V; preconcentration time, 300 seconds; 100 mm = 0.8 PA.

140 -

120-

100-

80- /+

0 02 04 06 08 10 12 14 16 28 2.0

and more. Finally, one common narrow peak appears at concentrations of 7 mg/L for Oxetal C114 and 10 mg/L for BRIJ-35. The overlap is caused by the shift of the wide peak toward the negative direction, while the narrow peak keeps its position ( - 1.81 and - 1.82 V, respectively).

Dependence of Height and Shape of Tensammetric Peaks on Preconcentration Time The results for the series investigating the dependence on peak height and shape on preconcentration time are

FIGURE 6. Changes in peak potential E, in the coordinate system €'p versus the logarithm of surfactant concentration. Preconcentration potential, - 1.20 V; preconcentration time, 300 seconds.

2.8

27

2.6

Lu' 2.5

2. L

-4

2.3

shown in Figure 7. Two concentrations were used: 0.2 mg/L (solid lines) and 1 mg/L (dashed lines). Almost all curves presented in Figure 7 are typical. These curves show the range of the growth of the peak with the increase of preconcentration time. Only the preconcentration times within this range can be used for purposes of quanti- tative analysis. Some deviation from the typical curve is observed for Oxetal T105 as early as 0.2 mg/L concell- tration.

The tensammetric curves obtained under conditions of various preconcentration times can supply additional

I -6 -5 -4 -3 -2

[g c

Adsorptive Stripping of Oxyethylated Alcohols: Part 1 21

FIGURE 7. Dependence of peak height on

Oxetal D104 (a, a'), Oxetal T105 (b), Oxetal C114 (c, c'), and BRIJ-35 (d, d'). Surfactant concentrations: 0.2 mg/L (a-d, dashed lines) and 1.0 mg/L (a', c', d', solid

potential, - 1.20 V.

11201

d' 0 preconcentration time for

X n lines). Preconcentration d ______------ C' n

C _ _ _ _ _ + _--- -------+

d ---- -___ _ _ _ _ _ _ _ _ _ _ _ 0 - _ _ _ _ _ _ --- :!!--====== = +

0 2 4 6 8 10 12 14 16 18 20 t. (mil)

information about appearance, shifting, overlap transfor- mation, and disappearance of specific peaks if their behav- ior is atypical. A very interesting conclusion can be drawn from the changes of tensammetric curves of Oxetal D104 performed for different preconcentration times if the con- centration of surfactant corresponds to the range of disap- pearance of the wide peak and growth of the narrow peak

FIGURE 8. Influence of preconcentration time on the shape and ratio of peaks of Oxetal D104 within the specific range of concentration corresponding to the transformation of the peaks. Solid points denote zero current for the corresponding curve. Surfactant concentration, 30 mg/L; preconcentration potential, -1.20 V. Preconcentration time (s): (a) 10, (b) 120, (c) 480, (d) 720, and (e) 1200.

2aomv - OlpA I

a

(see Figure 1). The series of curves obtained from the solution of 30 mg/L of surfactant is shown in Figure 8. Here the increasing preconcentration time causes the same effects as the increase in concentration (i.e., gradual disappearance of the initial peak and gradual growth of the second peak).

Also the complex nature of the wide peak of Oxetal C114 (see Figure 3, curves b and c) is better seen if different preconcentration times are used (Figure 9). Here the increase of preconcentration time, equivalent to the increase of surfactant concentration, causes a gradual shift of the broad peak toward the negative direction, so the further peaks are overlapped by this peak, with the exception of the more negatively located peak.

Dependence of Peak Height on Preconcentration Potential The effect of preconcentration potential was examined for all surfactants mentioned above, using a concentration of 0.5 mg/L and a preconcentration time of 300 seconds. The results are shown in Figure 10. Generally, the curves shown in Figure 10 are typical. The slight deviation from the ideal shape for Oxetal T105 (curve b), visible in the left-hand section of the figure, is not of great practical importance. All surfactants examined can be preconcen- trated within a wide potential range. This range is slightly narrower for Oxetals D104 and T105 than for the other surfactants, which is in agreement with theoretical expec- tations.

Mod@cation of the Conditions for Determination by Methanol Two of the surfactants investigated, Oxetal C114 and Oxe- tal T105, indicate poorly shaped tensammetric curves in the most interesting range of concentrations. The addition

22 Szymanski and Lukaszewski

9

FIGURE 9. Tensammetric curves illustrating the changes in number, shape, and height of the peaks of Oxetal C114 with increase in preconcentration time. Solid points denote zero current for the corresponding curve. Surfactant concentration (mg/L): (a) 0, (b-g) 0.2. Preconcentration time (s): (a) 300, (b) 60, (c) 180, (d) 300, (e) 480, (f) 720, and (9) 1200. Preconcentration potential, - 1.20 V.

of methanol to the solution substantially changes the shape of these peaks. The influence of different amounts of methanol on the peaks of Oxetal C114 and Oxetal T105 is shown in Figure 11. The peak of Oxetal C114 without methanol (curve a) indicates its complex nature. Three poorly separated peaks are visible. The addition of 8% of methanol (curve b) improves the shape of the peak, and the further increase of methanol concentration to 20% causes the appearance of the single well-shaped peak (curve c). It is interesting to note that the narrow peak also disappears. In the case of Oxetal T105 (curves d-f), the methanol causes the improvement of the shape of the peak, permitting a single peak to appear; but the decrease of the peak height is observed, as well (compare curves d and f).

Figure 12 shows the dependence of changes in peak height of Oxetal C114 and Oxetal T105 on surfactant con- centration in the presence of 20% methanol and in the absence of methanol. The peak height of Oxetal C114 changes slightly in the presence of methanol in the com- parison with the curve obtained in the absence of metha- nol (curves a and b). The influence of methanol on the height of the peak of Oxetal T105 is completely different. Here the peak is much lower in the presence of methanol. However, the abnormal shape of the calibration curve disappears in such a case.

DISCUSSION The number, position, and shape of the tensammetric peaks of the surfactants investigated depends on kind of hydrophobes, length of oxyethylated chain, concentration of surfactant, and preconcentration time.

All the surfactants investigated exhibit at least two tensammetric peaks within certain ranges of concentra- tion. Their behavior can be roughly simplified to the transformation of these two peaks. One of these peaks is wide and shifts toward the negative direction with in- creases of surfactant concentration and preconcentration time. In this preliminary treatment, the complex nature of this wide peak for the cases of Oxetal C114 and Oxetal T105 is neglected. The second peak is narrow and is located in vicinity of - 1.8 V, most frequently at - 1.82 V. As the wide peak moves toward the negative direction, it also gradually becomes more and more narrow. Eventu- ally, a common high, narrow peak appears, as a result of the gradual overlap of the two peaks (see Figures 1-4).

FIGURE 10. Dependence of peak height on preconcentration potential for Oxetal D104 (a), Oxetal TI05 (b), Oxetal C114 (c), and BRIJ-35 (d). Surfactant concentration, 0.5 rng/L; preconcentration time, 300 seconds.

lad

140 1601 120

E. M

Adsorptive Stripping of Oxyethylated Alcohols: Part 1 23

1 I 21

m e

FIGURE 11. Influence of methanol on the tensammetric curves of Oxetal C114 (a-c) and Oxetal T105 (d-f). Solid points denote zero current for the corresponding curve. Surfactant concentration, 0.2 mg/L; methanol concentration, (a and d) 0, (b and e) 8%, (c and f) 20%. Preconcentration potential, - 1.20 V; preconcentration time, 300 seconds.

In the cases of Oxetal D104 and Oxetal T105, only the wide peak appears within the wide range of their concentrations; the narrow peak appears only at relatively high concentrations. The wide peak is located at a compar- atively less negative potential and shifts toward the nega- tive direction as surfactant concentration increases. The behavior of oxyethylated straight-chain decanol and hexa- no1 is very similar to that of Oxetal D104 [ll, 131.

Basically, all the oxyethylated alcohols examined show the same order of transformation of the two peaks considered, although the particular cases differ with re- spect to the range of concentration or preconcentration time in which these transformations occur. This range is braodest for the case of Oxetal D104, where the peaks are well separated, which makes the observation of particular phases of h e transformation of the peaks much easier.

Among other effects, the shift of the potential of the wide peak toward the negative direction is visible due to the increase in its concentration, in agreement with the theoretically predicted dependence E: = f(lg C) [ 161 (see Figure 6), in which Ep is the potential of the peak and C is the concentration of surfactant. This dependence is fulfilled only for the lowest range of concentrations. The changes of peak potential are very small for concentra- tions higher than 0.4 mg/L. This value approximately cor- responds to the beginning of the plateau on the calibra- tion graph (see Figure 5, curve a), that is, the saturation of the electrode surface.

Another characteristic property of tensammetric curves that is fulfilled for the cases of Oxetals D104 and

T105 is the transformation of the wide peak into a vertical line. Such a transformation proceeds from the gradual narrowing of the wide peak, with the increase of surfactant concentration (Figures 1 and 2) or preconcentration time (Figure 8). The transformation of tensammetric peak into a vertical line is caused by two-dimensional condensation, in agreement with the theory of tensammetric curves

The behavior of the wide peak is similar to that of the peaks of model compounds under the conditions of classical tensammetry [2], while the narrow peak keeps its position, which is similar to the behavior of high molecu- lar weight polyethylene glycols [4]. Thus, the wide peak seems to belong to the monomeric form and the narrow one to the associated form of surfactant.

Except for the two peaks discussed, the presence of other peaks is visible on the tensammetric curves of Oxe- tal T105 and Oxetal C114 (Figures 2 and 3). However, in the case of Oxetal C114 they are not well-shaped separated peaks but rather bulges or very small peaks “growing” on the wide peak of the monomer form. These peaks are eventually overlapped by the wide peak, with increase of surfactant concentration or preconcentration time (Fig- ures 2 and 9). In the case of Oxetal T105, the wide peak consists of two overlapping peaks whose ratio changes depend on the concentration. The additional peaks con- sidered are probably connected with the existence of forms of surfactant intermediate between the monomer and the highly associated forms. This was suggested by Weiss [20] and Jehring [2] for the case of oxyethylated alcohol Prawozel W-OFC-100, examined by using classical tensammetry. However, the possibility exists that the peaks considered belong to the impurities (e.g., free non- oxyethylated alcohol or polyethylene glycol), because the surfactants investigated are commercial products.

The presence of methanol in the solution transforms the tensammetric curves of Oxetal C114 and Oxetal T105 in such a way that only the single peak corresponding to the wide peak of the monomer form remains. The substantial change of conditions for the association of surfactant in the presence of methanol seems to be the reason for such changes.

One additional important question arises in connec- tion with the association of surfactant: whether the associa- tion takes place primarily in the solution, and only already associated forms undergo adsorption on the electrode surface, or whether the formation of associated forms can occur on the electrode surface as a result of the accumulation of surfactant over the bulk concentration, even if the concentration of surfactant in the bulk of the solution is below the association threshold. Figure 8 illus- trates the transformation of the peaks due to the associa- tion, which increases with the prolongation of the accu- mulation time. There seems to be evidence for the gradual formation of the associated forms on the electrode surface because the surfactant concentration in the experiment discussed was much lower than the critical micelle con- centration value. On the other hand, important arguments were published earlier [9] in support of the thesis about the preconcentration of associates previously formed in

[17-191.

24 Szymanski and Lukaszewski

FIGURE 12. Calibration graphs I for Oxetal C114 (a and a’) and Oxetal T 105 (b and b’) in the absence (a and b) or presence and b’) of 20% methanol. Preconcentration potential, - 1.20 V; preconcentration tim 300 seconds.

b

a1

the solution. It is possible that both these versions can occur in parallel, and one series of experiments supports one option but the other series supports the second one.

The fundamental problem for quantitative analysis is the quality of the analytical signal (here the shape of the peak) and its response (peak height) on the concentration of the substance determined. The only such peak that may be considered to be the analytical signal is one that corresponds to the growing section on the calibration graph. The formation of the plateau on the calibration graph indicates the saturation of the electrode surface and thus the end of the accumulation of surfactant. Further transformations of the tensammetric curves, which corre- spond to the plateau of the calibration graph and were discussed above, have no significance for quantitative analysis, although they are important from the point of view of identification of surfactants or for the investigation of association precesses.

The surfactants investigated are characterized by the variety of their analytically useful signals. Oxetal D104 forms a single peak in the range of concentrations corre- sponding to the growing section of the calibration graph. This peak is a very useful analytical signal. Preliminary investigations indicate that oxyethylated n-decanol (10-6 and 10-10) and n-hexanol(6-10) behave similarly [ll, 131. BRIJ-35 forms a double peak within the concentration range of interest. However, this peak gives an excellent reaction to concentration increases and can be qualified as a very useful analyucal signal. Oxetal C114 forms a very complex, wide peak within the range of concentrations of analytical interest. Such behavior is useful from the point of view of identifying this surfactant; however, it can be the source of certain difficulties in quantitative determinations. Fortunately, the addition of methanol (20%) causes the transformation of the tensammetric curve of Oxetal C114 into a very simple one with a single peak. This addition of methanol changes the peak height only negligibly. Thus the addition of methanol is the best option in the case of Oxetal C114, although its determina- tion in water solutions without methanol is also possible.

The peak of Oxetal T105 as the analytical signal is the worst. It is a complex peak and additionally, its calibration

0.1 Q5

graph indicates deviation from the normal sigmoidal shape. The addition of methanol improves the situation, by permitting a single peak to appear on the tensammetric curve, such that the calibration graph becomes sigmoidal, but the peak height decreases in comparison with the water solution. These changes seem to be the price for improvement of the analytical signal.

The range of growth of the analytical signal with in- crease of surfactant concentration or preconcentration time is visible from Figures 5 and 7. The growing range takes place for concentrations lower than 0.1-0.3 mg/L, depending on the surfactant to be determined, under conditions of a 300 second preconcentration time. On the other hand, the range of such a response o f analytical signal on the length of preconcentration time is observed only for 2-4 minutes. The formation o f plateaus on both the calibration graphs as well as the curves o f the peak height versus accumulation time indicates the end o f accu- mulation ability due to saturation of the electrode surface. The increase of concentration or preconcentration time gives similar consequences. However, the range o f desir- able responses of the peak height on the increase of concentration can be shifted by changing the preconcen- tration time (i.e., prolonging the preconcentration time shifts the range of determinable concentration toward the lower concentration range, and decreasing the preon- centration time causes the opposite result).

For all the surfactants considered, as well as for ow- ethylated n-decanol (10-6 and 10-10) and n-hexanol (6- lo), the dependences of peak height on preconcentration potential have a theoretically predicted shape and reflect the influence of changes in the coverage of the electrode surface on changes of the electrode potential (Figure 10). This is fortunate because of very broad range of precon- centration potentials that can be used for the determina- tion of surfactants of group considered.

ACMVO WLEDGMENTS This work was supported by Research Program CPEP 01.17. B a n k are given for Dr. A. Ch. Swallow for helpSul d&cussions during the prqaration of this manz*FcTipt.

Adsorptive Stripping of Oxyethylated Alcohols: Part 1 25

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