Study of the adsorption of organic substances at a mercury electrode by the Kalousek commutator technique

Electroanalytical Chemistry aml Interfacial Electrochemistry, 46 (1973) 63-69

63

~DElsevierSequoia S.A., Lausanne - Printed in The Netherlands

STUDY OF THE ADSORPTION OF ORGANIC SUBSTANCES AT A MERCURY ELECTRODE BY THE KALOUSEK COMMUTATOR T EC HNI QUE

BOZENA COSOVIC and MARKO BRANICA Center for Marine Research, Rudjer Bogkoz,i( hzstitute. Zagreb, Croatia (Yugoslavia}

(Received30th January 1973)

INTRODUCTION The determination of traces of surface active substances in aqueous solutions, especially in samples of natural waters, is lately of great interest. Electrochemical methods can be generally applied for the determination of surface active substances on the basis of the adsorption effects on the electrodes. Adsorption phenomena on the mercury electrode such as a decrease of interfacial surface tension, changes in the capacity of the electrode double layer and suppression of the polarographic maxima are very often used for the determination and characterization of surface active substances. However not all of the methods are suitable for a simple and fast analysis of the organic substances in a dilute aqueous solution. Electrocapillary measurements with Lippmann's electrometer and drop time measurements are unsuitable for a low concentration range because of the long time necessary to reach the adsorption equilibrium 2. A.c. polarography with a small amplitude (tensammetry) and second harmonic measurements are very often and successfully used for a fast determination of various surface active substances even in dilute solutions 3- 5. The sensitivity of the hanging mercury drop electrode (HMDE) is about two orders of magnitude greater than for the dropping mercury electrode (DME) 6. The method of suppression of the polarographic maximum of oxygen by traces of surface active substances was proposed for a routine analysis of fresh water 7 and sea water 8 samples. M. Heyrovsk~,9 was the first who employed the polarographic method of discontinuously changed potential using a Kalousek switch in the study of the charging current. He supposed that the changes of capacity of the electrode double layer due to the adsorption or desorption of surface active substances produce changes on the charging curves which are comparable to polarographic reduction or oxidation waves. To find out the possibilities of the Kalousek commutator techniques for qualitative and/or quantitative determination of surface active substances we studied the corresponding charging current curves of tri-n-butyl phosphate (TBP), thymol and sodium lauryl sulphate at the DME and HMDE. General principle of the method

It is supposed that some organic substances, present even in very small amounts in aqueous solution, adsorb strongly at the surface of the mercury electrode. The adsorption causes a decrease in the interfacial surface tension which, for most neutral organic substances, is greatest at the potential of the electrocapillary maximum

64

B. COSOVIC, M. BRANICA

i/mAl

/ -/'"/

1

a

Eecm

E/V

Eecm

c ~/v

s

Fig. 1. (a) Typical electrocapillary curve for supporting electrolyte without ( ) and with (dashed curve) a surface active substance. (b) Time dependence of the polarizing voltage. (c) Charging current curve for supporting electrolyte without ( - - ) and with (dashed curve) an addition of a surface active substance.

(Fig. la). This potential is therefore selected as the constant or auxiliary potential in the Kalousek commutator technique. The polarization potential is thus discontinuously changed between the potential of the maximum adsorption (Eocm) and the potential which linearly changes from the desorption region at the positive side of the electrocapillary curve toward the desorption region at the negative side (Fig. lb). The mean charging current of the supporting electrolyte without surfactant, measured on the DME and recorded only during the first part of the cycle, is given as 9 :

i =fAE Co 77 wherefis the frequency, AE the amplitude of the polarization potential, Co the specific capacity, and q=0.51 m2/3t2/3 is the mean surface of the electrode. The charging current of the supporting electrolyte is schematically presented with a full line in Fig. lc. In the presence of surface active substances and in the potential region of adsorption the charging current is determined by the capacity C = Co(1-0)+CIO, where C1 is the specific capacity for organic molecules in the electrode double layer and 0 is the fraction of the covered electrode surface. In the potential region of desorption on both sides of the electrocapillary curve, 0 decreases rapidly to zero. This effect is followed by a sudden change of the charging current. At the potentials outside the adsorption region the charging current coincides with that of the supporting electrolyte. The charging current in the presence of the surface active substance is schematically presented in Fig. lc (dashed line). In order to analyse such curves the linear part of the charging current curve of the surfactant is prolonged toward the desorption regions. The current difference Ai= i' - 7 is measured at a certain potential, where ~ is the current value of the supporting electrolyte and i" the corresponding value on the prolonged linear part. For all concentrations of the given surfactant the current difference should be measured at the same potential, preferably in the desorption region. The value of the wave height At-obtained is proportional to 0. The concentration versus Ai curve is the adsorption isotherm of the investigated surfactant. EXPERIMENTAL

The measurements with the Kalousek commutator technique were performed using the home made Instrument for Characterization of Electrochemical Processes 10 in connection with a Hewlett Packard X-Y recorder model 7035B or strip chart recorder model 7101BM. The current was measured in both half cycles of the polarizing

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A D S O R P T I O N O F O R G A N I C SUBSTANCES A T Hg

voltage and the difference iIl--il was recordedlL The drop time of the dropping mercury electrode was regulated with an electromagnetic detacher. The HMDE was of the Kemula type. Each measurement was made with a new drop, which was formed after the extrusion of a few drops. The potential scan was applied after the electrode had been kept for several seconds at the starting potential. All potentials were referred to the saturated calomel electrode (SCE). All chemicals were of reagent grade. Tri-n-butyl phosphate (TBP) was purified as described previously12. Thymol and sodium lauryl sulphate (Kemika, Zagreb) were used without previous purification. The solutions were prepared with quadruply distilled water, the last two distillations being carried out in a quartz still. Bidistilled mercury was used throughout. Prepurified extra pure nitrogen was used for deaeration. RESULTS

The Kalousek technique was used for the study of the adsorption of tri-n-butyl phosphate (TBP) on the dropping mercury electrode from 0.1 M sodium perchlorate solution. The potential -0.6 V (SCE) was chosen as the auxiliary potential, because it is in the potential region of maximum adsorption and according to previous chronocoulometric measurement, it is very close to the potential of zero charge 13. The commutated curves were recorded only at potentials more negative than the auxiliary potential. The linear relationship between the charging current and the frequency was observed in the range from 4 to 256 Hz for all investigated solutions. The data obtained at 64 Hz and the drop time of 3 s are given for various concentrations of TBP in Fig. 2. When the adsorption equilibrium was not attained within the drop life for lower concentrations of TBP, the measurements were carried out with the HMDE. The curves obtained at 64 Hz are presented in Fig. 3. The charging current curves for various concentrations of TBP were analyzed as described above. With the assumption that the constant and maximum wave

-14 E/V

-12 vs. 5CE

-1.0

-0.8

-0.6

-06 V

-06 V

-06 V

-06 V

-06 V

Fig. 2. Charging current curve of 0.1 M NaC104 and tri-n-butyl phosphate : (1) 0, (2) 3 × 10- 5 (3) 5 x 10 5 (4) 8 × 10 -5, (5) 10 -4 and (6) 3 x 10 .4 M. Drop t i m e = 3 s , f = 6 4 Hz, auxiliary potential= - 0 . 6 V.

66

<

B. COSOVIC, M. BRANICA

3C

2O

o

S

0.5 I

-o.a

-1.2 E/V

I

I

-~.6

IL

--

vs. SCE

I

i

I

2 CONCN. T B P . 1 0 4 / M o I

[-I

3

Fig. 3. Charging current curve of 0.1 M NaC104 and tri-n-butyl phosphate: (1) 0, (2) 2 x 10 -6, (3) 5 x 10 6 (4) 8 × 10 6 (5) 10 -s, (6) 3× 10 - s and 17) 5 x 10 -5 M. Auxiliary potential= - 0 . 6 V , f = 6 4 Hz. Fig. 4. Adsorption isotherm of tri-n-butyl phosphate in 0.1 M NaC10,,: (1) HMDE, (2) DME.

~q

E/V

vs.

SCE

Fig. 5. Charging current curve of 0.1 M NaC104 and thymol : (1) 10 4 (2) 3 x 10 - 4 , (3) 5 x 10- 4, (4) 10- 3 and (5} 1.5 × l0 -3 M. Drop t i m e = 3 s, f = 6 4 Hz, auxiliary potential = - 0 . 6 V.

resulted from the maximum coverage of the electrode (0 = 1) the corresponding degree of electrode coverage (0< 1) for lower concentrations was obtained from the ratio of currents Ai/Aima x. These values plotted against the concentration are presented in Fig. 4. Charging current curves for various concentrations of thymol in aqueous solution of 0.1 M NaC104 obtained with the DME and the HMDE are given in Figs. 5 and 6, respectively. The charging current of thymol does not coincide with that of the supporting electrolyte for potentials more negative than desorption. The wave height of thymol was also measured from the charging current of the supporting electrolyte. The isotherm calculated from the data presented in Fig. 6 is shown in Fig. 7a. Figure 8 shows charging current waves for various concentrations of sodium lauryl sulphate in the same supporting electrolyte, 0.1 M NaC104, obtained with

h e i g h t Aima x

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A D S O R P T I O N OF ORGANIC SUBSTANCES AT Hg

(I

4 3 ,,,¢

0

20

0.5 6

3~~1 121

~ / 61

-0.8

-1.2

I

o--

1.0

30

123

b

/

I I

-~.6

2i

I

2I

CONCN.THYMOL.103/MoI[-I CONCN,Na-LAURYLSULPHATE'I04/Mo[

E/V vs. SCE

Fig. 6. Charging current curve of 0.1 M NaC10¢ and thymol: (1) 0, (2) 7 x 10 -s, (3) 10-", (4) 3 x 10 -4, (5) 7 × 10 -¢ and (6) 10 3 M. Auxiliary potential= - 0 . 6 V, f = 6 4 Hz. Fig. 7. Adsorption isotherm of (a) thymol and (b) sodium lauryl sulphate in 0.1 M NaCIO4; HMDE.

8O

00

4O 20 P -

0.7

-].I

E/V

vs.

I.5

SC E

Fig. 8. Charging current curve of 0.1 M NaCIO¢ and sodium lauryl sulphate: (1) 0, (2) 5 x 10 6 (3) 10 -s, (4) 2 x 10 s, (5) 4 x 10-5 and (6) 8 x 10-5 M. Auxiliary potential = - 0 . 5 V, f = 128 Hz.

H M D E and - 0.5 V as the auxiliary potential. The isotherm calculated on the basis of these data is given in Fig. 7b. DISCUSSION

The Kalousek commutator technique, used in this work for the study of adsorption of organic molecules at the mercury electrode, was found to be a promising method for simple and rapid analysis. The desorption waves can be measured at both sides of the electrocapillary curve. By selecting the desorption wave at potentials more negative than the electrocapillary maximum, the effects interfering at positive potentials can be avoided. Therefore the method can be applied for the investigation of organic substances in solutions containing adsorbable anions or anions which shift the anodic dissolution of mercury toward more negative potentials. The measured charging current is proportional to the amplitude of the polarization potential which amounts to several hundreds of millivolts in the desorption region. Thus, it is not necessary to work at the limit of sensitivity of the instrument and the faradaic current of possible impurities should not interfere. The concentration

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B. C'OSOVI(~, M. BRANICA

dependence of the wave height may serve as the basis for the analytical determination of a given surfactant in the aqueous solution. Various concentration ranges can be studied by selecting the appropriate drop time or stationary electrode as shown in Fig. 4. The sensitivity of the method could be increased for strongly adsorbable species by prolonging the time of waiting at the starting potential, as was reported for tensammetry on the hanging mercury drop electrode 6. However in that case it is necessary to use water and chemicals of very high purity. It should be stressed that the choice of the auxiliary potential influences the charging current curves in the presence of the surface active substance. If the auxiliary potential differs from the potential of zero charge the curve obtained shows a deviation from the corresponding curve of the supporting electrolyte alone in the potential region where the organic molecules do not adsorb. The charging current, in the presence of a surfactant, exceeds that of the supporting electrolyte for auxiliary potentials more negative than potential of zero charge, while for more positive potentials the current values are lower than those of the supporting electrolyte. As was observed for solutions of TBP, this effect depends also on the concentration of the surfactant, being negligible for low concentration. In order to obtain the adsorption isotherm of a given surfactant it is necessary to measure the wave height from the charging current of the supporting electrolyte. However, when the method is used for analytical determination ofa surfactant, the apparent isotherm may be applied. This can be obtained by selecting the auxiliary potential somewhere in the potential region of adsorption and measuring the wave height Ai from the charging current curve of the surfactant. Our further investigation will be extended to the adsorption~tesorption phenomena of mixtures of different surfactants. The problem arising is to distinguish the effects of particular substances in a mixture. This is also of interest for a study of natural aquatic systems, which may contain different surfactants, naturally occurring organic substances, and pollutants. ACKNOWLEDGEMENT

Grateful acknowledgement is made to the Republic Council for Scientific Research of Croatia for the support of this work under Grant No. I/6-72. SUMMARY

The polarographic method of discontinuously changing potential (4-128 Hz) known as the Kalousek commutator technique, was used for the study of adsorptiondesorption phenomena of tri-n-butyl phosphate, thymol and sodium lauryl sulphate on the mercury from aqueous solutions. The dropping mercury electrode and the hanging mercury drop electrode were used. The desorption waves were measured at potentials more negative than the electrocapillary maximum. The corresponding adsorption isotherms were obtained from the concentration dependence of the wave heights. The method is proposed for the analytical determination of surface active substances in dilute solutions. REFERENCES I B. B. Damaskin, O, A. Petrii and V. V. Batrakov, Adsorption of Organic Compounds on Electrodes, Plenum Press, New York, 1971.

ADSORPTION OF ORGANIC SUBSTANCES AT Hg

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2 J. KOta and I. Smoler, 23rd I.S.E. Meeting, Stockholm, 1972. 3 B. Breyer and H. H. Bauer, Alternating Current Polarography and Tensammetry, Interscience, New York, 1963. 4 H. Jehring, J. Electroanal. Chem., 21 (1969) 77. 5 H. Jehring and W. Stolle, Collect. Czech. Chem. Commun., 33 (1968) 1038. 6 H. Jehring and W. Stolle, Collect. Czech. Chem. Commun., 33 (1968) 1670. 7 K. Linhart, Tenside, 9 (1972) 241. 8 T. Zvonari6, V. Zuti6 and M. Branica, X X I I I e Congr~s-Assemblke pldnikre de la C.I.E.S.M., Athenes, 1972. 9 M. Heyrovsk~, Collect. Czech. Chem. Commun., 26 (1961) 3164. 10 J. Radej, D. Konrad, I. Ru~i6 and M. Branica, submitted for publication in J. ElectroanaL Chem. 11 J. Heyrovsk~ and J. Kfitm Principles of Polarography, Czechoslovak Academy of Sciences, Prague, 1965. 12 D. Krznari6, P. Cosovi6 and M. Branica, J. Electroanal. Chem., 33 (1971) 61. 13 D. Krznari6, B. (~osovi6 and M. Branica, to be published in J. Electroanal. Chem.