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Estimation of surfactant activity of polluted seawater by kalousek commutator technique
J. Electroanal. Chem., 68 (1976) 75--83
75
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
ESTIMATION OF SURFACTANT ACTIVITY OF POLLUTED SEAWATER BY KALOUSEK COMMUTATOR TECHNIQUE*
ZLATICA KOZARAC, BO~ENA COSOVIC and MARKO BRANICA
Center for Marine Research, "Rudjer Bo$kovid" Institute, Zagreb, Croatia (Yugoslavia) (Received 1st July 1975)
ABSTRACT The polarographic method of discontinuously changing potential, known as the Kalousek commutator technique, was applied to the estimation of surfactant activity of seawater samples from polluted area of a harbour. Adsorption isotherms of different detergents, lecithin and lauric acid at the HMDE have been used as calibration curves. The method is recommended for direct and rapid determination of the degree of seawater pollution by organic surface active substances.
INTRODUCTION
Dissolved organic matter in seawater comprises a complex mixture of naturally occurring substances, products of metabolism, degradation of organisms and artificial compounds brought into the sea by man's activities. The average concentration of dissolved organic matter in the open sea is in the range from 0.3 to 3 mg C 1-1 and up to 20 mg C 1-1 in coastal waters because of increased primary production and of pollution [ 1]. Various types of organic compounds like carbohydrates, proteins, amino acids, lipids, fatty acids, humic acids, biologically active substances, etc. are present in unpolluted seawater, though many of them in trace amounts [2]. Some of these substances are surface active substances and therefore concentrated in the upper layer i.e. at the air/water interface. Garrett [3] and Jarvis et al. [4] found that fatty acids, alcohols and esters represent the main components of the monomolecular layer taken from the surface of seawater by the screen technique [ 5]. Some authors presume the foaming properties of seawater to be connected with the presence of surface active substances (proteins, lipids and fatty acids) which, as they suppose, are of planktonic origin
[6]. It is very probable that synthetic detergents represent the most significant
* Taken in part from the M.Sc. thesis of Z. Kozarac in graduate study of " O c e a n o l o g y " at the University of Zagreb.
76 surface active compounds among various organic pollutants entering the sea. The main part of all detergents (about 80%) passing into natural waters are anionic. The general tendency of producers in last 15 years has been a changeover to readily biodegradable types, which has resulted in diminishing detergent concentration in natural waters. An example is the Illinois River, Ill., U.S.A., in which the anionic surfactants c o n t e n t had dropped from 0.5 mg 1-1 to less than 0.01 mg 1-1 from 1965--1968 [7]. At the same time the c o n t e n t of detergents in some rivers in West Germany [8,9] is still about 0.3 mg 1-1. Due to dilution the a m o u n t of surfactants in seawater is mainly lower than in rivers. Very high values can be observed in close vicinity of sewage outlets: in the polluted area of Marseille [10] 0.01 to 2.0 mg of anionic detergents per liter of seawater has been found. High values of surfactant activity have also been observed for samples of seawater from coastal stations in the North Adriatic [11,12]. The toxic effect of detergents on aquatic organisms is due to chemical a n d physical action. Surfactants accumulate on the gill epithelium and affect interfaces where exchange of gases, water and ions takes place. They also cause spasms, loss of equilibrium and affect swimming ability of fishes. According to Swedmark et al. [13] and Knauf [9] lethal and sublethal effects occur at concentrations over l m g 1-1. In the case of prolonged exposure time considerable effects of detergents upon fishes have been observed even at 0.5 mg1-1 [14]. The Kalousek c o m m u t a t o r technique (see previous paper [15] ) can be used for the determination of surface active substances in aqueous solutions by measuring adsorption effects of organic molecules at the mercury electrode surface. Adsorption effects of various types of organic substances, either naturally occurring in seawater or added to the sample, have been extensively studied. The aim of this work was to investigate applicability of the mentioned technique for estimation of the degree of seawater pollution by surface active substances. EXPERIMENTAL The measurements with the Kalousek c o m m u t a t o r technique were performed as described previously [ 15,16]. The hanging mercury drop electrode was used. Each measurement was made with a new drop, which was formed after the extrusion of a few drops. The potential scan was applied after accumulation of surface~active substances at the starting potential during 5 min. All potentials were referred to the saturated calomel electrode (SCE). The measurements have been carried out at room temperature (20 ° C) in the cell open to air, w i t h o u t any deaeration of sample. Sodium lauryl sulphate, lauric acid, N-cetyl-N,N,N-trimethyl a m m o n i u m bromide, sodium chloride (all " K e m i k a " , Zagreb) and hydrogencarbonate ("Merck", Darmstadt) were of reagent grade. Lecithin (BDH Chemicals Ltd.,
77
Poole) was used without prepurification. Triton-X-100 was obtained by courtesy of " R o h m and Haas", Milano. Seawater samples were taken from the depth of 0.5 m at several stations in the Rovinj harbour at the West Istrian coast. "Clean seawater", taken at 10 m depth and 1 N mile off the West Istrian coast, was used for preparation of standard surfactant solutions. RESULTS
Sodium lauryl sulphate (SLS) has been chosen as representative anionic detergent. Typical charging current--potential curves for 1.0 mg of SLS per
7 6
c
4
3
2
I
2 1
It
E/V vs. SCE Fig. 1. Charging current curve of " c l e a n s e a w a t e r " (1) and (a) SLS, 1 mg 1-1 with accum u l a t i o n of (2) 1, (3) 3 and (4) 5 min at HMDE; (b) TAB, (2) 0.1, (3) 0.3 and (4) 0.5 mg l~l; a c c u m u l a t i o n 5 min; (c) lecithin, (2) 3, (3) 6, (4) 7 and (5) 10 mg l - - l ; a c c u m u l a tion 5 min. Auxiliary potential = --0.6 V vs. SCE, f= 64 Hz.
78
liter of seawater, obtained with different times of accumulation of detergent at the electrode surface, are presented in Fig. la. The accumulation of 5 min was chosen as most convenient for increasing the sensitivity of the method. The height of the desorption wave Ai was measured at the potential --1.4 V. Typical charging current curves for various concentrations of cationic detergent N-cetyl-N,N,N-trimethyl a m m o n i u m bromide (TAB) in seawater are presented in Fig. lb. Adsorption of lipids and fatty acids, which are organic substances of biological origin, has also been studied, and corresponding curves for various con. centrations of lecithin dissolved in "clean seawater" are presented in Fig. lc.
6O
5O
~0
3O
b
2o
lO
i
i
Fig. 2. Charging current curve of "clean seawater" and T-X-100: (1) 0, (2) 0.1, (3) 0.3, (4) 0.5, (5) 0.9 and (6) 1.5 mg 1-1. Accumulation 5 rain, auxiliary potential = --0.6 V, f= 64 Hz. Measured separately at positive (a) and negative (b) potentials.
79 Adsorption of nonionic detergent Triton-X-100 on the m e r c u r y electrode p r o d u ced a lowering of charging current in the broad potential region and appearance of the well defined desorption wave at very negative potentials. Current--potential curves for rising concentrations of Triton-X-100 in seawater, obtained f r om positive potentials (dissolution of mercury) up to desorption of detergent at negative potentials, are given in Fig. 2. Measuring of charging current was p e r f o r m e d independently, starting from potential E = --0.6 V towards more positive values (a), as well as from --0.6 V towards more negative potentials (b). Because of precipitation of alkaline earth soaps the solubility of fat t y acids in seawater is limited (< 0.5 mg l - 1 ). Solutions of increasing concentrations of lauric acid were prepared in synthetic seawater (0.6 M NaC1, 1.5 X 10 -2 M NaHCO3, pH = 8) and were measured in the same way as described for detergents and lecithin. For quantitative estimation of surface active substances in seawater the adsorption isotherms were constructed and used as calibration curves. The Ai values for various types of surface active substances were det erm i ned graphically as shown in Fig. la, and pl ot t ed against the corresponding concentrations in seawater. Adsorption isotherms for detergents, lecithin and lauric acid are given in Fig. 3. ~ " The charging current and the Ai value measured by the Kalousek commu-
O.-O~O
o/d "°'~
A/A
2O <
/
./
10
J 0.1
t 1.0
I 10
CONCENTRATION / rng 1-1 Fig. 3. Adsorption isotherm of (1) SLS, (2) TAB, (3) T-X-IO0 and (5) lecithin in sea-
water and of (4) laurie acid in synthetic seawater. Accumulation 5 rain, auxiliary potential =--0.6 V, f= 64 Hz.
80
60
50
/.0
"~-~ 30
1
20
10
-0.8
I
I
-1.2
-1.6
.,,,_
E / V vs. SCE
Fig. 4. Charging current curves of "clean seawater" (1) and of polluted seawater from stations: (2) R4, (3) R1, (4) R 2 and (5) R 3. Accumulation 5 min, auxiliary potential = --0.6 V, f = 64 Hz. TABLE 1 Surfactant activity of seawater samples from the West Istrian Coast determined by Kalousek commutator technique (in equiv. SLS in mg 1-1) and spectrophotometrically [17 ] (in anionic detergents equiv. SLS in mg 1-1 ) Sampling data
Station
July July July July July
A1 R1 R2 R3 R4
0 0.21 0.77 ~>0.50 <0.10
0 0.022 0.620 0.028 0.022
A1 R1 R2 R3 R4
0 0 0.26 0.42 0.15
0 0 0.060 0.012 0.008
Oct. 12, 1973
R2
0.15
0.034
Oct. 27, 1973
R2
0.45
0.300
Sept. Sept. Sept. Sept. Sept.
3, 3, 3, 3, 3,
1973 1973 1973 1973 1973
14, 14, 14, 14, 14,
1973 1973 1973 1973 1973
Surfactant activity equiv. SLS/mg 1-1
Anionic detergents equiv. SLS/mg 1-1
81
tator technique are linearly dependent on the electrode surface area only if all other variables, such as: composition of supporting electrolyte, t y p e and concentration of surfactant, starting potential and frequency of discontinuously changed potential are kept constant. The Ai values have to be corrected for each working electrode for the constant value applied in the calibration curve. All isotherms in Fig. 3 are corrected to the same 0.026 cm 2 electrode surface area. Seawater samples from the Harbour were analysed for content of surface active substances by using the described technique. Typical charging current curves are presented in Fig. 4. Surfactant activity of samples evaluated as equivalent amount of anionic detergent sodium lauryl sulphate is given in Table 1. In the last column corresponding values for anionic detergents (equivalent to SLS) obtained in parallel measurements by spectrophotometric methylene blue method [17] are given. DISCUSSION
For measurement of organic substances in samples of seawater it was necessary to improve the Kalousek c o m m u t a t o r technique, which was described earlier [15]. The sensitivity of the method is increased (<1 ppm equivalent to detergents) with 5 rain accumulation of surfactant at the potential of maximum adsorption at the hanging mercury drop electrode, and better r e p r o d u c i b i l i t y and accuracy is achieved by elimination of sample deaeration. It is very probable that the deaeration procedure causes an impoverishment of sample on surface active substances. The presence of oxygen at the applied frequency of 64 Hz does not influence either the shape or the current--potential curve nor the measured current depression itself. Measured adsorption effects at the mercury electrode were observed in seawater samples from polluted harbour area, as shown in Fig. 3. The concentration of surface active substances in samples was expressed as equivalent to anionic detergent sodium lauryl sulphate and compared with values obtained by spectrophotometric determination of anionic detergents with methylene blue [17] (Table 1). The best correspondence of results by two m e t h o d s and the highest values for anionic detergents have been observed for samples from the station R2, which is placed in close vicinity of the city sewage outlet. Direct electroanalytical determination is influenced b y the total c o n t e n t of surface active substances in seawater sample. In addition to anionic detergents, which represent the main part of all detergents, the total a m o u n t of surface active substances in polluted seawater comprises also nonionics and other chemicals, as well as biologically active substances. The values obtained are therefore in principle higher than the corresponding content of anionic detergents. Very high values of surfactant activity obtained by the Kalousek commutator technique in relation to the values of anionic detergents (spectrophotometrically) are observed for samples from the station R3, which is placed in
82 vicinity of the fish factory outlet. The shape of the charging current curve is similar to those for lecithin and cationic detergent, which should be ascribed to the presence of a large a m o u n t of fish degradation products, which are mainly cationic charged surface active substances. In unpolluted seawater the quantity of naturally occurring substances is in most cases under the detection limit of the method described. All samples of seawater from an unpolluted area (offshore as well as some nearshore stations) showed no measurable effects. However, it was already reported that the content of lipids in coastal seawater can rise up to values of 9 mg l - 1 due to high biological activity [18], which could produce measurable effects. These effects may be amplified by contribution of other organic substances, such as proteins, polysaccharides, humic acids, etc., present in coastal water in significant amounts. Depression of charging current of seawater samples by the m e t h o d described indicates either pollution by detergents (conc. of ) 0 . 1 ppm) and/or presence of very large amounts of organic matter. In extreme cases, due to increased oxygen demand, the highest content of organic matter represents " p o l l u t i o n " of seawater too. The procedure described is a relatively fast m e t h o d for estimation of the degree of seawater pollution by surface active substances and can be recommended for application in field observations. It could be expected that, besides detergents, dissolved hydrocarbons from mineral oils also increase the surfactant activity of seawater [ 19]. In areas very polluted by oil seawater samples also show the influence on the surface characteristics of the mercury electrode. The aim of further studies is to estimate the solubility of commercial petroleum fractions in seawater and their effects on the results of surface activity. Taking into account that samples of natural and polluted seawater contain a mixture of various organic substances, it is of great interest to study adsorption effects of various mixtures at their very low concentrations on the mercury electrode. ACKNOWLEDGEMENTS Grateful acknowledgement is made to the Republic Council for Scientific Research of Croatia and to the National Bureau of Standards, Washington, D.C., U.S.A., for the support of this work under Grant NBS/IG/-191/JF. REFERENCES i J.P. Riley and R. Chester, Introduction to Marine Chemistry, Academic Press, London and New York, 1971. 2 E.K. Duursma in J.P. Riley and G. Skirrow (Eds.), Chemical Oceanography, Academic Press, London and New York, 1965. 3 W.D. Garrett, Deep-Sea Res., 14 (1967) 221. 4 N.L. Jarvis, W.D. Garrett, M.A. Scheiman and C.O. Timmons, Limnol. Oceanogr., 12 (1967) 88.
83 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
W.D. Garrett, Limnol. Oceanogr., 10 (1965) 602. W.B. Wilson and A. Collier, J. Marine Res., 30 (1972) 15. W.T. Sullivan and R.D. Swisher, Env. Sci. Tech., 3 (1969) 481. H.J. Heinz, paper presented June 6, 1973 at the Japanese Detergent Association, Osaka. W. Knauf, Tenside, 10 (1973) 251. A. Arnoux and F. Carnelle, in Marine Pollution and Sea Life, FAO, Fishing News (Books) Ltd., Enfitand, 1972. T. Zvonarid, V. Zutid and M. Branica, Thalassia Jugoslavica, 9 (1973) 65. T. Zvonarid, Z. Kozarac, V. ~utid, B. (~osovid and M. Branica, XXIVe Congr~sAssembl~e pl~ni~re de la C.I.E.S.M., Monaco, 6--14 d~cembre, 1974. M. Swedmark, B. Braaten, E. Emanuelsson and A. Granmo, Marine Biol., 9 (1971) 183. J.E. Bardach, M. Fujiya and A. Holl, Science, 148 (1965) 1605. B. (~osovid and M. Branica, J. Electroanal. Chem., 46 (1973) 63. J. Radej, I. Ru~tid, D. Konrad and M. Branica, J. Electroanal. Chem., 46 (1973) 261. Z. Kozarac, B. (~osovid and M. Branica, Marine Sci. Commun., 1, No. 2 (1975) in press L.M. Jeffrey, J. Amer. Oil Chem. Soc., 43 (1966) 211. Report of the joint group of experts on the scientific aspects of marine pollution (GESAMP) held at FAO headquarters, Rome, 22--27 February 1971.
75
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
ESTIMATION OF SURFACTANT ACTIVITY OF POLLUTED SEAWATER BY KALOUSEK COMMUTATOR TECHNIQUE*
ZLATICA KOZARAC, BO~ENA COSOVIC and MARKO BRANICA
Center for Marine Research, "Rudjer Bo$kovid" Institute, Zagreb, Croatia (Yugoslavia) (Received 1st July 1975)
ABSTRACT The polarographic method of discontinuously changing potential, known as the Kalousek commutator technique, was applied to the estimation of surfactant activity of seawater samples from polluted area of a harbour. Adsorption isotherms of different detergents, lecithin and lauric acid at the HMDE have been used as calibration curves. The method is recommended for direct and rapid determination of the degree of seawater pollution by organic surface active substances.
INTRODUCTION
Dissolved organic matter in seawater comprises a complex mixture of naturally occurring substances, products of metabolism, degradation of organisms and artificial compounds brought into the sea by man's activities. The average concentration of dissolved organic matter in the open sea is in the range from 0.3 to 3 mg C 1-1 and up to 20 mg C 1-1 in coastal waters because of increased primary production and of pollution [ 1]. Various types of organic compounds like carbohydrates, proteins, amino acids, lipids, fatty acids, humic acids, biologically active substances, etc. are present in unpolluted seawater, though many of them in trace amounts [2]. Some of these substances are surface active substances and therefore concentrated in the upper layer i.e. at the air/water interface. Garrett [3] and Jarvis et al. [4] found that fatty acids, alcohols and esters represent the main components of the monomolecular layer taken from the surface of seawater by the screen technique [ 5]. Some authors presume the foaming properties of seawater to be connected with the presence of surface active substances (proteins, lipids and fatty acids) which, as they suppose, are of planktonic origin
[6]. It is very probable that synthetic detergents represent the most significant
* Taken in part from the M.Sc. thesis of Z. Kozarac in graduate study of " O c e a n o l o g y " at the University of Zagreb.
76 surface active compounds among various organic pollutants entering the sea. The main part of all detergents (about 80%) passing into natural waters are anionic. The general tendency of producers in last 15 years has been a changeover to readily biodegradable types, which has resulted in diminishing detergent concentration in natural waters. An example is the Illinois River, Ill., U.S.A., in which the anionic surfactants c o n t e n t had dropped from 0.5 mg 1-1 to less than 0.01 mg 1-1 from 1965--1968 [7]. At the same time the c o n t e n t of detergents in some rivers in West Germany [8,9] is still about 0.3 mg 1-1. Due to dilution the a m o u n t of surfactants in seawater is mainly lower than in rivers. Very high values can be observed in close vicinity of sewage outlets: in the polluted area of Marseille [10] 0.01 to 2.0 mg of anionic detergents per liter of seawater has been found. High values of surfactant activity have also been observed for samples of seawater from coastal stations in the North Adriatic [11,12]. The toxic effect of detergents on aquatic organisms is due to chemical a n d physical action. Surfactants accumulate on the gill epithelium and affect interfaces where exchange of gases, water and ions takes place. They also cause spasms, loss of equilibrium and affect swimming ability of fishes. According to Swedmark et al. [13] and Knauf [9] lethal and sublethal effects occur at concentrations over l m g 1-1. In the case of prolonged exposure time considerable effects of detergents upon fishes have been observed even at 0.5 mg1-1 [14]. The Kalousek c o m m u t a t o r technique (see previous paper [15] ) can be used for the determination of surface active substances in aqueous solutions by measuring adsorption effects of organic molecules at the mercury electrode surface. Adsorption effects of various types of organic substances, either naturally occurring in seawater or added to the sample, have been extensively studied. The aim of this work was to investigate applicability of the mentioned technique for estimation of the degree of seawater pollution by surface active substances. EXPERIMENTAL The measurements with the Kalousek c o m m u t a t o r technique were performed as described previously [ 15,16]. The hanging mercury drop electrode was used. Each measurement was made with a new drop, which was formed after the extrusion of a few drops. The potential scan was applied after accumulation of surface~active substances at the starting potential during 5 min. All potentials were referred to the saturated calomel electrode (SCE). The measurements have been carried out at room temperature (20 ° C) in the cell open to air, w i t h o u t any deaeration of sample. Sodium lauryl sulphate, lauric acid, N-cetyl-N,N,N-trimethyl a m m o n i u m bromide, sodium chloride (all " K e m i k a " , Zagreb) and hydrogencarbonate ("Merck", Darmstadt) were of reagent grade. Lecithin (BDH Chemicals Ltd.,
77
Poole) was used without prepurification. Triton-X-100 was obtained by courtesy of " R o h m and Haas", Milano. Seawater samples were taken from the depth of 0.5 m at several stations in the Rovinj harbour at the West Istrian coast. "Clean seawater", taken at 10 m depth and 1 N mile off the West Istrian coast, was used for preparation of standard surfactant solutions. RESULTS
Sodium lauryl sulphate (SLS) has been chosen as representative anionic detergent. Typical charging current--potential curves for 1.0 mg of SLS per
7 6
c
4
3
2
I
2 1
It
E/V vs. SCE Fig. 1. Charging current curve of " c l e a n s e a w a t e r " (1) and (a) SLS, 1 mg 1-1 with accum u l a t i o n of (2) 1, (3) 3 and (4) 5 min at HMDE; (b) TAB, (2) 0.1, (3) 0.3 and (4) 0.5 mg l~l; a c c u m u l a t i o n 5 min; (c) lecithin, (2) 3, (3) 6, (4) 7 and (5) 10 mg l - - l ; a c c u m u l a tion 5 min. Auxiliary potential = --0.6 V vs. SCE, f= 64 Hz.
78
liter of seawater, obtained with different times of accumulation of detergent at the electrode surface, are presented in Fig. la. The accumulation of 5 min was chosen as most convenient for increasing the sensitivity of the method. The height of the desorption wave Ai was measured at the potential --1.4 V. Typical charging current curves for various concentrations of cationic detergent N-cetyl-N,N,N-trimethyl a m m o n i u m bromide (TAB) in seawater are presented in Fig. lb. Adsorption of lipids and fatty acids, which are organic substances of biological origin, has also been studied, and corresponding curves for various con. centrations of lecithin dissolved in "clean seawater" are presented in Fig. lc.
6O
5O
~0
3O
b
2o
lO
i
i
Fig. 2. Charging current curve of "clean seawater" and T-X-100: (1) 0, (2) 0.1, (3) 0.3, (4) 0.5, (5) 0.9 and (6) 1.5 mg 1-1. Accumulation 5 rain, auxiliary potential = --0.6 V, f= 64 Hz. Measured separately at positive (a) and negative (b) potentials.
79 Adsorption of nonionic detergent Triton-X-100 on the m e r c u r y electrode p r o d u ced a lowering of charging current in the broad potential region and appearance of the well defined desorption wave at very negative potentials. Current--potential curves for rising concentrations of Triton-X-100 in seawater, obtained f r om positive potentials (dissolution of mercury) up to desorption of detergent at negative potentials, are given in Fig. 2. Measuring of charging current was p e r f o r m e d independently, starting from potential E = --0.6 V towards more positive values (a), as well as from --0.6 V towards more negative potentials (b). Because of precipitation of alkaline earth soaps the solubility of fat t y acids in seawater is limited (< 0.5 mg l - 1 ). Solutions of increasing concentrations of lauric acid were prepared in synthetic seawater (0.6 M NaC1, 1.5 X 10 -2 M NaHCO3, pH = 8) and were measured in the same way as described for detergents and lecithin. For quantitative estimation of surface active substances in seawater the adsorption isotherms were constructed and used as calibration curves. The Ai values for various types of surface active substances were det erm i ned graphically as shown in Fig. la, and pl ot t ed against the corresponding concentrations in seawater. Adsorption isotherms for detergents, lecithin and lauric acid are given in Fig. 3. ~ " The charging current and the Ai value measured by the Kalousek commu-
O.-O~O
o/d "°'~
A/A
2O <
/
./
10
J 0.1
t 1.0
I 10
CONCENTRATION / rng 1-1 Fig. 3. Adsorption isotherm of (1) SLS, (2) TAB, (3) T-X-IO0 and (5) lecithin in sea-
water and of (4) laurie acid in synthetic seawater. Accumulation 5 rain, auxiliary potential =--0.6 V, f= 64 Hz.
80
60
50
/.0
"~-~ 30
1
20
10
-0.8
I
I
-1.2
-1.6
.,,,_
E / V vs. SCE
Fig. 4. Charging current curves of "clean seawater" (1) and of polluted seawater from stations: (2) R4, (3) R1, (4) R 2 and (5) R 3. Accumulation 5 min, auxiliary potential = --0.6 V, f = 64 Hz. TABLE 1 Surfactant activity of seawater samples from the West Istrian Coast determined by Kalousek commutator technique (in equiv. SLS in mg 1-1) and spectrophotometrically [17 ] (in anionic detergents equiv. SLS in mg 1-1 ) Sampling data
Station
July July July July July
A1 R1 R2 R3 R4
0 0.21 0.77 ~>0.50 <0.10
0 0.022 0.620 0.028 0.022
A1 R1 R2 R3 R4
0 0 0.26 0.42 0.15
0 0 0.060 0.012 0.008
Oct. 12, 1973
R2
0.15
0.034
Oct. 27, 1973
R2
0.45
0.300
Sept. Sept. Sept. Sept. Sept.
3, 3, 3, 3, 3,
1973 1973 1973 1973 1973
14, 14, 14, 14, 14,
1973 1973 1973 1973 1973
Surfactant activity equiv. SLS/mg 1-1
Anionic detergents equiv. SLS/mg 1-1
81
tator technique are linearly dependent on the electrode surface area only if all other variables, such as: composition of supporting electrolyte, t y p e and concentration of surfactant, starting potential and frequency of discontinuously changed potential are kept constant. The Ai values have to be corrected for each working electrode for the constant value applied in the calibration curve. All isotherms in Fig. 3 are corrected to the same 0.026 cm 2 electrode surface area. Seawater samples from the Harbour were analysed for content of surface active substances by using the described technique. Typical charging current curves are presented in Fig. 4. Surfactant activity of samples evaluated as equivalent amount of anionic detergent sodium lauryl sulphate is given in Table 1. In the last column corresponding values for anionic detergents (equivalent to SLS) obtained in parallel measurements by spectrophotometric methylene blue method [17] are given. DISCUSSION
For measurement of organic substances in samples of seawater it was necessary to improve the Kalousek c o m m u t a t o r technique, which was described earlier [15]. The sensitivity of the method is increased (<1 ppm equivalent to detergents) with 5 rain accumulation of surfactant at the potential of maximum adsorption at the hanging mercury drop electrode, and better r e p r o d u c i b i l i t y and accuracy is achieved by elimination of sample deaeration. It is very probable that the deaeration procedure causes an impoverishment of sample on surface active substances. The presence of oxygen at the applied frequency of 64 Hz does not influence either the shape or the current--potential curve nor the measured current depression itself. Measured adsorption effects at the mercury electrode were observed in seawater samples from polluted harbour area, as shown in Fig. 3. The concentration of surface active substances in samples was expressed as equivalent to anionic detergent sodium lauryl sulphate and compared with values obtained by spectrophotometric determination of anionic detergents with methylene blue [17] (Table 1). The best correspondence of results by two m e t h o d s and the highest values for anionic detergents have been observed for samples from the station R2, which is placed in close vicinity of the city sewage outlet. Direct electroanalytical determination is influenced b y the total c o n t e n t of surface active substances in seawater sample. In addition to anionic detergents, which represent the main part of all detergents, the total a m o u n t of surface active substances in polluted seawater comprises also nonionics and other chemicals, as well as biologically active substances. The values obtained are therefore in principle higher than the corresponding content of anionic detergents. Very high values of surfactant activity obtained by the Kalousek commutator technique in relation to the values of anionic detergents (spectrophotometrically) are observed for samples from the station R3, which is placed in
82 vicinity of the fish factory outlet. The shape of the charging current curve is similar to those for lecithin and cationic detergent, which should be ascribed to the presence of a large a m o u n t of fish degradation products, which are mainly cationic charged surface active substances. In unpolluted seawater the quantity of naturally occurring substances is in most cases under the detection limit of the method described. All samples of seawater from an unpolluted area (offshore as well as some nearshore stations) showed no measurable effects. However, it was already reported that the content of lipids in coastal seawater can rise up to values of 9 mg l - 1 due to high biological activity [18], which could produce measurable effects. These effects may be amplified by contribution of other organic substances, such as proteins, polysaccharides, humic acids, etc., present in coastal water in significant amounts. Depression of charging current of seawater samples by the m e t h o d described indicates either pollution by detergents (conc. of ) 0 . 1 ppm) and/or presence of very large amounts of organic matter. In extreme cases, due to increased oxygen demand, the highest content of organic matter represents " p o l l u t i o n " of seawater too. The procedure described is a relatively fast m e t h o d for estimation of the degree of seawater pollution by surface active substances and can be recommended for application in field observations. It could be expected that, besides detergents, dissolved hydrocarbons from mineral oils also increase the surfactant activity of seawater [ 19]. In areas very polluted by oil seawater samples also show the influence on the surface characteristics of the mercury electrode. The aim of further studies is to estimate the solubility of commercial petroleum fractions in seawater and their effects on the results of surface activity. Taking into account that samples of natural and polluted seawater contain a mixture of various organic substances, it is of great interest to study adsorption effects of various mixtures at their very low concentrations on the mercury electrode. ACKNOWLEDGEMENTS Grateful acknowledgement is made to the Republic Council for Scientific Research of Croatia and to the National Bureau of Standards, Washington, D.C., U.S.A., for the support of this work under Grant NBS/IG/-191/JF. REFERENCES i J.P. Riley and R. Chester, Introduction to Marine Chemistry, Academic Press, London and New York, 1971. 2 E.K. Duursma in J.P. Riley and G. Skirrow (Eds.), Chemical Oceanography, Academic Press, London and New York, 1965. 3 W.D. Garrett, Deep-Sea Res., 14 (1967) 221. 4 N.L. Jarvis, W.D. Garrett, M.A. Scheiman and C.O. Timmons, Limnol. Oceanogr., 12 (1967) 88.
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