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Rapid determination of surface-active substances by fast-scan differential-pulse voltammetry
Analytica Chimica Acta, 231 (1990) 233-236 Elsevier Science Publishers B.V.. Amsterdam
233
Short Communication
Rapid determination of surface-active substances by fast-scan differential-pulse voltammetry K.-H. Lubert Academy
**a
of Sciences of the G.D.R., Central Institute of Isotope and Radiation Research, Permoserstrasse 15, DDR-7050 L.eiptig (G.D.R.) M. Schnurrbusch
Academy of Sciences of the G. D. R., Research Institute of Chemical Toxicology, Permoserstrasse 1.5, DDR-7050 Leipzig (G. D. R.) (Received
24th November
1989)
Abstract Fast-scan differential-pulse voltammograms of air-saturated acetate buffer and mercury nitrate solutions are presented. The current-voltage curves obtained are markedly influenced by surface-active substances @AS). The peak potential of the voltammogram shifts to more positive potentials with increasing concentration of the SAS. This effect can be utilized for analytical purposes. Keywords: Surfactants
Surface-active substances (SAS) are adsorbed at mercury electrodes and this effect is the basis of some electroanalytical methods for the determination of SAS in aqueous solutions. First, the suppression of polarographic maxima by SAS is utilized in the so-called “adsorption analysis” (e.g., [1,2]). Second, a.c. polarograms are considerably influenced by SAS [3]. A comprehensive review of the determination of SAS by this technique (tensammetry) was given by Jehring [4]. Third, more recently voltammetric or pulse voltammetric determination of traces of SAS after their prior adsorptive preconcentration at a mercury electrode (adsorptive stripping analysis) has been proposed. These and related electroanalytical methods for the determination of SAS have been critically reviewed recently [5-71. However, these methods have some disadvantages. Adsorption analysis is relatively time A Present G.D.R.
address:
0003-2670/90/$03.50
Gottschedstrasse
3, DDR-7010,
Leipzig,
0 1990 - Elsevier Science
Publishers
consuming, and the detection limits of direct a.c. polarographic methods are mostly in the mg 1-l range. Adsorptive stripping analysis permits the determination of SAS down to the pg 1-l level but involves two or more steps. For these reasons, attempts have been made to develop a sensitive and rapid procedure for the determination of SAS by means of fast-scan differential-pulse (FSDP) voltammetry [8]. In this communication results on the influence of SAS on the FSDP voltammograms in the presence of oxygen are presented and preliminary attempts to utilize this effect for analytical purposes are described. Experimental The voltammograms were recorded with the PA 3 polarographic analyser (Laboratorni Pfistroje, Prague) equipped with an SMDE 1 static mercury drop electrode as the working electrode. An opening time of the needle valve of 160 ms was applied to form the stationary mercury drop. A platinum B.V.
234
K.-H.
wire electrode and a saturated silver/silver chloride electrode served as the auxiliary electrode and reference electrode, respectively. Electrode potentials are referred to the saturated Ag/AgCl electrode. The start potential was + 100 mV and the FSDP voltammograms were recorded with a scan in the negative direction. The amplitude of the negative pulses was 50 mV and the scan rate 50 mV s-i if not stated otherwise. The pulse width in the FSDP mode is 100 ms and the time between two pulses is also 100 ms. Acetate buffer [acetic acid-sodium acetate (1: l), pH 4.61 of different concentrations or mercury nitrate solution (2.5 x lop5 M) were used as the supporting electrolyte. Nonylphenol polyethylene glycol ether (“for tenside tests”, Merck, Darmstadt) was used as a model substance for SAS. All chemicals were of analytical-reagent grade. Triply distilled water was used throughout.
LUBERT
M. SCHNURRBUSCH
PT \
/
AND
-7
l-
I 5
0
P4
I
c
-
500 E (IN
-
V*e
Sata
1ooo
*o/elj
Fig. 2. FSDP voltammograms for different concentrations of air-saturated acetate buffer (pH 4.6): (a) 5 X 10W3;(b) 1 X 10m2; (c) 5 X 10W2 M buffer.
The solutions were before measuring.
I
1 P“
-
I
I
I
500
-1000
-1500
E
(m’J
vs.
aate
b/&cl)
Fig. 1. FSDP voltammograms for 2X10K3 M acetate buffer (1: 1; pH 4.6) in (a) air-saturated and (b) deaerated solution. Scan rate, 20 mV SC’.
bubbled
with
air for 1 min
Results and discussion The current-voltage curve obtained in airsaturated acetate buffer solution (as supporting electrolyte) in the FSDP mode is shown in Fig. la and that for the deaerated solution in Fig. lb. The FSDP voltammograms for different concentrations of acetate buffer (pH 4.6) are shown in Fig. 2. The characteristic shape for air-saturated solutions is a parabola (or a similar shape) for the first part of the curve followed by a sudden change which, at higher scan rates, is connected with a sharp minimum of the current. In the following attention is especially focused on the peak potential (characteristic potential, Er).
RAPID
DETERMINATION
OF SURFACE-ACTIVE
235
SUBSTANCES
-800
r 4
,’
s .
f
b
-400.
3
+\+-
w
-2oot I a
I
I
0
Pg
-1000 Ag/+l)
Fig. 3. FSDP voltammograms of air-saturated solutions: (a) 2x 10e3 M acetate buffer; (b) a+0.5 mg 1-l SAS; (c) a+ 1.0 mg I-’ SAS. Scan rate, 20 mV s-‘.
The influence of traces of SAS on the FSDP voltammograms of the air-saturated buffer solution is shown in Fig. 3. The potential E, shifts to more positive potentials after addition of the SAS.
,
I 100 ~‘9
750
Fig. 5. Dependence of peak potential, E,, on the concentration of added SAS (nonylphenol polyethylene glycol ether). Supporting electrolyte, 2.5 x 10-s M Hg(NO,),, air-saturated.
I
-500 E fmV vso sat-
250 500 Nonylphenolpolyethylensglycolether
I
,
I
1
500
Nonylphenolpolyethyleneglycolether
Fig. 4. Dependence of peak potential, Et,, on the concentration of added SAS (nonylphenol polyethylene glycol ether). Supporting electrolyte, 2 x 10m2 M acetate buffer, air-saturated.
The extent of the shift depends on the concentration of the SAS, and the concentration dependence is observed up to about 800 pg 1-i under the experimental conditions used. This dependence of E, on the concentration of the SAS can be utilized for the determination of SAS by FSDP voltammetry. The concentration dependences of E, for nonylphenol polyethylene glycol ether in air-saturated solutions of 1 X lo-* M acetate buffer and 2.5 X 10e5 M mercury(I1) nitrate are shown in the Figs. 4 and 5, respectively. It can be see that the shift of Ep is a monotonous function of the concentration of nonylphenol polyethylene glycol ether. The reason for the effect observed is not clear. Similar behaviour was observed, e.g., for the “Abbruchpotential” in d.c. polarography by von Stackelberg and Schlitz [9]. This effect was also utilized for the determination of surfactants. Conclusions The concentration dependence observed can be used for the determination of SAS. Analytical procedures based on this effect have the advantages that the procedure is simple and rapid, because the time-consuming deaeration step is not necessary, and the resulting one-step procedure allows the determination of SAS in a sample in about 1 min. A slight disadvantage is the non-linearity of the calibration graph.
K.-H.
236
The sensitivity is better than that of other onestep electroanalytical procedures (direct a.c. polarography, suppression of polarographic maxima, Kalousek’ polarography) for the determination of SAS (see Fig. 5 in [6]). The detection limit is < 100 I-181-l (e.g., see Fig. 4). The reproducibility of the method is G 5% (cf., [lo]). Attempts to determine various SAS by means of the observed effect and its application to natural waters are in progress and will be the subject of a subsequent paper [lo].
LUBERT
AND
M. SCHNURRBUSCH
REFERENCES 1 J. Heyrovsky, Polarographie, Springer, Vienna, 1941, pp. 414-418. 2 I.M. Kolthoff and J.J. Lingane, Polarography, Interscience, New York, London, 2nd ed., 1952, pp. 1844188. 3 B. Breyer and H.H. Bauer, Alternating Current Polarography and Tensammetry, Interscience, New York, London, 1963. 4 H. Jehring, Elektrosorptionsanalyse mit der Wechselstrompolarographie, Akadernie-Verlag, Berlin, 1974. 5 R. Kalvoda, Pure Appl. Chem., 59 (1987) 715. 6 P.M. Bersier and J. Bersier, Analyst, 113 (1988) 3. 7 J. Wang, in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 16, Plenum, New York, 1989, pp. l-88. 8 V. Gajda and K. Ho&, Anal. Chim. Acta, 134 (1982) 219. 9 M. von Stackelberg and H. Schlitz, Kolloid-Z., 105 (1943) 20. 10 M. Schnurrbusch and K.-H. Lubert, in preparation.
233
Short Communication
Rapid determination of surface-active substances by fast-scan differential-pulse voltammetry K.-H. Lubert Academy
**a
of Sciences of the G.D.R., Central Institute of Isotope and Radiation Research, Permoserstrasse 15, DDR-7050 L.eiptig (G.D.R.) M. Schnurrbusch
Academy of Sciences of the G. D. R., Research Institute of Chemical Toxicology, Permoserstrasse 1.5, DDR-7050 Leipzig (G. D. R.) (Received
24th November
1989)
Abstract Fast-scan differential-pulse voltammograms of air-saturated acetate buffer and mercury nitrate solutions are presented. The current-voltage curves obtained are markedly influenced by surface-active substances @AS). The peak potential of the voltammogram shifts to more positive potentials with increasing concentration of the SAS. This effect can be utilized for analytical purposes. Keywords: Surfactants
Surface-active substances (SAS) are adsorbed at mercury electrodes and this effect is the basis of some electroanalytical methods for the determination of SAS in aqueous solutions. First, the suppression of polarographic maxima by SAS is utilized in the so-called “adsorption analysis” (e.g., [1,2]). Second, a.c. polarograms are considerably influenced by SAS [3]. A comprehensive review of the determination of SAS by this technique (tensammetry) was given by Jehring [4]. Third, more recently voltammetric or pulse voltammetric determination of traces of SAS after their prior adsorptive preconcentration at a mercury electrode (adsorptive stripping analysis) has been proposed. These and related electroanalytical methods for the determination of SAS have been critically reviewed recently [5-71. However, these methods have some disadvantages. Adsorption analysis is relatively time A Present G.D.R.
address:
0003-2670/90/$03.50
Gottschedstrasse
3, DDR-7010,
Leipzig,
0 1990 - Elsevier Science
Publishers
consuming, and the detection limits of direct a.c. polarographic methods are mostly in the mg 1-l range. Adsorptive stripping analysis permits the determination of SAS down to the pg 1-l level but involves two or more steps. For these reasons, attempts have been made to develop a sensitive and rapid procedure for the determination of SAS by means of fast-scan differential-pulse (FSDP) voltammetry [8]. In this communication results on the influence of SAS on the FSDP voltammograms in the presence of oxygen are presented and preliminary attempts to utilize this effect for analytical purposes are described. Experimental The voltammograms were recorded with the PA 3 polarographic analyser (Laboratorni Pfistroje, Prague) equipped with an SMDE 1 static mercury drop electrode as the working electrode. An opening time of the needle valve of 160 ms was applied to form the stationary mercury drop. A platinum B.V.
234
K.-H.
wire electrode and a saturated silver/silver chloride electrode served as the auxiliary electrode and reference electrode, respectively. Electrode potentials are referred to the saturated Ag/AgCl electrode. The start potential was + 100 mV and the FSDP voltammograms were recorded with a scan in the negative direction. The amplitude of the negative pulses was 50 mV and the scan rate 50 mV s-i if not stated otherwise. The pulse width in the FSDP mode is 100 ms and the time between two pulses is also 100 ms. Acetate buffer [acetic acid-sodium acetate (1: l), pH 4.61 of different concentrations or mercury nitrate solution (2.5 x lop5 M) were used as the supporting electrolyte. Nonylphenol polyethylene glycol ether (“for tenside tests”, Merck, Darmstadt) was used as a model substance for SAS. All chemicals were of analytical-reagent grade. Triply distilled water was used throughout.
LUBERT
M. SCHNURRBUSCH
PT \
/
AND
-7
l-
I 5
0
P4
I
c
-
500 E (IN
-
V*e
Sata
1ooo
*o/elj
Fig. 2. FSDP voltammograms for different concentrations of air-saturated acetate buffer (pH 4.6): (a) 5 X 10W3;(b) 1 X 10m2; (c) 5 X 10W2 M buffer.
The solutions were before measuring.
I
1 P“
-
I
I
I
500
-1000
-1500
E
(m’J
vs.
aate
b/&cl)
Fig. 1. FSDP voltammograms for 2X10K3 M acetate buffer (1: 1; pH 4.6) in (a) air-saturated and (b) deaerated solution. Scan rate, 20 mV SC’.
bubbled
with
air for 1 min
Results and discussion The current-voltage curve obtained in airsaturated acetate buffer solution (as supporting electrolyte) in the FSDP mode is shown in Fig. la and that for the deaerated solution in Fig. lb. The FSDP voltammograms for different concentrations of acetate buffer (pH 4.6) are shown in Fig. 2. The characteristic shape for air-saturated solutions is a parabola (or a similar shape) for the first part of the curve followed by a sudden change which, at higher scan rates, is connected with a sharp minimum of the current. In the following attention is especially focused on the peak potential (characteristic potential, Er).
RAPID
DETERMINATION
OF SURFACE-ACTIVE
235
SUBSTANCES
-800
r 4
,’
s .
f
b
-400.
3
+\+-
w
-2oot I a
I
I
0
Pg
-1000 Ag/+l)
Fig. 3. FSDP voltammograms of air-saturated solutions: (a) 2x 10e3 M acetate buffer; (b) a+0.5 mg 1-l SAS; (c) a+ 1.0 mg I-’ SAS. Scan rate, 20 mV s-‘.
The influence of traces of SAS on the FSDP voltammograms of the air-saturated buffer solution is shown in Fig. 3. The potential E, shifts to more positive potentials after addition of the SAS.
,
I 100 ~‘9
750
Fig. 5. Dependence of peak potential, E,, on the concentration of added SAS (nonylphenol polyethylene glycol ether). Supporting electrolyte, 2.5 x 10-s M Hg(NO,),, air-saturated.
I
-500 E fmV vso sat-
250 500 Nonylphenolpolyethylensglycolether
I
,
I
1
500
Nonylphenolpolyethyleneglycolether
Fig. 4. Dependence of peak potential, Et,, on the concentration of added SAS (nonylphenol polyethylene glycol ether). Supporting electrolyte, 2 x 10m2 M acetate buffer, air-saturated.
The extent of the shift depends on the concentration of the SAS, and the concentration dependence is observed up to about 800 pg 1-i under the experimental conditions used. This dependence of E, on the concentration of the SAS can be utilized for the determination of SAS by FSDP voltammetry. The concentration dependences of E, for nonylphenol polyethylene glycol ether in air-saturated solutions of 1 X lo-* M acetate buffer and 2.5 X 10e5 M mercury(I1) nitrate are shown in the Figs. 4 and 5, respectively. It can be see that the shift of Ep is a monotonous function of the concentration of nonylphenol polyethylene glycol ether. The reason for the effect observed is not clear. Similar behaviour was observed, e.g., for the “Abbruchpotential” in d.c. polarography by von Stackelberg and Schlitz [9]. This effect was also utilized for the determination of surfactants. Conclusions The concentration dependence observed can be used for the determination of SAS. Analytical procedures based on this effect have the advantages that the procedure is simple and rapid, because the time-consuming deaeration step is not necessary, and the resulting one-step procedure allows the determination of SAS in a sample in about 1 min. A slight disadvantage is the non-linearity of the calibration graph.
K.-H.
236
The sensitivity is better than that of other onestep electroanalytical procedures (direct a.c. polarography, suppression of polarographic maxima, Kalousek’ polarography) for the determination of SAS (see Fig. 5 in [6]). The detection limit is < 100 I-181-l (e.g., see Fig. 4). The reproducibility of the method is G 5% (cf., [lo]). Attempts to determine various SAS by means of the observed effect and its application to natural waters are in progress and will be the subject of a subsequent paper [lo].
LUBERT
AND
M. SCHNURRBUSCH
REFERENCES 1 J. Heyrovsky, Polarographie, Springer, Vienna, 1941, pp. 414-418. 2 I.M. Kolthoff and J.J. Lingane, Polarography, Interscience, New York, London, 2nd ed., 1952, pp. 1844188. 3 B. Breyer and H.H. Bauer, Alternating Current Polarography and Tensammetry, Interscience, New York, London, 1963. 4 H. Jehring, Elektrosorptionsanalyse mit der Wechselstrompolarographie, Akadernie-Verlag, Berlin, 1974. 5 R. Kalvoda, Pure Appl. Chem., 59 (1987) 715. 6 P.M. Bersier and J. Bersier, Analyst, 113 (1988) 3. 7 J. Wang, in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 16, Plenum, New York, 1989, pp. l-88. 8 V. Gajda and K. Ho&, Anal. Chim. Acta, 134 (1982) 219. 9 M. von Stackelberg and H. Schlitz, Kolloid-Z., 105 (1943) 20. 10 M. Schnurrbusch and K.-H. Lubert, in preparation.