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Determination of traces of surfactants in distilled, potable and untreated waters and in supporting electrolytes by tensammetry with accumulation on the HMDE
J. Electroanal. Chem., 127 (1981) 241-253
241
Elsevier Sequoia S.A., Lausanne--Printed in The Netherlands
DETERMINATION OF TRACES OF SURFACTANTS IN DISTILLED, POTABLE AND UNTREATED WATERS AND IN SUPPORTING ELECTROLYTES BY TENSAMMETRY WITH ACCUMULATION ON THE HMDE
EWA BEDNARKIEWICZ, MIKOLAJ DONTEN and ZENON KUBLIK
Department of Chemistry, University of Warsaw, 02093 Warsaw, Pasteura 1 (Poland) (Received 17th November 1980; in revised form 31st March 1981)
ABSTRACT A simple and rapid raethod of determination of traces of surfactants in various waters and in pure solutions was developed. The method consists of an accumulation of surfactants on the surface of the H M D E at constant potential in stirred solutions. The ratio of the current decrease, attained by accumulation, to the maximum current is proportional to the surfactant concentration in the range 10-300/~g 1 I. Solutions with higher surfactant concentrations should be diluted prior to the determination. The total time of analysis does not exceed 15 rain. The proposed method permits the efficacy of different purification procedures to be compared quantitatively. Water with the lowest content of surfactants was obtained by combining two purification procedures based on various separation principles.
INTRODUCTION
The presence of organic, surface-active contaminants in distilled water and in "pure" supporting electrolytes prepared from such water can exert a harmful action on results obtained by anodic stripping voltammetry. This is because the conditions convenient for the accumulation of metals in the mercury phase are usually also convenient for the accumulation of surface-active contaminants on the mercury surface. The presence of surface-active contaminants in "pure" supporting electrolytes complicates electrochemical experiments performed not only at the HMDE [1-4] but also at platinum electrodes [5-7]. Conway et al. [5] and Hassan and Bruckenstein [6] defined several electrochemical purity criteria, on the basis of which the purity of the supporting electrolyte with respect to organic impurities can be characterized, but these purity tests are only qualitative in character. In a search for a useful method of detection and determination of traces of surface-active contaminants in distilled water and in pure supporting electrolytes, we took into account a procedure proposed for the determination of surface-active agents by Jehring and Stolle [8]. In this procedure the determination is performed tensammetrically after accumulation of surface-active agents on the HMDE in a 0022-0728/81/0000-0000/$02.50 © 1981 Elsevier Sequoia S.A.
242 quiet solution. Applying a 10 min accumulation time, the authors cited could determine 200 /~g 1-1 of polyethylene glycol added to a solution of lithium sulphate.The aim of the present work was to establish whether or not the procedure proposed by Jehring and Stolle may be used in the determination of surface-active contaminants occurring at significantly smaller concentrations in distilled water, and in "pure" supporting electrolytes used in anodic stripping voltammetry. To increase the sensitivity of this method we decided to stir the solutions during the accumulation step. EXPERIMENTAL Tensammetric curves were recorded with a Radelkis OH-105 polarograph with a three-electrode cell using an alternating voltage of 15 mV at 60 Hz. The reference electrode was an external calomel electrode or, in alkaline solutions, a 1 mol 1K O H mercury-mercury oxide electrode. However, all the potentials reported in this paper refer to the saturated calomel electrode. The counter electrode was a platinum foil with a surface area of c a . 4 cm 2. A H M D E of a type described by Kemula and Kublik [9], with surface area equal to 3.8 mm 2, was used as indicating electrode. Each new tensammetric curve was recorded after detachment of several mercury drops, because it was fou/ld that after detachment of the drop covered by surfaceactive agents some portion of these agents was transferred to the subsequent mercury drop. This effect was particularly evident when the mercury column distinctly receded into the capillary after detachment of the drop. The accumulation of surfactants was performed under stirring conditions at the potential of maximum adsorption. The stirring was performed by a magnetic stirrer. Two types of curves were recorded. The iac - E curves were recorded to identify the general adsorptive properties of the sample investigated, whereas the iac-t curves were recorded during the determination step; these curves can be recorded in the presence of dissolved oxygen, though after removal of oxygen they are more smooth. Measurements were performed in a constant-temperature room at ambient temperature 20 ~+ I°C. The supporting electrolytes used were prepared from analytical reagents and water purified by procedures given oelow. The following substances were used as the model surface-active agents: Rokafenol N-10 (Organika-Rokita, Poland), Triton X-100 (Loba, Austria) and sodium dodecylsulphate (Koch-Light). An activated charcoal (Riedel-DeHa~n Ag.) and Amberlite XAD 2 (B.D.H.) were used as adsorbents. Water was purified by the following procedures: (a) Tap water was distilled from a metallic, tin-lined apparatus working continuously with an efficiency of 5 1 h - l . After each 50 h of work the water in the distillation kettle was replaced by fresh water. (b) Singly distilled water was distilled further from a two-step quartz still working continuously. After each 50 h of work the water from the boiling flasks was replaced by a new sample of water purified as given in point (a). (c) Water prepurified by procedure (a) was distilled twice from a simple glass
243
apparatus enabling us to collect various fractions. (d) Singly distilled water was further distilled from an apparatus with a fractionating column connected to a reflux condenser. The column was filled with Rashig rings. The reflux ratio was about 7 to 1. (e) Procedure in point (d), but under diminished pressure produced by a water pump. Under the pressure used water boiled at 30°C. (f) Tap water was slowly run through a column filled with an activated charcoal. (g) Water prepurified as in point (f) was run slowly through a column filled with Amberlite XAD 2. (h) Water prepared as in point (g) was distilled twice, as in point (c). RESULTS
Determination of surface-active contaminants in supporting electrolytes Figure 1 shows a comparison of tensammetric curves obtained in dilute HC104 solution after various accumulation times. To avoid some losses of material accumulated at - 0 . 6 V, the curves were not recorded as usual from the positive potential, but each curve consisted of two segments recorded independently from the accumulation potential. The first, towards positive, and the second, after new accumulation, towards negative potentials. The shape of curve 1, obtained at a fresh mercury drop, is consistent with the literature data obtained with dropping mercury
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Fig. 1. Influence of variations in accumulation time on the shape of tensammetric curves obtained in 0.1 mol 1- i HCIO 4 solution. Times of accumulation in stirred solutions (rain): (1) 0; (2) 1; (3) 3; (4) 8; (5) as (3), but after addition of 75 ~g 1-1 of Triton. Vertical line defines an accumulation potential ( - 0 . 6 V) and starting point of recording.
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electrodes [10,11]. An increase of accumulation time led to essential changes in the shape of the curves. However, after times longer than 8 min the shape of the curve remains practically unaltered. In the presence of added surfactants (curve 5) the depression of the current proceeded more quickly. The character of the variations observed reveals, therefore, that the solution investigated is contaminated by surfaceactive agents, and that these contaminants are accumulated on the electrode surface. Owing to this effect the capacity of the double layer and, in turn, the alternating current measured, decrease with time. When a monolayer of surfactants becomes saturated the curve stops changing. The curves in Fig. 1 also show that in acidic solution no negative desorption peak can be observed prior to hydrogen evolution. The positive desorption peak appears only when the electrode is covered by greater amounts of surfactants. As shown in Fig. 2, similar effects were also obtained in neutral and alkaline solutions. In addition, in neutral media both negative and positive desorption peaks occur on the curve, whereas in alkaline media only the negative desorption peak is observed. As curve 3 shows, an addition of a small amount of Triton X-100 leads to a decrease of current in the central portion of the curve, but in the presence of this substance the negative desorption peak occurs at a more negative potential compared to the peak obtained in the solution investigated. On the other hand, the curves obtained in the presence of a small amount of sodium dodecylsulphate exhibited a negative desorption peak at nearly the same potential as that of the contaminants present in the solution tested. Such behavior indicates that the solution investigated is contaminated by anionic rather than by cationic surfactants.
245
Surface-active contaminants present in the solutions investigated can also be accumulated on the electrode surface in unstirred solution, as was suggested by Jehring and Stolle [8]. However, this type of accumulation under our experimental conditions was about 10 times less effective than accumulation in stirred solutions. Still less effective accumulation of surfactants was observed when the electrode was stored in the solution investigated with the electrical circuit disconnected. This latter phenomenon is of no significance when solutions with very low concentration of surfactants are analysed. On the other hand, at high concentration of surfactants this effect may lead to the accumulation of a quantity of surfactant during the formation of a new mercury drop, i.e. prior to the proper accumulation step. Therefore, the time between formation of a new drop and the beginning of an accumulation step should be as short as possible. The results presented above showed that tensamrnetry at the HMDE permits us to detect the presence of surface-active contaminants in "pure" solutions quite easily. However, this method does not permit us to determine a number of surfaceactive contaminants and to identify each particular surface-active constituent present in the solution. For such a complicated case, one can only perform a determination of the sum of the surface-active contaminants by expressing the results obtained in terms of an equivalent substance chosen rather arbitrarily. Triton X-100, Rokafenol N-10 and sodium dodecylsulphate were chosen as such equivalent substances. In principle, the determination can be performed on the basis of a decrease of the
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Fig. 3. Influence of variations in Triton X-100 concentration on the shape of tensammetric curves obtained in 0.1 mol 1-1 HC104 solution. Concentration of Triton (#g 1-1): 0,2) 0; (3) 50; (4) 100; (5) 200; (6) 300. Times of accumulation in stirred solutions (min): (1) 0; (2-6) 1. Vertical line defines accumulation potential ( - 0 . 6 V) and starting point of recording.
246
current at the potential of maximum adsorption or on the basis of variations in the height of the desorption peaks. In solutions containing higher concentration of a single surface-active agent the second procedure may be more convenient [8], but in a multi-component system with a very low concentration of surfactants the desorp-. tion peaks are low, broad and sometimes even split. Tensammetric curves shown in Fig. 3 illustrate the possibilities of the method proposed for the determination of a small amount of Triton X-100. It is evident that for a surfactant concentration equal to 50/~g 1 i the decrease of current at 0.6 V is well developed, whereas at the same curve the positive desorption peak does not occur at all. The value given above (50 /zg 1-1) is not a detection limit. The determination of smaller concentrations of Triton X-100 is possible by prolongation of the time of the accumulation step. In.practical determinations, instead of the iac-E curves the iac t curves were recorded at constant potential, and on the basis of such curves the ratios A i a c / i ama~ were determined. Figure4 presents several calibrating plots obtained for Triton X-100 and Rokafenol N-10 in perchloric acid and potassium hydroxide solutions after 1 rain accumulation. The plots are linear up to about 300/~g 1-i of Rokafenol and Triton in acidic solutions, and up to about 200 #g 1 i in alkaline medium. At higher concentrations of these substances the plots began to increase more slowly, or even become independent of further changes in concentrations of surface-active agents owing to coverage .of the electrode surface by a monolayer of surfactants. Similar linear calibration plots were also obtained for sodium dodecylsulphate. As the curves demonstrated show, the slope of the plots depends on the kind of surfactant used and on the alkalinity or acidity of solution.
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247 TABLE 1 Results of determinations of surface-active contaminants in different solutions. Triton and Rokafenol were used as equivalent substances. Accumulation time 1 min in stirred solution. Water purified by procedure (h). The values given are averages of two measurements Kind and concentration of solution [ ]/tool 1 f
Aiac/imax/%
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0.1 1.0 0.1 + NaC1040.3 0.1 + CH3COONa0.2 0.1 + Na2HPO4 0.1
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0.1 1.0 0.1 + mannitol 0.1 0.1 + NaK-tartrate 0.1
Concentrationof surfactants Triton//~g 1 i
Rokafenol//zg1 2
2.6 5.4 30.1 19.4 25.3
17 39 201 129 169
14 28 163 105 137
4.6 16.4 33.9 20.9
19 75 151 93
21 82 136 84
Variations in concentration of perchloric acid in the range 0.1-1 mol 1- i, as well as the addition to this solution of a quantity of neutral salts, led to some changes in the shape of the i - E curves and, in turn, to changes of the value of iamax measured at constant potential. However, even under such conditions the ratio Ai ac/iam ~ changed only slightly, and therefore the calibrating curve 1 can be used for the determination of surfactants in perchloric acid solution containing various concentrations of neutral salts. Similarly, the calibrating curve 3 can be exploited for determination of surfactants in various alkaline solutions. The slope of the calibrating curves also depend on the accumulation time. The longer the accumulation time, the higher the slope and simultaneously the smaller the range of linearity. The results of analyses of several pure solutions for surface-active contaminants are presented in Table 1. The data presented show that the usage of various substances for preparation of calibrating plots leads to differences in the results obtained. However, the occurrence of such differences is common for many procedures proposed for the determination of surface-active contaminants in solutions with unknown composition [12-15]. The procedure proposed enables the surfaceactive contaminants to be determined in solutions in the concentration range 10-300 /~g 1- ~. The detection limit does not result from the low sensitivity of the method but is limited by the purity of the solution used as a blank test. The upper limit is intrinsically bound to the proposed method. It reflects the fact that after coverage of the electrode surface with a monolayer of surfactant the formation of the second and the further layers is limited. The relative standard deviation obtained for multiple analyses of the same solution containing surfactant at the level 50/~g 1-1 was 7%. From the date given in Table 1 one may easily evaluate the contribution of surfactants introduced to the solution investigated with the distilled water and with the added reagent. For example, for 0.1 mol 1-1 perchloric acid solution prepared from the purest water attainable in this work, these contributions were 86% for water
248
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Fig, 5. Dependence.of the iac-t curves recorded at -0.6 V in stirred solutions on the modification of water-purification procedures. Water samples were acidified with HCIO 4 to a concentration of 0.1 tool 1 1. Purification procedures defined by the letters (a)-(h) are described in the experimental section.
a n d 14% for p e r c h l o r i c acid. T h o s e e v a l u a t e d for 0.1 m o l 1-1 of p o t a s s i u m h y d r o x i d e s o l u t i o n w e r e 66.8% for w a t e r a n d 33.2% for p o t a s s i u m h y d r o x i d e .
Investigation of the efficacy of the different modes of purification of tap water from surfactants T h e results g i v e n a b o v e s h o w t h a t s u r f a c t a n t s p r e s e n t in d i l u t e p e r c h l o r i c a c i d s o l u t i o n s t e m m a i n l y f r o m " p u r e " w a t e r . T h u s , the results o b t a i n e d for d i l u t e
TABLE 2 Results of determination of surfactants in water purified by various procedures. Triton and Rokafenol were used as equivalent substances. Concentration of perchloric acid 0.1 tool 1 i. Accumulation time I rain in stirred solution Kind of purification of water,
~5iac/imax/%
Conc. of surfactants Triton ~gl t
(a) (b) Still after cleaning (b') Still working 200 h without cleaning (c) (d) Middle fraction (d') First fraction (e) Middle fraction (e') First fraction (f) (g) (h)
Rokafenol ~gl -I
12.6 12.6
84 84
68 68
23.3 10.0 10.0 32.0 7,8 18.4 12.5 8.1 Z6
155 67 67 213 52 123 83 54 17
126 54 54 173 45 100 68 44 14
" More precise conditions given in the experimental section.
249
perchloric acid solution, after subtraction of the corresponding correction, may be recognized as the result obtained for purified water. In the experiments described in this section small portions of perchloric acid were added to the samples of waters purified by the procedures described in the experimental section in points (a)-(h). Several alternating current-time curves obtained in these experiments are given in Fig. 5. It is evident that the proposed procedure offers a simple method of investigating the efficacy of purification of water from surface-active contaminants. The quantitative data obtained on the basis of similar curves are summarized in Table 2. From the data obtained during the investigation of the dependence of the quality of water on the variations in purification procedures, the following conclusions can be drawn: (1) Multiple continuous distillations do not give water with high purity. (2) Usage of a continuously working still without frequent removal of water from the boiling flask leads to an enrichment of surfactants in the flask and, in turn, to the decrease of .purity of distillate. It should be noted that after long operation without removal of water from the boiling flask a marked foaming was observed during boiling. (3) The fractionated distillation at diminished pressure gives water with a lower surfactant content than distillation at atmospheric pressure. (4) The purest water was obtained by combining the purification procedures based on different separation principles. Determination of surfactants in untreated and potable waters Concentrations of surface-active agents in various waters differ significantly and very often they markedly exceed 300 #g l-i. Such samples could not be analysed directly by the method described above. In order to estimate quantitatively the
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HC10 4 to concentration of 0.1 mol l - l : (1) thrice-distilled water; (2) water from a mountain stream; (3) water from lowland forest stream; (4) tap water; (5) as 4, but after dilution 1 : 10.
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surfactant content in the sample of natural water, the whole iac-E curve should be recorded first. Comparison of such a curve with an appropriate blank test can help to answer the question whether or not a sample should be diluted. Such a comparison is demonstrated in Fig. 6. The shape of curve 2 shows that the water sample taken from the mountain stream has a low surfactant content. Thus, it can be analysed directly without dilution. On the other hand, the concentration of surfactants in potable water from Warsaw and in water from a lowland forest stream is higher. As curve 5 shows, after appropriate dilution, the potable water gave a tensammetric curve similar to the blank test. It should be noted that during investigation of untreated and potable waters in no case was the positive desorption peak observed on the tensammetric curve. A distinct shift of the first part of the curve towards negative potentials, occurring on curves 3 and 4, is caused by the presence of chloride ions in solution. In the course of analyses of samples with high surfactant concentration, the blank test was recorded first for 0.1 mol 1-~ HC10 4 solution prepared from very pure water. Next, an appropriate volume of blank solution was replaced by the same volume of the water to be investigated and the curve was recorded again. The 1 : 10 dilution means that one-tenth of the volume of the blank solution was replaced by the water to be investigated. The quantitative data obtained for various waters are given in Table 3. It is evident that the proposed method may also be used for the determination of surface-active agents in natural waters with a rather high surfactant content. DISCUSSION
In recent years the demand for a very pure water has considerably increased. Many workers have tried to find improved procedures enabling them to obtain pure water free from organic surfactants [5,7,12,16]. However, serious difficulties arise on comparison of the extent of contamination of water samples prepared by various purification procedures. It was clear ~to us that the finding of a simple analytical method convenient for determination of traces of surfactants in highly purified water and in pure supporting electrolytes is nowadays an even more important problem than finding a new purification procedure. The majority of the methods used hitherto for determination of traces of surfactants in solutions require a pre-separation or a pre-cor~centration step performed in a macro-scale. As a rule, this step is time-consuming and may lead to additional contamination of the sample investigated. Only electrochemical methods exploiting an adsorption of organic surfactants on platinum [5-7] or mercury electrodes [12-15,17] do not need any pre-concentration performed in the macro-scale. However, the polarographic method exploiting the suppression of the polarographic maxima, in spite of its convenient detection limit [12,14], has only a limited utility. Simply, a maximum proper for the determination of surfactants does not appear in every solution. In this respect the method proposed in the present paper may find a wide use because the dependence of the capacity of the double layer on the presence of surfactants in solution is a
252
more general phenomenon occurring in each solution. An opinion that measurements of the double-layer capacity may be exploited for the qualitative estimation of the purity of solution was expressed first by Barker [18]. He did not observe any change in double-layer capacity when the HMDE was held 24 h with the electrical circuit disconnected, in an unstirred solution purified by adsorption on the charcoal. On the basis of such experiments he stated that the solution investigated was free from surface-active contaminants. Next, the method was used by Jehring and Stolle [8] for the determination of polyethylene glycol added in small amounts to a solution of Li2SO4. In the work cited the accumulation step was performed in an unstirred solution. As shown in the present work, a stirring of the solution during the accumulation step leads to a marked increase in the sensitivity and, in turn, to a lowering of the detection limit of the method discussed. The detection limit attained permits us to determine easily the traces of surfactants in various laboratory waters and in "pure" supporting electrolytes. A disadvantage of the method discussed is the necessity to use an equivalent substance for the construction of a calibration plot. However, this disadvantage is encountered in all methods used for the determination of unknown substances present in analysed samples. Our attempts to obtain water free from organic surfactants by distillation were unsuccessful. This finding is in accordance with an opinion given by Conway et al. [5] that tap water is l~artially contaminated by surfactants which are steam volatile. Sorption on to activated charcoal was claimed to be a successful procedure for purification of water from organic surface active agents [12,19,20] although there are some doubts in the literature about its utility [5,6,21]. In the present work a distinct improvement of the purity of water was attained when the distillation step was preceded by an adsorptive step. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
W. Kemula and S. Glodowski, Rocz. Chem., 36 (1962) 1203. S.B. Cfasman and R.M.F. Salikhdzanova, Zavod. Lab., 30 (1964) 133. G.E. Batley and T.M. Florence, J. Electroanal. Chem., 72 (1976) 121. Z. Lukaszewski and M.K. Pawlak, J. Electroanal. Chem., 103 (1979) 225. B.E. Conway, H. Angerstein-Kozlowska, W.B.A. Sharp and E.E. Criddle, Anal. Chem., 45 (1973) 1331. M.Z. Hassan and S. Bruckenstein, Anal. Chem., 46 (1974) 1962. L. Formero and S. Trasatti, Electrochim. Acta, 12 (1967) 1457. H. Jehring and W. Stolle, Collect. Czech. Chem. Commun., 33 (1968) 1038. W. Kemula and Z. Kublik, Anal. Chim. Acta., 18 (1958) 104. G. Manohar and S. Sathyanarayana, J. Electroanal. Chem., 30 (1970) 301. R.S. Hansen, D.J. Kelsh and D.H. Grantham, J. Phys. Chem., 67 (1963) 2316. N.P. Berezina and N.V. Nikolaeva-Fedorovich, Elektrokhimiya, 3 (1967) 3. Z. Kozarec, V. Zuti~: and B. Cosovi~, Tenside Deterg., 13 (1976) 260. T.A. Kryukova, Zavod. Lab., 14 (1948) 765; 16 (1950) 134. K. [.inhard, Tenside Deterg., 9 (1972) 241. K.T. Stuck, Jr., L.L. Ciaccio (Ed.), Water and Water Pollution, Vol. 3, Marcel Dekker, New York, 1972, p. 816.
253 17 18 19 20 21
K, Schwartz, Z. Anal. Chem., 115 (1938) 161. G. Barker, Trans. Symp. Electrode Processes, Philadelphia 1959, Wiley, 1961, p. 366. G. Barker, Atomic Energy Research Establishment, C / R , Vol. 1954, p. 1563. R. Parsons and F.G.R. Zobel, Trans. Faraday Soc., 62 (1966) 351 I. D.A. Jenkins and C.J. Weedon, J. Electroanal. Chem., 31 (1971) 13.
241
Elsevier Sequoia S.A., Lausanne--Printed in The Netherlands
DETERMINATION OF TRACES OF SURFACTANTS IN DISTILLED, POTABLE AND UNTREATED WATERS AND IN SUPPORTING ELECTROLYTES BY TENSAMMETRY WITH ACCUMULATION ON THE HMDE
EWA BEDNARKIEWICZ, MIKOLAJ DONTEN and ZENON KUBLIK
Department of Chemistry, University of Warsaw, 02093 Warsaw, Pasteura 1 (Poland) (Received 17th November 1980; in revised form 31st March 1981)
ABSTRACT A simple and rapid raethod of determination of traces of surfactants in various waters and in pure solutions was developed. The method consists of an accumulation of surfactants on the surface of the H M D E at constant potential in stirred solutions. The ratio of the current decrease, attained by accumulation, to the maximum current is proportional to the surfactant concentration in the range 10-300/~g 1 I. Solutions with higher surfactant concentrations should be diluted prior to the determination. The total time of analysis does not exceed 15 rain. The proposed method permits the efficacy of different purification procedures to be compared quantitatively. Water with the lowest content of surfactants was obtained by combining two purification procedures based on various separation principles.
INTRODUCTION
The presence of organic, surface-active contaminants in distilled water and in "pure" supporting electrolytes prepared from such water can exert a harmful action on results obtained by anodic stripping voltammetry. This is because the conditions convenient for the accumulation of metals in the mercury phase are usually also convenient for the accumulation of surface-active contaminants on the mercury surface. The presence of surface-active contaminants in "pure" supporting electrolytes complicates electrochemical experiments performed not only at the HMDE [1-4] but also at platinum electrodes [5-7]. Conway et al. [5] and Hassan and Bruckenstein [6] defined several electrochemical purity criteria, on the basis of which the purity of the supporting electrolyte with respect to organic impurities can be characterized, but these purity tests are only qualitative in character. In a search for a useful method of detection and determination of traces of surface-active contaminants in distilled water and in pure supporting electrolytes, we took into account a procedure proposed for the determination of surface-active agents by Jehring and Stolle [8]. In this procedure the determination is performed tensammetrically after accumulation of surface-active agents on the HMDE in a 0022-0728/81/0000-0000/$02.50 © 1981 Elsevier Sequoia S.A.
242 quiet solution. Applying a 10 min accumulation time, the authors cited could determine 200 /~g 1-1 of polyethylene glycol added to a solution of lithium sulphate.The aim of the present work was to establish whether or not the procedure proposed by Jehring and Stolle may be used in the determination of surface-active contaminants occurring at significantly smaller concentrations in distilled water, and in "pure" supporting electrolytes used in anodic stripping voltammetry. To increase the sensitivity of this method we decided to stir the solutions during the accumulation step. EXPERIMENTAL Tensammetric curves were recorded with a Radelkis OH-105 polarograph with a three-electrode cell using an alternating voltage of 15 mV at 60 Hz. The reference electrode was an external calomel electrode or, in alkaline solutions, a 1 mol 1K O H mercury-mercury oxide electrode. However, all the potentials reported in this paper refer to the saturated calomel electrode. The counter electrode was a platinum foil with a surface area of c a . 4 cm 2. A H M D E of a type described by Kemula and Kublik [9], with surface area equal to 3.8 mm 2, was used as indicating electrode. Each new tensammetric curve was recorded after detachment of several mercury drops, because it was fou/ld that after detachment of the drop covered by surfaceactive agents some portion of these agents was transferred to the subsequent mercury drop. This effect was particularly evident when the mercury column distinctly receded into the capillary after detachment of the drop. The accumulation of surfactants was performed under stirring conditions at the potential of maximum adsorption. The stirring was performed by a magnetic stirrer. Two types of curves were recorded. The iac - E curves were recorded to identify the general adsorptive properties of the sample investigated, whereas the iac-t curves were recorded during the determination step; these curves can be recorded in the presence of dissolved oxygen, though after removal of oxygen they are more smooth. Measurements were performed in a constant-temperature room at ambient temperature 20 ~+ I°C. The supporting electrolytes used were prepared from analytical reagents and water purified by procedures given oelow. The following substances were used as the model surface-active agents: Rokafenol N-10 (Organika-Rokita, Poland), Triton X-100 (Loba, Austria) and sodium dodecylsulphate (Koch-Light). An activated charcoal (Riedel-DeHa~n Ag.) and Amberlite XAD 2 (B.D.H.) were used as adsorbents. Water was purified by the following procedures: (a) Tap water was distilled from a metallic, tin-lined apparatus working continuously with an efficiency of 5 1 h - l . After each 50 h of work the water in the distillation kettle was replaced by fresh water. (b) Singly distilled water was distilled further from a two-step quartz still working continuously. After each 50 h of work the water from the boiling flasks was replaced by a new sample of water purified as given in point (a). (c) Water prepurified by procedure (a) was distilled twice from a simple glass
243
apparatus enabling us to collect various fractions. (d) Singly distilled water was further distilled from an apparatus with a fractionating column connected to a reflux condenser. The column was filled with Rashig rings. The reflux ratio was about 7 to 1. (e) Procedure in point (d), but under diminished pressure produced by a water pump. Under the pressure used water boiled at 30°C. (f) Tap water was slowly run through a column filled with an activated charcoal. (g) Water prepurified as in point (f) was run slowly through a column filled with Amberlite XAD 2. (h) Water prepared as in point (g) was distilled twice, as in point (c). RESULTS
Determination of surface-active contaminants in supporting electrolytes Figure 1 shows a comparison of tensammetric curves obtained in dilute HC104 solution after various accumulation times. To avoid some losses of material accumulated at - 0 . 6 V, the curves were not recorded as usual from the positive potential, but each curve consisted of two segments recorded independently from the accumulation potential. The first, towards positive, and the second, after new accumulation, towards negative potentials. The shape of curve 1, obtained at a fresh mercury drop, is consistent with the literature data obtained with dropping mercury
1
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Fig. 1. Influence of variations in accumulation time on the shape of tensammetric curves obtained in 0.1 mol 1- i HCIO 4 solution. Times of accumulation in stirred solutions (rain): (1) 0; (2) 1; (3) 3; (4) 8; (5) as (3), but after addition of 75 ~g 1-1 of Triton. Vertical line defines an accumulation potential ( - 0 . 6 V) and starting point of recording.
244 i
8
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Fig. 2. Influence of accumulation on the shape of tensammetric curves obtained for: (1,1') 0.1 mol 1 1 NaC104; (2,2',3) 0.1 mol 1 - I KOH solution. Times of accumulation in stirred solution (rain): (1,2) 0; (1',2') 8; (3) 1 rain after addition of 75/~g 1- l of Triton. Vertical line defines the accumulation potential and starting point of recording. Current scales: (left) for curves 1 and 1'; (right) for curves 2, 2' and 3.
electrodes [10,11]. An increase of accumulation time led to essential changes in the shape of the curves. However, after times longer than 8 min the shape of the curve remains practically unaltered. In the presence of added surfactants (curve 5) the depression of the current proceeded more quickly. The character of the variations observed reveals, therefore, that the solution investigated is contaminated by surfaceactive agents, and that these contaminants are accumulated on the electrode surface. Owing to this effect the capacity of the double layer and, in turn, the alternating current measured, decrease with time. When a monolayer of surfactants becomes saturated the curve stops changing. The curves in Fig. 1 also show that in acidic solution no negative desorption peak can be observed prior to hydrogen evolution. The positive desorption peak appears only when the electrode is covered by greater amounts of surfactants. As shown in Fig. 2, similar effects were also obtained in neutral and alkaline solutions. In addition, in neutral media both negative and positive desorption peaks occur on the curve, whereas in alkaline media only the negative desorption peak is observed. As curve 3 shows, an addition of a small amount of Triton X-100 leads to a decrease of current in the central portion of the curve, but in the presence of this substance the negative desorption peak occurs at a more negative potential compared to the peak obtained in the solution investigated. On the other hand, the curves obtained in the presence of a small amount of sodium dodecylsulphate exhibited a negative desorption peak at nearly the same potential as that of the contaminants present in the solution tested. Such behavior indicates that the solution investigated is contaminated by anionic rather than by cationic surfactants.
245
Surface-active contaminants present in the solutions investigated can also be accumulated on the electrode surface in unstirred solution, as was suggested by Jehring and Stolle [8]. However, this type of accumulation under our experimental conditions was about 10 times less effective than accumulation in stirred solutions. Still less effective accumulation of surfactants was observed when the electrode was stored in the solution investigated with the electrical circuit disconnected. This latter phenomenon is of no significance when solutions with very low concentration of surfactants are analysed. On the other hand, at high concentration of surfactants this effect may lead to the accumulation of a quantity of surfactant during the formation of a new mercury drop, i.e. prior to the proper accumulation step. Therefore, the time between formation of a new drop and the beginning of an accumulation step should be as short as possible. The results presented above showed that tensamrnetry at the HMDE permits us to detect the presence of surface-active contaminants in "pure" solutions quite easily. However, this method does not permit us to determine a number of surfaceactive contaminants and to identify each particular surface-active constituent present in the solution. For such a complicated case, one can only perform a determination of the sum of the surface-active contaminants by expressing the results obtained in terms of an equivalent substance chosen rather arbitrarily. Triton X-100, Rokafenol N-10 and sodium dodecylsulphate were chosen as such equivalent substances. In principle, the determination can be performed on the basis of a decrease of the
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Fig. 3. Influence of variations in Triton X-100 concentration on the shape of tensammetric curves obtained in 0.1 mol 1-1 HC104 solution. Concentration of Triton (#g 1-1): 0,2) 0; (3) 50; (4) 100; (5) 200; (6) 300. Times of accumulation in stirred solutions (min): (1) 0; (2-6) 1. Vertical line defines accumulation potential ( - 0 . 6 V) and starting point of recording.
246
current at the potential of maximum adsorption or on the basis of variations in the height of the desorption peaks. In solutions containing higher concentration of a single surface-active agent the second procedure may be more convenient [8], but in a multi-component system with a very low concentration of surfactants the desorp-. tion peaks are low, broad and sometimes even split. Tensammetric curves shown in Fig. 3 illustrate the possibilities of the method proposed for the determination of a small amount of Triton X-100. It is evident that for a surfactant concentration equal to 50/~g 1 i the decrease of current at 0.6 V is well developed, whereas at the same curve the positive desorption peak does not occur at all. The value given above (50 /zg 1-1) is not a detection limit. The determination of smaller concentrations of Triton X-100 is possible by prolongation of the time of the accumulation step. In.practical determinations, instead of the iac-E curves the iac t curves were recorded at constant potential, and on the basis of such curves the ratios A i a c / i ama~ were determined. Figure4 presents several calibrating plots obtained for Triton X-100 and Rokafenol N-10 in perchloric acid and potassium hydroxide solutions after 1 rain accumulation. The plots are linear up to about 300/~g 1-i of Rokafenol and Triton in acidic solutions, and up to about 200 #g 1 i in alkaline medium. At higher concentrations of these substances the plots began to increase more slowly, or even become independent of further changes in concentrations of surface-active agents owing to coverage .of the electrode surface by a monolayer of surfactants. Similar linear calibration plots were also obtained for sodium dodecylsulphate. As the curves demonstrated show, the slope of the plots depends on the kind of surfactant used and on the alkalinity or acidity of solution.
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247 TABLE 1 Results of determinations of surface-active contaminants in different solutions. Triton and Rokafenol were used as equivalent substances. Accumulation time 1 min in stirred solution. Water purified by procedure (h). The values given are averages of two measurements Kind and concentration of solution [ ]/tool 1 f
Aiac/imax/%
HCl04
0.1 1.0 0.1 + NaC1040.3 0.1 + CH3COONa0.2 0.1 + Na2HPO4 0.1
KOH
0.1 1.0 0.1 + mannitol 0.1 0.1 + NaK-tartrate 0.1
Concentrationof surfactants Triton//~g 1 i
Rokafenol//zg1 2
2.6 5.4 30.1 19.4 25.3
17 39 201 129 169
14 28 163 105 137
4.6 16.4 33.9 20.9
19 75 151 93
21 82 136 84
Variations in concentration of perchloric acid in the range 0.1-1 mol 1- i, as well as the addition to this solution of a quantity of neutral salts, led to some changes in the shape of the i - E curves and, in turn, to changes of the value of iamax measured at constant potential. However, even under such conditions the ratio Ai ac/iam ~ changed only slightly, and therefore the calibrating curve 1 can be used for the determination of surfactants in perchloric acid solution containing various concentrations of neutral salts. Similarly, the calibrating curve 3 can be exploited for determination of surfactants in various alkaline solutions. The slope of the calibrating curves also depend on the accumulation time. The longer the accumulation time, the higher the slope and simultaneously the smaller the range of linearity. The results of analyses of several pure solutions for surface-active contaminants are presented in Table 1. The data presented show that the usage of various substances for preparation of calibrating plots leads to differences in the results obtained. However, the occurrence of such differences is common for many procedures proposed for the determination of surface-active contaminants in solutions with unknown composition [12-15]. The procedure proposed enables the surfaceactive contaminants to be determined in solutions in the concentration range 10-300 /~g 1- ~. The detection limit does not result from the low sensitivity of the method but is limited by the purity of the solution used as a blank test. The upper limit is intrinsically bound to the proposed method. It reflects the fact that after coverage of the electrode surface with a monolayer of surfactant the formation of the second and the further layers is limited. The relative standard deviation obtained for multiple analyses of the same solution containing surfactant at the level 50/~g 1-1 was 7%. From the date given in Table 1 one may easily evaluate the contribution of surfactants introduced to the solution investigated with the distilled water and with the added reagent. For example, for 0.1 mol 1-1 perchloric acid solution prepared from the purest water attainable in this work, these contributions were 86% for water
248
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Fig, 5. Dependence.of the iac-t curves recorded at -0.6 V in stirred solutions on the modification of water-purification procedures. Water samples were acidified with HCIO 4 to a concentration of 0.1 tool 1 1. Purification procedures defined by the letters (a)-(h) are described in the experimental section.
a n d 14% for p e r c h l o r i c acid. T h o s e e v a l u a t e d for 0.1 m o l 1-1 of p o t a s s i u m h y d r o x i d e s o l u t i o n w e r e 66.8% for w a t e r a n d 33.2% for p o t a s s i u m h y d r o x i d e .
Investigation of the efficacy of the different modes of purification of tap water from surfactants T h e results g i v e n a b o v e s h o w t h a t s u r f a c t a n t s p r e s e n t in d i l u t e p e r c h l o r i c a c i d s o l u t i o n s t e m m a i n l y f r o m " p u r e " w a t e r . T h u s , the results o b t a i n e d for d i l u t e
TABLE 2 Results of determination of surfactants in water purified by various procedures. Triton and Rokafenol were used as equivalent substances. Concentration of perchloric acid 0.1 tool 1 i. Accumulation time I rain in stirred solution Kind of purification of water,
~5iac/imax/%
Conc. of surfactants Triton ~gl t
(a) (b) Still after cleaning (b') Still working 200 h without cleaning (c) (d) Middle fraction (d') First fraction (e) Middle fraction (e') First fraction (f) (g) (h)
Rokafenol ~gl -I
12.6 12.6
84 84
68 68
23.3 10.0 10.0 32.0 7,8 18.4 12.5 8.1 Z6
155 67 67 213 52 123 83 54 17
126 54 54 173 45 100 68 44 14
" More precise conditions given in the experimental section.
249
perchloric acid solution, after subtraction of the corresponding correction, may be recognized as the result obtained for purified water. In the experiments described in this section small portions of perchloric acid were added to the samples of waters purified by the procedures described in the experimental section in points (a)-(h). Several alternating current-time curves obtained in these experiments are given in Fig. 5. It is evident that the proposed procedure offers a simple method of investigating the efficacy of purification of water from surface-active contaminants. The quantitative data obtained on the basis of similar curves are summarized in Table 2. From the data obtained during the investigation of the dependence of the quality of water on the variations in purification procedures, the following conclusions can be drawn: (1) Multiple continuous distillations do not give water with high purity. (2) Usage of a continuously working still without frequent removal of water from the boiling flask leads to an enrichment of surfactants in the flask and, in turn, to the decrease of .purity of distillate. It should be noted that after long operation without removal of water from the boiling flask a marked foaming was observed during boiling. (3) The fractionated distillation at diminished pressure gives water with a lower surfactant content than distillation at atmospheric pressure. (4) The purest water was obtained by combining the purification procedures based on different separation principles. Determination of surfactants in untreated and potable waters Concentrations of surface-active agents in various waters differ significantly and very often they markedly exceed 300 #g l-i. Such samples could not be analysed directly by the method described above. In order to estimate quantitatively the
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HC10 4 to concentration of 0.1 mol l - l : (1) thrice-distilled water; (2) water from a mountain stream; (3) water from lowland forest stream; (4) tap water; (5) as 4, but after dilution 1 : 10.
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251
surfactant content in the sample of natural water, the whole iac-E curve should be recorded first. Comparison of such a curve with an appropriate blank test can help to answer the question whether or not a sample should be diluted. Such a comparison is demonstrated in Fig. 6. The shape of curve 2 shows that the water sample taken from the mountain stream has a low surfactant content. Thus, it can be analysed directly without dilution. On the other hand, the concentration of surfactants in potable water from Warsaw and in water from a lowland forest stream is higher. As curve 5 shows, after appropriate dilution, the potable water gave a tensammetric curve similar to the blank test. It should be noted that during investigation of untreated and potable waters in no case was the positive desorption peak observed on the tensammetric curve. A distinct shift of the first part of the curve towards negative potentials, occurring on curves 3 and 4, is caused by the presence of chloride ions in solution. In the course of analyses of samples with high surfactant concentration, the blank test was recorded first for 0.1 mol 1-~ HC10 4 solution prepared from very pure water. Next, an appropriate volume of blank solution was replaced by the same volume of the water to be investigated and the curve was recorded again. The 1 : 10 dilution means that one-tenth of the volume of the blank solution was replaced by the water to be investigated. The quantitative data obtained for various waters are given in Table 3. It is evident that the proposed method may also be used for the determination of surface-active agents in natural waters with a rather high surfactant content. DISCUSSION
In recent years the demand for a very pure water has considerably increased. Many workers have tried to find improved procedures enabling them to obtain pure water free from organic surfactants [5,7,12,16]. However, serious difficulties arise on comparison of the extent of contamination of water samples prepared by various purification procedures. It was clear ~to us that the finding of a simple analytical method convenient for determination of traces of surfactants in highly purified water and in pure supporting electrolytes is nowadays an even more important problem than finding a new purification procedure. The majority of the methods used hitherto for determination of traces of surfactants in solutions require a pre-separation or a pre-cor~centration step performed in a macro-scale. As a rule, this step is time-consuming and may lead to additional contamination of the sample investigated. Only electrochemical methods exploiting an adsorption of organic surfactants on platinum [5-7] or mercury electrodes [12-15,17] do not need any pre-concentration performed in the macro-scale. However, the polarographic method exploiting the suppression of the polarographic maxima, in spite of its convenient detection limit [12,14], has only a limited utility. Simply, a maximum proper for the determination of surfactants does not appear in every solution. In this respect the method proposed in the present paper may find a wide use because the dependence of the capacity of the double layer on the presence of surfactants in solution is a
252
more general phenomenon occurring in each solution. An opinion that measurements of the double-layer capacity may be exploited for the qualitative estimation of the purity of solution was expressed first by Barker [18]. He did not observe any change in double-layer capacity when the HMDE was held 24 h with the electrical circuit disconnected, in an unstirred solution purified by adsorption on the charcoal. On the basis of such experiments he stated that the solution investigated was free from surface-active contaminants. Next, the method was used by Jehring and Stolle [8] for the determination of polyethylene glycol added in small amounts to a solution of Li2SO4. In the work cited the accumulation step was performed in an unstirred solution. As shown in the present work, a stirring of the solution during the accumulation step leads to a marked increase in the sensitivity and, in turn, to a lowering of the detection limit of the method discussed. The detection limit attained permits us to determine easily the traces of surfactants in various laboratory waters and in "pure" supporting electrolytes. A disadvantage of the method discussed is the necessity to use an equivalent substance for the construction of a calibration plot. However, this disadvantage is encountered in all methods used for the determination of unknown substances present in analysed samples. Our attempts to obtain water free from organic surfactants by distillation were unsuccessful. This finding is in accordance with an opinion given by Conway et al. [5] that tap water is l~artially contaminated by surfactants which are steam volatile. Sorption on to activated charcoal was claimed to be a successful procedure for purification of water from organic surface active agents [12,19,20] although there are some doubts in the literature about its utility [5,6,21]. In the present work a distinct improvement of the purity of water was attained when the distillation step was preceded by an adsorptive step. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
W. Kemula and S. Glodowski, Rocz. Chem., 36 (1962) 1203. S.B. Cfasman and R.M.F. Salikhdzanova, Zavod. Lab., 30 (1964) 133. G.E. Batley and T.M. Florence, J. Electroanal. Chem., 72 (1976) 121. Z. Lukaszewski and M.K. Pawlak, J. Electroanal. Chem., 103 (1979) 225. B.E. Conway, H. Angerstein-Kozlowska, W.B.A. Sharp and E.E. Criddle, Anal. Chem., 45 (1973) 1331. M.Z. Hassan and S. Bruckenstein, Anal. Chem., 46 (1974) 1962. L. Formero and S. Trasatti, Electrochim. Acta, 12 (1967) 1457. H. Jehring and W. Stolle, Collect. Czech. Chem. Commun., 33 (1968) 1038. W. Kemula and Z. Kublik, Anal. Chim. Acta., 18 (1958) 104. G. Manohar and S. Sathyanarayana, J. Electroanal. Chem., 30 (1970) 301. R.S. Hansen, D.J. Kelsh and D.H. Grantham, J. Phys. Chem., 67 (1963) 2316. N.P. Berezina and N.V. Nikolaeva-Fedorovich, Elektrokhimiya, 3 (1967) 3. Z. Kozarec, V. Zuti~: and B. Cosovi~, Tenside Deterg., 13 (1976) 260. T.A. Kryukova, Zavod. Lab., 14 (1948) 765; 16 (1950) 134. K. [.inhard, Tenside Deterg., 9 (1972) 241. K.T. Stuck, Jr., L.L. Ciaccio (Ed.), Water and Water Pollution, Vol. 3, Marcel Dekker, New York, 1972, p. 816.
253 17 18 19 20 21
K, Schwartz, Z. Anal. Chem., 115 (1938) 161. G. Barker, Trans. Symp. Electrode Processes, Philadelphia 1959, Wiley, 1961, p. 366. G. Barker, Atomic Energy Research Establishment, C / R , Vol. 1954, p. 1563. R. Parsons and F.G.R. Zobel, Trans. Faraday Soc., 62 (1966) 351 I. D.A. Jenkins and C.J. Weedon, J. Electroanal. Chem., 31 (1971) 13.