Sensitive Potentiometric Method for Determination of ...

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stimmt. Die Genauigkeit und Präzision wurde durch Ver- wendung der Standard-Additions-Methode bewertet. Die rela- tive Standardabweichung innerhalb der ...
ENVIRONMENTAL CHEMISTRY

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y

M. Sak-Bosnar1*, D. Madunic-Cacic2, R. Matesic-Puac1 and Z. Grabaric3

Sensitive Potentiometric Method for Determination of Micromolar Level of Polyethoxylated Nonionic Surfactants in Effluents The Metrohm NIO surfactant electrode has been used as endpoint indicator for potentiometric titration of low concentration level of polyethoxylated nonionic surfactants. This can be achieved by using of a diluted titrant concentration, thus reducing the amount of precipitate formed during titration and preventing the electrode deterioration. The solutions of low levels (down to 10–6 mol/L) of selected nonionic surfactants containing 5 to 23 EO groups were successfully titrated with diluted (as low as 10–4 mol/L) sodium tetraphenylborate as standard anionic titrant, increasing up to 20 times the sensitivity of the method. The low surfactant concentration has been determined in synthetic formulations of widely used detergent products and industrial waste waters. The titration end-point has been determined by applying extended Savitzky-Golay least-squares regression. The accuracy and precision has been evaluated by using the standard addition method. Relative standard deviation within results was between 3.4 and 12.8 % depending on the sample complexity and the surfactant concentration level. Key words: NIO surfactant electrode, polyethoxylated nonionic surfactant, potentiometric titration, detergent products, wastewater

Sensitive potentiometrische Methode zur Bestimmung mikromolarer Mengen polyethoxylierter nichtionischer Tenside in Industrieabwässern. Eine Metrohm NIO-Tensidelektrode wurde bei der potentiometrischen Titration polyethoxylierter nichtionischer Tenside in niedriger Konzentration zur Endpunktbestimmung verwendet. Das kann mit Hilfe einer verdünnten Titrant-Konzentration erreicht werden, wodurch die während der Titration gebildete Niederschlagsmenge reduziert und eine Elektrodenschädigung verhindert wird. Die Lösungen mit geringem Gehalt (bis zu 10–6 mol/L) an ausgewählten nichtionischen Tensiden, die 5 bis 23 EO-Gruppen enthalten, wurden mit verdünntem (bis zu 10–4 mol/L) Natriumtetraphenylborat als anionischer Standard-Titrant erfolgreich titriert. Die Empfindlichkeit der Methode wurde dadurch bis zu 20 mal erhöht. Die niedrige Tensidkonzentration wurde in synthetischen Formulierungen weit verbreiteter Wasch- und Reinigungsmitteln und in industriellen Abwässern bestimmt. Der Titrationsendpunkt wurde mittels einer erweiterten Savitzky-Golay-Regressionsanalyse bestimmt. Die Genauigkeit und Präzision wurde durch Verwendung der Standard-Additions-Methode bewertet. Die rela1

* 2 3

Department of Chemistry, Josip Juraj Strossmayer University of Osijek, F. Kuhaca 20, HR-31000 Osijek, Croatia To whom corespondence should be addressed. E-mail address: [email protected] Saponia Chemical, Pharmaceutical and Foodstuff Industry, M. Gupca 2, HR-31000 Osijek, Croatia Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, HR-10000 Zagreb, Croatia

Tenside Surf. Det. 44 (2007) 1

tive Standardabweichung innerhalb der Ergebnisse lag je nach Probenkomplexität und Tensidekonzentration zwischen 3,4 und 12,8 %. Stichwörter: NIO-Tensidelektrode, polyethoxyliertes nichtionisches Tensid, potentiometrische Titration, Wasch- und Reinigungsmittel, Industrieabwässer

1 Introduction

Nonionic surfactants are produced in large quantities. They hold second place in worldwide surfactants consumption, with a share of the total use of surfactants of about 35 %. Of the nonionic surfactants produced, the polyoxyethylene alcohols, which are often thought of as being synonymous with nonionic surfactants, hold third place after soap and alkyl benzene sulfonates (LAS) on the list of all the surfactants used in detergent industry. Because of their widespread use and their properties which allow facile transport between non-miscible interfaces (oil/water and water/biological membranes) these surfactants may be found anywhere in the environment. Although they are not classified as highly toxic substances, some of their metabolites are more toxic than the parent compounds. Currently, the increasing awareness of environmental problems imposes the obligation of analytical determination and self monitoring. Almost all the analytical methods for determination of polyethoxylated nonionic surfactants are based on the formation of tetraphenylborate (TPB) salts of pseudocationic complexes of nonionic surfactants with some metal cations (mainly barium). Levins and Ikeda (1) were among the first who exploited this precipitation reaction for potentiometric titration of nonionic surfactants using a metallic silver electrode as the indicator electrode. The stoichiometry of these complexes has been extensively studied (2). The complexes mentioned have been used as sensing materials in numerous different types of barium- and nonionic surfactant selective electrodes. The Vytras group (3 – 4) and those from Moody and Thomas (5 – 9) belong to the pioneers of the investigations in this area. The hexadecylpyridinium dodecylbenzenesulfonate ion-pair was also used as a sensing material for potentiometric titration of polyethoxylated nonionic surfactants (10). An extensive review of the use of polyethoxylate complexes in analytical chemistry has been presented by Okada (11). Barium-polyethoxylate-TPB complex was employed, besides nonionic surfactants determination (12 – 13), also for the determination of the number of ethoxy groups as well as polyethylene glycol content in nonionic surfactants (14). The commercial Orion surfactant electrode has been successfully applied for end-point detection in the

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M. Sak-Bosnar et al.: Sensitive potentiometric method for determination of micromolar level

2 Experimental

C12/C18E7, Mr = 515, 7 EO groups (Cognis, Germany); Genapol OA 080, polyethylene glycol fatty alcohol based, C14/C15E8, Mr = 440, 8 EO groups (Clariant; Germany); Genapol OX 100, polyethylene glycol fatty alcohol based, C12/C15E10, Mr = 645, 10 EO groups (Clariant; Germany); Genapol T 110, polyethylene glycol fatty alcohol based, C16/C18E11, Mr = 745, 11 EO groups (Clariant; Germany); Slovasol 458, polyethylene glycol fatty alcohol based, C14/ C15E8, Mr = 570, 8 EO groups (Sloveca, Slovakia); Slovasol 6811, polyethylene glycol fatty alcohol based, C16/C18E11, Mr = 738, 11 EO groups (Sloveca, Slovakia). Sodium tetraphenylborate (Fluka, Switzerland) was used as the titrant. Besides the standard solutions recommended by the electrode manufacturer (1 · 10–2 and 2 · 10–3 mol/L) (25), the following more diluted solutions were employed for low level potentiometric titration: 1 ·10–3, 5 · 10–4, 2.5 · 10–4, 2 · 10–4 and 1 · 10–4 mol/L. All standard tetraphenylborate solutions were buffered with borate buffer solution pH = 10.0. The stock tetraphenylborate solution (1 · 10–2 mol/L) contained 10 g polyvinyl alcohol per liter. Barium chloride (Kemika, Croatia) solution (c = 0.1 mol/L) was used for pseudo-ionic complex formation with nonionic surfactants investigated. To test the applicability of the method to modelled effluent solutions of formulated detergents, the following mixtures were used: mixture A (containing anionic surfactant, Na-tripolyphosphate, silicate, phosphonate, carboxymethylcellulose, water), mixture B (containing anionic surfactant, soap, Na-tripolyphosphate, Na2SO4, Na2CO3, Na-perborate, silicate, EDTA, antifoaming agent, carboxymethylcellulose, tetraacetylethylenediamine, optical brightener, enzymes cocktail) and mixture C (containing anionic surfactant, Na-tripolyphosphate, silicate, polycarboxylate, phosphonate, ethanolamines, enzymes cocktail, CaCl2, antifoaming agent, preservative, perfume, dyestuff, water). After addition of nonionic surfactant the mixtures represent typical frame formulations for anionic/nonionic based detergent products and were used after appropriate dilution. Redistilled water was employed in all the investigations.

2.1

2.3

automated titration of nonionic surfactants using NaTPB as titrant, accompanied by the corresponding complex-forming stoichiometry (15). A membrane ion-selective electrode based on the picrates of triphenylmetane or xanthene dyes has also been used for the titrimetric analysis of nonionic surfactants (16). The state of barium-polyethoxylated nonylphenol-TPB complexes in solvent-plasticiser medium has been studied conductometrically (17). Titration of low polyethoxylated nonionic surfactants after derivatization and introduction of an anionic group was described by Buschmann (18). An all-solid-state potentiometric sensor based on a combination of two polyethoxylate complexes and two different plasticisers as sensing material has been developed by Martinez-Barrachina et al. (19). The loss of electrode selectivity toward alkali metals caused by nonionic surfactants has been used as an idea for direct potentiometric determination of low level nonionic surfactant (0 – 5 ppm) (20). Potentiometric flow injection analysis was also employed for determination of nonionic surfactants in environmental samples (21 – 22). An on-line titration of nonionic surfactants, used in wastewater treatment plants has been described, too (23). Nonionic surfactants can be determined potentiometrically using liquid ion-selective electrodes (24). The serious drawback of almost all nonionic surfactant electrodes is the deterioration of the electrode performances caused by the formed precipitate of barium-polyethoxylatetetraphenylborate compound, which easily adheres to the electrode membrane. This is more evident in solutions containing higher concentration of nonionic surfactant. Therefore, the use of solutions of lower surfactant concentration may significantly prolonge the lifetime of electrode and lower the detection limit. The objective of the investigations described was to find out the lowest titrant concentration which still provides analytically usable titration curves, as well as to determine appropriate mathematical and statistical methods for their equivalent point evaluation.

Electrodes

NIO surfactant electrode, Metrohm, Herisau, Switzerland has been used as indicator for end-point detection. Between measurements, the electrode was kept in air. The lifetime of the electrode, according to the manufacturer, has been estimated at six months in pure surfactant solutions, but decreases in more complex solutions. After performing several titrations the electrode was rinsed with methanol or wiped with a tissue that was moistened with methanol. A silver/silver chloride reference electrode, SGJ, reference electrolyte c(KCl) = 3 mol/L (Metrohm, Switzerland) was used. 2.2

Reagents and solutions

The following ethoxylated nonionic surfactants were used in the investigations described: Brij 35, polyethylene glycol lauryl ether, C12E23, 30 % solution, Mr = 1200, 23 EO groups (Sigma-Aldrich, Switzerland); Triton X-100, octylphenol decaethylene glycol ether, C8E10, Mr = 647, 10 EO groups (Merck, Germany); PEG 200, polyethylene glycol, Mr = 200, 4 EO groups (ICN Biochemicals, Inc., USA), PEG 1000, polyethylene glycol, Mr = 1000, 22.3 EO groups (Fluka, Switzerland); Dehydol LT 5, polyethylene glycol fatty alcohol based, C12/C18E5, Mr = 420, 5 EO groups (Cognis, Germany); Dehydol LT 7, polyethylene glycol fatty alcohol based,

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Apparatus

The 751 GPD Titrino (Metrohm, Switzerland), an all-purpose titrator combined with Metrohm 806 Exchange unit (Metrohm, Switzerland) as a dosing element were used for the potentiometric titrations. The solutions were magnetically stirred during titrations using the 727 Ti Stand (Metrohm, Switzerland). 2.4

Procedure

The electrode response toward tetraphenylborate – and barium-ions has been investigated in the range 10–6 to 10–2 mol/L using incremental addition by means of a Metrohm dosing unit (accuracy 0.001 mL). The volume of solution used for titration varied between 20 and 100 mL, depending on sample nature and expected surfactant concentration. Ten milliliters of barium chloride solution (c = 0.1 mol/L) has been added into the solution before titration. All measurements and titrations were performed at room temperature while stirring, without ionic strength and pH adjustments, except where emphasized. The titrator was programmed to work in MET (Monotonic Equivalent point Titration) Mode with dosing increments of 0.1 or 0.2 mL. Equilibrium time was fixed at 30 or 60 s, depending on the titrant concentration and the nature of sample. Drift control was switched off.

Tenside Surf. Det. 44 (2007) 1

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M. Sak-Bosnar et al.: Sensitive potentiometric method for determination of micromolar level

3 Results and discussion

3.2

3.1

The influence of several inorganic cations on the electrode response characteristics was investigated. The reason for investigating their interfering effect is the ability of polyethoxylated nonionic surfactants for complexing of alkali and earth-alkali metals. The formed complexes reveal a pseudocationic character and are readily precipitated with tetraphenylborate. The influence of cations investigated on the electrode response was expressed as a selectivity factor and determined using matched potential method (MPM), which is independent of the Nikolskii-Eisenman equation, but strongly dependent on the experimental conditions, and is applicable in cases of non-Nernstian responses (26 – 27). By that, the selectivity factor is defined as the activity ratio of the primary ion and the interfering ion which gives the same potential change in a reference solution:

Response characteristics of the electrode

The response of the NIO surfactant electrode toward tetraphenylborate is shown in Figure 1. The electrode exhibited an almost theoretical response according to the Nernst equation E ¼ E 0 þ S  log aTPB , where E 0 = constant potential term, S = slope, aTPB = activity of tetraphenylborate anion. The electrode response to barium ions was also investigated (Figure 2) resulting in nearly Nernstian behaviour according to E ¼ E 0 þ S  log aBa2þ , where aBa2þ = activity of barium cation. The activity coefficients for the both, tetraphenylborate and barium, were calculated according to Davies equation. The response characteristics followed by the corresponding statistics are given in Table 1. The electrode indicated a sub-Nernstian response toward several selected polyethoxylated nonionic surfactants and polyethylene glycols in pure water (Figure 3). At the lower concentrations (below 4 · 10–5 mol/L) the electrode responded linearly to log cNS (cNS = concentration of the nonionic surfactant) with the slopes between 16 – 18 mV/decade and curvilinearly at the higher concentrations. There was practically no response from polyethylene glycols (PEG 200 and PEG 1000).

Interferences

kMPM ¼ IJ

DaI DaJ

ð1Þ

where aI and aJ are the activities of the primary and interferent ion respectively. To determine the selectivity factor, one would measure the change in potential while changing the primary ion activity. The interfering ion would then be added to an identical reference solution until the same potential change was obtained. Parameters

Ions investigated Tetraphenylborate

Barium

Slope/(mV/decade)

60.5 ± 1.1

26.8 ± 0.4

Intercept/mV

– 312 ± 5

131 ± 2

Correlation coefficient (r)

0.9993

0.9996

Detection limit/(mol/L)

1.5 · 10–6

2.5 · 10–6

Useful concentration range/(mol/L)

2.0 · 10–6 – 3.2 · 10–3 3.0 · 10–6 – 2.0 · 10–3

Table 1 Response characteristics of Metrohm NIO electrode toward tetraphenylborate and barium ion given together with ± confidence limits

Figure 1 Response characteristics of NIO surfactant electrode toward tetraphenylborate (TPB)

Figure 2 ium ion

Response characteristics of NIO surfactant electrode toward bar-

Tenside Surf. Det. 44 (2007) 1

Figure 3 Response characteristics of NIO surfactant electrode toward several selected ethoxylated nonionic surfactants and polyethylene glycols

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M. Sak-Bosnar et al.: Sensitive potentiometric method for determination of micromolar level

Figure 4 Titration curves of selected nonionic surfactants with NIO surfactant electrode using variable titrant (sodium TPB) concentrations (&, c(TPB) = 2 · 10–3 mol/L); (&, c(TPB) = 1 · 10–3 mol/L); (., c(TPB) = 5 · 10–4 mol/L); (*, c(TPB) = 2.5 · 10–4 mol/L). Here and in further figures some curves are displaced laterally or vertically for clarity

The selectivity factors were determined at the interferent concentration of 10–4 mol/L. For all measurements a constant initial background solution (c = 10–6 mol/L) of Triton X-100 was used as reference. The calculated selectivity factors are given in Table 2. The significant interfering effect of potassium can be attributed to the practically theoretical tetraphenylborate response of the electrode and the well known precipitation reaction of tetraphenylborate and potassium. Lead and barium (see Figure 2) interfere strongly, as well as cationic surfactants (not shown).

usable titration curves, enabling determination of the correspondingly low nonionic surfactant concentration, reducing the analyte volume and prolonging electrode lifetime. The formation of tetraphenylborate (TPB) salts of pseudocationic barium complexes of nonionics surfactants follows in two steps. The first step is complexing the barium ion with polyethoxylated nonionic surfactants according to the following scheme:  2þ Ba2þ þ xEONS Ð BaðEONSÞx ð2Þ

3.3

The “x” value varies depending on the number of ethoxy (EO) units in the surfactant molecule. It was found that one barium ion complexes 10 – 12 EO groups.

Potentiometric titration

The main application of the electrode used was to indicate the end-point in nonionic surfactant potentiometric titrations. The standard solutions of sodium tetraphenylborate (concentration range: 2 · 10–3 till 10–4 mol/L), used as titrants, have been applied for the titration of pseudocationic complexes of polyethoxylated nonionics surfactants with barium. The objective was to find out the solution of the minimum titrant concentration, which still gives analytically

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Interferent

Na+

K+

Ca2+

Mg2+

Pb2+

kij MPM

0.10

0.37

0.08

0.15

7.09

Table 2 Potentiometric selectivity factors (kij MPM ) for different inorganic cations obtained by Matched Potential Method (MPM) for NIO electrode

Tenside Surf. Det. 44 (2007) 1

M. Sak-Bosnar et al.: Sensitive potentiometric method for determination of micromolar level

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In the second step the hardly soluble compound is formed:  2þ   BaðEONSÞx þ 2TPB Ð BaðEONSÞx ðTPBÞ2 ð3Þ The stoichiometry of the above reactions depends on the chain length of the oxyethylene part (hydrophilic) of the non-ionic surfactant investigated, as well as on the nature of the rest of the surfactant molecule (hydrophobic part). The standard solutions (1 · 10–3, 5 · 10–4, 2.5 · 10–4, 2 · 10–4 and 1 · 10–4 mol/L) of TPB were used as titrants in the determinations of nonionic surfactants forming the water insoluble barium – nonionic surfactant – TPB complexes. The values of the stoichiometric factors, calculated from the investigations performed (Table 3), are in good agreement with the data from the papers from Vytras (3 – 4) and Gallegos (15). A series of polyethoxylated nonionic surfactants containing 5 to 23 EO groups, most frequently used in liquid and powdered household and industrial detergent formulations, have been titrated potentiometrically using the NIO surfactant electrode as end-point indicator. The resulting potentiometric titration curves of some selected nonionic surfactants, covering the entire range of EO groups investigated, are shown in Figure 4. The analyte concentration at these investigations was on the order of 10–6 mol/L. The results of these determinations are given in Table 4. The titration curves exhibited well defined and sharp inflexion for titrants concentrations of 2 · 10–3 and 1 · 10–3 mol/L, which facilitate a common end-point location using the first derivative method. By the titrations with the lower titrant concentrations (5 · 10–4, 2.5 · 10–4 and 2 · 10–4 mol/L) less pronounced inflexions are obtained. The equivalence point volumes of these titrations have been reliably calculated from the derivative curves by using the extension of the least-squares regression formalism of the Savitzky-Golay method, developed by Barak (28), with statistical testing of additional terms of polynomial degree leading to an adaptive-degree polynomial filter, which selects the lowest polynomial degree that is statistically justifiable. The application of this method has been exemplified in Figure 5. The potentiometric titration curve displayed (&), as well as its first derivative (&), reveals a poorly defined end point. By using of the modified Savitzky-Golay method mentioned, the strong pronounced minima on the first derivative curve has been obtained (~) which enabled reliable end-point location. The equivalence

Tenside Surf. Det. 44 (2007) 1

point can be more precisely located as the intersection of the second derivative curve with the x-axis (not shown). The use of an even lower titrant concentration of 10–4 mol/L was possible only in reduced sample size (20 mL, employed by titrating of effluents), as distinguished from sample volume of 100 mL used at titrations shown in Figure 4. 3.3.1 Titration of nonionic surfactant based detergent products used as modelled effluents

In this step, the more complex solutions of powdered and liquid detergent products, used as modelled effluents, were employed for investigations. In order to check the influence of ingredients from the formulated products, the known addition of selected nonionic surfactants was added in the product solution investigated. The corresponding potentiometric titration curves are shown in Figure 6. The titration curves of all the synthetic formulations exhibited satisfactory

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M. Sak-Bosnar et al.: Sensitive potentiometric method for determination of micromolar level

Surfactant investigated

Mean Mr declared

Mean number of EO groups

Stoichiometric constant, n(OEU)/n(BPh4–)* (found ± std.dev)

Dehydol LT 5

420

5

5.54 ± 0.34

Dehydol LT 7

515

7

5.77 ± 0.34

Slovasol 458

570

8

5.49 ± 0.23

Genapol OA 080

571

8

5.95 ± 0.16

Triton X-100

647

10

5.91 ± 0.27

Genapol OX 100

645

10

5.65 ± 0.24

Genapol T 110

745

11

5.30 ± 0.14

Slovasol 6811

738

11

5.31 ± 0.14

Brij 35

1200

23

6.30 ± 0.06

* average of 5 determinations  rN–1 Table 3 Results of stoichiometric constants calculated on the basis of potentiometric titrations of some selected commercial and analytical grade nonionic surfactants using different titrant concentrations

Sample

c(surfactant aded)/lM

c(surfactant found)*/lM

Recovery*/%

Dehydol LT 7, 7 EO

0.74 1.47 2.93 5.84

0.77 ± 0.04 1.46 ± 0.03 2.96 ± 0.08 5.87 ± 0.06

104.0 ± 5.4 99.4 ± 2.0 97.7 ± 2.6 100.5 ± 1.0

Genapol OX 100, 10 EO

0.59 1.18 2.33 4.66

0.59 ± 0.02 1.20 ± 0.04 2.31 ± 0.03 4.64 ± 0.10

100.4 ± 4.0 101.7 ± 3.4 98.7 ± 1.3 99.6 ± 2.1

Slovasol 6811, 11 EO

0.51 1.04 2.04 4.11

0.51 ± 0.01 1.03 ± 0.03 2.06 ± 0.03 4.05 ± 0.12

99.9 ± 2.0 99.1 ± 2.9 101.1 ± 1.5 98.3 ± 2.9

0.32 0.63 1.25 2.54

0.31 ± 0.07 0.63 ± 0.07 1.25 ± 0.07 2.54 ± 0.10

98.2 ± 2.0 99.6 ± 1.0 100.0 ± 0.5 99.8 ± 0.4

Brij 35, 23EO

* average of 5 determinations ± rN–1 Table 4 Results of potentiometric titrations of some selected ethoxylated nonionic surfactants of different concentrations

Figure 5 Titration curve of Dehydol LT 5 with NIO surfactant electrode using sodium TPB as titrant (&, c(TPB) = 2 · 10–4 mol/L); its first derivative (&); and first derivative curve obtained by using the extension of the least-squares regression formalism of the Savitzky-Golay method (~)

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Figure 6 Titration curves of several modelled powder and liquid formulations of detergent products (A, B and C) and with added known quantity of nonionic surfactant (NS) with NIO surfactant electrode and sodium TPB as titrant (A1 = A + 150 lg, B1 = B + 50 lg, B2 = B + 100 lg, B3 = B + 150 lg, C1 = C + 150 lg)

inflexion in the range of the equivalence point enabling reliable end-point detection using the methodology described in the former section. The added amount of surfactant, as low as 50 lg, exhibited an analytically usable inflexion shift compared to the blank titration. No significant difference has been seen by testing the influence of pH on the shape of titration curves in the range 2 – 12 on the synthetic formulations with known added nonionic surfactant. The effect of ionic strength on the form of titration curves has also been investigated (Figure 7). Salt concentrations (sodium chloride) up to 106 times higher than that of nonionic surfactants demonstrated no significant interferences. Cationic surfactants react with tetraphenylborate, too, by forming hardly soluble ion-pairs. Therefore in the presence of cationic surfactants two titrations

Sample

c(surfactant added)/lM

c(surfactant found)*/lM

Recovery*/%

Product A

0.203

0.209 ± 0.007

103.0 ± 3.4

Product B

0.291

0.296 ± 0.006

101.7 ± 2.1

Product C

0.291

0.307 ± 0.002

105.5 ± 0.7

* average of 5 determinations ± rN–1 Table 5 Results of potentiometric titrations of modelled effluents of variable complexity

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M. Sak-Bosnar et al.: Sensitive potentiometric method for determination of micromolar level

Sample

c(surfactant content)*/lM

c(surfactant added)/lM

c(surfactant found) */lM

Recovery*/%

1

0.112 ± 0.004

0.136

0.128 ± 0.002

94.1 ± 1.5

2

0.450 ± 0.002

0.230

0.240 ± 0.004

103.4 ± 1.7

3

0.450 ± 0.002

0.390

0.370 ± 0.007

94.7 ± 1.9

* average of 5 determinations ± rN–1 Table 6 Total nonionic surfactant recoveries found in industrial waste water on using the potentiometric titration at a spiked level from 0.136 to 0.390 lM nonionic surfactant

should be carried out. In the first one (without BaCl2 addition) only cationic surfactants are titrated; the second titration (with BaCl2 addition) comprises both cationic and nonionic surfactants. The nonionic surfactant content can be calculated from the difference. The concentration of nonionic surfactant in the solutions titrated was of the magnitude of 10–6 mol/L. The results of determinations are given in Table 5. 3.3.2 Titration of industrial waste waters

Eight waste water samples collected at different times and different places, originating from a manufacturer of surfactant based products, were used for investigation. When necessary, the sample was diluted and filtered. No pH or ionic strength adjustments were made. The pH values of samples varied between 6 and 9. The potentiometric titration curves of the wastewaters themselves with known addition of nonionic surfactant are shown in Figure 8. The results of nonionic surfactants recoveries are given in Table 6. It can be seen, that potentiometric titration curves for all the samples revealed an analytically usable inflexion, enabling the reliable equivalence point detection using the previously mentioned Savitzky-Golay-Barak method. The nonionic surfactant content in waste waters has been recalculated as Triton X-100 concentration, which is conventionally used as a reference nonionic surfactant in wastewater analysis. The corresponding results are shown in Table 7.

Sample

c(surfactant content)*/lM

Standard deviation* (rN–1)

1

0.047

0.006

2

0.114

0.012

3

0.072

0.007

4

0.075

0.005

5

0.096

0.005

6

0.112

0.006

7

0.179

0.006

8

0.151

0.009

* average of 5 determinations Table 7

Results of potentiometric titrations of industrial waste waters

4 Conclusions

The Metrohm NIO surfactant electrode has been used as an end-point detector for improved and more sensitive potentiometric titration of micromolar level of polyethoxylated nonionic surfactants. Even the very low concentration of standard sodium tetraphenylborate solutions (up to 10–4 mol/L), used as titrant, can be successfully applied for the titration of nonionic surfactants, thus increasing the sensitivity of the method up to 20 times. A series of polyethoxylated nonionic surfactant containing 5 to 23 EO groups has been titrated potentiometrically at different concentration levels (down to 10–6 mol/L). The values of the stoichiometric factors, calculated from the investigations performed, are in good agreement with the data from the literature. The titration end-point has been located by applying extended least-squares regression formalism of the Savitzky-Golay method. The accuracy and precision of the determination has been evaluated by using standard addition method. The pH value of the solution investigated does not influence the shape of titration curves in the range 2 – 12 on the modelled formulations with known added nonionic surfactant. There is no significant interference from the ionic strength magnitude on the form of titration curves either. The low surfactant concentration has been determined without preconcentration in the modelled formulations of widely used detergent products and industrial waste waters. The electrode described was used for the monitoring of nonionic surfactants in industrial wastewaters too, without pretreatment, except filtration when necessary. Acknowledgement

Figure 7 The influence of the ionic strength of the solution to the shape of titration curves at potentiometric titration of Dehydol LT 7 (data on the graph relate to the NaCl concentration in the solutions investigated)

Tenside Surf. Det. 44 (2007) 1

The authors greatfully acknowledge the support of Saponia Chemical, Pharmaceutical and Foodstuff Industry, especially to Mrs. D. Marijanovic, Head of Institut, Mr. G. Glavas, Quality Manager, as well as to Mrs. M. Ambreus, Mrs. K. Galovic and Mrs. B. Klobucar for sample preparation and useful discussion

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M. Sak-Bosnar et al.: Sensitive potentiometric method for determination of micromolar level

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during the experimental work. Financial help of the Croatian Ministry of Science, Technology and Sports given to the project no. 0058016 is greately appreciated. The authors are also indebted to Prof. Dr. Bozidar S. Grabaric for fruitful discussion and careful reading and correcting the final version of the typescript. References 1. Levins, R. J. and Ikeda, R. M.: Direct potentiometric titration of polyethylene glycols and their derivatives with sodium tetraphenylboron, Anal. Chem. 37 (1965) 671 – 675. 2. Delduca, P. G., Jaber, A. M. Y., Moody, G. J. and Thomas, J. D. R.: Tetraphenylborate salts of alkali and alkaline earth metal complex cations, J. Inorg. Nucl. Chem. 40 (1978) 187 – 193. 3. Vytras, K., Dvorakova, V. and Zeman, I.: Titrations of non-ionic surfactants with sodium tetraphenylborate using simple potentiometric sensors, Analyst 114 (1989) 1435 – 1441. 4. Vytras, K., Varmuzova, I. and Kalous, J.: A potentiometric study and determination of compounds containing poly(oxypropylene) chains, Electrochim. Acta 40 (1995) 3015 – 3020. 5. Jones, D. L., Moody, G. J. and Thomas, J. D. R.: Potentiometry of alkoxylates, Analyst 106 (1981) 439 – 447. 6. Jones, D. L., Moody, G. J., Thomas, J. D. R. and Birch, B. J.: Barium-polyethoxylate complexes as potentiometric sensors and their application to the determination of non-ionic surfactants, Analyst 106 (1981) 974 – 984. 7. Alexander, P. H. V., Moody, G. J. and Thomas, J. D. R.: Electrode membrane and solvent extraction parameters relating to the potentiometry of polyalkoxylates, Analyst 112 (1987) 113 – 120. 8. Moody, G. J. and Thomas, J. D. R.: Potentiometry of oxyalkylates in nonionic surfactants chemical analysis, M. Dekker Inc., New York (1987) 117 – 136. 9. Moody, G. J., Thomas, J. D. R., Lima, J. L. F. C. and Machado, A. S. C.: Characterisation of poly(vinyl chloride) barium ion-selective electrodes without an internal reference solution, Analyst 113 (1988) 1023 – 1027. 10. Ivanov, V. N. and Pravshin, Yu. S.: Determination of nonionogenic surfactants using ion-selective electrodes, J. Anal. Chem. 41 (1986) 291 – 295. 11. Okada, T.: Complexation of poly(oxyethylene) in analytical chemistry, A review, Analyst 118 (1993) 959 – 971. 12. Chernova, R. K., Kulapina, E. G., Materova, E. A., Kulapin, A. I. and Tret’yachenko, E. V.: Electrochemical and analytical properties of surfactant-selective electrodes, J. Anal. Chem. 50 (1995) 643 – 651. 13. Kulapina, E. G. and Apukhtina, L. V.: Selective electrodes based on Ba2+-polyethoxylate-tetraphenylborate, J. Anal. Chem. 52 (1997) 1151 – 1156. 14. Chernova, R. K., Kulapina, E. G., Materova, E. A., Chernova, M. A., Tret’yachenko, E. V., Novikova, L. V., Channova, G. K. and Ochneva, N. I.: Ionometric determination of the number of oxyethyl groups of nonionogenic surfactants, Zavod. Labor. 58 (1992) 6 – 8. 15. Gallegos, R. D.: Titration of non-ionic surfactants with sodium tetraphenylborate using the Orion surfactant electrode, Analyst 118 (1993) 1137 – 1141. 16. Khmel’nitskaya, E. Yu. and Kolokolov, B. N.: Determination of the number of oxyethyl groups in nonionic surfactants using ion-selective electrodes, J. Anal. Chem. 50 (1995) 1108 – 1110. 17. Kulapina, E. G. and Apukhtina, L .V.: A study of the state of electroactive compounds of nonionic surfactant-selective electrodes in dibutyl phtalate, J. Anal. Chem. 53 (1998) 140 – 143. 18. Buschmann, N. and Hülskötter, F.: Titration procedure for low ethoxylated nonionic surfactants, Tenside Surf. Det. 34 (1997) 8 – 11. 19. Martinez-Barrachina, S., Alonso, J., Matia, L., Prats, R. and del Valle, M.: Allsolid-state potentiometric sensors sensitive to nonionic surfactants based on ionophores containing ethoxylate units, Talanta 54 (2001) 811 – 820. 20. Giannetto, M., Minari, C. and Mori, G.: Potentiometric determination of nonionic surfactants by liquid membrane electrodes, Electroanalysis 15 (2003) 1598 – 1600. 21. Martinez-Barrachina, S., del Valle, M., Matia, L., Prats, R. and Alonso, J.: Potentiometric flow injection system for the determination of polyethoxylate nonionic surfactants using tubular ion-selective electrodes, Anal. Chim. Acta 438 (2001) 305 – 313.

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Correspondence to Dr. Milan Sak-Bosnar Department of Chemistry Josip Juraj Strossmayer University of Osijek F. Kuhaca 20 HR-31000 Osijek, Croatia E-mail: [email protected]

The authors of this paper Milan Sak-Bosnar obtained his Ph.D. in 1982. His main research areas are surfactant analyses, electroanalytical methods and chemical sensors. At present he is employed at the Department of Chemistry, Josip Juraj Strossmayer University of Osijek. Dubravka Madunic-Cacic obtained her B.Sc. at the Faculty of Food Technology, Josip Juraj Strossmayer University of Osijek. Since 1995 she is employed in Saponia Chemical Industry, Osijek, at the Department for Analytical Methods Development (electrochemical and enzymatic methods). Her main research interests are development and application of chemical and electrochemical sensors for surfactant and enzyme based products analysis. Ruzica Matesic-Puac received her diploma in food technology from Faculty of Food Technology, Josip Juraj Strossmayer University of Osijek, M.Sc. degree from Faculty of Science and her Ph.D. from University of Zagreb. Currently she is assistant lecturer in analytical chemistry at the Faculty of Food Technology, Josip Juraj Strossmayer University of Osijek. Her main research areas are ion-selective electrodes, surfactants and electroanalytical methods. Prof. Dr. Zorana Grabaric is a full professor of chemistry at the Faculty of Food Technology and Biotechnology, University of Zagreb, teaching General and Inorganic Chemistry and Instrumental analysis (electrochemistry). Her main research interests are development and application of chemometric methods for resolution of analytical signals and development of chemical sensors and biosensors for application in food analyses and biomedicine.

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