Determination of cationic surfactants in pharmaceutical disinfectants using a new sensitive potentiometric sensor

Talanta 76 (2008) 259–264

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Determination of cationic surfactants in pharmaceutical disinfectants using a new sensitive potentiometric sensor ˇ cic´ a , Milan Sak-Bosnar b,∗ , Olivera Galovic´ b , ´ Caˇ Dubravka Madunicb ´ Nikola Sakaˇc , Ruˇzica Mateˇsic-Puaˇ cb a b

Saponia Chemical, Pharmaceutical and Foodstuff Industry, M.Gupca 2, HR-31000 Osijek, Croatia Department of Chemistry, Josip Juraj Strossmayer University of Osijek, F. Kuhaˇca 20, HR-31000 Osijek, Croatia

a r t i c l e

i n f o

Article history: Received 21 November 2007 Received in revised form 18 February 2008 Accepted 20 February 2008 Available online 4 March 2008 Keywords: Surfactant sensor PVC membrane Cationic surfactant Potentiometric titration Pharmaceuticals

a b s t r a c t A new sensitive potentiometric surfactant sensor was prepared based on a highly lipophilic 1,3didecyl-2-methyl-imidazolium cation and a tetraphenylborate antagonist ion. This sensor was used as a sensing material and incorporated into the plasticized PVC-membrane. The sensor responded fast and showed a Nernstian response for investigated surfactant cations: cetylpyridinium chloride (CPC), hexadecyltrimethylammonium bromide (CTAB) and Hyamine with slope 59.8, 58.6 and 56.8 mV/decade, respectively. The sensor served as an end-point detector in ion-pair surfactant potentiometric titrations using sodium tetraphenylborate as titrant. Several technical grade cationic surfactants and a few commercial disinfectant products were also titrated, and the results were compared with those obtained from a two-phase standard titration method. The sensor showed satisfactory analytical performances within a pH range of 2–11, and exhibited excellent selectivity performance for CPC compared to all of the organic and inorganic cations investigated. The influence of the nonionic surfactants on the shape of titration curves was negligible if the mass ratio of ethoxylated nonionic surfactants and cationic surfactants (EONS:CS) was not greater than 5. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Cationic surfactants account for only 5–6% of the total surfactant production. However, they are extremely effective for some specific uses because of their peculiar properties. Their positive charge allows them to adsorb on negatively charged substrates, while most solid surfaces are at neutral pH. This capacity confers to them an antistatic behavior and a softening action for fabric and hair rinsing. The positive charge enables them to operate as flotation collectors, hydrophobating agents, corrosion inhibitors, and solid particle dispersants. Many cationic surfactants are also used as bactericides. They are used to clean and aseptize surgery hardware, to formulate heavy-duty disinfectants for domestic and hospital use, and to sterilize food bottles or containers, particularly in the dairy and beverage industries. Cationic and anionic surfactants have usually been determined by two-phase titration [1], but this technique suffers from a large number of drawbacks such as the limitation of application to strongly colored and turbid samples, the toxicity of the organic chlorinated solvent used, the formation of emulsion

∗ Corresponding author. Tel.: +385 31 495 530; fax: +385 31 495 549. E-mail address: [email protected] (M. Sak-Bosnar). 0039-9140/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2008.02.023

during titration that can disturb visual end-point detection, and the numerous matrix interferences. These limitations are overcome by the use of ion-selective electrodes in direct potentiometry or as indicators in potentiometric surfactant titration. Potentiometric titrations involve reaction with an oppositely charged ion (ion-pair formation). An Al-wire electrode coated with plasticized PVC membrane was developed for potentiometric titration of cationic and ampholytic surfactants using the solution of sodium tetraphenylborate as titrant [2,3]. Denter et al. investigated a commercial liquid membrane PVC body electrode for the determination of cationic surfactant in a comparative study [4]. Buschmann and Schulz proposed a method for the potentiometric titration of cationic and zwitterionic surfactants with sodium tetraphenylborate (TPB) using the PVC membranebased electrode as an end-point indicator [5]. Due to increasing use of cationic surfactants (corrosion inhibitors, flotation reagents, softeners in textile and detergent industries, disinfectants in pharmacy, medicine and cosmetics, etc.), considerable interest for the application of cationic surfactant-sensitive electrodes has been seen in recent years [6–13]. Several ion-pair based sensing materials have been used for potentiometric determination of cationic surfactants in some pharmaceutical formulations. Cetylpyridinium–iodomercurate PVC

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membrane ion selective electrode was used for the determination of cetylpyridinium cation in mouthwash [14]. Amodiaquine, an antimalarial drug prophylactic, can be potentiometrically determined using a polymeric membrane electrode based on the cationic drug and sodium tetraphenylborate or potassium tetrakis(4chlorophenyl) borate ion-pair [15]. A PVC-membrane sensor based on s-benzylthiuronium tetraphenylborate was used as an endpoint indicator by potentiometric titration of CPC in some mouth wash preparations [16]. The capillary electrophoresis method was developed for the determination of cationic surfactants and benzethonium and cetylpyridinium ions, which are commonly used as preservatives in various pharmaceutical and cosmetic products [17]. The surfactant to dye binding degree (SDBD) method was extended to the determination of cationic surfactants in pharmaceuticals [18–20]. Cationic surfactants can be determined in environmental samples using ion-sensitive field effect transistor (ISFET) devices [21], flow injection analysis [22], and spectrophotometry [23]. Spectrofluorimetry [24] and optical sensors [25] can also be used for their determination. In this study, a PVC-plasticized liquid type ionic surfactant sensitive membrane was prepared based on 1,3-didecyl-2methylimidazolium–tetraphenylborate (DMI–TPB) as a sensing ion-pair element and o-nitrophenyloctylether (o-NPOE) as a plasticizer. The sensor was applied for potentiometric titration of the commonly used pure and technical grade cationic surfactants, as well as for cationic surfactant content in several pharmaceutical and household disinfectant formulations. 2. Experimental 2.1. Reagents and materials The standard solutions of sodium dodecylsulfate (DDS) (c = 4 mM) and sodium TPB (c = 5 mM) were used as titrants. Hyamine 1622 (benzethonium chloride, diisobutylphenoxyethoxyethyldimethylbenzylammonium chloride), cetylpyridinium chloride (CPC), 1,3-didecyl-2-methylimidazolium chloride (DMIC), and hexadecyltrimethylammonium bromide (CTAB) were used for sensor response characteristics measurements and for potentiometric titrations. All of the above-mentioned chemicals were of reagent grade quality and supplied by Fluka. The following technical grade cationic surfactants were used for investigations too: alkyldimethylbenzylammonium chloride (ADBAC); alkyldimethylbenzylammonium bromide (ADBAB); didecyldimethylammonium chloride (DDAC); triethanolamine di-esterquat methosulfate (TDM); dihydrogenated tallowethyl hydroxyethylammonium methosulfate (DTHM). The sensor membrane was composed of onitrophenyloctylether (o-NPOE) as plasticizer, high molecular weight poly(vinyl chloride) (PVC) (both the o-NPOE and PVC were from Fluka, Switzerland), and an isolated ion-exchange complex as a sensing material (1.0%).

dried with anhydrous sodium sulfate. The solvent was evaporated at room temperature and the precipitate was dissolved in 10-mL mixture of diethylether:methanol (1:1) by use of an ultrasonic bath. The solvent was evaporated at −18 ◦ C and the isolated precipitate was used as a sensing material for membrane preparation. 2.2.2. Sensor preparation The plasticizer:PVC ratio was 3:2. One hundred and eighty milligrams of the mixture were dissolved in 2-mL tetrahydrofuran using an ultrasonic bath for homogenization. Then the clear solution was carefully poured into a glass ring (i.d. 24 mm, lower side ground flat, Fluka) which was fixed tightly on a glass plate. After curing, small disks (i.d. 7 mm) were punched from the cast film and mounted in a Philips electrode body IS-561 (Glasblaeserei Moeller, Zurich, Switzerland). A mixture of sodium chloride (c = 0.1 M) and sodium dodecylsulfate solution (c = 0.001 M) was employed as the internal filling solution. A few preliminary titrations were used as a preconditioning procedure before using the sensor as an indicator in quantitative titrations. Between measurements, the sensor was kept in distilled water. The lifetime of the sensor was several months. A silver/silver(I) chloride reference electrode (Metrohm, Switzerland) in sodium chloride solution (c = 3 M) was used as one reference. 2.3. Apparatus The all-purpose titrators 751 GPD Titrino and 794 Basic Titrino (Metrohm, Switzerland), combined with Metrohm 806 Exchange units (Metrohm, Switzerland), were used as dosing elements to perform potentiometric titrations. The solutions during titrations were magnetically stirred using the 727 Ti Stand (Metrohm, Switzerland). The Titronic Basic piston burette was combined with the Handylab pH 12 (both manufactured by Schott Geraete GmbH, Germany) and controlled by a PC using self-programmed software to measure the response characteristics of dynamic response time and interferences. 2.4. Procedure The sensor response toward Hyamine, DMIC, CPC and CTAB was investigated at constant ionic strength (0.01 M Na2 SO4 ) using incremental analyte addition. The volume of solution used for titration varied between 25 and 50 mL, depending on the sample nature and expected surfactant concentration. All measurements and titrations were performed at room temperature using a magnetic stirrer without ionic strength and pH adjustment. The titrator was programmed to work in MET (monotonic equivalent point titration) Mode with dosing increments of 0.1 mL, equilibrium time 20 s, and signal drift 5 mV/min. The titrator was also programmed to work in DET (dynamic equivalent point titration) Mode with signal drift 5 mV/min and equilibrium time 60 s. The wait time before the start of titration varied between 30 and 60 s, depending on the sample nature and surfactant concentration.

2.2. Preparation of the PVC membrane sensor

3. Results and discussion

2.2.1. Preparation of the DMI–TPB ion-exchange complex The DMI–TPB ion-exchange complex was prepared by dropwise addition of 10 mL of a 0.01 M sodium tetraphenylborate solution to 10 mL of a 0.01 M DMIC solution. The mixture was magnetically stirred for 30 min. The white precipitate was extracted with three portions of dichloromethane, 30 mL each. The extracts were collected and washed with three portions of water, of 50 mL each, and

3.1. Response characteristics The electromotive force of the membrane sensor assembly when dipped in the solution of cationic surfactant (CS) being investigated is given by the Nernst equation: E = E 0 + S log aCS+

(1)

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Fig. 1. Response characteristics of DMI–TPB surfactant sensor toward Hyamine (), CPC () and CTAB ().

where E0 is the constant potential term, S is the sensor slope, and aCS+ is the activity of surfactant cation. The response characteristics of a DMI–TPB surfactant sensor in solutions of Hyamine, CTAB and CPC are shown in Fig. 1. The slope values and correlation coefficients were calculated from the linear region of the calibration graphs on the five series of measurements using linear regression analysis. The detection limits were estimated according to the IUPAC recommendations [26]. The sensor showed Nernstian response to all of the tested cationic surfactants. The deviations from linearity at the lower concentration level are caused by gradual dissolution of the ion-pair complex from the membrane and by the micelle formation at the higher concentration level. Agreement between several sensor coating procedures was satisfactory concerning analytical performances of the sensors. Statistical evaluation of the sensor characteristics is given in Table 1. The dynamic response of the DMI–TPB surfactant sensor was also evaluated. As shown in Fig. 2, the sensor reached 95% of its equilibrium response for CPC ion within 13 s for the concentration change 1 × 10−6 M → 1 × 10−5 M, 5 s for the concentration change 1 × 10−5 M → 1 × 10−4 M, and 2 s for the concentration change 1 × 10−4 M → 1 × 10−3 M. The corresponding values for the Hyamine and CTAB cations for the same concentration changes were 36, 6 and <3 s and 10, 4 and <3 s, respectively.

Fig. 2. Dynamic response characteristics of DMI–TPB surfactant sensor in Hyamine, CPC and CTAB solutions.

as follows: DMIC > CPC > CTAB > Hyamine. However, those obtained using sodium TPB exhibited higher potential jump at the equivalence point, thus sodium TPB was used in further investigations. The large potential changes at the equivalence point (300–400 mV, depending on the analyte nature) using TPB as titrant enabled high accuracy and sensitivity in cationic surfactant determination.

3.2.2. Titration of technical grade cationic surfactants The main application of the sensor described was for indication of the end-point in ion-pair surfactant potentiometric titrations. The cationic surfactant (CS+ = analyte determined) reacts during titration with the anionic surfactant (AS− = titrant), accompanied by formation of a water insoluble (1:1) ion-pair CS+ AS− (ionexchange or ion-pair complex), which dissociates as follows: CS+ AS−  CS+ + AS−

(2)

For the above equilibrium, the solubility product is defined as: Ksp = a(CS+ ) a(AS− )

(3)

3.2. Potentiometric titration 3.2.1. Titrant selection The standard solutions of DDS and TPB, both of c = 4 mM, were tested as titrants for potentiometric titration of cationic surfactants. The following pure cationic surfactants were used as analytes: Hyamine 1622; CPC; DMIC; CTAB. The resulting titration curves exhibited sharp inflexions for both titrants (Fig. 3), with magnitudes

where a(CS+ ) and a(AS− ) are activities of the corresponding surfactant ions. Before the equivalence point, the change (decrease) of sensor potential responded to the change of cationic surfactant concentration as shown in Eq. (1). After the equivalence point (all the cationic surfactant is precipitated), the increase of anionic surfactant concentration in solution is evident.

Table 1 Response characteristics of DMI–TPB-based liquid membrane sensor selective to the cationic surfactants given together with ±95% confidence limits Parameters

Slope (mV/decade) Correlation coefficient (r) Detection limit (M) Useful conc. range (M)

Cationic surfactants CTAB

CPC

Hyamine 1622

58.6 ± 0.4 0.9999 1.4 × 10−6 2 × 10−6 to 1 × 10−4

59.8 ± 0.9 0.9992 4.6 × 10−6 5 × 10−6 to 1 × 10−4

56.8 ± 1.2 0.9972 2.9 × 10−6 4 × 10−6 to 4 × 10−4

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Fig. 4. The influence of pH value on the potentiometric response of DMI–TPB surfactant sensor in the solutions of CPC: () 5 mM; () 0.5 mM.

Fig. 3. Potentiometric titration curves and their first derivatives of a few cationic surfactants solutions (c = 4 × 10−3 M) with sodium tetraphenylborate () and sodium dodecylsulfate () using DMI–TPB surfactant sensor as indicator. Here and in later figures some curves are displaced laterally or vertically for clarity.

From Eq. (3), a(CS+ ) = Ksp /a(AS− ), and after insertion into Eq. (1), the following sensor response is obtained: E = E 0 + S log

Ksp a(AS− )

products investigated were CPC and ADBAC. Disinfectant for hospital use C contained a synergistic mixture of n-octyl-dimethylbenzylammonium chloride, Hyamine 1622 and methylbenzethonium chloride. The results of cationic surfactant content in the disinfectants for hospital use B and C were expressed in M due to lack of the corresponding average molecular mass data, although the quantitative composition of product C (each cationic surfactant content) was declared. No pH or ionic strength adjustment was made. The pH values of samples varied between 6 and 9. Their potentiometric titration curves and the corresponding first derivatives exhibited clearly defined inflection and enabled unambiguous equivalence point detection. The results (Table 3) were compared to those obtained using a commercial surfactant electrode and exhibited satisfactory mutual agreement.

(4)

which after rearrangement yields: E = const − S log a(AS− )

(5)

const = E0

+ S log Ksp . where From Eq. (5) it follows that after the equivalence point, the sensor responds to the change of anionic titrant (anionic response). Further addition of anionic titrant after the equivalence point causes further decrease of the sensor potential E. It can be also concluded from Eq. (5) that the magnitude of the inflexion at the equivalence point is strongly dependent on the solubility product value. The lower Ksp value causes the higher potential change at the equivalence point, resulting in a more sensitive surfactant determination. The standard solution of TPB was used as the titrant in the determination of cationic surfactants forming the water insoluble (1:1) complexes. The several technical grade cationic surfactants were titrated potentiometrically using the new DMI–TPB membrane sensor as a surfactant sensor for end-point determination. The results of the determinations of some of the most frequently used surfactants, compared with those obtained using a commercial surfactant sensor and two-phase titration method, showed no significant difference (Table 2). The equivalence points for all potentiometric titrations were calculated from the derivative curves. 3.2.3. Titration of commercial products Six cationic surfactant-based commercial disinfectant products were potentiometrically titrated using the new DMI–TPB sensor as an indicator. Cationic surfactants declared in the

Fig. 5. Influence of EONS nature (EO groups number) at fixed CPC:EONS molar ratio 1:2 on the shape of titration curves of CPC using TPB as titrant and DMI–TPB surfactant sensor as indicator. EONS investigated: () CPC only; () 5 EO; () 10 EO; () 12 EO.

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Table 2 Results of potentiometric titrations of some technical grade cationic surfactants using sodium TPB (c = 5 × 10−3 M) as titrant and DMI–TPB surfactant sensor as an indicator, in comparison with the results obtained with a commercial surfactant sensor and standard two-phase titration method Surfactant used

Surfactant contenta DMI–TPB sensor

DDAC ADBAC 1 ADBAC 2 Hyamine 1622 CTAB CPC a b

Two-phase titrationb

Commercial sensor

Found (%)

R.S.D. (%)

Found (%)

R.S.D. (%)

Found (%)

52.83 50.03 53.25 98.36 95.41 97.52

0.51 0.99 0.79 0.58 0.54 0.24

51.50 51.11 53.67 98.27 95.68 97.36

0.71 0.31 0.48 0.54 0.57 0.22

52.42 50.87 53.94 98.59 95.90 98.23

Average of five determinations. Average of three determinations.

Table 3 Results of potentiometric titrations of six disinfectant products using sodium TPB (c = 5 × 10−3 M) as titrant and DMI–TPB surfactant sensor as an indicator, in comparison with the results obtained with commercial sensor Product

Cationic surfactant contenta DMI–TPB sensor

Mouthwash A Mouthwash B Disinfectant for food industry Disinfectant for hospital use A

Disinfectant for hospital use B Disinfectant for hospital use C a

Commercial sensor

Found (%)

R.S.D. (%)

Found (%)

R.S.D. (%)

0.0554 0.0584 4.3360 5.0230

0.99 0.94 0.96 0.34

0.0536 0.0564 4.3690 5.0040

1.02 0.97 0.89 0.12

Found (M)

R.S.D. (%)

Found (M)

R.S.D. (%)

0.1510 0.00373

0.23 0.79

0.1505 0.00351

0.19 0.73

Average of five determinations.

3.3. Interferences 3.3.1. The influence of pH The sensor potential stability was investigated over a wide pH range in order to simulate the practical titration conditions of different formulated products of varying acidity and alkalinity. The investigations were performed in Hyamine 1622 and CPC solutions at two concentration levels: 0.4 and 4 mM. The pH values were adjusted with solutions of NaOH and H2 SO4 (c = 1, 0.1 and 0.01 M). The surfactant solutions investigated contained 0.1 M of Na2 SO4 to provide measurements at constant ionic strength. The sensor potential readings were maintained within ±1 mV. There were no significant sensor potential deviations within the pH range of 2–11, which indicates the applicability of the sensor in strongly acidic and alkaline conditions (Fig. 4). The shapes of titration curves and the magnitude of the potential change at the inflection point, at different pH values, further confirmed the above statement (not shown). 3.3.2. The influence of nonionic surfactants The ethoxylated nonionic surfactants (EONSs) can be a component part of cationic surfactants (CSs) based on formulated products. The widely used class of nonionics are alkoxylated alcohols, which under certain circumstances may exhibit slightly anionic character. Therefore its influence on the potentiometric titration of cationic surfactants was investigated. Five different EONS containing 5, 6, 10, 12 and 25 ethoxy groups (EO), were separately added in different molar proportions to the solution of CPC, one of the most frequently used cationic surfactants. It can be concluded that the titration curves are seriously disturbed for the EONS:CS molar ratio greater than 3 (corresponding mass ratio 6.6). This value depends on the number of EO groups in the EONS molecule. The EONS with higher EO content influence the shape of the titration curves more strongly (Fig. 5). Practi-

cal surfactant-based products are rarely formulated with EONS:CS mass ratios greater than 2. Therefore, it can be concluded that EONS do not interfere seriously with potentiometric titration of CS in common products. 3.3.3. Determination of selectivity coefficients The influence of the interferents on the response of the cationic surfactant sensor described is defined by the Nikolskii–Eisenman equation: 0 E = ECS + + det

RT pot ln[aCS+ + K + + aCS+ ] CS CS F det int det int

pot CS+ CS+

where K

det

int

(6)

is the selectivity coefficient, aCS+ and aCS+ are det

int

the activities of analyte (determined) ion (det) and interfering ion (int), respectively. Table 4 Potentiometric selectivity coefficients for different inorganic and organic cations measured with DMI–TPB surfactant sensor pot

Interference, M

KCS,M

Ammonium Sodium Potassium Magnesium Calcium Zinc Monoethanolamine Triethanolamine Tetraethylammonium Tetrabutylammonium Benzyltrimethylammonium Benzyltriethylammonium

3.1 × 10−5 5.1 × 10−5 2.8 × 10−5 7.7 × 10−5 4.8 × 10−5 2.1 × 10−5 Less than 10−5 Less than 10−5 1.3 × 10−5 2.6 × 10−3 1.8 × 10−5 7.1 × 10−5

CPC was used as the primary (analyte) ion, concentration of the interfering cation was c = 0.02 M.

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The mixed solution method [27] was used for measurement of selectivity coefficients, because it yields more realistic data than the separate solution method for the systems investigated. The sensor response was measured in a series of solutions of varying primary (determined) ion activity aCS+ and fixed interfering ion activity det

aCS+ . The selectivity coefficients were then estimated graphically, int

which is a very subjective and rough method. The more reliable method involved fitting the Nikolskii–Eisenman equation (used as a model) to the experimental data obtained by the mixed solution method. By using Solver, an analysis tool incorporated into Microsoft Excel, the minimal sum of squared residuals was calcupot lated by varying the values of E0 , S and K + + . CS

det

CS

int

The selectivity coefficients of some potentially interfering inorganic and organic cations that are usually present in products were determined for the DMI–TPB ion-pair-based sensor (Table 4). For all selectivity measurements, CPC was used as the primary ion in the range of 10 ␮M to 10 mM, while the concentration of the interfering ion was 20 mM. The new DMI–TPB ion-pair-based sensor exhibited excellent selectivity performances for CPC for all organic and inorganic cations investigated. 4. Conclusions

sensor displayed satisfactory analytical performances within a pH range of 2–11. The influence of the widely used class of nonionic surfactants on the shape of titration curves was negligible if the mass ratio of EONS:CS was not greater than 5. The selectivity coefficients were determined by fitting the Nikolskii–Eisenman equation to the experimental data obtained by the mixed solution method. The sensor exhibited excellent selectivity performances for CPC over all organic and inorganic cations investigated. Acknowledgements The authors gratefully acknowledge the financial support of the Croatian Ministry of Science, Education and Sports given to project No. 291-0580000-0169. The authors are also indebted to Prof. Dr. Bozidar Grabaric for fruitful discussion, careful reading, and correction of the final version of the type-script. References [1] [2] [3] [4] [5] [6] [7]

A new liquid membrane potentiometric surfactant sensitive sensor was prepared. It was based on a highly lipophilic 1,3-didecyl2-methyl-imidazolium cation and tetraphenylborate as antagonist ion and was used as a sensing material and incorporated into the plasticized PVC-membrane. The sensor showed a Nernstian response for all the investigated surfactant cations: 59.8 mV/decade between 5 × 10−6 and 1 × 10−4 M for CPC; 58.6 mV/decade between 2 × 10−6 and 1 × 10−4 M for CTAB; 56.8 mV/decade between 4 × 10−6 and 4 × 10−4 M for Hyamine. The sensor responded very fast and reached 95% of its equilibrium response for the concentration change 1 × 10−5 M → 1 × 10−4 M (the range usually used in potentiometric titration) within 4–6 s for all the cationic surfactants investigated. The main application of the sensor described is the indication of the end-point in ion-pair surfactant potentiometric titrations. The frequently used cationic surfactants of analytical and technical grades were successfully titrated and the results were evaluated using a commercial surfactant electrode. Several commercial disinfectant products containing various cationic surfactants were potentiometrically titrated using the new DMI–TPB sensor as an indicator. The results, compared to those obtained using a commercial surfactant electrode with the standard two-phase titration method, exhibited satisfactory mutual agreement. The resulting potentiometric titration curves from all the investigations revealed analytically usable inflexions, enabling reliable equivalence point detection using the first derivative method. The

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