Using Potentiometric Electrodes Based on Nonselective Polymeric

Article Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

pubs.acs.org/jchemeduc

Using Potentiometric Electrodes Based on Nonselective Polymeric Membranes as Potential Universal Detectors for Ion Chromatography: Investigating an Original Research Problem from an Inquiry-Based-Learning Perspective María Cuartero* and Gastoń A. Crespo*

J. Chem. Educ. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/24/18. For personal use only.

Applied Physical Chemistry Division, School of Engineering Science in Chemistry, Biotechnology and Health, Royal Institute of Technology, KTH, Teknikringen 30, SE-100 44 Stockholm, Sweden S Supporting Information *

ABSTRACT: Because traditional laboratory practices in advanced chemistry education are being replaced by inquirybased approaches, we present herein a new laboratory activity based on a small research project that was designed and executed by students. The laboratory project aims at answering a well-defined research question: how far can potentiometric electrodes based on nonselective polymeric membranes be used as universal detectors in ion chromatography (IC)? Hence, the experiments were designed and conducted to explore the analytical performances of potentiometric electrodes based on different commercial membranes that are typically used in electrodialysis. The nonselective behavior shown by the electrodes permits a critical evaluation of their further implementation as a universal detector of anions in regular IC. Thus, the students were able to integrate a nonselective potentiometric sensor to analyze several anions in flow mode, mimicking the signal that is to be obtained using such electrodes as an IC detector. The proposed practice covers different pedagogical purposes: (i) to develop competence toward “thinking like a scientist” through reflective teaching; (ii) to promote argumentation skills and critical decision making; (iii) to improve students’ research-planning and experimental-design skills; (iv) to refresh conceptual knowledge about analytical detectors, which typically goes unnoticed in laboratory practices; and (v) to reinforce students’ knowledge about the basis of potentiometry. Furthermore, the present document may serve as an easy guide to develop other laboratory practices based on potentiometric sensors. KEYWORDS: Analytical Chemistry, Inquiry-Based/Discovery Learning, Problem Solving/Decision Making, Ion Selective Electrodes, Laboratory Equipment/Apparatus, Graduate Education/Research

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toward a profitable practice. One tool to realize this purpose is the generation of argumentation by proposing reflective questions to the students.3,5,6 Consequently, the aim of the instructor is to bring about metacognitive activities, both in personal and group learning processes, in which each student is involved, rather than stating facts and delivering information related to the practice.7,8 In this work, we describe a new laboratory practice that was designed on the basis of inquiry-based learning to be part of a course related to analytical-separations science. The students were encouraged to carry out a new experimental project with the aim of answering the following research question: how far can potentiometric electrodes based on nonselective polymeric membranes be used as universal detectors in ion chromatography (IC)? Thus, the main goal of the project is to explore the

aboratory practice is a fundamental piece of education in chemistry, especially during upper-level courses in bachelor’s and master’s degree programs, in which students may conduct guided experimental projects. Pedagogical strategies used in these kinds of activities are continuously evolving to ensure the fostering of the appropriate competencies in students’ scientific skills while the students work independently.1 Thus, traditional approaches based on established experimental recipes are being replaced by inquiry-based practices that enable students to further understand experimental design, literature searches, required methods, data analysis, and discussion.2,3 On the one hand, inquiry-based laboratory practice may induce in students an engagement with the research that has to be conducted to achieve a motivated aim and answer welldefined research questions. However, on the other hand, the student may feel some frustration facing some struggles at different steps of the activity.4 Therefore, the decisive role of the instructor is to focus the students’ work in the right direction © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: June 21, 2018 Revised: October 2, 2018

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DOI: 10.1021/acs.jchemed.8b00455 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Article

Supporting Information for more details). Figure 1b shows a picture of the experimental setup, which is composed of the electrochemical cell (reference and working electrodes), stirrer, potentiometer, and computer. Aliquots of the stock solutions were added with micropipettes to the initial 100.0 mL of Milli-Q water to achieve an activity range from 10−6 to 10−1.5. Note that activity is a dimensionless quantity that is calculated with a twoparameter Debye−Hückel approximation from the experimental concentrations.11 Each logarithmic activity was then plotted against the average steady-state potential (registered over 30 s) achieved after each addition. The data were fitted to the Nernst equation (eq 1):

analytical performances of several potentiometric electrodes prepared with different nonselective polymeric membranes and then discuss their potential use as universal detectors for anions in regular IC instrumentation according to the obtained results. This project involves new research never reported before, and therefore, the students are offered the opportunity to design and conduct laboratory experiments as “true scientists”. The practice is designed for a specific number of hours and sessions to fit within the schedule of a subject named “Analytical Separations”, which is framed within a series of chemistry-based courses offered at the master and Ph.D. levels.9 However, it should be noted that the activity is flexible and can be applied to other experimental courses and other kinds of requirements. In addition, this paper may serve as an easy guide to develop other laboratory projects based on potentiometric sensors. Furthermore, the presented practice may contribute not only to underpinning the understanding of potentiometry but also to illustrating the importance of detection after use of a separation column.10 This is a relevant issue because insights about detectors used in chromatography-based instruments usually go slightly unnoticed by the students. Hence, it is necessary to promote the use of laboratory practices aimed at providing students with knowledge about detectors from a practical point of view, apart from traditional theoretical courses.

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EMF = E 0 +

2.303RT log(ai) z iF

(1)

where E0 is the standard potential of the cell, R is the gas constant, T is the temperature, zi is the charge of the primary ion, and F is the Faraday constant.12 Accordingly, the calibration parameters E0 and the slope

(

2.303RT z iF

) were obtained.

The limit of detection of the potentiometric electrodes was calculated as the activity related to the cross point between the extrapolation of the lines defining the nonresponsive range and linear-response range of the electrode.12 The response time of the potentiometric electrodes was herein calculated as t95, which is the time that elapses between the instant at which the activity of the ion of interest is changed in the solution and the first instant at which the EMF becomes equal to 95% of the EMF at steady state.12 Although this approach was accepted by the IUPAC in 1994, more recent studies point out that the utilization of the “differential criterion” (eq 2) is more convenient for exploiting polymeric-membrane electrodes.13

EXPERIMENTAL SECTION

Procedure for Calibration and Data Treatment

Dynamic calibration curves were constructed by measuring the electromotive force (EMF) of the electrochemical cell every 1 s at increasing activity of the different anions. The electrochemical cell was composed of the working electrode, which was based on a piece of the corresponding nonselective polymeric membrane (Figure 1a), and the reference electrode (see Appendix 1 in the

i ΔE yz zz t * = t ′jjj k Δt {

(2)

where t* is the response time of the electrode; t′ is the reading time, corresponding to the recorded time distance between two additions; and ΔE is the potential increment over a certain period of time; Δt is the time needed to reach steady state after each activity change. However, this criterion is not being widely used today by the electrochemistry community. The response drift of a potentiometric electrode is defined as the nonrandom change in time in the EMF in a solution of constant composition and temperature. Its determination in this paper was carried out by linear-curve fitting of the data set that was collected in a given period of time. The slope of this EMF versus time trace corresponds to the electrode-response drift.12 Procedure to Study the Selectivity of Membranes

The selectivity was evaluated using the separate-solution method (SSM) according to IUPAC recommendations.14,15 Accordingly, individual calibration graphs were constructed for the primary and interfering anions. The logarithmic selectivity coefficients were calculated on the basis of eq 3, using Cl− as the primary ion (A) and setting the activities of the primary and interfering ion (B) equal to 10−1.5, which is the maximum activity used in the experiments.

Figure 1. (a) Picture of the components of an ISE. (b) Picture of the experimental setup for potentiometric measurements. (c) Picture of the experimental setup for potentiometric measurements in flow mode. (1) Cap, (2) membrane, (3) adaptor, (4) screw, (5) body for the inner filling solution, (6) software for potentiometric measurements, (7) potentiometer, (8) electrodes (the reference electrode is in the middle of the six potentiometric electrodes placed in the holder and therefore not visible in the picture), (9) beaker with the sample solution, (10) stirrer, (11) reference electrode, (12) electrode based on a FAPQ membrane, (13) inlet of the flow cell, (14) outlet of the flow cell, (15) pump.

pot log KA,B =

B

(E B − EA)zAF z zy ji + jjj1 − A zzzlog(aA ) j 2.303RT z B z{ k

(3)



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measure low sample volumes.20 In addition, it is sometimes preferable that the detector responds in a similar way to all (or as many as possible) analytes. Those detectors are called universal detectors. Specifically, among all the possible ion detectors that can be used in IC, potentiometric electrodes provide all the mentioned characteristics,21 and additionally, the use of nonselective potentiometric membranes may provide a universal detector. Furthermore, potentiometric sensors are affordable from economic and ease-of-use points of view. Because, as far as we know, nonselective potentiometric sensors have never been explored as universal detectors in IC, this is the main motivation for raising the research question to be solved by the students in the developed laboratory activity. To achieve this purpose, the laboratory was scheduled in different sessions: one session of 2 h for the introduction of the project followed by a guided literature search (Session 1); six sessions of 6 h dedicated to performing the experiments accompanied by an evaluation and discussion of the results (Sessions 2−7), and one last session of 2 h in which the students presented the results of the research project (Session 8). Session 1. The objective and research question of the laboratory practice were presented to the students (Appendix 2, Supporting Information). Then, the students carried out a guided literature search (Appendix 3, Supporting Information). Sessions 2−7. The students prepared a working plan for the research project during the beginning of the second session and discussed the chart with the instructor. The agreed working plan was executed by the students with certain flexibility to introduce justified modifications. More specifically, to answer the research question of the project, it is important that the following tasks are included in the working plan: Task 1. Construction of a calibration graph of Cl− using electrodes based on the FAB membrane. Task 2. Treatment of data; fitting to the Nernst equation; and calculation of slope, E0, limit of detection, response time, drift, and reproducibility. Task 3. Study of the selectivity of the FAB membrane toward F−, Cl−, NO3−, Br−, I−, ClO4−, and SCN−, considering Cl− as the primary ion and using SSM. Task 4. Comparison of the obtained results with those already published by Grygolowicz-Pawlak et al.19 Task 5. Construction of calibration graphs for F−, Cl−, NO3−, Br−, I−, ClO4−, and SCN− using electrodes based on FAD, FAPQ, and FAS. Task 6. Comparison of the analytical performances of all the electrodes and, accordingly, selection of the optimal membrane. Task 7. Study of the performance of an electrode based on the optimal membrane in flow conditions, calibration for Cl−, and study of selectivity. Task 8. Discussion of the potentiality of the developed electrode as a universal detector of anions in IC. Answering the question, “What are the next steps toward addressing the research question?” Session 8. The students presented the project both in written (scientific-article style) and oral formats (20 min presentation with 10 min of questions). Regarding the written report, the instructor was allowed to provide two sets of feedback, after which the students submitted their final versions.

Among all the available methods to calculate potentiometricselectivity coefficients, the SSM is recommended to avoid the calculation of bias values for ions that do not show a Nernstian response.16,17 In this case, the calibration graph is more representative of the electrode selectivity than the calculation of a numerical coefficient. In addition, some authors prefer the use of the standard potential (i.e., the extrapolation of the Nernstian response to activity equal to 1) rather than the potential observed for an equal activity of the primary and interfering ion when applying the SSM.18 Consequently, eq 4 is used. pot KA,B =

aA aBzA / z B

exp

{

}

E B − EA zAF RT

| l o o E B0 − EA0 o = expo z F m } A o RT o o o n ~

(4)

For simplicity reasons, because the students that were involved in the present practice did not possess a high-level background in electrochemistry compared with experienced researchers, we decided to follow the procedure proposed by the IUPAC for the implementation of the SSM, as explained above. Procedure for Flow Potentiometric Measurements

Figure 1c shows a picture of the experimental setup for the flow experiments. The potentiometric flow cell comprised the reference electrode, working electrode (potentiometric sensor based on different anion-exchange membranes), and inlet and outlet connections for the solution. To obtain the calibration graph of Cl− in flow conditions, separate solutions of increasing activity of this ion (10−5 to 10−1.5) were flowed at 1.70 mL min−1 (speed 20.0 of the pump) through the fluidics setup while the potential was continuously registered. Finally, solutions of the same activity (10−3) of all the selected anions (F−, Cl−, NO3−, Br−, I−, ClO4−, and SCN−) were also measured to mimic the signal obtained in case the sensor is implemented as an IC detector.

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IMPLEMENTATION OF THE ACTIVITY IN THE LABORATORY

Description of the Activity

Polymeric ion-exchange membranes used for electrodialysis applications exhibit highly reduced selectivity toward different ions when they are incorporated into potentiometric sensors, as recently demonstrated by Grygolowicz-Pawlak et al.19 Thus, a response study of two cation-exchange membranes (Nafion and FKL) and one anion-exchange membrane (FAB) showed nonselective patterns, making the fabrication of a universal ion detector for IC, among other applications, an attractive prospect. The present work extends the mentioned study by exploring, for the first time, the analytical performance of potentiometric sensors based on different anion-exchange membranes that are commercially available (FAB, FAD, FAPQ, and FAS), with the aim of answering the following research question: how far can potentiometric electrodes based on nonselective polymeric membranes be used as universal detectors in ion chromatography (IC)? The analytical detector is responsible for measuring the different analytes that are present in the sample after passage through a chromatographic column. Thus, an ideal detector has to fulfill certain analytical requirements, including having adequate sensitivity, long-term stability and reproducibility, a wide linear range of response, a wide range of working temperatures, a fast response time, and the possibility of miniaturization to

Task Justifications

Following the mentioned piece of work by Grygolowicz-Pawlak et al. as an example19 and after the guided literature search C



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Figure 2. For a FAB-based electrode: (a) Dynamic response for Cl−-activity additions. Numbers close to the signal trace are referred to as the logarithmic values of increasing chloride activity. (b) Average calibration graph observed for three electrodes. The calculation of the limit of detection is illustrated. Error bars represent the standard deviation of the steady-state potential observed for three different electrodes. (c) Calculation of the response time associated with an activity change from 10−3.5 to 10−3.0. (d) Calculation of the drift using a response of the ISE with 10−2.5 activity of chloride over 20 min (short-term drift).

videos as introductory material in a future variant or extension of the practice proposed herein. Overall and considering the established inquiry rubric scales,23−25 the proposed activity is framed between guided inquiry (level 3) and open inquiry (level 4). Thus, students were provided with details about electrode preparation, experimental setup, and needed software, but they were required to design their own protocols to gather and analyze data. In the present paper, we focused on the description of the implementation of the activity in the laboratory (mainly based on the working plan), obtained results, data treatment, and discussion.

(Session 1), a working plan containing the experiments necessary to answer the research question was designed by the students (Session 2). Importantly, it is convenient that the plan begins with training to acquire the appropriate skills in potentiometric experiments and data treatment using the FAB membrane. Therefore, the students could evaluate if they were properly dealing with experiments, data treatment, and the discussion of outcomes by comparing their results with those already published in the literature.19 Consequently, the practice started with a deep study of the analytical performances (slope, intercept, limit of detection, linear range of response, response time, drift, reproducibility, and selectivity) of electrodes based on the FAB membrane (Sessions 2 and 3). After that, the project planned by the students considered the evaluation and comparison of the analytical performances of electrodes based on all the commercial anion-exchange membranes provided to them (FAD, FAPQ, and FAS; Sessions 4−6). Finally, the students selected the best electrode to test in the flow cell to mimic detection conditions after column separation of a sample that contains a mixture of several anions, including F−, Cl−, NO3−, Br−, I−, ClO4−, and SCN−, in IC (Sessions 6 and 7). During the activity development in the laboratory, the students were first introduced to all the instrumentation needed, electrode preparation, software management, and data treatment by the instructor. After attending to relevant explanations and live demonstrations in the laboratory about assembling the whole experimental setup, the students were able to develop the project by themselves but always with the appropriate guidance of the instructor. Interestingly, other pedagogic resources, such as a set of videos, may be used as an alternative to the live demonstration. Indeed, it has been recently demonstrated that using videos in laboratory instruction is an effective resource for students attempting to complete experiments.22 One advantage of this approach is that the students can watch the videos at any time in case they have a question on how to proceed in the shown protocols. In addition, the video could contain embedded questions that make them reflect on some relevant aspects of the practice. This is also compatible with live demonstrations by the instructors. We do not discount the incorporation of illustrative

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RESULTS AND DISCUSSION

Exploring the Potentiometric Response toward Chloride Ions in Order to Acquire General Skills about Potentiometric Electrodes

After the described introductory part, the students started with the experiments. Calibration graphs for Cl− were obtained using three electrodes based on three different cuts of the same FAB membrane (Session 2). Figure 2a shows the dynamic response of one of the three electrodes as an example. Then, the corresponding calibration curve was plotted as explained in the Experimental Section, fitting the average steady-state potential at each activity to the Nernst equation (eq 1, Figure 2b) and calculating the intercept (E0) and slope. For this purpose, the students may use different software according to their background: MATLAB, Mathematica, Origin, Sigmaplot, and Excel, among others. Other parameters, such as the limit of detection (Figure 2b), linear-response range, response time within the linear-response range (Figure 2c), and drift (Figure 2d), were also calculated as explained in the Experimental Section. For clarification, Figure 3b−d additionally illustrates the approaches used in each case. Then, the students discussed the analytical performances of the FAB-based electrode by presenting the data in a scientific format (figures and tables similar to those presented in this paper). Values close to the theoretical Nernstian slope (−59.2 mV for monocharged ions), limits of detection at the micromolar level, D



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the instructor suggested to the students a search in the literature to see if there were any approaches for improving the analytical performances of potentiometric electrodes to achieve, for instance, lower limits of detection.27−29 An active discussion with the instructor about the application of these strategies to nonselective potentiometric electrodes was generated as a consequence of students’ reflective learning process, after inspection of the corresponding papers.8 Regarding reproducibility, which was evaluated considering the response of the three electrodes (n = 3) based on the FAB membrane, it was excellent at higher activities and slightly worse at lower values. This was clearly manifested in the standard deviations observed for the measured potential at each activity (Figure 2b) and also for the slope and intercept (E0, Table 1). Exploring the Potentiometric Responses toward Anions of Different Lipophilicities

A selectivity study of the FAB membrane was accomplished using the SSM by preparing individual calibration graphs for each anion (Session 3): F−, Cl−, NO3−, Br−, I−, ClO4−, and SCN−. Figure 3 shows the obtained fittings, and the calculated logarithmic selectivity coefficients were collected in Table 1. Several interesting features were observed in this interference study:

Figure 3. Calibration graphs for different anions (F−, Cl−, NO3−, Br−, I−, ClO4−, and SCN−) obtained with the FAB-based electrode.

linear-response ranges of at least three activity decades, fast response times, and acceptable drifts were obtained.26 The observed results are within the expected values commonly displayed by traditional ion-selective electrodes.26 At this point,

(i) Cl−, which is the primary ion of the FAB membrane as it was conditioned in NaCl, presented a Nernstian slope.

Table 1. Analytical Performances Obtained for Potentiometric Electrodes Based on FAB, FAS, FAD, and FAPQ Membranes Membrane FAB

FAS

FAD

FAPQ

Ion (B) −

F Cl− NO3− Br− I− ClO4− SCN− F− Cl− NO3− Br− I− ClO4− SCN− F− Cl− NO3− Br− I− ClO4− SCN− F− Cl− NO3− Br− I− ClO4− SCN−

Slope (mV) −49.5 ± 0.8 −57.2 ± 0.2 −62.5 ± 0.1 −62.7 ± 0.2 −61.8 ± 0.3 −66.9 ± 0.1 −67.1 ± 0.1 −49.8 ± 0.3 −58.9 ± 0.2 −58.5 ± 0.2 −58.8 ± 0.3 −59.8 ± 0.1 −60.2 ± 0.1 −62.5 ± 0.1 −49.6 ± 0.5 −56.2 ± 0.3 −59.0 ± 0.3 −58.7 ± 0.1 −59.8 ± 0.2 −59.9 ± 0.2 −60.1 ± 0.1 −50.5 ± 0.3 −58.2 ± 0.2 −58.9 ± 0.4 −59.9 ± 0.1 −60.1 ± 0.3 −60.9 ± 0.1 −61.8 ± 0.2

E0 (mV) 65.1 ± 3.5 31.0 ± 3.8 −7.2 ± 2.2 −1.8 ± 2.5 4.3 ± 1.8 −29.0 ± 2.0 −32.4 ± 2.3 60.0 ± 2.5 25.3 ± 2.3 30.2 ± 2.3 20.5 ± 3.5 15.1 ± 1.0 15.1 ± 1.2 15.1 ± 1.8 57.1 ± 5.2 30.0 ± 4.9 30.3 ± 2.4 29.1 ± 2.2 27.0 ± 3.7 30.7 ± 4.4 27.5 ± 3.1 57.3 ± 1.5 35.7 ± 2.3 33.2 ± 4.1 29.2 ± 3.1 25.6 ± 2.8 30.4 ± 1.9 25.0 ± 1.1

LDa

LRRa −5

(2.1 ± 1.1) × 10 (2.3 ± 0.5) × 10−6 (1.7 ± 0.3) × 10−5 (3.2 ± 1.1) × 10−6 (6.3 ± 0.8) × 10−6 (6.5 ± 0.5) × 10−6 (6.6 ± 0.3) × 10−6 (6.1 ± 1.0) × 10−5 (4.7 ± 0.7) × 10−6 (3.2 ± 0.1) × 10−5 (1.0 ± 0.3) × 10−5 (5.3 ± 0.7) × 10−6 (5.5 ± 0.9) × 10−6 (1.6 ± 0.3) × 10−5 (1.1 ± 0.9) × 10−5 (5.3 ± 1.5) × 10−6 (3.2 ± 0.8) × 10−5 (9.2 ± 1.0) × 10−6 (9.7 ± 0.5) × 10−6 (7.9 ± 0.5) × 10−6 (1.6 ± 0.5) × 10−5 (1.1 ± 1.5) × 10−5 (2.0 ± 0.6) × 10−6 (1.6 ± 0.1) × 10−5 (5.0 ± 0.8) × 10−6 (5.5 ± 0.7) × 10−6 (6.2 ± 0.5) × 10−6 (2.6 ± 0.5) × 10−6

−4.0

−1.5

10 to 10 10−4.5 to 10−1.5 10−4.0 to 10−1.5 10−4.5 to 10−1.5 10−4.5 to 10−1.5 10−4.5 to 10−1.5 10−4.5 to 10−1.5 10−4.5 to 10−1.5 10−4.5 to 10−1.5 10−4.0 to 10−1.5 10−4.5 to 10−1.5 10−4.5 to 10−1.5 10−4.5 to 10−1.5 10−4.5 to 10−1.5 10−5.0 to 10−1.5 10−4.5 to 10−1.5 10−4.0 to 10−1.5 10−4.5 to 10−1.5 10−4.5 to 10−1.5 10−4.5 to 10−1.5 10−4.5 to 10−1.5 10−4.5 to 10−1.5 10−5.0 to 10−1.5 10−4.5 to 10−1.5 10−5.0 to 10−1.5 10−5.0 to 10−1.5 10−5.0 to 10−1.5 10−4.5 to 10−1.5

t95 (s)b

Drift (μV s−1)c

log KA,B

8 7 5 5 5 6 3 9 9 10 8 9 8 9 7 7 8 6 9 9 8 6 5 5 3 3 5 4

10 5 8 5 8 5 6 10 8 12 10 7 7 8 10 8 10 8 8 7 7 5 4 4 3 3 3 5

−0.51d 0 −0.30 (3.1)e 0.01 (1.9)e 0.25 (3.5)e 0.62 (5.5)e 0.68 (3.8)e −0.28d 0 −0.07 0.08 0.14 0.12 0.05 −0.25d 0 −0.10 −0.07 −0.07 −0.14 −0.09 −0.11d 0 0.02 0.05 0.11 0.01 0.12

a

The limit of detection (LD) and linear range of response (LRR) are expressed as activity. bCalculated from the potentiometric response corresponding to an activity change from 10−3.5 to 10−3.0. cShort-term drift calculated in a solution of activity equal to 10−2.5 over 20 min. dNote that even though fluoride showed a slope value that is ∼15% lower than the Nernstian value, the logarithmic selectivity coefficients were calculated for comparative purposes. ePotentiometric-selectivity coefficients observed by Cuartero et al.17 for a polymeric membrane based on an anion exchanger with no ionophore. E



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Then, the slope was sub-Nernstian for F− and slightly super-Nernstian for the rest of anions. (ii) The lowest limit of detection was obtained for the primary ion, Cl−. Then, for Br−, I−, ClO4−, and SCN−, they were slightly higher. For F− and NO3−, they were 1 order of magnitude higher. (iii) The response time, linear range of response, and drift were similar for all the tested anions. (iv) Regarding the selectivity, the response order found for FAB seems to be similar to those of membranes based on anion exchangers, therefore following the order of the hydrodynamic radii (or Stokes’s radii). However, the magnitude of this effect is dramatically reduced in the FAB membranes. Calculated logarithmic selectivity coefficients were in the range from −0.51 (for F−) to 0.68 (for SCN−), which pointed to a nonselective pattern of the FAB membrane. (v) Compared with a traditional membrane based on the anion-exchanger tridodecylmethylammonium chloride,17 the observed potentiometric response gradually increased with the anion lipophilicity. For instance, the selectivity coefficient calculated for ClO4− using the SSM and considering Cl− as the primary ion was roughly 5.5.17 This means that ClO4− is preferred by 5.5 orders of magnitude over Cl−. However, in the case of the FAB membrane, this value is reduced to less than 1 order of magnitude. Therefore, with FAB-based electrodes, chloride ions may be analyzed in samples with less interference from the rest of the anions compared with that from traditional membranes based on anion-exchange compounds. Remarkably, Grygolowicz-Pawlak et al. used FAB-based electrodes to detect chloride ions in blood.19 Because the primary anion (Cl−) was the only one presenting a slope close to Nernstian behavior, and in addition, it presented the lowest limit of detection, it could be said that both membrane conditioning and the nature of the inner filling solution likely influenced the membrane potentiometric behavior. Similarly, Grygolowicz-Pawlak et al. observed certain changes in the potentiometric responses of the analogous cation-exchange membranes Nafion and FKL according to the membrane conditioning used.19 These membranes responded to divalent cations (Ca2+ and Mg2+) with a monovalent slope when the membrane was conditioned in NaCl solution with 10 mM NaCl as the inner filling solution. Furthermore, a change from a monovalent to a divalent slope (i.e., from 59.2 to 29.7 mV) was observed in the Nafion-based electrodes from an activity equal to the concentration used in the inner filling solution (10 mM). When the Nafion and FKL membranes were conditioned in a divalent cation (for instance CaCl2), a divalent slope was found for both divalent and monovalent cations. Both in the case of cation-exchange membranes (Nafion and FKL) and in the case of the anion-exchange membrane studied here (FAB), further experiments are necessary to explain the observed behavior. Although this is interesting from a fundamental point of view, it is outside of the aim of the present practice, and the most relevant observation is that the FABbased electrode responded with a nonselective pattern toward all the anions tested.

were also characterized by the students in a similar way (Sessions 4−6). Because the maximum number of electrodes that could be incorporated together with the reference electrode in the holder was eight and every membrane was tested in triplicate, the students decided to first run the experiments with FAS and FAD membranes and then with the FAPQ membrane, before finally comparing and discussing all the observations. The analytical performances displayed by all the membranes are shown in Table 1, together with those for the FAB-based electrodes. Although similar trends were observed for all the membranes, lower limits of detection, wider linear ranges of response, slightly faster response times, and lower drifts were obtained for the FAPQ membrane. In addition, the potentiometric responses for all the anions were more similar in the case of the FAPQ membrane, and therefore, the difference between the lowest and highest selectivity coefficient was lower (i.e., 0.2 for FAPQ and 1.2, 0.4, and 0.4 for FAB, FAS, and FAD, respectively). In other words, the FAPQ membrane presented a more marked nonselective pattern than the rest of the membranes, as well as slightly better analytical performances for all the anions tested. Consequently, this membrane was selected by the students for further studies based on flow measurements (Sessions 6 and 7). Potentiometric Response under Flow Conditions

One electrode based on the FAPQ membrane was inserted into the flow cell together with the reference electrode (Figure 1c). A calibration graph for Cl− was obtained by flowing solutions of increasing activity in the system (Figure 4a). Compared with the calibration graph observed in the beaker, a slightly higher slope, −60.0 ± 0.3 mV; a similar limit of detection, (1.5 ± 0.8) × 10−6; and equal linear range of response were obtained in the flow mode (Figure 4b). This behavior was totally expected after implementing the potentiometric electrode in flow mode. Moreover, sometimes a lower limit of detection may be achieved.30,31 The membrane selectivity was also explored (Figure 4c) and the logarithmic selectivity coefficients were calculated for all the pot pot tested anions: log Kpot Cl,F = −0.13, log KCl,NO3 = −0.05, log KCl,Br = pot pot pot −0.02, log KCl,I = −0.01, log KCl,ClO4 = −0.1, and log KCl,SCN = −0.04. It is here manifested again that the electrode possesses a nonselective response pattern and also that the response to the primary ion (Cl−) is somehow slightly favored over the rest of the anions. The response of the potentiometric sensor was totally reversible facing activity changes (from lower to higher and vice versa), as observed in Figure 4c. Thus, the potentiometric response always showed the same value for Cl− activity of 10−5 after measuring each anion. This is an important characteristic, considering the final application of the nonselective potentiometric sensor as a universal detector in IC. However, there are many other aspects to consider before real inline coupling of the electrode after a separation column. Although individual calibration graphs are normally accomplished for all the ions to be determined in a sample by using IC, one of the advantages of the use of nonselective potentiometric sensors could be the utilization of a general calibration graph for all the ions tested. In our case, if the calibration graph observed for chloride ions (eq 5) is considered in this way, absolute errors of 0.27 mM for F− (26%), 0.10 for NO3− (9%), 0.03 mM for Br− (3%), 0.03 mM for I−(3%), 0.05 mM for ClO4− (5%), and 0.09 mM for SCN− (9%) are obtained in the calculation of 1 mM concentration with the registered potential (Figure 4c). Although the error in the concentration calculation for the case of F− is out of the usual tolerance limits for accuracy

Comparing the Potentiometric Responses of Several Nonselective Polymeric Membranes

Having evaluated the analytical performances of FAB-based electrodes, the rest of the membranes (FAS, FAD, and FAPQ) F



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Figure 4. (a) Dynamic response for solutions of increasing Cl− activity. Numbers close to the signal trace are referred to as the logarithmic values of the increasing chloride activity. (b) Average calibration graph (n = 3 using the same electrode). (c) Dynamic responses for solutions of equal activity (10−3) of the different anions. The initial baseline corresponds to water, whereas a chloride solution of 10−5 activity was used between the injections of the different anions.

disparate sensitivities to the different ions. For instance, the anion-selective electrode responded four times more to NO3− than to Cl−, and the cation-selective electrode responded more than double to K+ compared with its responses to Na+ and NH4+. What is really remarkable is the potentiometric cell,33 and an analogous design may be considered in future steps toward the real implementation of nonselective potentiometric electrodes as IC detectors. For this purpose, it is important to adapt the electrodes developed here to a solid-contact configuration. These electrodes may be prepared by drop-casting solutions of appropriate polymers, such as commercially available liquid Nafion or Fumion (Fumatech), thereby forming nonselective membranes. This direction in the research is undoubtedly very interesting as part of a student project similar to the one presented herein. To improve the students’ knowledge of analytical detectors used in chromatographic techniques, it would be very interesting to develop a systematic study of the quality parameters of the nonselective potentiometric sensor once it is implemented in an appropriate flow cell for coupling with the IC. This study includes evaluation of the sensitivity, stability, reproducibility, linear range of response, working-temperature range, and response time. In addition, it is important to accomplish an exhaustive validation with standard solutions of all the analytes to define the precision of the sensor. The final step would be the coupling of the detector with IC and analysis of the chromatographic peaks. In principle, because the sensor responds equally to all the ions, the adequate separation of the chromatographic peaks will be a matter of the conditions selected for the separation event, which includes the nature, particle size, and length of the column as well as the eluent composition and type of elution gradient, among others.23 Overall, the results shown in this paper are very promising toward a universal potentiometric IC detector. Answering the research question that was proposed at the beginning of the laboratory practice (“How far can potentiometric electrodes based on nonselective polymeric membranes be used as universal detectors in ion chromatography?”), the main steps and

requirements (10−20%, depending on specific analytical applications),32 the error percentage is always lower than 10% for the other anions tested. It would be convenient to use a correction in the calculation of the anion concentration based on a calibration factor, at least for the anions that presented the higher error levels. E (mV) = −46.4 − 60.0 log(aCl )

(5)

Addressing the Research Question on the Basis of Experimental Observations: Planning Further Experiments

At this point, the students critically evaluated the features of the explored electrode as an IC detector and the future steps needed to accomplish the final implementation. For this purpose, a deeper study of relevant papers published in the literature that describe real uses of ion-selective electrodes as IC detectors was crucial.33−39 The main reason for searching for an IC detector different from the conductivity detector, which is the IC detector most widely used, is mainly related to the use of suppression columns, because the detector response strongly depends on the eluent’s background conductivity.33 Therefore, other approaches have been explored for this purpose, including potentiometry. However, the majority of the examples found in the literature are based on the selective measurement of relevant targets rather than the simultaneous analysis of several ions, for which it is necessary to have nonselective detectors. Isildak et al. have been working for several years on this topic and have recently published a work based on the simultaneous analysis of monovalent anions (Cl− and NO3−) and cations (Na+, K+, and NH4+) with a potentiometric detector for IC.33 The detector was composed of a microfluidic cell with a sample volume of 2 μL containing a reference electrode, one electrode based on an anion exchanger, and another electrode based on a cation exchanger. These electrodes were of the all-solid-state type and microsized. The system was successfully applied to the detection of Cl−, NO3−, Na+, K+, and NH4+ in saliva, sweat, and environmental water samples. However, the selectivity pattern displayed by the electrodes, which is based on the ions’ lipophilicities, means that the electrodes responded with very G



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requirements to be addressed for the realization of nonselective potentiometric sensors as universal IC detectors have been discussed. Hence, there is still open research that could be accomplished in subsequent laboratory activities designed and executed by and for students, following an inquiry-based approach similar to the one presented in this paper.

understanding and hypothesis development was successfully generated by promoting student argumentation through reflective questions by the teacher. In this context and compared with traditional laboratory practices based on detailed recipes, it was already observed that problemsolving and active engagement in asking crucial questions were appropriate strategies for promoting metacognitive skills in students.43 Indeed, the students’ perception (100%) was that the inquiry-based experiments promoted understanding, and they reported a deep appreciation for the new experience as “independent scientists”. They felt involved with the entire project, from the planning to the evaluation of the activity. (v) In the oral presentation of the research project, the students followed all the suggestions made by the instructors, mainly concerning the selection of data to be shown and the method of discussing the results. The presentation was clear, concise, and well-argued, even using evidence already published in the literature. Each member of the group was in charge of one of the parts of the presentation: (1) introduction based on the establishment of the project aim and a brief description of potentiometry fundamentals, (2) explanation of the experiments and data treatment, (3) results and discussion concerning the electrode development and characterization, and (4) implementation of the best electrode in flow mode and a discussion of future steps and conclusions. Although task division is a common tool in a working team, there is a risk of the isolation of knowledge and, therefore, of profound ignorance of other tasks by each student.44 To avoid this, the instructor posed this issue to the students and proposed to them the holding of a team meeting to share and discuss each task together until the complete understanding and agreement of all the members.45 In addition, the students were asked individual questions after the oral presentation that did not involve the part that they were in charge of. The aim was to verify the effectiveness of the metacognitive skills acquired on the basis of a well-organized teamwork. (vi) Regarding the written report, the students (100%) pointed out that they considered the two feedbacks provided by the instructor to be very useful. Importantly, they had the initiative to postpone the deadline by 1 day to present the report in order to incorporate any additional conclusions from the round of questions by the instructor and the rest of audience, which was composed of the rest of the students and other teachers involved in the subject, after the oral presentation. According to our evaluation criteria, the specification of which is outside the aim of the present paper, the written report was of excellent quality. The majority of the statements included in the data discussion were linked to literature references to provide evidence of their veracity. In addition, the students provided a critical evaluation of the possible implementation of the developed potentiometric sensor as an IC detector together with some future steps in this direction.

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EVALUATION OF THE PEDAGOGICAL EFFECTIVENESS OF THE ACTIVITY The evaluation of the pedagogical effectiveness of the presented inquiry-based laboratory activity was accomplished using three different sources of information: (1) daily observations of the instructors during the development of the laboratory, (2) evaluation of the oral presentation and written report of the results and discussion, and (3) blind individual interviews with the students. Note that the presented laboratory activity was accomplished once with four students because it was an original research project that was proposed to the students, and therefore, we have based the evaluation of the pedagogical effectiveness on collected information from only this laboratory. However, to have a really meaningful statistical comparisons and fully assess the pedagogical effectiveness of the developed activity, it would be necessary to have a larger batch of observations. Indeed, it is crucial both to use a robust and rigorous survey tool to analyze the interviews, for instance an approach based on data codification and Mayring’s analysis,40 and to establish the learning effectiveness by comparing the present inquiry-based approach with a control (recipe-based) experiment, which could be accomplished analyzing lab reports by the Toulmin’s model.41 Following, we remark on some important conclusions from this evaluation. The percentage of students supporting the statement is given in brackets, when convenient. (i) The students (100%) explicitly stated that they found the guided literature search really useful for refreshing fundamental concepts and planning the experimental protocol for the project to be developed. This type of activity seems to really promote the critical thinking of the students, as already shown by other authors.42 (ii) The instructors pointed out that the first proposal of the working plan was slightly different than expected. However, the feedback from the instructor generated active brainstorming in the student team that resulted in a good plan in terms of resources and time. It was observed that the guided student argumentation brought about a well-designed, feasible, and flexible working plan. (iii) The students (50%) suggested that live demonstration of the required instrumentation, electrode preparation, software management, and data treatment could likely be substituted with video materials that the students could review at any time. In addition, reflective questions could be embedded in the video to encourage experimental skills. Consequently, for future experimental projects included in the same course as the one presented here, it could be interesting to construct an online platform with videos and materials about all the background information and protocols that the students could use even outside the laboratory. (iv) The instructors observed that most of the questions that the students were asking during the laboratory sessions involved some concerns about experimental setup as well as data presentation. On the other hand, conceptual

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CONCLUSIONS The laboratory activity presented here describes the fundamental exploration of nonselective potentiometric sensors as potential detectors in IC. Several anion-exchange membranes H



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implementation. We spent a great time together in the laboratory.

used in electrodialysis applications (FAB, FAS, FAD, and FAPQ) were incorporated as sensing materials in potentiometric electrodes for the first time. An intensive study of their responses revealed good analytical performances and nonselective patterns for the detection of the anions F−, Cl−, NO3−, Br−, I−, ClO4−, and SCN−. These electrodes are especially attractive for applications in which their nonselective anionic responses supply unique features, such as in the analysis of abundant hydrophilic anions in complex samples (i.e., chloride in blood)19 and as detectors in analytical-separation techniques. This latter application was explored by the implementation of the electrode based on the FAPQ membrane in a flow cell. The students were successfully able to design the working plan for the research and were helped by a guided literature search and instructor feedback. In addition, an introductory session with live demonstrations gave the students the needed background information and protocols. The use of video materials for online learning is foreseen for future laboratory practices to maximize the effectiveness of this introductory session. According to the observations of the instructors, both during the laboratory and during the result-presentation session, the following pedagogical objectives were achieved by the students: (i) they developed competence toward “thinking like a scientist”, (ii) they developed argumentation and critical decision-making skills, (iii) they reinforced research-planning and experimentaldesign skills, (iv) they refreshed their conceptual knowledge about detectors used in analytical separation techniques, and (v) they improved fundamental knowledge about potentiometry. Furthermore, the present paper may serve as a simple guide to developing other laboratory practices based on potentiometric sensors.

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(1) Kipnis, M.; Hofstein, M. The Inquiry Laboratory as a Source for Development of Metacognitive Skills. Int. J. Sci. Math. Educ. 2008, 6 (3), 601−627. (2) Bowen, R. S.; Picard, D. R.; Verberne-Sutton, S.; Brame, C. J. Incorporating Student Design in an HPLC Lab Activity Student Metacognition and Argumentation. J. Chem. Educ. 2018, 95 (1), 108− 115. (3) Wheeler, L. B.; Clark, C. P.; Grisham, C. M. Transforming a Traditional Laboratory to an Inquiry-Based Course: Importance of Training TAs when Redesigning a Curriculum. J. Chem. Educ. 2017, 94 (8), 1019−1026. (4) Ural, E. The Effect of Guided-Inquiry Laboratory Experiments on Science Education Students’ Chemistry Laboratory Attitudes, Anxiety and Achievement. J. Educ. Train. Stud. 2016, 4 (4), 217−227. (5) Towndrow, P. A.; Ling, T. A.; Venthan, A. M. Promoting Inquiry Through Science Reflective Journal Writing. EURASIA J. Math., Sci. Technol. Ed 2008, 4 (3), 279−283. (6) Bathgate, M.; Crowell, A.; Schunn, C.; Cannady, M.; Dorph, R. The Learning Benefits of Being Willing and Able to Engage in Scientific Argumentation. Int. J. Sci. Educ. 2015, 37 (10), 1590−1612. (7) Bodner, G. M. Constructivism - A theory of knowledge. J. Chem. Educ. 1986, 63 (10), 873−878. (8) Zohar, A.; Ben David, A. Paving a clear path in a thick forest: a conceptual analysis of a metacognitive component. Metacognition and Learning 2009, 4 (3), 177−195. (9) KD2330 Analytical Separations 7.5 credits. KTH. www.kth.se/ student/kurser/kurs/KD2330?l=en (accessed Oct 2018). (10) Buchberger, W. W.; Haddad, P. R. Advances in detection techniques for ion chromatography. J. Chromatograp. A 1997, 789 (1− 2), 67−83. (11) Meier, P. C. Two-parameter Debye-Huckel approximation for the evaluation of mean activity coefficients of 109 electrolytes. Anal. Chim. Acta 1982, 136, 363−368. (12) Buck, R. P.; Lindner, E. Recommendations for Nomenclature of Ion-Selective Electrodes - (Iupac Recommendations 1994). Pure Appl. Chem. 1994, 66 (12), 2527−2536. (13) Macca, C. Response time of ion-selective electrodes - Current usage versus IUPAC recommendations. Anal. Chim. Acta 2004, 512 (2), 183−190. (14) Umezawa, Y.; Umezawa, K.; Buhlmann, P.; Hamada, N.; Aoki, H.; Nakanishi, J.; Sato, M.; Xiao, K. P.; Nishimura, Y. Potentiometric selectivity coefficients of ion-selective electrodes Part II. Inorganic anions - (IUPAC technical report). Pure Appl. Chem. 2002, 74 (6), 923−994. (15) Umezawa, Y.; Buhlmann, P.; Umezawa, K.; Tohda, K.; Amemiya, S. Potentiometric selectivity coefficients of ion-selective electrodes Part I. Inorganic cations - (Technical report). Pure Appl. Chem. 2000, 72 (10), 1851−2082. (16) Bakker, E. Determination of unbiased selectivity coefficients of neutral carrier-based cation-selective electrodes. Anal. Chem. 1997, 69 (6), 1061−1069. (17) Cuartero, M.; Ortuno, J. A.; Garcia, M. S.; Sanchez, G.; MasMontoya, M.; Curiel, D. Benzodipyrrole derivates as new ionophores for anion-selective electrodes: Improving potentiometric selectivity towards divalent anions. Talanta 2011, 85 (4), 1876−1881. (18) Bakker, E.; Pretsch, E.; Buhlmann, P. Selectivity of potentiometric ion sensors. Anal. Chem. 2000, 72 (6), 1127−1133. (19) Grygolowicz-Pawlak, E.; Crespo, G. A.; Ghahraman Afshar, M.; Mistlberger, G.; Bakker, E. Potentiometric Sensors with Ion-Exchange Donnan Exclusion Membranes. Anal. Chem. 2013, 85 (13), 6208− 6212. (20) Skoog, D. A.; Holler, F. J.; Crouch, S. R. Principles of Instrumental Analysis; Harcourt: Paris, France, 1998. (21) Bakker, E.; Pretsch, E. Modern Potentiometry. Angew. Chem., Int. Ed. 2007, 46 (30), 5660−5668.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00455. Appendix 1. Chemicals, materials, and instruments (PDF, DOCX) Appendix 2. Project proposal (PDF, DOCX) Appendix 3. Guided literature search (PDF, DOCX)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

María Cuartero: 0000-0002-3858-8466 Gastón A. Crespo: 0000-0002-1221-3906 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the KTH Royal Institute of Technology (Starting Grant Programme K-2017-0371), the Swedish Research Council (Project Grant VR-2017-4887), WPCRN at KTH (Scholarship K-2017-0804), and the Wenner-Gren Foundation (Scholarship UPD20170220). We thank Yu Ren Tan, Lotta Gustavsson, Vera Boor, and Kok Foo Tay for their excellent feedback about the practice I



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Using Potentiometric Electrodes Based on Nonselective Polymeric

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