Direct Potentiometric Sensing of Anion Concentration (Not Activity)

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Direct Potentiometric Sensing of Anion Concentration (Not Activity)

GAO, Wenyue, XIE, Xiaojiang, BAKKER, Eric

Abstract

Potentiometric probes used in direct potentiometry are attractive sensing tools. They give information on ion activities, which is often uniquely useful. If, instead, concentrations are desired as sensor output, the ionic strength of the sample must be precisely known, which is often not possible. Here, for the first time, direct potentiometry can be made to report concentrations, rather than activities. It is demonstrated for the detection of monovalent anionic species by using a self-referencing Ag/AgI pulstrode as the reference element instead of a traditional reference electrode. This reference pulstrode releases a discrete quantity of iodide ions from the electrode and the resulting reference potential varies with the activity coefficient of iodide. The effects of activity coefficient on the indicator and reference electrode are therefore compensated and the observed cell potential may now be described in a Nernstian manner against anion concentration, rather than activity. Theoretical simulations and experimental results support the validity of this approach. For most monovalent anions of practical relevance, the [...]

GAO, Wenyue, XIE, Xiaojiang, BAKKER, Eric. Direct Potentiometric Sensing of Anion Concentration (Not Activity). ACS Sensors , 2020, vol. 5, no. 2, p. 313-318

DOI : 10.1021/acssensors.9b02523

Available at:

http://archive-ouverte.unige.ch/unige:136736

Disclaimer: layout of this document may differ from the published version.

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Direct Potentiometric Sensing of Anion Concentration (Not Activity)

Wenyue Gao,

^†,‡

Xiaojiang Xie

^‡,

* and Eric Bakker

^†,

*

† Department of Inorganic and Analytical Chemistry, University of Geneva, Quai Ernest-Ansermet 30, CH-1211 Gene- va, Switzerland

‡ Department of Chemistry, Southern University of Science and Technology, Shenzhen, 518055, China Keywords: direct potentiometry; concentration and activity; pulstrodes; reference electrode; local iodide release Supporting Information Placeholder

ABSTRACT: Potentiometric probes used in direct potentiometry are attractive sensing tools. They give information on ion activities, which is often uniquely useful. If, instead, concentrations are desired as sensor output, the ionic strength of the sample must be precisely known, which is often not possible.

Here, for the first time, direct potentiometry can be made to report concentrations, rather than activities. It is demonstrated for the detection of monovalent anionic species by using a self-referencing Ag/AgI pulstrode as reference element instead of a traditional reference electrode. This reference pulstrode releases a discrete quantity of iodide ions from the electrode and the resulting reference potential varies with the activity coefficient of iodide. The effects of activity coefficient on the indicator and reference electrode are therefore compensated and the observed cell potential may now be described in a nernstian manner against anion concentration, rather than activity. Theoretical simulations and experimental results support the validity of this approach. For most monovalent anions of practical relevance, the potential difference between this approach and from a traditional activity coefficient calculation is less than 0.5 mV. The concept is validated with an all-solid-state nitrate sensor as well as a commercial fluoride-selective electrode, giving nernstian responses in different ionic strength backgrounds against concentration without the need for correcting activity coefficients or liquid junction potentials.

Ion activity, symbolized as a, represents the effective con- centration of a species in a non-ideal solution and directly dictates the change in chemical potential. The relationship between activity and molar concentration of the (uncomplexed) species is given by the activity coefficient, γ, defined as the ratio of activity to concentration.¹ The assessment of ion activity is often very valuable, as it best reflects the physiolog- ical action of the ion, as with sodium, potassium or calcium in blood.² Potentiometric sensing probes (ion-selective electrodes) fundamentally respond to ion activity and are unique sensing tools in this regard.

Still, in some cases, the reporting of ion concentration is de- sirable because the values may be more easily cross- correlated and validated with other analytical techniques that all tend to report concentrations.³ Concentrations are for example reported when evaluating environmental water quality

with important ions such as nitrate, phosphate, sulfate, copper, cadmium and lead, although the distinction between total and so-called free (uncomplexed) concentration remains very important in speciation analysis.^4,5

To obtain ion concentration information from direct potentiometry, the observed potentials must be corrected for changes at the liquid junction of a conventional reference element by the Henderson equation. The resulting activity is then further converted to concentration by some variation of the Debye-Hückel equation.⁶ For each of the two steps, however, the electrolyte concentration of the sample solution must be precisely known, which is not generally the case.

Methods have been used with potentiometric sensors to give concentration, such as the application of ionic strength adjust- ers to eliminate variations of activity coefficients.⁷ Ionic strength adjustments may introduce undesirable ionic inter- ferences and are not ideal for on-site analysis.

The development of sensors for anions is important because the significant roles anions play in chemical, biochemi- cal and environmental systems.^8,9 Potentiometric sensors offer several advantages for anion detection, including the possibility for real-time detection and remote sensing in aquatic environments.^10,11 This work uses the direct potentiometric detection of nitrate and fluoride as model examples.

Nitrate is an important nutrient in aquatic ecosystems^12-15 and an indicator for water quality and the extent of anthropogenic environmental changes,^16-18 while fluoride is well known for preventing dental cavities and therefore a useful additive in oral care products such as toothpaste and mouthwash.^19-26

We propose here a potentiometric anion sensor that directly reports concentration instead of activity and that avoids the calculation of liquid junction potentials and activity coefficients. The innovation is in the use of the reference electrode, which is not of the traditional type but uses a recently intro- duced self-referencing Ag/AgI pulstrode.²⁷ It involves three consecutive pulse steps: 1) the release of iodide ions under a controlled reductive current pulse, 2) the immediate open- circuit potential measurement that serves as signal, and 3) the recapture of the previously released iodide at or around the open circuit potential determined before release. Iodide is normally not present at significant concentration in most environmental samples, so the key interfering species for this electrode is expected to be sulfide because of the significantly smaller solubility product with silver ions than iodide.

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The essence of the approach is that the current pulse gives a defined concentration of iodide at the electrode surface,²⁷ but the resulting potential is sensitive to iodide activity. There exists therefore a fundamental symmetry with the indicator electrode that allows one to eliminate activity coefficients from the final sensor output. The change of the two activity coefficients at both electrodes remains, within small error, indifferent of background electrolyte, and the sensor directly responds to anion concentration instead of activity. To our knowledge, this marks the first time that an equilibrium type potentiometric sensing probe is designed to directly report ion concentration.

EXPERIMENTAL SECTION

Materials and Instruments. Tridodecylmethylammonium nitrate (TDMAN), poly(vinyl-chloride) (PVC, high molecular weight), multi-walled carbon nanotubes (MWCNTs), bis(2- ethylhexyl) phthalate (DEHP), tetrakis(4-chlorophenyl)borate tetradodecylammonium salt (ETH-500), sodium iodide, sodium chloride, sodium nitrate, sodium fluoride, TISAB IV solution, and tetrahydrofuran (THF) were purchased from Sigma- Aldrich (analytical grade). A platinum electrode (Model 6.0331.010) purchased from Metrohm (Switzerland) was used as counter electrode. A glassy carbon (GC) electrode with a diameter of 3.0 mm (Model 6.1204.300, Metrohm, Switzer- land) was used as inner electrode for solid-contact ion- selective electrode (ISE). A commercial fluoride-selective electrode with crystal membrane (Model 6.0502.150, Metrohm, Switzerland) was used for fluoride detection. The electrochemical measurements were carried out with an Autolab PGSTAT128N (Metrohm Autolab, Utrecht, The Netherlands) controlled by a personal computer using Nova 2.1.4 software (supplied by Autolab). Measurements based on Ion Chroma- tography (IC) method were accomplished using an IC Conduc- tivity Detector from Metrohm (850 Professional IC detector, cell volume 0.8 µL).

Preparation of electrodes. The Ag/AgI electrode was pre- pared by electrochemically depositing AgI layer on the surface of an Ag electrode with a diameter of 3.0 mm (Model 6.1204.330, Metrohm, Switzerland) in a solution of 0.1 M NaI for one hour at a constant current density of 0.5 mA cm^-2. The solid-state nitrate ISE was prepared using MWCNTs as ion- electron transducer according to Yuan et al.²⁸ Solid-state ISEs based on MWCNTs work well for anion detection, showing good long-term stability and selectivity compared with that based on conducting polymers.²⁸ The solution of 1 mg/mL of MWCNTs in THF was deposited onto the GC discs by drop- casting (20 μLx5). After a few hours of drying, a membrane cocktail solution for nitrate determination was drop cast onto the GC-MWCNTs electrode. The membrane cocktail solution was prepared by dissolving 1.2 mg of TDMAN (10 mmol/kg), 3.4 mg of ETH-500 (15 mmol/kg), 65.1 mg of PVC and 130.2 mg of DEHP in 2 mL THF.²⁹ A volume of 150 μL of the cocktail (50 μLx3) was pipetted and drop cast on the GC-MWCNTs electrode. Before use, the prepared nitrate ISE was condi- tioned in 1.0 mM of NaNO3 solution for at least 12 hours.

Electrochemical protocols. A three-electrode system was used, with the counter electrode to avoid current passing through the indicator electrode consisting of the reference pulstrode. The self-referencing Ag/AgI electrode was applied as a reference electrode connected to the working electrode input to allow current to pass through. The nitrate ISE or fluoride ISE was connected to the reference input of the potenti-

ostat. The working principle of the Ag/AgI pulstrode was in- troduced and detailed in our recent paper.²⁷ The initial open- circuit potential was recorded before electrochemical measurements. Triple pulse steps were then executed during each measurement: the local release of iodide ions under a constant cathodic current pulse of -5 µA for 5 s; the EMF reading for 1 s with intervals of 25 ms; the recapture of iodide ions under a constant potential based on the initial OCP value for 30 s. Since the reference potential depends on the activity of the released iodide at the electrode/solution interface, and also depends on the diffusion of iodide, all the measurements were done in quiescent solution to avoid convections. To keep with standard practice, the sign of the obtained data was sub- sequently reversed. All the measurements were carried out in triplicate.

A certain mass of toothpaste sample was weighed and ho- mogenized in TISAB IV solution (pH 5.6) by stirring for 5 min and then heated to boiling for 5 min. The liquid part extracted from the suspension by centrifugation and ﬁltration was used as the toothpaste stock solution and diluted when needed. The mouthwash and mineral water samples were used as re- ceived, and the river water sample was filtered before use.

RESULTS AND DISCUSSION

The schematic configuration and working principle of the potentiometric sensor with the Ag/AgI reference electrode is shown in Figure 1. According to the Nernst equation, the potential of an ideally responding electrode is a function of the primary ion activity. Thus, the potential of the indicator electrode 𝐸_!"# (responsive to ions A^–) and the Ag/AgI reference electrode 𝐸_$%& (responsive to iodide ions, I^–) is written as:

𝐸_!"#= 𝐸_!"#^' + 𝑠₍𝑙𝑜𝑔𝑎₍ (1) 𝐸_$%&= 𝐸_()*/()^' + 𝑠_*𝑙𝑜𝑔𝑎_* (2) where 𝑠( and 𝑠* respectively represent the Nernstian slope for the two electrodes. For the reference element, a timed gal- vanostatic experiments keeps the iodide concentration at the electrode surface practically constant. The potential of the electrochemical cell with respect to the Ag/AgI reference electrode is:

𝐸,%--= 𝐸!"#− 𝐸$%&

= 𝐸_!"#^' + 𝑠₍𝑙𝑜𝑔𝑎₍− /𝐸_()*/()^' + 𝑠_*𝑙𝑜𝑔𝑎_* 0 (3)

Figure 1. The schematic configuration of the potentiometric sensor for anions (A^.) with the Ag/AgI pulstrode as reference electrode.

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Ion activity is related to concentration through the activity coefficient (𝛾_/) for any ion j as follows:

𝑎_!= 𝛾_!𝑐_! (4) The potential of the electrochemical cell is now rewritten as:

𝐸_,%--= 𝐸_!"#^' − 𝐸_()*/()^' + log (𝛾₍⁰^!/𝛾_*⁰^") + 𝑠₍𝑙𝑜𝑔𝑐₍− 𝑠_*𝑙𝑜𝑔𝑐_* (5) For monovalent anions, 𝑠₍ is ideally −59.2 mV/decade. The concentration of released iodide ions (𝑐_*) is constant for a given amplitude and time of the applied current. Theoretical- ly, it is calculated to be around 0.5 mM for an applied current density of ‒70 µA cm^-2 for 5 s.²⁷ This value may vary with the dimensions and hydrodynamics of the cell that should be kept constant during measurement. Ideally, therefore, for monovalent anions:

𝐸,%--= 𝐶 − 59.2 𝑙𝑜𝑔𝑐(− 59.2 log (𝛾(/𝛾*) (6) where C is a constant:

𝐶 = 𝐸_!"#^' − 𝐸_()*/()^' − 𝑠_*𝑙𝑜𝑔𝑐_* (7) If the activity of the target anion is used to fit the Nernst equation, the activity coefficient of the released iodide should be corrected. Instead, if the ratio of the activity coefficients between the target anion and iodide is close to unity, concentrations can be used directly instead of activities in the Nernst equation. This makes the measurement independent of ionic strength of the solution, which is highly desired for many applications.

The mean activity coefficient is calculated according to the extended Debye-Hückel theory:³⁰

𝑙𝑜𝑔𝛾±= −A|𝑧2𝑧.|√𝐼

1 + B√𝐼 + C𝐼 (8) where A is a constant (A = 0.5108) that is a function of tem- perature, and B and C are two parameters that depend on the electrolyte under study, while 𝐼 represents the ionic strength for all ions j present in solution:

𝐼 = 0.5 G 𝑧_/³𝑐_/

/

(9) The relevant single ion activity coefficient is calculated from the mean activity coefficient on the basis of the simplified Debye-Hückel convention:

𝑙𝑜𝑔𝛾₂= H𝑧2

𝑧_.H log𝛾_± (10) 𝑙𝑜𝑔𝛾_.= H𝑧.

𝑧₂H log𝛾_± (11)

Calculations were made based on these equations to show the potential difference caused by the activity coefficient ratio between different target anions and iodide, as shown in Fig- ure 2. Different anions cause potential differences that depend on their valency and hydrated radii differences with iodide. In a background of monovalent electrolyte M²N^., increasing the concentration of the target anion, the potential difference (error) is a few 100 µV for concentrations lower than 10 mM.

A higher background concentration leads to a higher ionic strength, and thus causes a larger activity coefficient difference between the target anion and iodide. Still, even in a 10 mM background electrolyte, the potential difference caused by a variable activity coefficient ratio is less than 0.5 mV when the target anion concentration is 10 mM. The potential difference is predictable for any ions once the ionic strength of the sample solution is known and the parameters used in the De- bye-Hückel formalism are available. Since the parameters B and C in Equation 8 are not known for divalent anions, a simplified Debye-Hückel equation³¹ was used to simulate activity coefficients in that case:

𝑙𝑜𝑔𝛾₍= − 0.51⨉𝑧₍³√𝐼

1 + 3.3⨉𝑎(̇ √𝐼 (12) where 0.51 and 3.3 are constant parameters appropriate for an aqueous solution at 25℃, 𝑧( is the charge of the anion A, 𝑎(̇ is the hydrated ion’s effective diameter in nanometers.³¹ This equation is appropriate for ionic strength less than 0.1 M but gives larger errors than Equation 8 at higher ionic strength.^32-

34 As the focus of this work is fresh water analysis both equations should here be appropriate. As shown in Figure S1, in different concentrations of a monovalent background M²N^., the potential differences caused by the ionic strength effect are larger for divalent anions compared with that of monovalent anions. But with a given background, the potential difference remains constant in a wide concentration range up to 1 mM. It means that for a sample solution with known ionic strength, the potential difference for divalent anions caused by the ionic strength effect can be calculated and corrected, and concentration still can be used instead of activity in a rela- tively low concentration range. Still, the approach is not suitable for the sensing of divalent anions in an unknown sample, especially at high ionic strength. For most monovalent anions, however, it is convincing that concentration can be used directly instead of activity for analysis in moderately dilute electrolyte background. For the detection of divalent anions, cations, or other multi-charged ions, the potential difference caused by the effect of ionic strength will require correction.

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Figure 2. Calculated potential differences caused by the activity coefficient ratio of different target anions A and iodide in different concentrations of a monovalent background electrolyte M²N^..

To evaluate the approach by experiment, this protocol was applied to fluoride and nitrate detection with a commercial fluoride-selective electrode and a solid-state nitrate-selective probe as indicator electrode, respectively. The Nernstian response of the commercial fluoride-selective electrode against the Ag/AgI pulstrode is shown in Figure S2, which is comparable to a conventional double junction Ag/AgCl reference electrode. In a constant concentration of 1.0 mM NaF solution, the potential variations in the absence and presence of increasing concentrations of NaCl background is shown in Fig- ure 3. Consistent with the theoretically simulated value, the potential difference based on the Ag/AgI pulstrode is negligible at lower ionic strength, and increases when the ionic strength is higher than 10 mM. With a classical reference electrode of Ag/AgCl/3 M KCl/1 M LiOAc, the expected potential difference caused by the ionic strength effect and liquid junc- tions is dramatically larger than that using the pulstrode protocol, shown as solid blue line. Analogous results for nitrate are shown in Figure S3.

Figure 3. Experimental potential differences for the fluoride- selective electrode with increasing concentrations of NaCl in 1.0 mM acetate buffer (pH 5.6) and a constant concentration of 1.0 mM of NaF. The red line shows the theoretical expecta- tion with the Ag/AgI pulstrode as reference electrode, while the blue line is the expected behavior based on a classical reference electrode of Ag/AgCl/3 M KCl/1 M LiOAc and an uncorrected liquid junction potential.

Applied to fluoride or nitrate detection, the Nernstian responses against the Ag/AgI pulstrode in different concentrations of NaCl background are shown in Figure 4 (for fluoride) and Figure S4 (for nitrate). When the activities of fluoride or nitrate were used for the Nernstian curve, the EMF values were also corrected by the activity coefficient of iodide according to Equation 7. When concentrations were used, the EMF values directly reflect the experimental readings. For different background conditions, the Nernstian responses are practically indistinguishable whether using activity or concentration of the target anion. The Nernstian slope of EMF against concentration is just 0.1 mV lower than that against activity, which is within experimental error. The small difference may be attributed to the slight increase of the activity coefficient ratio between the target anion and iodide at higher ionic strength. is within experimental error.

Figure 4. Nernstian responses for fluoride with respect to the Ag/AgI reference electrode in different concentrations of NaCl background with 1.0 mM of acetate as buffer solution (pH 5.6). The symbol of a and c represents activity and concentration of fluoride, respectively.

The potential differences for different background concentrations is negligible for fluoride detection (see Figure 4d).

The difference of Nernstian linear range and slope for nitrate detection in different concentrations of NaCl background (Figure S4) originates in the interference from chloride, which is consistent with the reported selectivity coefficient for nitrate over chloride (log𝐾₄₅_#_,7-= −2.1).³⁵ The results demon- strate the feasibility and simplicity of using concentrations instead of activities in direct potentiometric analysis with anion-selective electrodes. It is noted that the released iodide might eventually interfere with the nitrate ISE depending on how much iodide will diffuse to the nitrate ISE. This may be eliminated or reduced by minimizing the diffusion of iodide, including the iodide releasing time and EMF reading period.

Here, all measurements were done in quiescent solution to avoid excessive convection of the released iodide. While no apparent interference was observed, this possible limitation should be considered in practice.

To further evaluate the practical applicability of this protocol, we applied these sensors to the detection of fluoride and nitrate in real world samples. The results were compared to those based on a conventional double junction reference electrode and to ion chromatography. The fluoride concentration in toothpaste and mouthwash, and the nitrate concentration in mineral water (Henniez, Switzerland) and river water (Hermance River, Switzerland) were detected (Table S1). The obtained concentrations using the proposed Ag/AgI pulstrode protocol were comparable with the results based on a conventional protocol, within experimental error. To evaluate the influence of ionic strength on the results, ionic strength adjuster was applied. For fluoride detection, ionic strength strongly influences the concentration readout with conventional reference electrode compared with the newly proposed

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system. For nitrate determination, ionic strength adjuster is not suitable since the added concentration of chloride will cause serious interference. With just 10 mM of added NaCl chloride interference was still apparent, suggesting that the detection in unmodified samples is advantageous. Samples with high concentration of iodide may also influence the analysis. However, iodide is not commonly distributed in environmental samples as it is it is thermodynamically unstable.³⁶ In surface ocean waters, for example, iodide concentration approaches 100-200 nM³⁷, much lower than that locally released here (around 0.5 mM).

In conclusion, using the solid-state Ag/AgI pulstrode as reference electrode, the direct potentiometric sensing of anion concentration instead of activity was achieved. The uncertain- ty of using concentrations instead of activities is given by the activity coefficient ratio between the ion of interest and the released iodide from the Ag/AgI reference electrode. For most monovalent anions of practical relevance, the difference is negligible. The approach is not yet applicable for the detection of divalent anions, cations, or other multi-charged ions, which is the topic of current research in our laboratory. It is noted that the reference potential depends on the release and recapture of iodide, which might be invasive to samples and interfere with indicator electrodes. This could be eliminated or reduced by a protective gel layer on the electrode surface and optimizing the iodide release and recapture processes. We believe that having the option to directly calibrate potentiometric sensing probes against concentrations is highly desired for many applications.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website.

Theoretically simulated potential differences for divalent anions; Nernstian responses of the commercial fluoride-selective electrode; the potential differences between the absence and presence of increasing concentrations of NaCl background in a constant concentration of nitrate solution; Nernstian responses for nitrate with respect to the Ag/AgI reference electrode in different concentrations of NaCl background (PDF)

AUTHOR INFORMATION Corresponding Author

* Eric Bakker, E-mail: eric.bakker@unige.ch.

* Xiaojiang Xie, E-mail: xiexj@sustech.edu.cn.

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT

This project was ﬁnancially supported by the National Natural Science Foundation of China (21874063) and the Swiss Na- tional Science Foundation (200021_175622).

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(35) Cuartero, M.; Crespo, G. A.; Bakker, E. Tandem Electrochemical Desalination-Potentiometric Nitrate Sensing for Seawater Analysis, Anal. Chem. 2015, 87, 8084-8089.

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7 For TOC only

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S-1

Supporting Information For:

Direct Potentiometric Sensing of Anion Concentration (Not Activity)

Wenyue Gao,

^†,‡

Xiaojiang Xie

^‡,

* and Eric Bakker

^†,

*

† Department of Inorganic and Analytical Chemistry, University of Geneva, Quai Ernest-Ansermet 30, CH-1211 Geneva, Switzerland

‡ Department of Chemistry, Southern University of Science and Technology, Shenzhen, 518055, China

* E-mail: xiexj@sustech.edu.cn; eric.bakker@unige.ch.

Table of Contents:

Figure S1. Theoretically simulated potential differences for divalent anions………...……S-2 Figure S2. Nernstian responses of the commercial fluoride-selective electrode….…………S-2

Figure S3. The potential differences for nitrate between the absence and presence of

increasing concentrations of NaCl background………...…...………S-3

Figure S4. Nernstian responses for nitrate with respect to the Ag/AgI reference electrode in

different concentrations of NaCl background……..………S-3

Table S1. Anion detection in real samples using the Ag/AgI pulstrode as reference electrode

compared with other methods……….…..………S-4

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S-2

Figures ：

Figure S1. Theoretically simulated potential differences caused by the activity coefficient ratio

between different target divalent anions A and iodide in different concentrations of monovalent electrolyte (M

^!N^"

) background.

Figure S2. Nernstian responses of the commercial fluoride-selective electrode against the Ag/AgI

pulstrode (black line) and with the conventional Ag/AgCl/3 M KCl/1 M LiOAc reference electrode

(red line). Background electrolyte: TISAB IV solution at pH 5.6.

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S-3

Figure S3. Measurements with a nitrate-selective electrode. The potential differences with

increasing concentrations of NaCl in a constant concentration of 0.1 mM NaNO

3

solution. The black dots and red line represent respectively the experimental data points and the theoretically simulated curve with the Ag/AgI pulstrode as reference electrode, while the blue line is the theoretical curve based on a classical electrode of Ag/AgCl/3 M KCl/1 M LiOAc and an uncorrected liquid junction potential.

Figure S4. Nernstian responses of the solid-state nitrate ISE with respect to the Ag/AgI reference

electrode in different concentrations of NaCl background. The symbol of

a and c represents

activity and concentration of nitrate, respectively. The value of the slope depends on the

concentration range chosen for the calculation. The electrode slopes determined for the five

highest concentrations (log a (or c) = -4 to -2) are closer at -60.5 (a), -61.7 (b), -60.6 (c), -55.5 (d).

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S-4

Table S1. Anion detection in real samples using the Ag/AgI pulstrode as reference electrode compared with a commercial reference electrode and ion chromatography.

Target Anion Sample Ag/AgI CRE^a IC^b

NO^c IS^d NO^c IS^d

Fluoride (ppm)

toothpaste 1447 ± 29 1401 ± 23 1413 ± 14 1063 ± 21 1358±11

mouthwash 251 ± 8 243 ± 9 249 ± 7 168 ± 3 256±4

Nitrate (µM)

mineral water 270 ± 8 303 ± 5 268 ± 6 312 ± 10 257±5 river water 188 ± 6 234 ± 10 191 ± 4 245 ± 6 168±2 a: commercial reference electrode of Ag/AgCl/3 M KCl/1 M LiOAc

b: Ion chromatography

c: without ionic strength adjuster

b: with ionic strength adjuster: for fluoride, TISAB IV was used; for nitrate, 10 mM NaCl was added to adjust ionic strength.