Dynamic Electrochemical Measurement of Chloride Ions

Summary

Dynamic measurement of chloride ions is presented. Transition time of an Ag/AgCl electrode, during a chronopotentiometric technique, can give the concentration of chloride ions in electrolyte. This method does not require a stable conventional reference electrode.

Abstract

This protocol describes the dynamic measurement of chloride ions using the transition time of a silver silver chloride (Ag/AgCl) electrode. Silver silver chloride electrode is used extensively for potentiometric measurement of chloride ions concentration in electrolyte. In this measurement, long-term and continuous monitoring is limited due to the inherent drift and the requirement of a stable reference electrode. We utilized the chronopotentiometric approach to minimize drift and avoid the use of a conventional reference electrode. A galvanostatic pulse is applied to an Ag/AgCl electrode which initiates a faradic reaction depleting the Clˉ ions near the electrode surface. The transition time, which is the time to completely deplete the ions near the electrode surface, is a function of the ion concentration, given by the Nernst equation. The square root of the transition time is in linear relation to the chloride ion concentration. Drift of the response over two weeks is negligible (59 µM/day) when measuring 1 mM [Clˉ]using a current pulse of 10 Am^-2. This is a dynamic measurement where the moment of transition time determines the response and thus is independent of the absolute potential. Any metal wire can be used as a pseudo-reference electrode, making this approach feasible for long-term measurement inside concrete structures.

Introduction

A chloride ion sensor based on the transition time measurement of an Ag/AgCl electrode is presented. The aim is to avoid the inherent drifts during the long-term continuous monitoring of the chloride ions in the electrolyte. Chronopotentiometric measurement, which is a dynamic measurement approach, of an Ag/AgCl electrode is used for this purpose. Here a rate of change of potential of an Ag/AgCl electrode is measured during a stimulus (galvanostatic pulse). The advantage of this approach is demonstrated by eluding the liquid-junction reference electrode and instead using any metal wire as a pseudo-reference electrode, therefore allowing the detection of Clˉ ions concentration for long-term (years) and in situ applications, such as measurement inside concrete structures.

Chloride ions in concrete structures is one of the major causes of degradation^1,2. It initiates pitting corrosion in the reinforcement steel and results in the ultimate failure of the structure³. Therefore, measuring Clˉ ions in concrete is inevitable to predict the service life and maintenance cycle of a structure^4,5. Different sensing principles have been reported for chloride ion measurement in concrete such as electrochemical^6,7, optical^8,9 and electromagnetic^10,11. However, optical and electromagnetic methods have bulky setups, are difficult to integrate as a stand-alone system and have issues with selectivity¹². In electrochemical technique, potentiometric measurement of an Ag/AgCl electrode is the state of the art approach^6,7,13. Despite promising results, this approach is limited to lab-scale measurement since the drifts in reference potential and diffusion potential drop results in flawed data^14,15. A transition time approach based on the dynamic electrochemical measurement (DEM) could alleviate the problem due to potential drift¹⁶.

In the DEM, a system's response to an applied stimulus is measured^17-19. The example of such a system is chronopotentiometry. Here an applied current pulse is used as a stimulus depleting ions near the electrode surface and the corresponding potential response is measured. An anodic current at an Ag/AgCl electrode initiates a faradaic reaction (Ag + Clˉ AgCl + eˉ) resulting in a depletion of Clˉ ions near electrode surface. The potential change is a function of the applied current and the concentration of the (selective) ions in the electrolyte^12,20. The moment these ions deplete completely near the electrode surface the rate of change of potential rises rapidly, giving an inflection point²¹. The inflection point on the potential-time response curve (chronopotentiogram) shows the transition time and can be determined from the maximum of the first derivative of the potential response²². The transition time is a characteristic of the ion concentration. This approach has been used to determine different ions concentration¹⁷ and the pH of electrolytes^23,24. In case of an Ag/AgCl electrode as a working electrode (to which current is applied) the depleting ions will be chloride ions¹⁷. Therefore measuring its transition time will determine its concentration.

Protocol

1. Chip Fabrication

Note: The chip consists of an Ag/AgCl working electrode (WE), an Ag/AgCl pseudo-reference electrode (pseudo-RE) and a platinum counter electrode on a glass chip. The silver metal is deposited on a glass chip, using standard cleanroom processes¹⁶. It is then chloridized in 0.1 M FeCl₃ solution for 30 sec to form an AgCl layer over the surface. The Ag/AgCl WE (area = 9.812 mm²) is located in the center, surrounded by the Ag/AgCl pseudo-RE as shown in Figure 1.

1. Deposit the silver metal on glass to form a planar silver electrode. The step-by-step chip fabrication in a cleanroom is given as follows:
   1. Lithography
      1. For wafer cleaning, immerse the glass wafer first into conc. HNO₃ bath to remove any organic contamination and then into 1 M KOH to clean the surface.
      2. Apply a positive photo resist (200 nm thick) to the wafer by spin coating and prebake the resist at 95 °C for 15 min.
      3. Expose the wafer to UV light for 5 sec through a glass mask.
      4. Develop the photoresist in developer solution. This will expose the area of glass where electrodes will be deposited and the remaining area will be covered by the unexposed resist.
   2. Glass Etching
      1. Immerse the developed glass wafer in the KOH bath to etch the exposed area of the glass.
   3. Metal Deposition (Sputtering)
      1. Deposit silver metal on the exposed glass area.
         1. First deposit a 20 nm titanium (Ti) layer (by sputtering) as an adhesion layer for glass.
         2. Then deposit 70 nm palladium (Pd) layer (by sputtering) as a diffusion barrier between silver and Ti.
         3. Finally deposit 500 nm of silver metal. This is the thickness of the deposited metal. The area of the working electrode is 4 mm².
         4. Remove the remaining resist by lift-off process in an acetone bath.
      2. Dice the wafer into the single chip size. The chip is now ready to be chloridized to form an on-chip Ag/AgCl electrode.
2. Place the chip in an in-house designed Polytetrafluoroethylene chip holder, as shown in Figure 1. The holder provides easy electrical connections via push-pins (or spring-loaded pins) and contains an electrochemical cell.
3. Chloridize the silver electrode to form an Ag/AgCl electrode on the chip. For that, pour 0.1 M FeCl₃ solution in the cell for 30 sec. or till the silver color of the electrode becomes dark grayish. Longer immersion is not critical whereas shorter immersion time can lead to no deposition of Ag/AgCl.
4. Rinse the chip with deionized water to clean the remaining FeCl₃.The chip is now ready to be used for the chronopotentiometric measurement.

2. Electrolyte Preparation

Note: The electrolytes were prepared with different concentration of potassium chloride in 0.5 M KNO₃.

1. Prepare 50 ml of 50 mM KCl electrolyte stock solution (0.186 g of KCl in 50 ml of solution).
2. In the same solution add 2.52 g of KNO₃ to form a 0.5 M KNO₃ background electrolyte.
3. Keep the starting concentration of chloride to zero and KNO₃ to 0.5 M. In other words add 2.52 g of KNO₃ in a 50 ml of deionized water.
4. Add 50 mM KCl to increase the chloride ion concentration (from 1 mM to 6 mM) of the electrolyte, systematically, as shown in the following steps.
   1. Add 100 µl of the 50 mM KCl into a 4.9 ml of 0.5 M KNO₃ to get 1 mM [Clˉ].
   2. Add 104 µl of the 50 mM KCl solution into the solution of step 2.4.1 to get 2 mM [Clˉ].
   3. Add 108.41 µl of the 50 mM KCl solution into the solution of step 2.4.2 to get 3 mM [Clˉ].
   4. Add 113.19 µl of the 50 mM KCl solution into the solution of step 2.4.3 to get 4 mM [Clˉ].
   5. Add 118.2 µl of the 50 mM KCl solution into the solution of step 2.4.4 to get 5 mM [Clˉ].
   6. Add 123.3 µl of the 50 mM KCl solution into the solution of step 2.4.5 to get 6 mM [Clˉ].

3. Experimental Setup

Note: The measurements were performed using a potentiostat (Biologic Science Instruments, France).

1. Place the chip in the chip holder and connect the corresponding terminal of the potentiostat.
2. Connect the working electrode terminal of the potentiostat to the Ag/AgCl electrode.
3. Connect the reference electrode terminal of the potentiostat to another Ag/AgCl electrode.
4. Connect the counter electrode terminal of the potentiostat to the platinum electrode.
5. Place the complete experiment setup in a faraday cage to avoid ambient noise.

4. Instrument Operational Settings

Note: The operational parameters of the electrochemical cell are controlled via the user interface of the potentiostat. The potential response of the Ag/AgCl WE during an applied current pulse is measured for different concentrations of chloride ion.

1. In the potentiostat program, open the technique chronopotentiometry and set the operational parameters.
   1. Set the following parameter according to the requirements:
      1. Current (I_s)
      2. Time of applied current (t_s)
   2. Start this technique by clicking the play button in the potentiostat program.
      Note: The potentiostat program displays the potential response as a function of time under the applied current pulse.

5. Measurements and Data Analysis

1. Draw the calibration curve of the transition time vs. Clˉ ion concentration in electrolyte.
   1. Change the Clˉ concentration in the KCl electrolyte, systematically.
      1. Start the measurement with 5 ml of 1 mM KCl with 0.5 M KNO₃ background electrolyte in the electrochemical cell.
      2. Start the chronopotentiometric experiment by using the potentiostat and apply a current of 10 mA/cm^-2 for 10 sec and store the data.
      3. Systematically change the concentration to 6 mM with 1 mM increments and repeat the measurements.
   2. Store the measured data as a.mpt file and analyze the data file in data processing program.
      1. Analyze the data using an in-house developed data processing program to calculate the first derivative of the potential response and calculate the peak of the first derivative. The peak of the first derivative is the transition time.
         Note: The complete coding of data processing program is given in the supplementary information of Abbas et al., 2014.
   3. Repeat the measurement three times with the interval of 1 hr each.
   4. Open all the data files in data processing program and calculate the transition time for each measurement. Detail steps of transition time calculation is given below:
      1. Plot the potential difference vs. time obtained from the data files of chronopotentiometric measurement.
      2. Calculate the first derivative of the potential response.
      3. Indicate the maximum of the first derivative and the time of it. The time of the maximum of first derivative is the transition time.
   5. For the calibration curve, plot the square root of the transition time with respect to the concentration of Clˉ ions.
   6. Along the measured data plot the theoretical curve based on the Sand equation²⁵. Calculate back the diffusion coefficient from the data plot.
      1. In the measured data plot between transition time and the [Clˉ] take any point on the line and record the value of transition time and the [Clˉ]. The Sand equation is given as:
         τ = (FC^*/2j(1-t_Cl^-))² Dπ
         Here, C^* is the bulk Clˉ ion concentration, F is the Faraday constant, j is the current density, t_Cl^- is the transport number and D is the diffusion coefficient. Neglect the t_Cl^- since it approaches zero for higher background electrolyte concentration.
      2. The only remaining variable in the Sand equation in the diffusion coefficient; put the value of transition time, the chloride ion concentration and the applied current in the Sand equation getting the value of the diffusion coefficient.
2. Drift Measurement
   1. Pour 5 ml of 1 mM of the KCl in the electrochemical cell.
   2. In the potentiostat, set the applied current to be 10 Am^-2 and time to be 10 sec.
   3. Measure the potential response for 2 weeks with three measurement each day with an interval of 3 hr between measurements.
   4. Refresh the electrolyte, i.e., 1 mM KCl, every day before performing the measurements.
   5. Plot the transition time over two weeks of measurements. The change in the transition time per day is the drift of the transition time response.
3. Effect of the pseudo-reference electrode on the transition time measurements
   Note: Various pseudo-references such as a Ag/AgCl wire, a platinum wire and a steel wire are tested to measure the transition time.
   1. Pour 4 mM of KCl electrolyte in the electrochemical cell.
   2. Use Ag/AgCl as a pseudo-reference electrode and connect it to the reference electrode terminal of the potentiostat. Due to anodic current pulse the Clˉ ions are depleted near the Ag/AgCl working electrode.
   3. Perform the chronopotentiometric measurement by applying a current density of 15 Am^-2 for 10 sec.
   4. Repeat the measurements with platinum as a pseudo-reference electrode and record the data.
   5. Repeat the measurements with steel bar as a pseudo-reference electrode and record the data.
   6. Plot the measured transition time for various pseudo-reference electrodes used.
   7. Repeat the complete set of measurement with 5 mM of KCl and plot the transition time using various pseudo-reference electrodes.

Results

The Ag/AgCl electrode is fabricated on a glass chip (Figure1) using a standard cleanroom process. The chronopotentiometric measurement setup (Figure 2) was used and response was measured using a potentiostat. To observe the effect of Clˉ ion concentration on the transition time, solutions containing 4, 5 and 6 mM of Clˉ ions in a 0.5 M KNO₃ background are measured (Figure 3). The calibration curve of the square root of the tran...

Discussion

The transition time is the moment of inflection; it is theoretically independent of the reference potential i.e., the reference electrode. Therefore any metal wire can be used as a pseudo-reference electrode for transition time measurements. In contrast to the existing potentiometric measurement of chloride ions in concrete this method enables a long-term and calibration free measurement. Furthermore the sensitivity and the detection range of concentration can be tuned by adjusting the applied current pulse. For...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Platinum wire (≥99.99% trace metals)	Sigma Aldrich, the Netherlands	EP1330-1EA
Potassium chloride (BioXtra, ≥99.0%)	Sigma Aldrich, the Netherlands	P9333-500G
Potassium hydroxide (90% pure reagent grade)	Sigma Aldrich, the Netherlands	484016-1KG
Ferric chloride	Sigma Aldrich, the Netherlands	451649-1G
potassium nitrate (>99% reagent grade)	Sigma Aldrich, the Netherlands	P6083-500G
Ag/AgCl liquid junction reference electrode	BASi, USA	model MF-2079
VSP potentiostat	Biologic Science Instruments, France	VSP 300
Steel wire	Microlab TU Delft
Silver wire	Sigma Aldrich, the Netherlands