Name: dynamic electrochemical measurement of chloride ions
Uploaded: 2016-02-05T13:00:00
Description: Watch this Scientific Journal Video about Dynamic Electrochemical Measurement of Chloride Ions at JoVE.com

This work is a part of the STW project "Integral solution for sustainable construction (IS2C, Fleur van Rossem for her support during the chip fabrication, Justyna Wiedemair for the chip design and Allison Bidulock for her support during the manuscript preparation.

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 sho...

     

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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...

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 Cl&#713; 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 degradation1,2. It initiates pitting corrosion in the reinforcement steel and results in the ultimate failure of the structure3. Therefore, measuring Cl&#713; ions in concrete is inevitable to predict the service life and maintenance cycle of a structure4,5. Different sensing principles have been reported for chloride ion measurement in concrete such as electrochemical6,7, optical8,9 and electromagnetic10,11. 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 approach6,7,13. Despite promising results, this approach is limited to lab-scale measurement since the drifts in reference potential and diffusion potential drop results in flawed data14,15. A transition time approach based on the dynamic electrochemical measurement (DEM) could alleviate the problem due to potential drift16.
In the DEM, a system&#39;s response to an applied stimulus is measured17-19. 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 + Cl&#713; <img alt="Figure 1" src="/files/ftp_upload/53312/arrow.jpg" />AgCl + e&#713;) resulting in a depletion of Cl&#713; ions near electrode surface. The potential change is a function of the applied current and the concentration of the (selective) ions in the electrolyte12,20. 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 electrolytes23,24. 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.

<table border="0" cellpadding="0" cellspacing="0" width="1009">
	<tbody>
		<tr height="25">
			<td height="25" style="height:25px;width:373px;">Platinum wire (&#8805;99.99% trace metals)</td>
			<td style="width:292px;">Sigma Aldrich, the Netherlands</td>
			<td style="width:172px;">EP1330-1EA</td>
			<td style="width:172px;"></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">Potassium chloride (BioXtra, &#8805;99.0%)</td>
			<td>Sigma Aldrich, the Netherlands</td>
			<td>P9333-500G</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">Potassium hydroxide (90% pure reagent grade)</td>
			<td>Sigma Aldrich, the Netherlands</td>
			<td>484016-1KG</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">Ferric chloride&#160;</td>
			<td>Sigma Aldrich, the Netherlands</td>
			<td>451649-1G</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">potassium nitrate (&#62; 99% reagent grade)&#160;</td>
			<td>Sigma Aldrich, the Netherlands</td>
			<td>P6083-500G</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">Ag/AgCl liquid junction reference electrode&#160;</td>
			<td>BASi, USA</td>
			<td>model MF-2079</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">VSP potentiostat</td>
			<td>Biologic Science Instruments, France</td>
			<td>VSP 300</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">Steel wire</td>
			<td>Microlab TU Delft</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">Silver wire&#160;</td>
			<td>Sigma Aldrich, the Netherlands</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

dynamic electrochemical measurement of chloride ions

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 Cl&#713; 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 &#181;M/day) when measuring 1 mM [Cl&#713;]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.

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 Cl&#713; ion concentration on the transition time, solutions containing 4, 5 and 6 mM of Cl&#713; ions in a 0.5 M KNO3 background are measured (Figure 3). The calibration curve of the square root o...

Watch this Scientific Journal Video about Dynamic Electrochemical Measurement of Chloride Ions at JoVE.com

Dynamic Electrochemical Measurement of Chloride Ions

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.

This protocol describes the dynamic measurement of chloride ions using the transition time of a silver silver chloride (Ag/AgCl) electrode. Silver silver ...

The overall goal of this experiment is to demonstrate the measurement of chloride ion concentration in aqueous electrolyte, using a chronopotentiometric technique which is a dynamic measurement technique and does not need a long term stable reference electrode. This technique will help us answering key questions in the field of electrochemistry and concrete application. For example, measuring chloride ion concentration inside concrete.The development of this technology is quite relevant for the future application of sensing chloride ion concentration in concrete. Because only with this technology, we can be able to make senses that can measure for a long term in concrete in the future. Prior to starting this procedure, deposit silver metal on a glass chip to form a planar silver electrode.Place the strip in a polytetrafluoroethylene chip holder, which has electrical connections and contains an electrochemical cell. To chloridize the silver electrode to form a silver silver chloride electrode on the chip, pour a 0.1 molar iron chloride solution in the cell until the silver color of the electrode becomes dark grayish. Following this, rinse the chip with deionized water to remove the excess iron chloride.After preparing the electrolyte and setting up the potentiostat, pour five milliliters of one millimolar potassium chloride solution with a 0.5 molar potassium nitrate background electrolyte in the electrochemical cell. Next, start the chronopotentiometric experiment by using the potentiostat and apply a current of ten milliamps per centimeter squared for ten seconds. Then, store the data.Systematically change the concentration to six millimolar with one millimolar increments of potassium chloride. Then, repeat the measurements. After storing the measured data as an A.MPT file, analyze the data using an in-house developed data processing program and calculate the peak of the first derivative.At this point, repeat the measurement three times with an interval of one hour between each measurement. After opening all the data files in the data processing program, calculate the transition time for each measurement by plotting the potential difference versus time obtained. Calculate the first derivative of the potential response.Then, indicate the maximum of the first derivative and the time, which is the transition time. For the calibration curve, plot the square root of the transition time with respect to the concentration of chloride ions. Along the measured data, plot the theoretical curve based on the sand equation.Then, calculate back the diffusion coefficient from the data plot. For the drift measurement, pour five milliliters of one millimolar potassium chloride solution in the electrochemical cell. In the potentiostat, set the applied current as 10 ampere per meter squared and the time as 10 seconds.Measure the potential response for two weeks with three measurements each day, with an interval of three hours between measurements. Refresh the electrolyte every day before performing the measurements. Plot the transition time over two weeks of measurements.To study the effect of a pseudo-reference electrode on the transition-time measurements, pour four millimolar of the potassium chloride electrolyte in the electrochemical cell. Connect a silver silver chloride pseudo-reference electrode to the reference electrode terminal of the potentiostat. Following this, perform the chronopotentiometric measurement by applying a current density of 15 ampere per meter squared for ten seconds.Repeat the measurements with platinum and a steel bar as pseudo-reference electrodes. Record the data for each experiment. Finally, plot the measured transition time for the various pseudo-reference electrodes used.The measured transition times for four, five, and six millimolar chloride ion concentrations are 2.69, 4.28, and 5.92 seconds respectively. The time instant of the peak shifts to higher values, which is expected as more chloride ions present in the bulk electrolyte means it will take longer to completely deplete the ions near the working electrode surface. The square root of the transition time is in linear correlation to the chloride ion concentration, as predicted by the sand equation.The apparent diffusion coefficient of chloride ions is in good agreement with the theoretical value. The decreasing trend in transition time over the measurement is small and could be attributed to handling errors, changing chloride ion concentration due to evaporation, change in apparent current density, and temperature variation. It is therefore difficult to give a conclusion about the drift of the senser.Either there is no inherent drift, or the drift is small. For various pseudo-references, the response does not change significantly, and transition time varies within 80 milliseconds. Therefore, the reference system has no systematic effect, and any metal wire can be used as a pseudo-reference electrode for the measurements.Once mastered, this technique can be done in 30 minutes if performed properly. While applying this technique, it's important to remember that the current applied should not be really high, as it will change the surface morphology of the electrodes. After its development, this technique will pave the way for unlikely chemists to explore the area of a reference-free system and dynamic system inside concrete.After watching this video, you should have a good idea of determining chloride ion concentration using dynamic electrochemical techniques such as chronopotentiometry.

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