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 conce....

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 ....

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