Dynamic Electrochemical Measurement of Chloride Ions

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.