Comparison of Voltammetric Techniques for the Electroanalysis of Dissolved Solids and Gases in Water, Acetonitrile and Room Temperature Ionic Liquids (RTILs): Unusual Behaviour for Gases

Abstract

Square wave voltammetry (SWV) and differential pulse voltammetry (DPV) are commonly used in electroanalysis, typically in aqueous solvents, as they are usually an order of magnitude more sensitive than linear sweep voltammetry (LSV)/cyclic voltammetry (CV), (e.g. detection limits as low as nanomolar are possible using SWV). These more sensitive techniques are often applied in the development of electrochemical sensors and biosensors over the last ten years due to the negligible capacitive current compared to faradaic current (in protic solvents).1,2 Over the last two decades there has been great attention focused on the improvement of electrochemical gas sensors to monitor toxic gases using amperometric gas sensors (AGSs), which typically employ water/sulphuric acid electrolytes as solvents. Room temperature ionic liquids (RTILs) have been attracting great attention3 as replacement electrolytes in AGSs due to their unique physical properties such as low volatility and negligible vapor pressure, wide electrochemical windows, high ionic conductivity, high chemical and thermal stability and their ability to dissolve a wide range of gases. To the best of our knowledge, there are limited studies on the sensitivity of SWV, DPV and LSV for the sensing of gaseous analytes in RTIL solvents. In this presentation, a comparison of three different electrochemical techniques (SWV, DPV and LSV) will be employed for two gases (ammonia and oxygen) in RTILs. As a control, oxygen gas will also be studied in acetonitrile and water solvents, and dissolved solid analytes are also studied (K3Fe(CN)6in water, ferrocene in acetonitrile and ferrocene in RTILs). For all dissolved solid analytes, SWV gave the highest current response compared to DPV and LSV, respectively. For dissolved gases, the behavior was different. For oxygen, LSV gave larger currents (although relatively comparable) to SWV, with DPV giving the lowest currents. For ammonia, LSV was far superior to both SWV and DPV and was consistent throughout eight different RTIL solvents. This suggests that the generally accepted methods for low concentration analyte detection may not apply for dissolved gases.