# Monitoring Electrochemical Reactions in Situ with Low Field NMR: A Mini-Review
## 3 TD NMR Coupled to Electrochemistry  
Electrochemical measurements are quite limited when it comes to discriminating how much of each compound involved in the reaction is consumed or produced, since they measure the total current that flows through the system, which includes not only the target reaction but also parallel reactions such as gas evolution. Thus, it is not possible to exactly determine how much of the current has been used to produce a certain compound based on these measurements alone. It is for this reason that coupling NMR to electrochemical methods is useful, as NMR (more specifically high-field NMR) can provide information regarding the presence and concentration of the different soluble molecules involved in the reaction. On the other hand, TD NMR can be used to follow the consumption of paramagnetic ions in the solution through measurements of either \(T_{1}\) or \(T_{2}\). Taking advantage of this fact, in 2010 [PERSON] et al. [10] used an electrochemical cell with a height of 6 cm and 3 cm of diameter (Figure 2) in a 2.1 T superconductor magnetic resonance imaging system to follow in situ the electrodeposition reaction of copper ions in a solution containing sorbitol as a complexing agent. However, they could not observe a substantial difference in the values of \(T_{2}\), which could be linked to the large cell volume. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Magnet Type** & **LOD (mol\(\cdot\)L\({}^{-1}\))** & **LOQ (mol\(\cdot\)L\({}^{-1}\))** \\ \hline H-type & \(4\times 10^{-6}\) & \(2\times 10^{-5}\) \\ Unilateral & \(1\times 10^{-3}\) & \(5\times 10^{-3}\) \\ Uv/Vis & \(6\times 10^{-4}\) & \(2\times 10^{-3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Average limits of detection and quantification (LOD and LOQ) of the different spectrometers for the tested paramagnetic ions [22]. Figure 2: Schematic diagram of the EC-NMR cell used by [PERSON] et al. WE, CE and RE refer to the working, counter and reference electrodes, respectively. Reproduced with permission from [10]. Copyright (2010), The Electrochemical Society. Introducing an electrochemical cell in the NMR detection region does bring with it some challenges. First and foremost, introducing metal electrodes in the detection region of the NMR spectrometer reduces the signal-to-noise ratio of the NMR experiments by introducing noise and by reducing the intensity of the signal due to distortions caused to the static magnetic field, which reduces the homogeneity of the magnetic field. Typical ways to deal with these problems include using radio-frequency filters (chokes), the use of non-metallic electrodes and even the use of metallic thin films [25]. In 2012 [PERSON] et al. [7] published the first article showing that it is possible to monitor the electrodeposition reaction of Cu\({}^{2+}\) ions by coupling the electrochemical experiment to a low-field bench-top NMR spectrometer (0.23 T). They measured the \(T_{2}\) of the solution using the CPMG pulse sequence and were able to track the consumption of the copper ions in real-time (Figure 3) despite the fact that the noise in the NMR signals was increased. The authors placed the electrodes in the NMR detection region and made no use of radio-frequency filters. However, they did isolate the cables connecting the electrochemical cell to the potentiostat to reduce the interference that the electrodes cause on the NMR signal. In 2018, [PERSON] et al. [26] showed that it is possible to increase the NMR signal-to-noise ratio of the TD NMR experiments performed during in situ electrochemical experiments by using a secondary coil which is wrapped directly on the electrochemical cell. The system that was used consisted of an electrochemical cell built into a 5 mm standard NMR tube which contained a Pt wire as CE and an Ag/AgCl wire as RE positioned above the secondary electrode and a WE consisting of a Pt disc placed at the bottom of the tube (Figure 4A).