# Monitoring Electrochemical Reactions in Situ with Low Field NMR: A Mini-Review
## 4 TD NMR for the Study of Magnetoelectrolysis
demonstrated that the increase in the conversion rate (determined through the measurement of the electric current that flows during the experiment) is proportional to **B**[27]. Another force that may play an important role in the magnetoelectrolysis effect is the force of the magnetic field gradient, also known as [PERSON]'s force, **F\({}_{\
abla\textbf{B}}\)**, which is more influent the less homogeneous the magnetic field is, as shown by Equation (3). \[\textbf{F}_{\
abla\textbf{B}}=\frac{1}{2\mu_{0}}c\chi_{m}2(\textbf{B}\cdot \
abla)\textbf{B} \tag{3}\]  
where \(\mu_{0}\) is the magnetic permeability of free space in units of T m A\({}^{-1}\), \(c\) is the concentration of paramagnetic species in solution given in mol m\({}^{-3}\), \(\chi_{m}\) is the molar magnetic susceptibility, **B** is the external magnetic field in T, \(\
abla\) is the Del operator and **F\({}_{\
abla\textbf{B}}\)** is given in units of N m\({}^{-3}\). This force can alter the morphology of metallic deposits as well as contribute to the magnetic agitation of the solution. In 2015, [PERSON] et al. [9] again showed that it is possible to follow the electrochemical deposition of copper ions, this time using a unilateral, homebuilt, sensor. To do this, however, a Faraday cage was built around the system to isolate it from ambient radio-frequencies which are driven into the NMR detection region by the electrodes and completely destroy the NMR signal. The cage consisted of a simple box whose inner walls were lined with aluminum foil. The reactions performed in situ (they again tested both **B\(\perp\)j** and **B\(\parallel\)j**) showed that after the 3h reaction 40% of copper had been  
Figure 6: (**A**) Electric double-layer thickness (\(\delta\)) during electrochemical experiments performed in the absence of an external magnetic field, **B**. The bulk concentration of ionic species is \(c_{0}\). (**B**) Electric double-layer thickness when **B**\(\
eq\) 0, which induces a tangential velocity component, **U**, in the bulk solution which in turn creates a hydrodynamic boundary layer with thickness \(\delta_{0}\). **i** is the current flowing in the system. Adapted with permission from [12]. Copyright (2001) American Chemical Society. deposited while only 12% had been deposited during the ex situ reaction, the former value being independent of the orientation between **j** and **B**. The authors also investigated and estimated the magnetic forces at play during the in situ experiments and attributed the higher deposition rate of the in situ reactions to the effects of **F\({}_{\textbf{B}}\)** and **F\({}_{\
abla\textbf{B}}\)**, the latter being quite important as the magnet produces a highly inhomogeneous magnetic field (the gradient was estimated to be 6 T m\({}^{-1}\)). Just as with the work from 2014, the reaction rate remained the same whether **j** and **B** were parallel or not (Figure 7), for the same reasons. The works from [PERSON] et al. from 2014 and 2015 highlight the importance of taking the effect of the magnetic field into account whenever EC-NMR experiments are performed in situ, since the reaction rate may differ from the rate observed in an ex situ reaction and this difference is proportional to the intensity of the applied magnetic field as shown in Equation (2). Thus, before initiating any in situ EC-NMR experiments the influence of the external magnetic field must be assessed to avoid inaccurate conclusions. It is worth pointing out that the effect of the magnetic field is especially evident on reactions that are limited by mass-transport because it has the effect of reducing the thickness of the electric double-layer due to magnetic stirring.