Fig. 2

The experimental setup. (a) Schematic of the setup showing the hydraulic, control, sensor, and interface and acquisition subsystems. The cell (light-blue rectangle) is connected through flexible plastic tubing to the Solution Tank, which is in thermal contact with the Thermal bath tank. A solution of \({\text{NaHCO}_3}\) or \({\text{Na2CO}_{3}}\) is circulated by a centrifugal pump, \({\text{P}}_1\). \({\text{CaCl}}_{2\mathrm{(aq)}}\) is added using a peristaltic pump, \({\text{P}}_2\). Various probes are positioned either within the cell, along the pipeline, or within the solution tank: conductivity (\(\sigma _\text{i}\)), pH (\(\text{pH}_\text{i}\)), temperature (\({\text{T}}_\text{i}\)), manometer (P), flow meter (FM), and a pair of electrodes (Cu-Cu, Stainless Steel-Stainless Steel (SS-SS), Al-Al, or Brass-Brass). We observed that energy losses associated with flow restriction and increased pressure drop—due to the tubing or the insertion of auxiliary probes—lead to an increase in the temperature of the main solution beyond the capacity of our thermal bath. However, this temperature increase, which is at most \(18^\circ \text {C}\), was found to have little influence on the reaction kinetics. The electrode pairs (5 mm diameter, mounted in the middle of the cell with an approximate \(\omega \approx 5\) cm separation) are linked to a 10-channel voltage scanner. All input and control signals are routed via an Arduino and GPIB interface to or from a desktop computer; through the latter, all automation and data processing are managed. (b) The cell is placed between the poles (10 cm in diameter and 6 cm apart) of the electromagnet with \(|\vec {{\text{H}}}| \le 10\) kOe. This arrangement allows large liquid volume to be exposed to higher magnetic dose⁵. (c) Schematic cross-section illustrating the arrangement of four bar magnets positioned around a cylinder with a 5.0 cm diameter. (d.a) The field distribution along the cell axis, the x-axis, with the origin being at the center. (d.b) The field gradient manifesting an extremum around 9.5 cm away from the center. (e) Photograph of the non-conductive transparent polycarbonate cylinder. (f) Photograph of the cell with a stainless steel cylinder, all fit within the electromagnet poles, within the homogeneous part of the magnetic field (see panel b).