Cyclic voltammetry (CV) was used to investigate both the anodic and cathodic behaviour of 8AQ. Experiments were performed in solutions with pH values ranging from 3 to 10 in order to follow relationship between 8AQ dissociation equilibrium and its redox properties. Cyclic voltammetric curves of 8AQ were recorded in the potential range from 0.2 to 0.9 V with scan rate 0.1 V/s (Figure A, B) and 0.02 V/s (Figure ). In this potential range, we have registered two oxidation peaks, the first one (Eox1) is a shoulder of the second oxidation peak (Eox2). The presence of two peaks was more pronounced in the pH range of 3 to 8. At higher pH values, a broadening of the peaks was observed, which caused the signals to merge (Figure ). Importantly, peak potentials always shifted to less positive values with increasing pH because it is more difficult to oxidise a protonated aromatic nitrogen in acidic medium. The first anodic peak (Eox1) was only visible in the first cycle recorded on a well-cleaned GC electrode while in the second and third cycle only the second oxidation signal (Eox2) was present (see SI Figure ). The shift of the second peak towards more positive potentials with a decreasing current value suggests adsorption of oxidation products which block the GC surface. When we increased the number of measurement cycles from 3 to 30, blocking the surface was evident (see SI Figure .2). The two anodic signals in CV were better seen for lower scan rate values e.g., 0.02 V/s (Figure ) as well as when pulse techniques (differential pulse voltammetry (DPV) and square wave voltammetry (SWV)) were applied (Figure and SI Figure .3). It seems that the presence of two peaks (especially the first one (Eox1)) is related to the measurement conditions. Barek and co-workers observed, for all AQs, only one well-developed oxidation peak in a Britton-Robinson buffer solution at carbon paste electrodes , while Stevic et al. observed two oxidation peaks for 6AQ at a GC electrode but only for certain pH values. In order to determine the type of electrochemical process for both peak Eox1 and Eox2 we performed a series of cyclic voltammograms with increasing scan rate from 0.01V/s to 0.1 V/s and based on results we plotted the peak currents vs. square root scan rate or scan rate (Figure SI 1.4). Based on these results, oxidation (Eox2) process can be described as diffusion-controlled since a linear dependence of the peak currents vs. the square root of the scan rate was observed in the range of pH investigated. Similar results were published for 6AQ and aminoquinoline based drugs . As the first oxidation peak is located as a shoulder on Eox2 it was difficult to determine the peak heights accurately. However, we estimated that the current values increase roughly proportionally to the scan rate, which suggests that the process associated with Eox1 can be described as adsorption-controlled. The position of this peak also indicates substrate adsorption. To confirm this assumption, we have performed transfer experiments. A GC electrode was carefully cleaned, and was immersed for 1 minute (or 5 minutes) in 0.5 mM 8AQ solution (pH 7.4). After this, the electrode was rinsed with water and CV was measured in phosphate buffer at pH 7.4 from 0.2 to 0.9 V. A clear anodic signal was observed on the recorded voltammogram at a potential corresponding to Eox1, confirming that this peak is associated with adsorption of the test compound on the electrode surface. (SI Figure .5) Based on the form distribution diagram (Figure ), we calculated that at pH 3 there is ~90% monopotonated aminoquinoline (8AQH + ) and about 9% non-protonated (8AQ). An increase of pH to a value 4 results in a 1:1 ratio of both forms. As the pH increases to 5, the percentage of the deprotonated form (8AQ) rises to 90%. At pH 6, almost 99% of the tested compound exits as 8AQ and only 1% as 8AQH + . The change in the distribution of the two forms significantly affects the position of the oxidation peaks. The potential of peak Eox2 is shifted to a more negative value with increasing pH from 3 to 10, (Figure ). In Figure , the position of the Eox2 is plotted vs. pH (■), two separate linear regions can be distinguished, the first one between pH 3 and 5 (~ 33 mV per pH unit) and the second between pH 6 to 10 (~ 44 mV per pH unit). This indicates that the oxidation mechanism in aqueous media is changing for these two forms (8AQ and 8AQH + ).