## 1 Introduction  
The thermodynamic properties of bioactive molecules in aqueous solutions are of significant scientific and practical interest. In this context, the acidic constants are important physicochemical parameters of a molecule, as they reflect its reactivity, solubility, affinity for a macromolecule and/or ability to establish intermolecular interactions. In addition, the acid and basic functional groups of a molecule strongly influence its pharmacokinetics and toxicity. The experimental evaluation of the pK\({}_{\alpha}\) values of the molecules of biological interest, which could be hygroscopic and very unstable in an extreme pH range, is still unsolved, as it is related to the conditions in which the experiments are performed. The most useful and widely employed conventional methods are potentiometric and spectrophotometric titrations and conductometry [1; 2; 3]. However, only a few of them have referred to measurements conducted in aqueous media, where they show low water solubility.  
In this framework, the present study intends to evaluate the acidic constant values of quercetin, luteolin and L-ascorbic acid (Figure 1a-c), three natural antioxidants (generically H\({}_{n}\)L) whose beneficial properties are well known, in aqueous solution. The experimental approach provides measurements via potentiometric and UV-vis spectrophotometric titrations in a wide pH range, at 37 \({}^{\circ}\)C and ionic strengths of 0.16 M, using NaCl as a background electrolyte. The constant ionic medium method has proved to be indispensable in equilibrium studies of complex ionic reactions as it is necessary to minimize activity coefficient variation, even though the modification is in the concentration of the reagent. The method, which consists of using concentrated aqueous solutions of inert salts as the solvent, allows for the use of concentrations instead of activities in the evaluation of equilibrium constants. Additionally, it was useful to minimize the liquid junction potential, owing to the hydrogen ion concentration, which was varied in a wide pH range to assess the acidic constants of H\({}_{n}\)L.  
Quercetin and luteolin belong to flavonoids that are secondary plant metabolites of the phenolic family. The most common flavonoids contain three rings, indicated as A, B and C (Figure 1a,b), and the oxidation state of C ring provides their classification into several subgroups, as quercetin and luteolin belong to flavonoid and flavone subclasses, respectively. L-ascorbic acid is a water-soluble vitamin required for the prevention of scurvy and is known to be active as a cellular hydrogen transfer carrier for the redox enzyme in live cells [4]. Structurally, ascorbic acid is a \(\gamma\)-lactone and an ene-diol (Figure 1c).  
All these plant-derived natural compounds are found in significantly high quantities in fruits and vegetables and are considered valuable health-promoting compounds [4; 5; 6; 7; 8; 9]. Choosing these natural antioxidants is also related to their well-known capability to chelate several metal ions of relevant importance for biological systems, and they have attracted our research interest in the past 10 years [10; 11; 12; 13; 14; 15; 16; 17]. Their ability to bind cations depends on their structure, characterized by functional groups such as phenol, enol andenedi moieties that can undergo deprotonation via metal interaction, whatever their p\(K_{\text{a}}\) value. In this context, the accurate knowledge of the acidity of -OH is fundamental to understanding how these molecules can counteract the metal-related damage [18; 19; 20; 21; 22; 23; 24; 25]. Furthermore, their antioxidant ability could be correlated to the relative acidity of -OH groups.