## 1 Introduction  
Anaerobic digestion (AD) is attracting increasing attention due to its environmental benefits such as the production of renewable energy (biogas) and the valorization of organic biomass. Four key microbial transformation steps have been identified: hydrolysis, acidogenesis and methanogenesis. During the acidogenesis stage, the molecules produced at the end of the hydrolysis stage are degraded by acidogenic microorganisms to produce volatile fatty acids (VFAs) such as acetic, propionic and butyric acids. The VFAs are then consumed by acetogenic and methanogenic microorganisms for the production of biogas and, thus, methane. The acidic nature of VFAs leads to the acidification of reactors by lowering the pH, which is a crucial parameter for the success of AD, particularly for methanogenic activities. As shown in Table S1 in the Supplementary Materials, the acidity constants (pKa) of the main VFAs found in a digester are relatively close to each other. For this reason, the VFA concentration is often expressed in an equivalent acetic acid content (usually expressed in g HAc\(\cdot\)L\({}^{-1}\)). Depending on the digestion method and the nature of substrates, the VFA concentration can reach 3 g HAc\(\cdot\)L\({}^{-1}\) or much more [1]. When VFAs are not consumed and, therefore, accumulate in reactors, the alkalinity characterizing the buffering capacity of reactors becomes an important indicator for ensuring a stable and optimal pH (between 6.5 and 7.5) for AD processes. Theoretically, the term \"total alkalinity\" (TAC) is defined as the total proton acceptance capacity of an aqueous system by weak acids such as carbonic acid (H\({}_{2}\)CO\({}_{3}\)/HCO\({}_{3}\)\({}^{-}\)/CO\({}_{3}\)\({}^{2}\)), phosphoric acid (H\({}_{3}\)PO\({}_{4}\)/H\({}_{2}\)PO\({}_{4}\)\({}^{-}\)/HPO\({}_{4}\)\({}^{2}\)\({}^{-}\)/PO\({}_{4}\)\({}^{3-}\)), ammonia (NH\({}_{4}\)\({}^{+}\)/NH\({}_{3}\)) and OH\({}^{-}\) ions. In an anaerobic digester, the buffering effect of H\({}_{2}\)CO\({}_{3}\)/HCO\({}_{3}\)\({}^{-}\), H\({}_{3}\)PO\({}_{4}\)\({}^{-}\)/H\({}_{2}\)PO\({}_{4}\)\({}^{-}\), H\({}_{2}\)PO\({}_{4}\)\({}^{-}\)/HPO\({}_{4}\)\({}^{2}\)\({}^{-}\)/PO\({}_{4}\)\({}^{3-}\) and NH\({}_{4}\)\({}^{+}\)/NH\({}_{3}\) is generally negligible compared to that of HCO\({}_{3}\)\({}^{-}\)/CO\({}_{3}\)\({}^{2-}\) due to their low presence in the operational pH of AD [2]. This is justified by the pKa values of different compounds constituting alkalinity (see Table S1 in Supplementary Materials). Alkanity is, thus, often presented in an equivalent carbonate alkalanity and usually expressed in g CaCO\({}_{3}\)\(\cdot\)L\({}^{-1}\). Many methods for determining TAC and VFA concentration in a complex solution have been proposed since the 1960s. The simplest ones rely on the acid-base character of the compounds and are based on titration, a simple and inexpensive method. Since the pK\({}_{a}\) of the H\({}_{2}\)CO\({}_{3}\)/HCO\({}_{3}\)\({}^{-}\) couple differs from that of acetic acid, titration with a strong acid (HCl or H\({}_{2}\)SO\({}_{4}\)) by following the pH evolution is one of the methods developed and widely used. For a pH between 6 and 8, as shown in Figure 1, most protons are consumed by bicarbonate. However, at a pH less than 6, the VFAs (in most cases acetate) are titrated as well. A total of 36.5% of acetate is transformed into acetic acid at pH = 5, while more than 95% of bicarbonate is titrated. Between pH = 5 and pH = 4, acetate becomes the main target of titration--about 50% more acetate (i.e., 86.5% in total) is titrated. Since less than 5% of the inorganic carbon is consumed at this moment, bicarbonate has a limited effect on the determination of VFAs. Hence, the difficulty in interpreting the titration result arises from the fact that VFAs, mainly in their dissociated form (e.g., acetate) at a pH between 5 and 6, are also titrated and taken into account as the bicarbonate alkalinity.