Likely, the decreased value stems from the ionic strength of the employed electrolytes between 50 to 500 mM. Regarding the proton acceptor and donor species, we used a linear regression (see Fig. ) to quantify the relationship between the pH and the limiting current. From the linear regression we can determine the exponent 𝑥 in Eq. 38c and 38d and the diffusion coefficient at a specified pH value. For the proton donor, we chose pH 4 and for the proton acceptor pH 10, as most of their respective measurement points are available around these values and we identify the proton donor and acceptor species simply as hydronium and hydroxyl ions from hereon. According to the Levich equation, 𝑥 should be exactly one, reflecting a linear relationship between bulk concentration and limiting current, so that the diffusion coefficient is independent of pH. We can indeed confirm such a relationship for hydronium ions and find a diffusion coefficient that is close to the theoretical value of 1•10 -4 cm 2 /s (see Fig. ), virtually unaffected by the ionic strength of the electrolyte. In contrast, the value of 𝑥 for hydroxyl ions deviates significantly from one. This deviation is more pronounced in electrolytes with greater ionic strength and more pronounced in the presence of lithium than sodium or potassium (Fig. ). The origin of this effect is unclear to us at this moment and challenges the evaluation of the diffusion coefficient of the hydroxyl ions, using the Levich equation. We hypothesize that this is a consequence of the proton hopping contribution (Grotthus mechanism) to the transport of hydroxyl ions in water (Fig 7b ), accounting for ~60%