The hydrated proton is very acidic: at 25 °C, its pK is approximately 0. The values commonly given for pK(HO) are 0 or –1.74. The former uses the convention that the activity of the solvent in a dilute solution (in this case, water) is 1, while the latter uses the value of the concentration of water in the pure liquid of 55.5 M. Silverstein has shown that the latter value is thermodynamically unsupportable. The disagreement comes from the ambiguity that to define pK of HO in water, HO has to act simultaneously as a solute and the solvent. The IUPAC has not given an official definition of pK that would resolve this ambiguity. Burgot has argued that HO(aq) + HO (l) ⇄ HO (aq) + HO (aq) is simply not a thermodynamically well-defined process. For an estimate of pK(HO), Burgot suggests taking the measured value pK(HO) = 0.3, the pK of HO in ethanol, and applying the correlation equation pK = pK – 1.0 (± 0.3) to convert the ethanol pK to an aqueous value, to give a value of pK(HO) = –0.7 (± 0.3). On the other hand, Silverstein has shown that Ballinger and Longs experimental results  support a pK of 0.0 for the aqueous proton. Neils and Schaertel provide added arguments for a pK' of 0.0 
The aqueous proton is the most acidic species that can exist in water (assuming sufficient water for dissolution): any stronger acid will ionize and yield a hydrated proton. The acidity of (aq) is the implicit standard used to judge the strength of an acid in water: strong acids must be better proton donors than (aq), as otherwise a significant portion of acid will exist in a non-ionized state (i.e.: a weak acid). Unlike (aq) in neutral solutions that result from water's autodissociation, in acidic solutions, (aq) is long-lasting and concentrated, in proportion to the strength of the dissolved acid.
pH was originally conceived to be a measure of the hydrogen ion concentration of aqueous solution. Virtually all such free protons are quickly hydrated; acidity of an aqueous solution is therefore more accurately characterized by its concentration of (aq). In organic syntheses, such as acid catalyzed reactions, the hydronium ion () is used interchangeably with the  ion; choosing one over the other has no significant effect on the mechanism of reaction.