Note that in the pure water case, the occurence of OSAZ is thermodynamically slightly more preferred over H3SiO4 -(eqs A3 and A4 in table ) and this preference is enhanced when NaCl is present (eqs B2 and B3 in table ). These local thermodynamic preferences are crucial in determining which mechanism predominates because while the slow step in the OSAZ mechanism is the formation of OSAZ itself, the slow step in the next competing mechanism is the attack by H3SiO4 -on another monomer and is therefore directly dependent on the concentration of H3SiO4 -. This is why the latter mechanism predominates in higher pH ranges.