Pseudo-first-order reaction

In a second-order reaction $\mathrm{A}+\mathrm{B}⟶\mathrm{P}$ with rate law $-d[\mathrm{A}]/dt=k_2[\mathrm{A}][\mathrm{B}]$, a large excess of B can keep $[\mathrm{B}]\approx [\mathrm{B}{]}_0$ during the observation window. ThenThis is a pseudo-first-order reaction: the approximate first-order reaction describes that window, while changing the excess B concentration changes $k_1$. Its units are ${\mathrm{s}}^{-1}$, distinct from the second-order reaction rate constant $k_2$. For one-to-one stoichiometry, the maximum fractional depletion of B is $[\mathrm{A}{]}_0/[\mathrm{B}{]}_0$; an original $100:1$ initial excess bounds its change by $1\%$. A twofold excess permits $50\%$ depletion and does not provide the same control.^[ref]