  Interstellar complex organic molecules (COMs) are commonly observed during
star formation, and are proposed to form through radical chemistry in icy grain
mantles. Reactions between ions and neutral molecules in ices may provide an
alternative cold channel to complexity, as ion-neutral reactions are thought to
have low or even no energy barriers. Here we present a study of a the kinetics
and mechanisms of a potential ion-generating acid-base reaction between
NH$_{3}$ and HCOOH to form the salt NH$_{4}^{+}$HCOO$^{-}$. We observe salt
growth at temperatures as low as 15K, indicating that this reaction is feasible
in cold environments. The kinetics of salt growth are best fit by a two-step
model involving a slow "pre-reaction" step followed by a fast reaction step.
The reaction energy barrier is determined to be 70 $\pm$ 30K with a
pre-exponential factor 1.4 $\pm$ 0.4 x 10$^{-3}$ s$^{-1}$. The pre-reaction
rate varies under different experimental conditions and likely represents a
combination of diffusion and orientation of reactant molecules. For a
diffusion-limited case, the pre-reaction barrier is 770 $\pm$ 110K with a
pre-exponential factor of $\sim$7.6 x 10$^{-3}$ s$^{-1}$. Acid-base chemistry
of common ice constituents is thus a potential cold pathway to generating ions
in interstellar ices.
