The acid-base neutralization reaction between formic acid (HCHO2) and sodium hydroxide (NaOH) is
HCHO2(aq) + NaOH(s) --> NaCHO2(aq) + H2O(l)
Formic acid is a weak acid. So, the salt formed here is a basic salt, NaCHO2 or Sodium Formate. A buffer solution is when the system contains either a weak acid and its conjugate strong base or a weak base and its conjugate strong acid. In this case, formic acid is the weak base, while sodium formate is the conjugate strong base. So, this is a buffer system. By definition, a buffer resists pH changes with small addition of either a base or an acid. So, when NaOH is added, it would produce more of sodium formate. As long as formic acid is not depleted, the buffer system would be stable.
