  We report on higly accurate absolute frequency measurement against a
femtosecond frequency comb of 6 saturated absorption lines of formic acid
(HCOOH) with an accuracy of 1 kHz. We also report the frequency measurement of
17 other lines with an accuracy of 2 kHz. Those lines are in quasi coincidence
with the 9R(36) to 9R(42) CO$_2$ laser emission lines and are probed either by
a CO$_2$ or a widely tunable quantum cascade laser phase locked to a master
CO$_2$ laser. The relative stability of two HCOOH stabilized lasers is
characterized by a relative Allan deviation of 4.5 10$^{-12}$ $\tau^{-1/2}$.
They give suitable frequency references for H$_2^+$ Doppler free two-photon
spectroscopy.
