  We report the detection of hydrogen chloride, HCl, in diffuse molecular
clouds on the line of sight towards the star-forming region W31C (G10.6-0.4).
The J = 1-0 lines of the two stable HCl isotopologues, H35Cl and H37Cl, are
observed using the 1b receiver of the Heterodyne Instrument for the
Far-Infrared (HIFI) aboard the Herschel Space Observatory. The HCl line is
detected in absorption, over a wide range of velocities associated with diffuse
clouds along the line of sight to W31C. The analysis of the absorption strength
yields a total HCl column density of few 10^13 cm^-2, implying that HCl
accounts for ~0.6 % of the total gasphase chlorine, which exceeds by a factor
of ~6 the theoretical model predictions. This result is comparable to those
obtained from the chemically-related species H2Cl+ and HCl+, for which large
column densities have also been reported on the same line of sight. The source
of discrepancy between models and observations is still unknown; however, the
detection of these Cl-bearing molecules, provides key constraints for the
chlorine chemistry in the diffuse gas.
