  We study the correlation of the velocity dispersion of the coexisting
molecules H13CN and H13CO+ and the turbulent energy dissipation scale in the
DR21(OH) star-forming region. The down-shift of the H13CO+ spectrum relative to
H13CN is consistent with the presence of ambipolar diffusion at dissipation
length scales that helps the process of turbulent energy dissipation, but at a
different cut-off for ions compared to the neutrals. We use our observational
data to calculate a turbulent ambipolar diffusion length scale L'\simeq17 mpc
and a strength of B_{pos}\simeq1.7 mG for the plane of the sky component of the
magnetic field in DR21(OH).
