This work presents high spectral resolution observations of the CII line at 158 micron, one of the major cooling lines of the interstellar medium, taken with the HIFI heterodyne spectrometer on the Herschel satellite. In BCLMP 691, an HII region far north (3.3 kpc) in the disk of M 33, the CII and CO line profiles show similar velocities within $0.5 kms$, while the HI line velocities are systematically shifted towards lower rotation velocities by $sim 5kms$. Observed at the same $12$ angular resolution, the CII lines are broader than those of CO by about 50% but narrower than the HI lines. The CII line to far-infrared continuum ratio suggests a photoelectric heating efficiency of 1.1%. The data, together with published models indicate a UV field $G_0 sim 100$ in units of the solar neighborhood value, a gas density $n_H sim 1000 cc$, and a gas temperature $Tsim 200$ K. Adopting these values, we estimate the C$^+$ column density to be $N_{C^+} approx 1.3 times 10^{17} cmt$. The CII emission comes predominantly from the warm neutral region between the HII region and the cool molecular cloud behind it. From published abundances, the inferred C$^+$ column corresponds to a hydrogen column density of $N_H sim 2 times 10^{21} cmt$. The CO observations suggest that $N_H = 2 N_{H_2} sim 3.2 times 10^{21} cmt$ and 21cm measurements, also at $12$ resolution, yield $N_HI approx 1.2 times 10^{21} cmt$ within the CII velocity range. Thus, some H$_2$ not detected in CO must be present, in agreement with earlier findings based on the SPIRE 250 -- 500 $mu$m emission.
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