The simplest way to determine /CN/2 might be GLC or HPLC in its agueous solution. In the case of GLC you can test Chromosorb 101, Carbopack or Porapack fillings with N2 or He as a carrier gas. Detection: TCD or possibly FID /in case if you observe reliable response/. Temperature of thermostat up to 100 oC, injector slightly over 100 oC.

In case of HPLC you might test column with low or medium polarity filling, isocratic mode of separation, refractive index detection /RI/. As elution medium might serve CH3CN or its mixture with water, also THF could be tested.

Concentation of cyanogene in your sample may vary from 0 to 9.72 mg /ml, as it follows from solubility of this gas under normal conditions. Maximum amount of /CN/2 dissolved in H2O in this case is 4.0 l per 1 liter of water /or approx. 8.64 g per 1 l of water. Such amounts of /CN/2 should be easy to detect.   

Notice: cyanogene undergoes slow hydrolysis yielding ammonium oxalate  when stored in water, especially when exposed to UV portion of the day light.   

Good luck.

Try out this approach:

www.ncbi.nlm.nih.gov/pubmed/11128214

Good luck!

Regina