The decomposition of NaCN in water One method for making NaCN is the reaction of $\ce {2K4FeCN6 + 7H2SO4 = 12HCN + 2FeSO4 +2KHSO4 +K2SO4}$. The $\ce {HCN}$ is distilled into 50% $\ce {NaOH}$...after awhile the solution turns dark brown and you can smell ammonia.The reaction is likely $\ce {8NaCN +3H2CO3}$ (from water and $\ce {CO2}$)$\ce {=2NH3 + 6NaOCN + Na2CO3 + 2C}$.
What can be done to reverse this reaction? Is there any way to get rid of the dark brown color? The Merck Index says water solutions of $\ce {NaCN}$ rapidly decompose...if that is true then how are any $\ce {S_N2}$ reactions done with $\ce {NaCN}$ in water--ethanol solution? The internet has next to nothing on this problem.
Sodium cyanide is slowly decomposed by water, but the implication is that it can still be used over short time periods provided that acids are kept away and proper safety practices are followed.  See here.