The determination of cyanide was based on discoloration of the [Cu(BCA)2]3− complexdue to reaction with cyanide.
This reaction is fast and thermodynamically favored, Kf[Cu(CN)2]−= 1.0 × 1024.
The [Cu(BCA)2]3−complex was on-line formed at the acceptor stream (it is stable for at least 15 min) and long pathlength spectrophotometry (LPS) was exploited to increase sensitivity.