Several solutions (15–120 mmol L−1 NH4OH, 20 mmol L−1 NaOH and the [Cu(BCA)2]3− complex formed on-line) were evaluated as acceptor stream aiming to maximize HCN diffusion by fixing cyanide.
The reference signal was 3-fold lower when NaOH wasthe acceptor stream due to copper hydrolysis, which impaired the [Cu(BCA)2]3− formation.
Similar results were obtained with the complex and ammonium hydroxide solutions and the former was selected to simplify the flow manifold.
The effect of acidity of the acceptor stream is shown in Fig. 2c.
The analytical signal tends to increase at higher pH due to HCN fixation in alkaline medium (pK(HCN) = 9.2).
However, complex formation (stable within pH 7.0–9.0) was hindered in more alkaline medium also due to cop-per hydrolysis.
The [Cu(BCA)2]3− complex solution in ammonium acetate pH 7.0 was then selected to avoid this drawback and min-imize interference from sulfide (pK2(H2S) = 13.8) [9]. With this acceptor solution, HCN fixation is mainly due to formation of highly stable complexes, such as [Cu(CN)2]−.