Inspection of Cyanide in Water Samples by ¹⁹F-NMR with Fluorinated Derivatization

An aliquot of 0.50 mL of 69.8 mg·L^-1 NaCN solution was taken and its acidity was adjusted to pH 12 with 0.25 mol·L^-1 NaOH solution. The solution was then evaporated to dryness under reduced pressure. The residue was taken up with 0.5 mL of DMSO, and then CN^- in the solution was derivatized with 10 μL of TEAP and 20 μL of acetic anhydride at room temperature for 45 min to form 1-cyano-2,2,2-trifluoro-1-phenylethyl acetate (CTPA). An aliquot of 0.30 mL of the solution was taken and used for ¹⁹F-NMR analysis under the specified instrumental condition after 0.2 mL of 7.78 mmol·L^-1 3-fluoro-benzyl acetate-CDCl₃ solution was added as internal standard. As shown experimentally, the detection limit of CN^- after derivatization was found 0.7 mg·L^-1. The method was found to be highly specific, as interferences of many co-existing anions, e.g., CO₃^2-, B₄O₇^2-, Cl^-, SO₄^2- and etc. were avoided. Three water samples of various origins (i.e., tap water, river water and pure water) were derivatized and determined by the present method, giving values of recovery of 74.0%, 74.0% and 75.0% respectively, showing no significant difference. Values of RSDs (n=6) found for the 3 samples were 2.1%, 2.4% and 3.0% respectively. The present method has been used satisfactorily for determination of CN^- in some water samples taken from the incidental site of the 8·12 explosion in Tianjin.