Quantification of free and weakly bound cyanide in water using infrared spectroscopy

We report a simple method for the detection of polyatomic anions in water. The approach involves precipitation of the polyatomic anion using simple monoatomic cations followed by collection of the particles on an infrared transparent membrane for quantification by infrared spectroscopy. The approach is demonstrated with the detection of free and weakly bound cyanide in water. The free CN^- is converted into AgCN precipitates by addition of Ag⁺ ions. An IR spectrum of the captured particles on a membrane is then recorded directly through the membrane in transmission mode. From the intensity of the CN stretching band at 2165 cm^-1 band, the concentration of the free cyanide in solution is determined. The detection range was 0.2-5.0 mgL^-1 cyanide with high linearity (R² = 0.998) and precision (%RSD for nine different concentrations was 22%). Key to our method is spiking the sample with a fixed amount of ¹³CN^- as this enabled collection of particles of sufficient size for capture on the membrane. It is shown that anions such as S^-2, OH^-, Cl^- do not interfere with the detection of the cyanide. Moreover, addition of S^2- to samples liberates weakly bound CN^- for detection by our method.