Development of an on-line procedure for the indirect determination of iodide by flow-injection cold-vapor atomic absorption spectrometry

A commercially available mercury-analyzer based on flow-injection cold-vapor atomic absorption spectrometry (FI–CVAAS) was modified so as to enable an indirect determination of iodide. The technique makes use of the interference caused by iodide in the determination of Hg by FI–CVAAS and allows it to obtain a calibration curve with a negative slope. An on-line technique was realized by providing an additional loop for a Hg solution introduced in the FI–CV system. The loop was provided by installing a further magnetic valve and by using the fourth channel of the peristaltic pump. Software methods for filling and operating the system were developed. The concentrations of the reducing agent SnCl2 and of the HNO3 were optimized with regard to the analytical figures of merit. A linear calibration curve (R2=0.9959) for 13 solutions with different concentrations of iodide between 0 and 60 ng/ml and a detection limit of 3.6 ng/ml were obtained. The analytical precision in terms of the R.S.D. is at the 1% level. The method developed could be successfully applied to the determination of iodide in a brine sample at the 2-ng/ml level. Efforts to improve the power of detection by using preconcentration by gold amalgamation at the optimum concentration of Hg were hampered by the higher standard deviations for the blank values which could not be compensated by the increase in sensitivity. This resulted in a detection limit of 9.6 ng/ml. The procedure with preconcentration was found to lead to the same results as the procedure without preconcentration for the same analysis brine sample.