Modification of a continuous flow method for analysis of trace amounts of nitrate in iron-rich sediment pore-waters of mine pit lakes

Nitrate was analysed in pore-waters with high ionic strength. Extremely high concentrations of dissolved ferrous iron interfere common analytical methods. The automatic photometrical procedure based on the cadmium reduction method is often used for analysis of nitrate in water samples (continuous flow analysis CFA). An integrated dialysis usually serves for sample dilution and (or) sample purification (from particles). Iron was precipitated as iron hydroxide due to the imidazole buffer system (pH 7.5). The dialysis membrane is an effective barrier for iron hydroxide particles to prevent interferences within the cadmium column or the flow-cell. However, dialysis membrane is blocked successively after analysis of several iron-rich pore-water samples by agglomeration of precipitated iron. The blocking of nitrate diffusion through the dialysis membrane is tantamount to a decrease of analytical sensitivity to recognise by decreasing photometrical extinction. Minimising the iron deposition within the dialyser solved the problem. A simple modification of the CFA apparatus was found to keep the analytical sensitivity nearly constant: The mixing coil for the imidazole buffer was considerably elongated. Nearly all iron hydroxide was deposited at the glass coil surface installed before the dialyser. At least 50 iron-rich samples could be analysed within one sample queue with a loss of sensitivity <10%. The recovery of nitrate was about 95%, demonstrated by spiking experiments.